Science Matters

by Robert M. Hazen

  Many magmas fail to make it to the surface. Geologists call these molten masses that cool deep underground intrusive rocks, to distinguish them from more visible extrusive lava flows. Because they form deep underground, it may take many millions of years for the material above an intrusive rock formation to be lifted up and weathered away, so that the rock can finally appear at the surface. But given enough time, intrusive rocks can be uncovered to create prominent landmarks like Mount Rushmore in the Black Hills of South Dakota, Stone Mountain in Georgia, and the highest peaks of the Colorado Rockies.

  Devils Tower in northeastern Wyoming, which figured so prominently in the movie Close Encounters of the Third Kind, is a classic example of a volcano that didn’t quite make it. The magma penetrated hundreds of feet upward through overlying sandstone, but it never breached the surface. Cooling in place, the intrusion developed long vertical cracks as it contracted. Now, millions of years later, the soft sandstone has weathered away, leaving behind the spectacular plug of igneous rock with its graceful rock columns.

  Sedimentary Rocks

  Imagine the early Earth. Jagged volcanic peaks rose from the steaming oceans, only to be battered and broken by wind and waves. Small chips of rock broke off and were washed down to the sea. Soon sandy beaches buffered land from sea. River valleys and lake bottoms gradually filled with sediments from the debris of weathered rocks. Given time, thick deposits of sediments—layer upon layer of igneous rock fragments—were themselves buried, baked, and turned to stone. Rocks formed in this way are called sedimentary rocks.

  Weathering, the process that generates sediments by destroying rocks, takes many forms. Ocean tides, flowing rivers and streams, and windblown sand contribute to physical weathering, as does the wedge-like effect of water freezing in cracks and pores. Rocks can dissolve by chemical weathering, and they can be attacked by the actions of living organisms, the way that tree roots and grass can gradually break up a sidewalk. All of these processes provide the raw material for the formation of sedimentary rocks.

  Over time, the accumulation of sediment may bury beach sand deep underground. Pressure and the heat of Earth’s interior, together with minerals deposited by water, cement the grains together into a rock called sandstone. Later, mountain-building activity may lift this rock up and the weathering process will start again. Eventually each grain in the sandstone will be broken off and transported to another, as yet unimagined beach. Many of the grains of sand at your favorite beach have been on other beaches in the distant past, and many will someday reappear on beaches of the future.

  For the past three billion years, living organisms have themselves contributed directly to the formation of sedimentary rocks. Plants die and their stems, leaves, and trunks accumulate in swamps to form layers of coal, while microscopic organisms in the ocean die and contribute their skeletons to the accumulation of material at the bottom—material that will eventually become the sedimentary rock we call limestone.

  Because they form from material filtering down to ocean and lake bottoms, sedimentary rocks usually appear to be layered. They