23. TPO 1 Reading 1
Groundwater
Groundwater is the word used to describe water that saturates the ground, filling all the available spaces. By far the most abundant type of groundwater is meteoric water; this is the groundwater that circulates as part of the water cycle. Ordinary meteoric water is water that has soaked into the ground from the surface, from precipitation (rain and snow) and from lakes and streams. There it remains, sometimes for long periods, before emerging at the surface again. At first thought it seems incredible that there can be enough space in the “solid” ground underfoot to hold all this water.
The necessary space is there, however, in many forms. The commonest spaces are those among the particles—sand grains and tiny pebbles—of loose, unconsolidated sand and gravel. Beds of this material, out of sight beneath the soil, are common. They are found wherever fast rivers carrying loads of coarse sediment once flowed. For example, as the great ice sheets that covered North America during the last ice age steadily melted away, huge volumes of water flowed from them. The water was always laden with pebbles, gravel, and sand, known as glacial outwash, that was deposited as the flow slowed down.
The same thing happens to this day, though on a smaller scale, wherever a sediment-laden river or stream emerges from a mountain valley onto relatively flat land, dropping its load as the current slows: the water usually spreads out fanwise, depositing the sediment in the form of a smooth, fan-shaped slope. Sediments are also dropped where a river slows on entering a lake or the sea; the deposited sediments are on a lake floor or the seafloor at first, but will be located inland at some future date, when the sea level falls or the land rises; such beds are sometimes thousands of meters thick.
In lowland country almost any spot on the ground may overlie what was once the bed of a river that has since become buried by soil; if they are now below the water’s upper surface (the water table), the gravels and sands of the former riverbed, and its sandbars, will be saturated with groundwater.
So much for unconsolidated sediments. Consolidated (or cemented) sediments, too, contain millions of minute water-holding pores. This is because the gaps among the original grains are often not totally plugged with cementing chemicals; also, parts of the original grains may become dissolved by percolating groundwater, either while consolidation is taking place or at any time afterwards. The result is that sandstone, for example, can be as porous as the loose sand from which it was formed.
Thus a proportion of the total volume of any sediment, loose or cemented, consists of empty space. Most crystalline rocks are much more solid; a common exception is basalt, a form of solidified volcanic lava, which is sometimes full of tiny bubbles that make it very porous.
The proportion of empty space in a rock is known as its porosity. But note that porosity is not the same as permeability, which measures the ease with which water can flow through a material; this depends on the sizes of the individual cavities and the crevices linking them.
Much of the water in a sample of water-saturated sediment or rock will drain from it if the sample is put in a suitable dry place. But some will remain, clinging to all solid surfaces. It is held there by the force of surface tension without which water would drain instantly from any wet surface, leaving it totally dry. The total volume of water in the saturated sample must therefore be thought of as consisting of water that can, and water that cannot, in away.
The relative amount of these two kinds of water varies greatly from one kind of rock or sediment to another, even though their porosities may be the same. What happens depends on pore size. If the pores are large, the water in them will exist as drops too heavy for surface tension to hold, and it will drain away; but if the pores are small enough, the water in them will exist as thin films, too light to overcome the force of surface tension holding them in place: then the water will be firmly held.
22. TPO 3 Reading 2
Depletion of the Ogallala Aquifer
The vast grasslands of the High Plains in the central United States were settled by farmers and ranchers in the 1880’s. This region has a semiarid climate, and for 50 years after its settlement, it supported a low-intensity agricultural economy of cattle ranching and wheat farming. In the early twentieth century, however, it was discovered that much of the High Plains was underlain by a huge aquifer (a rock layer containing large quantities of groundwater). This aquifer was named the Ogallala aquifer after the Ogallala Sioux Indians, who once inhabited the region.
The Ogallala aquifer is a sandstone formation that underlies some 583,000 square kilometers of land extending from northwestern Texas to southern South Dakota. Water from rains and melting snows has been accumulating in the Ogallala for the past 30,000 years. Estimates indicate that the aquifer contains enough water to fill Lake Huron, but unfortunately, under the semiarid climatic conditions that presently exist in the region, rates of addition to the aquifer are minimal, amounting to about half a centimeter a year.
The first wells were drilled into the Ogallala during the drought years of the early 1930’s. The ensuing rapid expansion of irrigation agriculture, especially from the 1950’s onward, transformed the economy of the region. More than 100,000 wells now tap the Ogallala. Modern irrigation devices, each capable of spraying 4.5 million liters of water a day, have produced a landscape dominated by geometric patterns of circular green islands of crops. Ogallala water has enabled the High Plains region to supply significant amounts of the cotton, sorghum, wheat, and corn grown in the United States. In addition, 40 percent of American grain-fed beef cattle are fattened here.
This unprecedented development of a finite groundwater resource with an almost negligible natural recharge rate—that is, virtually no natural water source to replenish the water supply—has caused water tables in the region to fall drastically. In the 1930’s, wells encountered plentiful water at a depth of about 15 meters; currently, they must be dug to depths of 45 to 60 meters or more. In places, the water table is declining at a rate of a meter a year, necessitating the periodic deepening of wells and the use of ever-more-powerful pumps. It is estimated that at current withdrawal rates, much of the aquifer will run dry within 40 years. The situation is most critical in Texas, where the climate is driest, the greatest amount of water is being pumped, and the aquifer contains the least water. It is projected that the remaining Ogallala water will, by the year 2030, support only 35 to 40 percent of the irrigated acreage in Texas that it supported in 1980.
The reaction of farmers to the inevitable depletion of the Ogallala varies. Many have been attempting to conserve water by irrigating less frequently or by switching to crops that require less water. Others, however, have adopted the philosophy that it is best to use the water while it is still economically profitable to do so and to concentrate on high-value crops such as cotton. The incentive of the farmers who wish to conserve water is reduced by their knowledge that many of their neighbors are profiting by using great amounts of water, and in the process are drawing down the entire region’s water supplies.
In the face of the upcoming water supply crisis, a number of grandiose schemes have been developed to transport vast quantities of water by canal or pipeline from the Mississippi, the Missouri, or the Arkansas rivers. Unfortunately, the cost of water obtained through any of these schemes would increase pumping costs at least tenfold, making the cost of irrigated agricultural products from the region uncompetitive on the national and international markets. Somewhat more promising have been recent experiments for releasing capillary water (water in the soil) above the water table by injecting compressed air into the ground. Even if this process proves successful, however, it would almost triple water costs. Genetic engineering also may provide a partial solution, as new strains of drought-resistant crops continue to be developed. Whatever the final answer to the water crisis may be, it is evident that within the High Plains, irrigation water will never again be the abundant, inexpensive resource it was during the agricultural boom years of the mid-twentieth century.
21. TPO 5 Reading 3
The Cambrian Explosion
The geologic timescale is marked by significant geologic and biological events, including the origin of Earth about 4.6 billion years ago, the origin of life about 3.5 billion years ago, the origin of eukaryotic life-forms (living things that have cells with true nuclei) about 1.5 billion years ago, and the origin of animals about 0.6 billion years ago. The last event marks the beginning of the Cambrian period. Animals originated relatively late in the history of Earth—in only the last 10 percent of Earth’s history. During a geologically brief 100-million-year period, all modern animal groups (along with other animals that are now extinct) evolved. This rapid origin and diversification of animals is often referred to as “the Cambrian explosion.”
Scientists have asked important questions about this explosion for more than a century. Why did it occur so late in the history of Earth? The origin of multicellular forms of life seems a relatively simple step compared to the origin of life itself. Why does the fossil record not document the series of evolutionary changes during the evolution of animals? Why did animal life evolve so quickly? Paleontologists continue to search the fossil record for answers to these questions.
One interpretation regarding the absence of fossils during this important 100-million-year period is that early animals were soft bodied and simply did not fossilize. Fossilization of soft-bodied animals is less likely than fossilization of hard-bodied animals, but it does occur. Conditions that promote fossilization of soft-bodied animals include very rapid covering by sediments that create an environment that discourages decomposition. In fact, fossil beds containing soft-bodied animals have been known for many years.
The Ediacara fossil formation, which contains the oldest known animal fossils, consists exclusively of soft-bodied forms. Although named after a site in Australia, the Ediacara formation is worldwide in distribution and dates to Precambrian times. This 700-million-year-old formation gives few clues to the origins of modern animals, however, because paleontologists believe it represents an evolutionary experiment that failed. It contains no ancestors of modern animal groups.
A slightly younger fossil formation containing animal remains is the Tommotian formation, named after a locale in Russia. It dates to the very early Cambrian period, and it also contains only soft-bodied forms. At one time, the animals present in these fossil beds were assigned to various modern animal groups, but most paleontologists now agree that all Tommotian fossils represent unique body forms that arose in the early Cambrian period and disappeared before the end of the period, leaving no descendants in modern animal groups.
A third fossil formation containing both soft-bodied and hard-bodied animals provides evidence of the result of the Cambrian explosion. This fossil formation, called the Burgess Shale, is in Yoho National Park in the Canadian Rocky Mountains of British Columbia. Shortly after the Cambrian explosion, mud slides rapidly buried thousands of marine animals under conditions that favored fossilization. These fossil beds provide evidence of about 32 modern animal groups, plus about 20 other animal body forms that are so different from any modern animals that they cannot be assigned to any one of the modern groups. These unassignable animals include a large swimming predator called Anomalocaris and a soft-bodied animal called Wiwaxia, which ate detritus or algae. The Burgess Shale formation also has fossils of many extinct representatives of modern animal groups. For example, a well-known Burgess Shale animal called Sidneyia is a representative of a previously unknown group of arthropods (a category of animals that includes insects, spiders, mites, and crabs).
Fossil formations like the Burgess Shale show that evolution cannot always be thought of as a slow progression. The Cambrian explosion involved rapid evolutionary diversification, followed by the extinction of many unique animals. Why was this evolution so rapid? No one really knows. Many zoologists believe that it was because so many ecological niches were available with virtually no competition from existing species. Will zoologists ever know the evolutionary sequences in the Cambrian explosion? Perhaps another ancient fossil bed of soft-bodied animals from 600-million-year-old seas is awaiting discovery.
20. TPO 5 Reading 1
Minerals and Plants
Research has shown that certain minerals are required by plants for normal growth and development. The soil is the source of these minerals, which are absorbed by the plant with the water from the soil. Even nitrogen, which is a gas in its elemental state, is normally absorbed from the soil as nitrate ions. Some soils are notoriously deficient in micro nutrients and are therefore unable to support most plant life. So-called serpentine soils, for example, are deficient in calcium, and only plants able to tolerate low levels of this mineral can survive. In modern agriculture, mineral depletion of soils is a major concern, since harvesting crops interrupts the recycling of nutrients back to the soil.
Mineral deficiencies can often be detected by specific symptoms such as chlorosis (loss of chlorophyll resulting in yellow or white leaf tissue), necrosis (isolated dead patches), anthocyanin formation (development of deep red pigmentation of leaves or stem), stunted growth, and development of woody tissue in an herbaceous plant. Soils are most commonly deficient in nitrogen and phosphorus. Nitrogen-deficient plants exhibit many of the symptoms just described. Leaves develop chlorosis; stems are short and slender; and anthocyanin discoloration occurs on stems, petioles, and lower leaf surfaces. Phosphorus-deficient plants are often stunted, with leaves turning a characteristic dark green, often with the accumulation of anthocyanin. Typically, older leaves are affected first as the phosphorus is mobilized to young growing tissue. Iron deficiency is characterized by chlorosis between veins in young leaves.
Much of the research on nutrient deficiencies is based on growing plants hydroponically, that is, in soilless liquid nutrient solutions. This technique allows researchers to create solutions that selectively omit certain nutrients and then observe the resulting effects on the plants. Hydroponics has applications beyond basic research, since it facilitates the growing of greenhouse vegetables during winter. Aeroponics, a technique in which plants are suspended and the roots misted with a nutrient solution, is another method for growing plants without soil.
While mineral deficiencies can limit the growth of plants, an overabundance of certain minerals can be toxic and can also limit growth. Saline soils, which have high concentrations of sodium chloride and other salts, limit plant growth, and research continues to focus on developing salt-tolerant varieties of agricultural crops. Research has focused on the toxic effects of heavy metals such as lead, cadmium, mercury, and aluminum; however, even copper and zinc, which are essential elements, can become toxic in high concentrations. Although most plants cannot survive in these soils, certain plants have the ability to tolerate high levels of these minerals.
Scientists have known for some time that certain plants, called hyperaccumulators, can concentrate minerals at levels a hundredfold or greater than normal. A survey of known hyperaccumulators identified that 75 percent of them amassed nickel; cobalt, copper, zinc, manganese, lead, and cadmium are other minerals of choice. Hyperaccumulators run the entire range of the plant world. They may be herbs, shrubs, or trees. Many members of the mustard family, spurge family, legume family, and grass family are top hyperaccumulators. Many are found in tropical and subtropical areas of the world, where accumulation of high concentrations of metals may afford some protection against plant-eating insects microbial pathogens.
Only recently have investigators considered using these plants to clean up soil and waste sites that have been contaminated by toxic levels of heavy metals—an environmentally friendly approach known as phytoremediation. This scenario begins with the planting of hyperaccumulating species in the target area, such as an abandoned mine or an irrigation pond contaminated by runoff. Toxic minerals would first be absorbed by roots but later relocated to the stem and leaves. A harvest of the shoots would remove the toxic compounds off site to be burned or composted to recover the metal for industrial uses. After several years of cultivation and harvest, the site would be restored at a cost much lower than the price of excavation and reburial, the standard practice for remediation of contaminated soils. For example, in field trials, the plant alpine pennycress removed zinc and cadmium from soils near a zinc smelter, and Indian mustard, native to Pakistan and India, has been effective in reducing levels of selenium salts by 50 percent in contaminated soils.
19. TPO 7 Reading 1
The Geologic History of the Mediterranean
In 1970 geologists Kenneth J. Hsu and William B. F. Ryan were collecting research data while aboard the oceanographic research vessel Glomar Challenger. An objective of this particular cruise was to investigate the floor of the Mediterranean and to resolve questions about its geologic history. One question was related to evidence that the invertebrate fauna (animals without spines) of the Mediterranean had changed abruptly about 6 million years ago. Most of the older organisms were nearly wiped out, although a few hardy species survived. A few managed to migrate into the Atlantic. Somewhat later, the migrants returned, bringing new species with them. Why did the near extinction and migrations occur?
Another task for the Glomar Challenger’s scientists was to try to determine the origin of the domelike masses buried deep beneath the Mediterranean seafloor. These structures had been detected years earlier by echo-sounding instruments, but they had never been penetrated in the course of drilling. Were they salt domes such as are common along the United States Gulf Coast, and if so, why should there have been so much solid crystalline salt beneath the floor of the Mediterranean?
With questions such as these clearly before them, the scientists aboard the Glomar Challenger proceeded to the Mediterranean to search for the answers. On August 23, 1970, they recovered a sample. The sample consisted of pebbles of hardened sediment that had once been soft, deep-sea mud, as well as granules of gypsum and fragments of volcanic rock. Not a single pebble was found that might have indicated that the pebbles came from the nearby continent. In the days following, samples of solid gypsum were repeatedly brought on deck as drilling operations penetrated the seafloor. Furthermore, the gypsum was found to possess peculiarities of composition and structure that suggested it had formed on desert flats. Sediment above and below the gypsum layer contained tiny marine fossils, indicating open-ocean conditions. As they drilled into the central and deepest part of the Mediterranean basin, the scientists took solid, shiny, crystalline salt from the core barrel. Interbedded with the salt were thin layers of what appeared to be windblown silt.
The time had come to formulate a hypothesis. The investigators theorized that about 20 million years ago, the Mediterranean was a broad seaway linked to the Atlantic by two narrow straits. Crustal movements closed the straits, and the landlocked Mediterranean began to evaporate. Increasing salinity caused by the evaporation resulted in the extermination of scores of invertebrate species. Only a few organisms especially tolerant of very salty conditions remained. As evaporation continued, the remaining brine (salt water) became so dense that the calcium sulfate of the hard layer was precipitated. In the central deeper part of the basin, the last of the brine evaporated to precipitate more soluble sodium chloride (salt). Later, under the weight of overlying sediments, this salt flowed plastically upward to form salt domes. Before this happened, however, the Mediterranean was a vast desert 3,000 meters deep. Then, about 5.5 million years ago came the deluge. As a result of crustal adjustments and faulting, the Strait of Gibraltar, where the Mediterranean now connects to the Atlantic, opened, and water cascaded spectacularly back into the Mediterranean. Turbulent waters tore into the hardened salt flats, broke them up, and ground them into the pebbles observed in the first sample taken by the Challenger. As the basin was refilled, normal marine organisms returned. Soon layers of oceanic ooze began to accumulate above the old hard layer.
The salt and gypsum, the faunal changes, and the unusual gravel provided abundant evidence that the Mediterranean was once a desert.
gypsum: a mineral made of calcium sulfate and water
18. TPO 15 Reading 3
Glacier Formation
Glaciers are slowly moving masses of ice that have accumulated on land in areas where more snow falls during a year than melts. Snow falls as hexagonal crystals, but once on the ground, snow is soon transformed into a compacted mass of smaller, rounded grains. As the air space around them is lessened by compaction and melting, the grains become denser. With further melting, refreezing, and increased weight from newer snowfall above, the snow reaches a granular recrystallized stage intermediate between flakes and ice known as firn. With additional time, pressure, and refrozen meltwater from above, the small firn granules become larger, interlocked crystals of blue glacial ice. When the ice is thick enough, usually over 30 meters, the weight of the snow and firn will cause the ice crystals toward the bottom to become plastic and to flow outward or downward from the area of snow accumulation.
Glaciers are open systems, with snow as the system’s input and meltwater as the system’s main output. The glacial system is governed by two basic climatic variables: precipitation and temperature. For a glacier to grow or maintain its mass, there must be sufficient snowfall to match or exceed the annual loss through melting, evaporation, and calving, which occurs when the glacier loses solid chunks as icebergs to the sea or to large lakes. If summer temperatures are high for too long, then all the snowfall from the previous winter will melt. Surplus snowfall is essential for a glacier to develop. A surplus allows snow to accumulate and for the pressure of snow accumulated over the years to transform buried snow into glacial ice with a depth great enough for the ice to flow. Glaciers are sometimes classified by temperature as faster-flowing temperate glaciers or as slower-flowing polar glaciers.
Glaciers are part of Earth’s hydrologic cycle and are second only to the oceans in the total amount of water contained. About 2 percent of Earth’s water is currently frozen as ice. Two percent may be a deceiving figure, however, since over 80 percent of the world’s fresh water is locked up as ice in glaciers, with the majority of it in Antarctica. The total amount of ice is even more awesome if we estimate the water released upon the hypothetical melting of the world’s glaciers. Sea level would rise about 60 meters. This would change the geography of the planet considerably. In contrast, should another ice age occur, sea level would drop drastically. During the last ice age, sea level dropped about 120 meters.
When snow falls on high mountains or in polar regions, it may become part of the glacial system. Unlike rain, which returns rapidly to the sea or atmosphere, the snow that becomes part of a glacier is involved in a much more slowly cycling system. Here water may be stored in ice form for hundreds or even hundreds of thousands of years before being released again into the liquid water system as meltwater. In the meantime, however, this ice is not static. Glaciers move slowly across the land with tremendous energy, carving into even the hardest rock formations and thereby reshaping the landscape as they engulf, push, drag, and finally deposit rock debris in places far from its original location. As a result, glaciers create a great variety of landforms that remain long after the surface is released from its icy covering.
Throughout most of Earth’s history, glaciers did not exist, but at the present time about 10 percent of Earth’s land surface is covered by glaciers. Present-day glaciers are found in Antarctica, in Greenland, and at high elevations on all the continents except Australia. In the recent past, from about 2.4 million to about 10,000 years ago, nearly a third of Earth’s land area was periodically covered by ice thousands of meters thick. In the much more distant past, other ice ages have occurred.
17. TPO 19 Reading 3
Discovering the Ice Ages
In the middle of the nineteenth century, Louis Agassiz, one of the first scientists to study glaciers, immigrated to the United States from Switzerland and became a professor at Harvard University, where he continued his studies in geology and other sciences. For his research, Agassiz visited many places in the northern parts of Europe and North America, from the mountains of Scandinavia and New England to the rolling hills of the American Midwest. In all these diverse regions, Agassiz saw signs of glacial erosion and sedimentation. In flat plains country, he saw moraines (accumulations of earth and loose rock that form at the edges of glaciers) that reminded him of the terminal moraines found at the end of valley glaciers in the Alps. The heterogeneous material of the drift (sand, clay, and rocks deposited there) convinced him of its glacial origin.
The areas covered by this material were so vast that the ice that deposited it must have been a continental glacier larger than Greenland or Antarctica. Eventually, Agassiz and others convinced geologists and the general public that a great continental glaciation had extended the polar ice caps far into regions that now enjoy temperate climates. For the first time, people began to talk about ice ages. It was also apparent that the glaciation occurred in the relatively recent past because the drift was soft, like freshly deposited sediment. We now know the age of the glaciation accurately from radiometric dating of the carbon-14 in logs buried in the drift. The drift of the last glaciation was deposited during one of the most recent epochs of geologic time, the Pleistocene, which lasted from 2.6 million to 11,700 years ago. Along the east coast of the United States, the southernmost advance of this ice is recorded by the enormous sand and drift deposits of the terminal moraines that form Long Island and Cape Cod.
It soon became clear that there were multiple glacial ages during the Pleistocene, with warmer interglacial intervals between them. As geologists mapped glacial deposits in the late nineteenth century, they became aware that there were several layers of drift, the lower ones corresponding to earlier ice ages. Between the older layers of glacial material were well-developed soils containing fossils of warm-climate plants. These soils were evidence that the glaciers retreated as the climate warmed. By the early part of the twentieth century, scientists believed that four distinct glaciations had affected North America and Europe during the Pleistocene epoch.
This idea was modified in the late twentieth century, when geologists and oceanographers examining oceanic sediment found fossil evidence of warming and cooling of the oceans. Ocean sediments presented a much more complete geologic record of the Pleistocene than continental glacial deposits did. The fossils buried in Pleistocene and earlier ocean sediments were of foraminifera—small, single-celled marine organisms that secrete shells of calcium carbonate, or calcite. These shells differ in their proportion of ordinary oxygen (oxygen-16) and the heavy oxygen isotope (oxygen-18). The ratio of oxygen-16 to oxygen-18 found in the calcite of a foraminifer’s shell depends on the temperature of the water in which the organism lived. Different ratios in the shells preserved in various layers of sediment reveal the temperature changes in the oceans during the Pleistocene epoch.
Isotopic analysis of shells allowed geologists to measure another glacial effect. They could trace the growth and shrinkage of continental glaciers, even in parts of the ocean where there may have been no great change in temperature—around the equator, for example. The oxygen isotope ratio of the ocean changes as a great deal of water is withdrawn from it by evaporation and is precipitated as snow to form glacial ice. During glaciations, the lighter oxygen-16 has a greater tendency to evaporate from the ocean surface than the heavier oxygen-18 does. Thus, more of the heavy isotope is left behind in the ocean and absorbed by marine organisms. From this analysis of marine sediments, geologists have learned that there were many shorter, more regular cycles of glaciation and deglaciation than geologists had recognized from the glacial drift of the continents alone.
16. TPO 21 Reading 1
Geothermal Energy
Earth’s internal heat, fueled by radioactivity, provides the energy for plate tectonics and continental drift, mountain building, and earthquakes. It can also be harnessed to drive electric generators and heat homes. Geothermal energy becomes available in a practical form when underground heat is transferred by water that is heated as it passes through a subsurface region of hot rocks (a heat reservoir) that may be hundreds or thousands of feet deep. The water is usually naturally occurring groundwater that seeps down along fractures in the rock; less typically, the water is artificially introduced by being pumped down from the surface. The water is brought to the surface, as a liquid or steam, through holes drilled for the purpose.
By far the most abundant form of geothermal energy occurs at the relatively low temperatures of 80° to 180° centigrade. Water circulated through heat reservoirs in this temperature range is able to extract enough heat to warm residential, commercial, and industrial spaces. More than 20,000 apartments in France are now heated by warm underground water drawn from a heat reservoir in a geologic structure near Paris called the Paris Basin. Iceland sits on a volcanic structure known as the Mid-Atlantic Ridge. Reykjavik, the capital of Iceland, is entirely heated by geothermal energy derived from volcanic heat.
Geothermal reservoirs with temperatures above 180° centigrade are useful for generating electricity. They occur primarily in regions of recent volcanic activity as hot, dry rock; natural hot water; or natural steam. The latter two sources are limited to those few areas where surface water seeps down through underground faults or fractures to reach deep rocks heated by the recent activity of molten rock material. The world’s largest supply of natural steam occurs at The Geysers, 120 kilometers north of San Francisco, California. In the 1990s enough electricity to meet about half the needs of San Francisco was being generated there. This facility was then in its third decade of production and was beginning to show signs of decline, perhaps because of over development. By the late 1990s some 70 geothermal electric-generating plants were in operation in California, Utah, Nevada, and Hawaii, generating enough power to supply about a million people. Eighteen countries now generate electricity using geothermal heat.
Extracting heat from very hot, dry rocks presents a more difficult problem: the rocks must be fractured to permit the circulation of water, and the water must be provided artificially. The rocks are fractured by water pumped down at very high pressures. Experiments are under way to develop technologies for exploiting this resource.
Like most other energy sources, geothermal energy presents some environmental problems. The surface of the ground can sink if hot groundwater is withdrawn without being replaced. In addition, water heated geothermally can contain salts and toxic materials dissolved from the hot rock. These waters present a disposal problem if they are not returned to the ground from which they were removed.
The contribution of geothermal energy to the world’s energy future is difficult to estimate. Geothermal energy is in a sense not renewable, because in most cases the heat would be drawn out of a reservoir much more rapidly than it would be replaced by the very slow geological processes by which heat flows through solid rock into a heat reservoir. However, in many places (for example, California, Hawaii, the Philippines, Japan, Mexico, the rift valleys of Africa) the resource is potentially so large that its future will depend on the economics of production. At present, we can make efficient use of only naturally occurring hot water or steam deposits. Although the potential is enormous, it is likely that in the near future geothermal energy can make important local contributions only where the resource is close to the user and the economics are favorable, as they are in California, New Zealand, and Iceland. Geothermal energy probably will not make large-scale contributions to the world energy budget until well into the twenty-first century, if ever.
15. TPO 27 Reading 2
The Formation of Volcanic Islands
Earth’s surface is not made up of a single sheet of rock that forms a crust but rather a number of “tectonic plates” that fit closely, like the pieces of a giant jigsaw puzzle. Some plates carry islands or continents; others form the seafloor. All are slowly moving because the plates float on a denser semiliquid mantle, the layer between the crust and Earth’s core. The plates have edges that are spreading ridges (where two plates are moving apart and new seafloor is being created), subduction zones (where two plates collide and one plunges beneath the other), or transform faults (where two plates neither converge nor diverge but merely move past one another). It is at the boundaries between plates that most of Earth’s volcanism and earthquake activity occur.
Generally speaking, the interiors of plates are geologically uneventful. However, there are exceptions. A glance at a map of the Pacific Ocean reveals that there are many islands far out at sea that are actually volcanoes—many no longer active, some overgrown with coral—that originated from activity at points in the interior of the Pacific Plate that forms the Pacific seafloor.
How can volcanic activity occur so far from a plate boundary? The Hawaiian Islands provide a very instructive answer. Like many other island groups, they form a chain. The Hawaiian Island Chain extends northwest from the island of Hawaii. In the 1840s American geologist James Daly observed that the different Hawaiian Islands seem to share a similar geologic evolution but are progressively more eroded, and therefore probably older, toward the northwest. Then in 1963, in the early days of the development of the theory of plate tectonics, Canadian geophysicist Tuzo Wilson realized that this age progression could result if the islands were formed on a surface plate moving over a fixed volcanic source in the interior. Wilson suggested that the long chain of volcanoes stretching northwest from Hawaii is simply the surface expression of a long-lived volcanic source located beneath the tectonic plate in the mantle. Today’s most northwestern island would have been the first to form. Then, as the plate moved slowly northwest, new volcanic islands would have formed as the plate moved over the volcanic source. The most recent island, Hawaii, would be at the end of the chain and is now over the volcanic source.
Although this idea was not immediately accepted, the dating of lavas in the Hawaiian (and other) chains showed that their ages increase away from the presently active volcano, just as Daly had suggested. Wilson’s analysis of these data is now a central part of plate tectonics. Most volcanoes that occur in the interiors of plates are believed to be produced by mantle plumes, columns of molten rock that rise from deep within the mantle. A volcano remains an active “hot spot” as long as it is over the plume. The plumes apparently originate at great depths, perhaps as deep as the boundary between the core and the mantle, and many have been active for a very long time. The oldest volcanoes in the Hawaiian hot-spot trail have ages close to 80 million years. Other islands, including Tahiti and Easter Island in the Pacific, Reunion and Mauritius in the Indian Ocean, and indeed most of the large islands in the world’s oceans, owe their existence to mantle plumes.
The oceanic volcanic islands and their hot-spot trails are thus especially useful for geologists because they record the past locations of the plate over a fixed source. They therefore permit the reconstruction of the process of seafloor spreading, and consequently of the geography of continents and of ocean basins in the past. For example, given the current position of the Pacific Plate, Hawaii is above the Pacific Ocean hot spot. So the position of the Pacific Plate 50 million years ago can be determined by moving it such that a 50-million-year-old volcano in the hot-spot trail sits at the location of Hawaii today. However, because the ocean basins really are short-lived features on geologic time scales, reconstructing the world’s geography by backtracking along the hot-spot trail works only for the last 5 percent or so of geologic time.
14. TPO 28 Reading 1
Groundwater
Most of the world’s potable water—freshwater suitable for drinking—is accounted for by groundwater, which is stored in the pores and fractures in rocks. There is more than 50 times as much freshwater stored underground than in all the freshwater rivers and lakes at the surface. Nearly 50 percent of all groundwater is stored in the upper 1,000 meters of Earth. At greater depths within Earth, the pressure of the overlying rock causes pores and cracks to close, reducing the space that pore water can occupy, and almost complete closure occurs at a depth of about 10 kilometers. The greatest water storage, therefore, lies near the surface.
Aquifers, Porosity, and Permeability
Groundwater is stored in a variety of rock types. A groundwater reservoir from which water can be extracted is called an aquifer. We can effectively think of an aquifer as a deposit of water. Extraction of water depends on two properties of the aquifer: porosity and permeability. Between sediment materials (such as sand or small rocks) that are deposited by water, wind, or glacial ice grains are spaces that can be filled with water. This pore space is known as porosity and is expressed as a percentage of the total rock volume. Porosity is important for water-storage capacity, but for water to flow through rocks, the pore spaces must be connected. The ability of water, or other fluids, to flow through the interconnected pore spaces in rocks is termed permeability. Fractures and joints have very high permeability. In the intergranular spaces of rocks, however, fluid must flow around and between grains in a tortuous path; this winding path causes a resistance to flow. The rate at which the flowing water overcomes this resistance is related to the permeability of rock.
Sediment sorting and compaction influence permeability and porosity. The more poorly sorted or the more tightly compacted a sediment is, the lower its porosity and permeability. Sedimentary rocks—the most common rock type near the surface—are also the most common reservoirs for water because they contain the most space that can be filled with water. Sandstones generally make good aquifers, while finer-grained mudstones are typically impermeable. Impermeable rocks are referred to as aquicludes. Igneous and metamorphic rocks are more compact, commonly crystalline, and rarely contain spaces between grains. However, even igneous and metamorphic rocks may act as groundwater reservoirs if extensive fracturing occurs in such rocks and if the fracture system is interconnected.
The Water Table
The water table is the underground boundary below which all the cracks and pores are filled with water. In some cases, the water table reaches Earth’s surface, where it is expressed as rivers, lakes, and marshes. Typically, though, the water table may be tens or hundreds of meters below the surface. The water table is not flat but usually follows the contours of the topographythe shape of a surface such as Earth’s, including the rise and fall of such features as mountains and valleys . Above the water table is the vadose zone, through which rainwater percolates. Water in the vadose zone drains down to the water table, leaving behind a thin coating of water on mineral grains. The vadose zone supplies plant roots near the surface with water.
Because the surface of the water table is not flat but instead rises and falls with topography, groundwater is affected by gravity in the same fashion as surface water. Groundwater flows downhill to topographic lows. If the water table intersects the land surface, groundwater will flow out onto the surface at springs, either to be collected there or to subsequently flow farther along a drainage. Groundwater commonly collects in stream drainages but may remain entirely beneath the surface of dry stream-beds in arid regions. In particularly wet years, short stretches of an otherwise dry stream-bed may have flowing water because the water table rises to intersect the land surface.
[Glossary]
Sediment: materials (such as sand or small rocks) that are deposited by water, wind, or glacial ice.
Topography: the shape of a surface such as Earth’s, including the rise and fall of such features as mountains and valleys.
13. TPO 31 Reading 1
Speciation in Geographically Isolated Populations
Evolutionary biologists believe that speciation, the formation of a new species, often begins when some kind of physical barrier arises and divides a population of a single species into separate subpopulations. Physical separation between subpopulations promotes the formation of new species because once the members of one subpopulation can no longer mate with members of another subpopulation, they cannot exchange variant genes that arise in one of the subpopulations. In the absences of gene flow between the subpopulations, genetic differences between the groups begin to accumulate. Eventually the subpopulations become so genetically distinct that they cannot interbreed even if the physical barriers between them were removed. At this point the subpopulations have evolved into distinct species. This route to speciation is known as allopatry ("alio" means "different", and "patria" means "homeland").
Allopatric speciation may be the main speciation route. This should not be surprising, since allopatry is pretty common. In general, the subpopulations of most species are separated from each other by some measurable distance. So even under normal situations the gene flow among the subpopulations is more of an intermittent trickle than a steady stream. In addition, barriers can rapidly arise and shut off the trickle. For example, in the 1800s a monstrous earthquake changed the course of the Mississippi River, a large river flowing in the central part of the United States of America. The change separated populations of insects now living along opposite shore, completely cutting off gene flow between them.
Geographic isolation also can proceed slowly, over great spans of time. We find evidence of such extended events in the fossil record, which affords glimpses into the breakup of formerly continuous environments. For example, during past ice ages, glaciers advanced down through North America and Europe and gradually cut off parts of populations from one another. When the glacier retreated, the separated populations of plants and animals came into contact again. Some groups that had descended from the same parent population were no longer reproductively compatible - they had evolved into separate species. In other groups, however, genetic divergences had not proceeded so far, and the descendants could still interbreed - for them, reproductive isolation was not completed, and so speciation had not occurred.
Allopatric speciation can also be brought by the imperceptibly slow but colossal movements of the tectonic plates that make up Earth's surface. About 5 million years ago such geologic movements created the land bridge between North America and South America that we call the Isthmus of Panama. While previously the gap between the continents had allowed a free flow of water, now the isthmus presented a barrier that divided the Atlantic Ocean from the Pacific Ocean. This division set the stage for allopatric speciation among populations of fishes and other marine species.
In the 1980s, John Graves studied two populations of closely related fishes, one population from the Atlantic side of isthmus, the other from the Pacific side. He compared four enzymes found in the muscles of each population. Graves found that all four Pacific enzymes function better at lower temperatures than the four Atlantic versions of the same enzymes. This is significant because Pacific seawater is typically 2 to 3 degrees cooler than seawater on the Atlantic side of isthmus. Analysis by gel electrophoresis revealed slight differences in amino acid sequence of the enzymes of two of the four pairs. This is significant because the amino acid sequence of an enzyme is determined by genes.
Graves drew two conclusions from these observations. First, at least some of the observed differences between the enzymes of the Atlantic and Pacific fish populations were not random but were the result of evolutionary adaption. Second, it appears that closely related populations of fishes on both sides of the isthmus are starting to genetically diverge from each other. Because Graves's study of geographically isolated populations of isthmus fishes offers a glimpse of the beginning of a process of gradual accumulation of mutations that are neutral or adaptive, divergences here might be evidence of allopatric speciation in process.
12. TPO 35 Reading 1
Earth's Age
One of the first recorded observers to surmise a long age for Earth was the Greek historian Herodotus, who lived from approximately 480 B.C. to 425 B.C. He observed that the Nile River Delta was in fact a series of sediment deposits built up in successive floods. By noting that individual floods deposit only thin layers of sediment, he was able to conclude that the Nile Delta had taken many thousands of years to build up. More important than the amount of time Herodotus computed, which turns out to be trivial compared with the age of Earth, was the notion that one could estimate ages of geologic features by determining rates of the processes responsible for such features, and then assuming the rates to be roughly constant over time. Similar applications of this concept were to be used again and again in later centuries to estimate the ages of rock formations and, in particular, of layers of sediment that had compacted and cemented to form sedimentary rocks.
It was not until the seventeenth century that attempts were made again to understand clues to Earth’s history through the rock record. Nicolaus Steno (1638–1686) was the first to work out principles of the progressive depositing of sediment in Tuscany. However, James Hutton (1726–1797), known as the founder of modern geology, was the first to have the important insight that geologic processes are cyclic in nature. Forces associated with subterranean heat cause land to be uplifted into plateaus and mountain ranges. The effects of wind and water then break down the masses of uplifted rock, producing sediment that is transported by water downward to ultimately form layers in lakes, seashores, or even oceans. Over time, the layers become sedimentary rock. These rocks are then uplifted sometime in the future to form new mountain ranges, which exhibit the sedimentary layers (and the remains of life within those layers) of the earlier episodes of erosion and deposition.
Hutton’s concept represented a remarkable insight because it unified many individual phenomena and observations into a conceptual picture of Earth’s history. With the further assumption that these geologic processes were generally no more or less vigorous than they are today, Hutton’s examination of sedimentary layers led him to realize that Earth’s history must be enormous, that geologic time is an abyss and human history a speck by comparison.
After Hutton, geologists tried to determine rates of sedimentation so as to estimate the age of Earth from the total length of the sedimentary, or stratigraphic, record. Typical numbers produced at the turn of the twentieth century were 100 million to 400 million years. These underestimated the actual age by factors of 10 to 50 because much of the sedimentary record is missing in various locations and because there is a long rock sequence that is older than half a billion years that is far less well defined in terms of fossils and less well preserved.
Various other techniques to estimate Earth’s age fell short, and particularly noteworthy in this regard were flawed determinations of the Sun’s age. It had been recognized by the German philosopher Immanuel Kant (1724–1804) that chemical reactions could not supply the tremendous amount of energy flowing from the Sun for more than about a millennium. Two physicists during the nineteenth century both came up with ages for the Sun based on the Sun’s energy coming from gravitational contraction.Under the force of gravity, the compression resulting from a collapse of the object must release energy. Ages for Earth were derived that were in the tens of millions of years, much less than the geologic estimates of the time.
It was the discovery of radioactivity at the end of the nineteenth century that opened the door to determining both the Sun’s energy source and the age of Earth. From the initial work came a suite of discoveries leading to radioisotopic dating, which quickly led to the realization that Earth must be billions of years old, and to the discovery of nuclear fusion as an energy source capable of sustaining the Sun’s luminosity for that amount of time. By the 1960s, both analysis of meteorites and refinements of solar evolution models converged on an age for the solar system, and hence for Earth, of 4.5 billion years.
11. TPO 36 Reading 1
Soil Formation
Living organisms play an essential role in soil formation. The numerous plants and animals living in the soil release minerals from the parent material from which soil is formed, supply organic matter, aid in the translocation (movement) and aeration of the soil, and help protect the soil from erosion. The types of organisms growing or living in the soil greatly influence the soil’s physical and chemical characteristics. In fact, for mature soils in many parts of the world, the predominant type of natural vegetation is considered the most important direct influence on soil characteristics. For this reason, a soil scientist can tell a great deal about the attributes of the soil in any given area simply from knowing what kind of flora the soil supports. Thus prairies and tundra regions, which have characteristic vegetations, also have characteristic soils.
The quantity and total weight of soil flora generally exceed that of soil fauna. By far the most numerous and smallest of the plants living in soil are bacteria. Under favorable conditions, a million or more of these tiny, single-celled plants can inhabit each cubic centimeter of soil. It is the bacteria, more than any other organisms, that enable rock or other parent material to undergo the gradual transformation to soil. Some bacteria produce organic acids that directly attack parent material, breaking it down and releasing plant nutrients. Others decompose organic litter (debris) to form humus (nutrient-rich organic matter). A third group of bacteria inhabits the root systems of plants called legumes. These include many important agricultural crops, such as alfalfa, clover, soybeans, peas, and peanuts. The bacteria that legumes host within their root nodules (small swellings on the root) change nitrogen gas from the atmosphere into nitrogen compounds that plants are able to metabolize, a process, known as nitrogen fixation, that makes the soil more fertile. Other microscopic plants also are important in soil development. For example, in highly acidic soils where few bacteria can survive, fungi frequently become the chief decomposers of organic matter.
More complex forms of vegetation play several vital roles with respect to the soil. Trees, grass, and other large plants supply the bulk of the soil’s humus. The minerals released as these plants decompose on the surface constitute an important nutrient source for succeeding generations of plants as well as for other soil organisms. In addition, trees can extend their roots deep within the soil and bring up nutrients from far below the surface. These nutrients eventually enrich the surface soil when the tree drops its leaves or when it dies and decomposes. Finally, trees perform the vital function of slowing water runoff and holding the soil in place with their root systems, thus combating erosion. The increased erosion that often accompanies agricultural use of sloping land is principally caused by the removal of its protective cover of natural vegetation.
Animals also influence soil composition. The faunal counterparts of bacteria are protozoa. These single-celled organisms are the most numerous representatives of the animal kingdom, and, like bacteria, a million or more can sometimes inhabit each cubic centimeter of soil. Protozoa feed on organic matter and hasten its decomposition. Among other soil-dwelling animals, the earthworm is probably the most important. Under exceptionally favorable conditions, up to a million earthworms (with a total body weight exceeding 450 kilograms) may inhabit an acre of soil. Earthworms ingest large quantities of soil, chemically alter it, and excrete it as organic matter called casts. The casts form a high-quality natural fertilizer. In addition, earthworms mix soil both vertically and horizontally, improving aeration and drainage.
Insects such as ants and termites also can be exceedingly numerous under favorable climatic and soil conditions. In addition, mammals such as moles, field mice, gophers, and prairie dogs sometimes are present in sufficient numbers to have significant impact on the soil. These animals primarily work the soil mechanically. As a result, the soil is aerated, broken up, fertilized, and brought to the surface, hastening soil development.
10. TPO 43 Reading 2
The Origin of Petroleum
Petroleum is defined as a gaseous, liquid, and semisolid naturally occurring substance that consists chiefly of hydrocarbons (chemical compounds of carbon and hydrogen). Petroleum is therefore a term that includes both oil and natural gas. Petroleum is nearly always found in marine sedimentary rocks. In the ocean, microscopic phytoplankton (tiny floating plants) and bacteria (simple, single-celled organisms) are the principal sources of organic matter that is trapped and buried in sediment. Most of the organic matter is buried in clay that is slowly converted to a fine-grained sedimentary rock known as shale. During this conversion, organic compounds are transformed to oil and natural gas.
Sampling on the continental shelves and along the base of the continental slopes has shown that fine muds beneath the seafloor contain up to 8 percent organic matter. Two additional kinds of evidence support the hypothesis that petroleum is a product of the decomposition of organic matter: oil possesses optical properties known only in hydrocarbons derived from organic matter, and oil contains nitrogen and certain compounds believed to originate only in living matter. A complex sequence of chemical reactions is involved in converting the original solid organic matter to oil and gas, and additional chemical changes may occur in the oil and gas even after they have formed.
It is now well established that petroleum migrates through aquifers and can become trapped in reservoirs. Petroleum migration is analogous to groundwater migration. When oil and gas are squeezed out of the shale in which they originated and enter a body of sandstone or limestone somewhere above, they migrate readily because sandstones (consisting of quartz grains) and limestones (consisting of carbonate minerals) are much more permeable than any shale. The force of molecular attraction between oil and quartz or carbonate minerals is weaker than that between water and quartz or carbonate minerals. Hence, because oil and water do not mix, water remains fastened to the quartz or carbonate grains, while oil occupies the central parts of the larger openings in the porous sandstone or limestone. Because oil is lighter than water, it tends to glide upward past the carbonate- and quartz-held water. In this way, oil becomes segregated from the water; when it encounters a trap, it can form a pool.
Most of the petroleum that forms in sediments does not find a suitable trap and eventually makes its way, along with groundwater, to the surface of the sea. It is estimated that no more than 0.1 percent of all the organic matter originally buried in a sediment is eventually trapped in an oil pool. It is not surprising, therefore, that the highest ratio of oil and gas pools to volume of sediment is found in rock no older than 2.5 million years—young enough so that little of the petroleum has leaked away—and that nearly 60 percent of all oil and gas discovered so far has been found in strata that formed in the last 65 million years. This does not mean that older rocks produced less petroleum; it simply means that oil in older rocks has had a longer time in which to leak away.
How much oil is there in the world? This is an extremely controversial question. Many billions of barrels of oil have already been pumped out of the ground. A lot of additional oil has been located by drilling but is still waiting to be pumped out. Possibly a great deal more oil remains to be found by drilling. Unlike coal, the volume of which can be accurately estimated, the volume of undiscovered oil can only be guessed at. Guesses involve the use of accumulated experience from a century of drilling. Knowing how much oil has been found in an intensively drilled area, such as eastern Texas, experts make estimates of probable volumes in other regions where rock types and structures are similar to those in eastern Texas. Using this approach and considering all the sedimentary basins of the world, experts estimate that somewhere between 1,500 and 3,000 billion barrels of oil will eventually be discovered.
9. TPO 51 Reading 2
Surface Fluids on Venus and Earth
A fluid is a substance, such as a liquid or gas, in which the component particles (usually molecules) can move past one another. Fluids flow easily and conform to the shape of their containers. The geologic processes related to the movement of fluids on a planet's surface can completely resurface a planet many times. These processes derive their energy from the Sun and the gravitational forces of the planet itself. As these fluids interact with surface materials, they move particles about or react chemically with them to modify or produce materials. On a solid planet with a hydrosphere and an atmosphere, only a tiny fraction of the planetary mass flows as surface fluids. Yet the movements of these fluids can drastically alter a planet. Consider Venus and Earth, both terrestrial planets with atmospheres.
Venus and Earth are commonly regarded as twin planets but not identical twins. They are about the same size, are composed of roughly the same mix of materials, and may have been comparably endowed at their beginning with carbon dioxide and water. However, the twins evolved differently largely because of differences in their distance from the Sun. With a significant amount of internal heat, Venus may continue to be geologically active with volcanoes, rifting, and folding. However, it lacks any sign of a hydrologic system (water circulation and distribution): there are no streams, lakes oceans or glaciers. Space probes suggest that Venus may have started with as much water as Earth, but it was unable to keep its water in liquid form. Because Venus receives more heat from the Sun, water released from the interior evaporated and rose to the upper atmosphere where the Sun's ultraviolet rays broke the molecules apart. Much of the freed hydrogen escaped into space, and Venus lost its water. Without water, Venus became less and less like Earth and kept an atmosphere filled with carbon dioxide. The carbon dioxide acts as a blanket, creating an intense greenhouse effect and driving surface temperatures high enough to melt lead and to prohibit the formation of carbonate minerals. Volcanoes continually vented more carbon dioxide into the atmosphere. On Earth, liquid water removes carbon dioxide from the atmosphere and combines it with calcium, from rock weathering, to form carbonate sedimentary rocks. Without liquid water to remove carbon from the atmosphere, the level of carbon dioxide in the atmosphere of Venus remains high.
Like Venus, Earth is large enough to be geologically active and for its gravitational field to hold an atmosphere. Unlike Venus, it is just the right distance from the Sun so that temperature ranges allow water to exist as a liquid, a solid, and a gas. Water is thus extremely mobile and moves rapidly over the planet in a continuous hydrologic cycle. Heated by the Sun, the water moves in great cycles from the oceans to the atmosphere, over the landscape in river systems, and ultimately back to the oceans. As a result, Earth's surface has been continually changed and eroded into delicate systems of river valleys - a remarkable contrast to the surfaces of other planetary bodies where impact craters dominate. Few areas on Earth have been untouched by flowing water. As a result, river valleys are the dominant feature of its landscape. Similarly, wind action has scoured fine particles away from large areas, depositing them elsewhere as vast sand seas dominated by dunes or in sheets of loess (fine-grained soil deposits). These fluid movements are caused by gravity flow systems energized by heat from the Sun. Other geologic changes occur when the gases in the atmosphere or water react with rocks at the surface to form new chemical compounds with different properties. An important example of this process was the removal of most of Earths carbon dioxide from its atmosphere to form carbonate rocks. However, if Earth were a little closer to the Sun, its oceans would evaporate; if it were farther from the Sun, the oceans would freeze solid. Because liquid water was present, self-replicating molecules of carbon, hydrogen, and oxygen developed life early in Earth's history and have radically modified its surface, blanketing huge parts of the continents with greenery. Life thrives on this planet, and it helped create the planet's oxygen- and nitrogen-rich atmosphere and moderate temperatures.
8. TPO 56 Reading 1
Alfred Wegener’s Theory of Continental Drift
In 1912, the German geologist Alfred Wegener proposed that Earth's continents are mobile rafts of lighter crust that have shifted over time by plowing their way through the denser crust of the oceans. The theory, called continental drift, was partly motivated by the apparent fit, like puzzle pieces, of the coastlines of South America and Africa. Wegener first presented his theory of continental drift at a meeting and in a paper, and then as a book. The Origin of Continents and Oceans, published in 1915. He continued to write updated versions of this work until his death in an ill-fated expedition to Greenland in 1930. Wegener maintained that Earth is composed of concentric shells of increasing density from crust to core. The outermost shell is not continuous but made of continental blocks of lighter rock called sial (an acronym for silica- and aluminum-rich rock) floating in the denser sima (silica- and magnesia-rich rock) underlying the oceans. All the continents had been joined in the supercontinent of Pangaea. As the continent broke up, the pieces moving apart left bits behind, explaining the presence of nonvolcanic islands and island chains, according to Wegener. Where the moving pieces collided, mountains formed. They were thrust up either by the plowing of the continents through the sima, as in the case of the Andes, or by the colliding of two blocks of sial, as in the case of the Himalayas. As for the force driving continental drift, Wegener initially invoked Polfluchtkraft-a force causing flight from the poles as a result of Earth’s rotation-and later the tidal force resulting from the gravitational attraction between Earth and the Moon and Sun.
One of the most influential geologists to join the mobilist camp, as the drifters school became known, was Arthur Holmes. Holmes recognized the importance of radioactive heat-which had recently been discovered-and realized that there must be a mechanism to remove it from Earth’s interior. That mechanism, he argued, is convection-the rising of less dense material and the sinking of more dense material. He went on to propose that the mantle (the part of Earth’s interior below the outer crust and above the core) convects in large, circulating patterns, and that this motion carries the continents across Earth’s surface. He also related crustal movement and mantle convection to the evolution of mountain belts. Wegener adopted Holmes's mechanism in the last rendition of his theory Holmes, for his part, presented his grand concept of a dynamic Earth in his influential and popular text Principles of Physical Geology, published in 1944.
Although it was eventually supplanted by the theory of plate tectonics, Wegener’s theory of continental drift influenced science because it explained disparate observations, because it was placed in the context of existing theories, and because it offered a coherent view of Earth's evolution. For example, Wegener showed not only that the coastlines on opposite sides of the Atlantic fit together, but that geologic features on the different continents fit as well. He asserted that the Appalachians, which can be traced northward through the Canadian Maritime Provinces, match the Caledonian Mountains in Scotland and Norway. He marshaled evidence from the distributions of fossil and living species to argue that land bridges joining continents were less likely than a single continent. The example commonly cited is that of Mesosaurus, a shoreline scavenger reptile that lived in the Permian period and is found as fossils in rocks on both sides of the South Atlantic Ocean. Mesosaurus was thought not to be a great swimmer, certainly not able to cross an ocean.
Wegener also found supporting evidence in ancient climates. Mounting observations indicated that the past climate of many regions was much different from the present climate. In the tropics, geologists had found sand and gravel left by ancient glaciers, and in rainy regions they had located prehistoric deserts. Then there was the discovery, by a British expedition, of plant fossils only 600 kilometers (370 miles) from the South Pole. Particularly puzzling was evidence suggesting that widely different climates in different regions had occurred at the same time, so one could not account for different climates by claiming simply that the whole of Earth was once hotter or once cooler than now. Wegener solved this dilemma by showing that observations of paleoclimate could be explained if the positions of the continents had shifted relative to those of the poles.
7. TPO 60 Reading 3
Earth’s Core
Knowledge of Earth’s deep interior is derived from the study of the waves produced by earthquakes, called seismic waves Among the various kinds of seismic waves are primary waves (P-waves) and secondary waves (S-waves). Primary and secondary waves pass deep within Earth and therefore are the most instructive. Study of abrupt changes in the characteristics of seismic waves at different depths provides the basis for a threefold division of Earth into a central core; a thick, overlying mantle: and a thin, enveloping crust. Sudden changes in seismic wave velocities and angles of transmission are termed discontinuities.
One of the discontinuities is the Gutenberg discontinuity, which is located nearly halfway to the center of Earth at a depth of 2,900 kilometers and marks the outer boundary of Earth's core. At that depth, the S-waves cannot propagate, while at the same time P-wave velocity is drastically reduced S-waves are unable to travel through fluids. Thus, if S-waves were to encounter a fluidlike region of Earth’s interior, they would be absorbed there and would not be able to continue. Geophysicists believe this is what happens to S-waves as they enter the outer core. As a result, the S-waves generated on one side of Earth fail to appear at seismograph stations on the opposite side of Earth, and this observation is the principal evidence of an outer core that behaves as a fluid. Unlike S-waves, P-waves are able to pass through liquids. They are, however, abruptly slowed and sharply refracted (bent) as they enter a fluid medium. Therefore, as P-waves encounter the molten outer core of Earth, their velocity is reduced and they are refracted downward
The radius of the core is about 3,500 kilometers. The inner core is solid and has a radius of about 1,220 kilometers, which makes this inner core slightly larger than the Moon. Most geologists believe that the inner core has the same composition as the outer core and that it can only exist as a solid because of the enormous pressure at the center of Earth. Evidence of the existence of a solid inner core is derived from the study of hundreds of records of seismic waves produced over several years. These studies showed that the inner core behaves seismically as if it were a solid.
Earth has an overall density of 5.5 grams per cubic centimeter, yet the average density of rocks at the surface is less than 3.0 grams per cubic centimeter. This difference indicates that materials of high density must exist in the deep interior of the planet to achieve the 5.5 grams per cubic centimeter overall density. Under the extreme pressure conditions that exist in the region of the core, iron mixed with nickel would very likely have the required high density. Laboratory experiments, however, suggest that a highly pressurized iron-nickel alloy might be too dense and that minor amounts of such elements such as silicon, sulfur, carbon, or oxygen may also be present to lighten the core material.
Support for the theory that the core is composed of iron (85 percent) with lesser amounts of nickel has come from the study of meteorites. Many of these samples of solar system materials are iron meteorites that consist of metallic iron alloyed with a small percentage of nickel. Some geologists suspect that iron meteorites may be fragments from the core of a shattered planet. The presence of iron meteorites in our solar system suggests that the existence of an iron-nickel core for Earth is plausible.
There is further evidence that Earth may have a metallic core. Anyone who understands the functioning of a compass is aware that Earth has a magnetic field. The planet itself behaves as if there was a great bar magnet embedded within it. A magnetic field is developed by the flow of electric charges and requires good electrical conductors. Silicate rocks, such as those in the mantle and crust, do not conduct electricity very well, whereas metals such as iron and nickel are good conductors. Heat-driven movements in the outer core, coupled with movements induced by Earth’s spin, are thought to provide the necessary flow of electrons (very small particles that carry a negative charge) around the inner core that produces the magnetic field. Without a metallic core, Earth’s magnetic field would not be possible.
6. TPO 61 Reading 2
Conditions on Earth When Life Began
In the 1920s, Aleksandr Oparin, a Russian biochemist, proposed and developed the idea that life originated in the warm, watery environment of early Earth’s surface, under an atmosphere mostly composed of methane. The early seas were believed by Oparin to be rich in simple organic molecules, which reacted to form more complex molecules, eventually leading to proteins and life. Then, almost 30 years after Oparin published his ideas, Stanley Miller demonstrated that amino acids, the building blocks of the proteins necessary for life, could form under conditions thought to prevail on early Earth. Miller’s experiment was elegant. He passed electric discharges through a mixture of methane, hydrogen, ammonia, and steam, and when he analyzed the results, found that he had made amino acids. The discharges were a proxy for lightning, the gas mixture an educated guess about what the early atmosphere may have been like. Amino acids cannot replicate themselves, and are not themselves alive. Nevertheless, this experiment has long been recognized as a landmark for understanding a process that must have been one of the important steps in the evolution of life on Earth, the natural synthesis of amino acids. However, it now seems likely that Miller’s experiments may not be directly applicable to the events of the early Archean (that is, early in the geologic eon that lasted from Earth’s formation until about 2.5 billion years ago).
One of the problems hindering understanding of the origin of life is that environmental conditions on early Earth are not known with any certainty. It is possible to make only reasoned estimates. For example, for some fairly long period of time after formation, perhaps as much as several hundred million years, the surface must have been much hotter than it is today. Continued impacts of meteorites, large and small, would have added further heat energy, and in the earliest part of Earth history the larger impacting bodies may have broken through the cooling crust to expose underlying molten material. Large quantities of volcanic gases would have been released into the atmosphere as lavas erupted onto the surface, producing a greenhouse effect far more severe than anything likely to result from human activity. It is quite possible that the early atmosphere was many times as dense as today’s, and that the seas and oceans were hot. Some have even suggested that because of the high atmospheric pressure, the oceans could have been hotter than the boiling point of water today. However, life as we know it is quite sensitive to temperature, and no modern organisms are known to survive much above 100°C It is unlikely that life became established until surface temperatures had decreased to this level, or lower.
Although we do not know the precise composition of the early atmosphere, there has been enough progress made on this subject in recent years that it is possible to say with some certainty that the methane-rich composition envisioned by Oparin, and the methane-ammonia-hydrogen mixture used by Miller in his experiments, are probably not very realistic. Based on studies of our closest neighbor planets, Mars and Venus, and also considering evidence from Earth’s sedimentary rocks, it seems probable that Earth’s early atmosphere was rich in carbon dioxide rather than methane. On both Mars and Venus, carbon dioxide is by far the most abundant gas in the atmosphere. On Earth it is a minor constituent. But there is an enormous amount of this compound buried in the sedimentary rocks of Earth’s crust, enough so that, if it were all released, our atmosphere would be much more like those of our neighboring planets. How did carbon dioxide gas end up in the crust? The answer lies in what geologists refer to as the carbon cycle. Through a series of chemical reactions, carbon dioxide from the atmosphere finds itself, in dissolved form, in the oceans. In seawater it combines with calcium to precipitate as calcium carbonate, the main constituent of limestone Over geologic time so much carbon dioxide from the atmosphere has been converted to limestone in this fashion that there is more than 100,000 times as much stored as limestone as there is in the atmosphere.
5. TPO 61 Reading 1
Physical Properties of Minerals
A mineral is a naturally occurring solid formed by inorganic processes. Since the internal structure and chemical composition of a mineral are difficult to determine without the aid of sophisticated tests and apparatus, the more easily recognized physical properties are used in identification.
Most people think of a crystal as a rare commodity, when in fact most inorganic solid objects are composed of crystals. The reason for this misconception is that most crystals do not exhibit their crystal form: the external form of a mineral that reflects the orderly internal arrangement of its atoms. Whenever a mineral forms without space restrictions, individual crystals with well-formed crystal faces will develop. Some crystals, such as those of the mineral quartz, have a very distinctive crystal form that can be helpful in identification. However, most of the time, crystal growth is interrupted because of competition for space, resulting in an intergrown mass of crystals, none of which exhibits crystal form.
Although color is an obvious feature of a mineral, it is often an unreliable diagnostic property. Slight impurities in the common mineral quartz, for example, give it a variety of colors, including pink, purple (amethyst), white, and even black. When a mineral, such as quartz, exhibits a variety of colors, it is said to possess exotic coloration. Exotic coloration is usually caused by the inclusion of impurities, such as foreign ions, in the crystalline structure, Other minerals-for example, sulfur, which is yellow, and malachite, which is bright green-are said to have inherent coloration because their color is a consequence of their chemical makeup and does not vary significantly.
Streak is the color of a mineral in its powdered form and is obtained by rubbing a mineral across a plate of unglazed porcelain. Whereas the color of a mineral often varies from sample to sample, the streak usually does not and is therefore the more reliable property.
Luster is the appearance or quality of light reflected from the surface of a mineral. Minerals that have the appearance of metals, regardless of color, are said to have a metallic luster. Minerals with a nonmetallic luster are described by various adjectives, including vitreous (glassy), pearly, silky, resinous, and earthy (dull).
One of the most useful diagnostic properties of a mineral is hardness, the resistance of a mineral to abrasion or scratching. This property is determined by rubbing a mineral of unknown hardness against one of known hardness, or vice versa. A numerical value can be obtained by using Mohs' scale of hardness, which consists of ten minerals arranged in order from talc, the softest, at number one, to diamond, the hardest, at number ten. Any mineral of unknown hardness can be compared with these or with other objects of known hardness. For example, a fingernail has a hardness of 2.5, a copper penny 5, and a piece of glass 5.5. The mineral gypsum, which has a hardness of two, can be easily scratched with your fingernail. On the other hand, the mineral calcite, which has a hardness of three, will scratch your fingernail but will not scratch glass Quartz, the hardest of the common minerals, will scratch a glass plate.
The tendency of a mineral to break along planes of weak bonding is called cleavage. Minerals that possess cleavage are identified by the smooth, flat surfaces produced when the mineral is broken. The simplest type of cleavage is exhibited by the micas. Because the micas have excellent cleavage in one direction, they break to form thin, flat sheets. Some minerals have several cleavage planes, which produce smooth surfaces when broken, while others exhibit poor cleavage, and still others exhibit no cleavage at all. When minerals break evenly in more than one direction, cleavage is described by the number of planes exhibited and the angles at which they meet. Cleavage should not be confused with crystal form. When a mineral exhibits cleavage, it will break into pieces that have the same configuration as the original sample does. By contrast, quartz crystals do not have cleavage, and if broken, would shatter into shapes that do not resemble each other or the original crystals. Minerals that do not exhibit cleavage are said to fracture when broken Some break into pieces with smooth curved surfaces resembling broken glass. Others break into splinters or fibers, but most fracture irregularly.
4. TPO 66 Reading 2
Visions of the Land
Successive generations of North Americans have viewed their continent's natural environment in different ways. From the vantage point of the present it is clear that perceptions of the land have changed dramatically from the first years of settlement to the Civil War. Not only have such visions often shifted, but also different peoples have used their particular perspective to reshape the land itself and make it fit their own sense of what nature should be. If the consequences of some changes, such as cutting forests and filling in lowlands, have been deliberate and purposeful-to open the landscape and create sweeping vistas, for example-other human undertakings, such as mining and dam building, have brought results neither anticipated nor intended. Native peoples, no less than the first colonists and subsequent immigrants to North America, have reshaped the natural environment to meet their physical wants and spiritual needs Indeed, much of the landscape we know today reflects patterns of use and abuse that began several centuries ago.
Long before the first European settlers reached the continent's eastern shores, native peoples had developed agricultural practices that had changed the face of the land. By cutting away the bark to kill trees selectively, Indians in the Virginia tidewater (low, coastal land) and much of the Northeast had cleared space to plant small gardens of corn, squash, beans, and melons Although the first English immigrants described the countryside as almost entirely wooded, the forests provided canopies of large, well-spaced trees under which a horse and rider could pass unhindered. By frequently moving their garden plots to find more-fertile soil and by periodically burning the undergrowth, Indians had further opened the land, in this way facilitating their hunting of deer and other game Native American visions of the landscape not surprisingly featured people living in harmony with nature, whose riches they celebrated in seasonal rituals and through time- honored practices.
In contrast, the European colonists who intruded on this harmonious world often viewed it as alien and menacing; some called it, in the language of the Bible, a "howling wilderness." The newcomers to America brought with them agricultural practices and preconceptions about nature based on their experiences in England. They saw uncultivated lands as "wastes" that needed to be "broken," "dressed." and "improved" . In New England, transplanted English settlers attempted to subdue what they considered a fearsome wilderness by mapping the countryside, draining marshlands clearing pastures, fencing particular parcels, and planting wheat and other familiar crops. Within twenty years of the initial Puritan settlement, Edward Johnson boasted of the newcomers' achievements: "This remote, rocky, barren, bushy, wild-woody wilderness, a receptacle for lions, wolves, bears, foxes, racoons, beavers, otters, and all kind of wild creatures, a place that never afforded the Natives better than the flesh of a few wild creatures and parched Indian corn inched out with chestnuts and bitter acorns, now through the mercy of Christ [has] become a second England for fertility in so short a space, that it is indeed the wonder of the world".
So, rather than adapting to their new land, the English either changed it by cutting trees, building farms, and plowing or searched for soil and landscape features that reminded them of the English countryside. Seeking to tame the land and to conquer their fear of it. generations of seventeenth- and eighteenth-century settlers nevertheless failed to gain the mastery they desired. In part, this failure resulted from their custom of settling along waterways. To expedite travel and facilitate the shipment of agricultural produce, newcomers invariably built their homes along rivers. Despite the colonists' attempts to control waterways through dams, rivers never failed to remind them of nature's unpredictability and power. Rivers could and often did, change course abruptly or flood during sudden rainstorms. Not until the middle of the eighteenth century did the colonists begin to discard their negative view of the landscape as a wilderness to be feared and controlled, and to substitute the idea that nature could be as much useful as fearsome.
3. TPO 68 Reading 2
Predicting Volcanic Eruptions
Volcanoes are the landforms created when molten rock, or magma, escapes from vents in the Earth's surface and then solidifies around these vents. In any given year, roughly 50 of Earth's active volcanoes erupt - usually with some warning. Before they blow, they typically shake,swell, warm up and belch a variety of gases. Scientists from the Volcano Disaster Assistance Program (VDAP)-part of the US. Geological Survey and based at the US. Geologicai Survey's Cascades Volcano Observatory in Vancouver, Washington -are always on call, ready when summoned to rush at a moment's notice to an awakening volcano anywhere in the worid; armed with the latest in lasers, sesmometers, and other monitoring devices, they can assess the volcano's potential for violence and predict when it might ignite.
Geologists have enjoyed fair success in predicting individual eruptive episodes when they concentrate on a specific volcano after an eruptive phase has begun. These monitoring efforts involve carefully measuring changes in a volcano's surface temperature, watching for the slightest expansion in its slope, and keeping track of regional earthquake activity. A laboratory at the University of Washington in Seattle is staffed 24 hours a day to monitor the rumblings of Mount St. Helens. Even with the advances brought by today's technology, however, the art of volcano prediction has not been fully mastered. The US. Geological Survey missed the call on Mount St. Helens' 1980 blast despite the fact that the mountain was being watched closely by a large team of scientists armed with the latest in prediction technology. lt did successfully predict the eruption of Mount Pinatubo in the Philippines evacuating virtually everyone within 25 kilometers (15 miles) before the volcano's powerful blast on May 17,1991.
Before a volcano erupts, hot magma rises toward the surface, so any local manifestation of increasing heat may signal an impending event. Ongoing surveys can identify new surface hot springs and take the temperature of the water and steam in existing ones. lf the escaping steam isn't much hotter than the boiling point of water, then surface water is probably seeping into the mountain and being heated by contact with hot subsurface rocks, and all is well for the time being. If the steam is superheated, with temperatures as high as 500°C(900°F),then it probably derives from shallow watern'ch magma, a sign that an eruption may be brewing. As magma rises, the volcanic cone itself begins to heat up.The overall temperatur of a volcanic cone can be monitored from an orbiting satellite equipped with infrared heat sensors to detect the slightest change in surface temperature. This high-altitude technology serves as a simultaneous early-warning system for most of Earths 600 or so active volcanoes. lmpending eruptions may also be predicted by increased gas emissions from rising magmas. For this reason, volcanologists continuously monitor sulfur dioxide and carbon dioxide emissions from potentially active volcanoes.
Active volcanoes expand in volume as they acquire new supplies of magma from below. As a result, an increase in the steepness or bulging of a volcano's slope may signal an impending eruption. To detect the inflation of a volcanic cone, a tilt meter, a device like a carpenter's level, is used. As magma rises, it pushes aside fractured rock,enlarging the fractures as it moves. Because this type of fracturing causes earthquakes, eruptions are often preceded by a distinctive pattern of earthquake activity called harmonic tremors, a continuous rhythmic rumbling. Sensitive equipment that monitors the location where these tremors occur can measure the increased height of rising magma. The rate at which the magma rises provides an estimate of when an eruption may occur. Indeed, it served as the principal means by which scientists accurately predicted recent eruptions of Mount St. Helens.
Efforts to predict eruptions are thwarted, however, when we are unaware of a site's volcanic potential. Occasionally, a new volcano appears suddenly and rather unexpectedly, as was the case in 1943, when the volcano Paricutin developed literally aovernight in the Mexican state of Michoacan, 320 kilometers (200 miles) west of Mexico City. The surrounding area was known to be volcanic because of its geologic zone, but it was not possible to predict that the volcano would appear at this particular site. Our ability to predict volcanic eruptions continues to improve but is not yet as accurate as we need it to be.
2. TPO 70 Reading 2
Nineteenth-Century Theories of Mountain Formation
One of the central scientific questions of nineteenth-century geology was the origin of mountains. How were they formed? What process squeezed and folded rocks like bread dough? What made Earth’s surface move? Most theories invoked terrestrial contraction as a causal force. It was widely believed that Earth had formed as a hot, incandescent body and had been steadily cooling since the beginning of geological time. Because most materials contract as they cool, it seemed logical to assume that Earth had been contracting as it cooled, too. As it did, its surface would have deformed, producing mountains.
In Europe, Austrian geologist Eduard Suess (1831–1914) popularized the image of Earth as a drying apple: as the planet contracted, its surface wrinkled to accommodate the diminished surface area. Suess assumed that Earth’s initial crust was continuous but broke apart as the interior shrank. The collapsed portions formed the ocean basins, the remaining elevated portions formed the continents. With continued cooling, the original continents became unstable and collapsed to form the next generation of ocean floor, and what had formerly been ocean now became dry land. Over the course of geological history, there would be a continual interchange of land and sea, a periodic rearrangement of the landmasses.
The interchangeability of continents and oceans explained a number of other perplexing geological observations, such as the presence of marine fossils on land (which had long before puzzled Leonardo da Vinci) and the extensive interleaving of marine and terrestrial sediments in the stratigraphic record. Suess’s theory also explained the striking similarities of fossils in parts of Africa and South America. Indeed, in some cases the fossils seemed to be identical, even though they were found thousands of miles apart. These similarities had been recognized since the mid-nineteenth century, but they had been made newly problematic by Darwin’s theory of evolution. If plants and animals had evolved independently in different places within diverse environments, then why did they look so similar? Suess explained this conundrum by attributing these similar species to an early geological age when the continents were contiguous in an ancient supercontinent called Gondwanaland.
Suess’s theory was widely discussed and to varying degrees accepted in Europe, but in North America geologist James Dwight Dana (1813–1895) had developed a different version of contraction theory. Dana suggested that the continents had formed early in Earth history, when low-temperature minerals such as quartz and feldspar had solidified. Then the globe continued to cool and contract, until the high-temperature minerals such as olivine and pyroxene finally solidified–on the Moon, to form the lunar craters; on Earth, to form the ocean basins. As contraction continued after Earth was solid, its surface began to deform. The boundaries between continents and oceans were most affected by the pressure, and so mountains began to form along continental margins. With continued contraction came continued deformation, but with the continents and oceans always in the same relative positions. Although Dana’s theory was a version of contraction, it came to be known as permanence theory, because it viewed continents and oceans as globally permanent features.
In North America permanence theory was linked to the theory of subsidence (or sinking) of sedimentary basins along continental margins. This idea was developed primarily by paleontologist James Hall (1811–1898), who noted that beneath the forest cover, the Appalachian Mountains of North America were built up of folded layers of shallowwater sedimentary rocks, thousands of feet thick. How did these sequences of shallow-water deposits form? How were they folded and uplifted into mountains? Hall suggested that materials eroded off the continents accumulated in the adjacent marginal basins, causing the basins to subside.  Subsidence allowed more sediment to accumulate, causing more subsidence, until finally the weight of the pile caused the sediments to be heated, converted to rock, and then uplifted into mountains. Dana modified Hall’s view by arguing that thick sedimentary piles were not the cause of subsidence but the result of it.Either way, the theory provided a concise explanation of how thick sequences of shallow-water rocks could accumulate, but was vague on the question of how they were transformed into mountain belts.
1. TPO 71 Reading 1
Electrical Energy from the Ocean
Solar energy reaching Earth is responsible for differential heating of the atmosphere and thus air circulation as wind.Some of the energy of wind is transferred to the oceans, where it causes waves and is partly responsible for oceanic currents,although Earth's rotation also plays a role in currents. Gravitational attraction between Earth and the Sun and Moon generates tides and,along with Earth's rotation, causes most coastal areas to experience a twice-daily rise and fall of sea level In short, the oceans possess a tremendous reservoir of largely untapped energy.
If we could effectively harness the energy possessed by the oceans, an almost limitless, largely nonpolluting energy supply would be ensured. Unfortunately, ocean energy is diffuse, meaning that the amount of energy for a given volume of water is small and thus difficult to concentrate and use. Several ways of using ocean energy are being considered or are under development,and one is currently in use, although it accounts for only a tiny proportion of all energy production. Of the several sources of ocean energy-temperature differences with depth,currents,waves,and tides-only the last shows much promise for the near future.
Ocean water at depth might be as much as 25° C colder than surface water, a difference that allows for ocean thermal energy conversion (OTEC). OTEC exploits this temperature difference to run turbines and generate electricity.The amount of energy available is enormous, but a number of practical problems must be solved before it can be used For one thing, any potential site must be close to land and also have a sufficiently rapid change with depth to result in the required temperature difference. Furthermore, enormous quantities of warm and cold seawater would have to circulate through an electrical-generating plant, thus requiring that large surface areas be devoted to this purpose.
The concept of OTEC is more than a century old, but despite several decades of research,no commercial OTEC plants are operating or even under construction, although small experimental ones have been tested in Hawaii and Japan.
Wind-generated ocean currents, such as the Gulf Stream,which flows along the east coast of North America,also possess energy that might be tapped to generate electricity.Unlike streams that can be dammed to impound a reservoir, any electrical generating facility exploiting oceanic currents would have to concentrate currents' diffuse energy and contend with any unpredictable changes in direction. In addition, whereas hydroelectric generating plants on land depend on the rapid movement of water from a higher elevation to the turbines,the energy of ocean currents comes from their flow velocity, which is at most a few kilometers per hour.
The most obvious form of energy in the oceans lies in waves. Harnessing wave energy and converting it to electricity is not a new idea, and it has been used on an extremely limited scale.Unfortunately, the energy possessed by a wave is distributed along its crest and is difficult to concentrate. Furthermore, any facility would have to be designed to withstand the effects of storms and saltwater corrosion.The Japanese have developed wave-energy devices to power lighthouses and buoys, and a facility capable of providing power to about 300 homes began operating in Scotland during September 2000.
Perhaps tidal power is the most promising form of ocean energy. In fact, it has been used for centuries in some coastal areas to run mills, but its use at present for electrical generation is limited.Most coastal areas experience a twice-daily rise and fall of tides, but only a few areas are suitable for exploiting this energy source. One limitation is that the tidal range must be at least five meters, and there must also be a coastal region where water can be stored following high tide.
Suitable sites for using tidal power are limiled not only by tidal range but also by location. Many areas along the U. S. Gulf Coast would certainly benefit from lidal power plants, but a tide range of generally less than one meter precludes the possibility of development. Even areas with an appropriate tidal range such as the Arctic islands of Canada offer little potential because of their great distances from population centers.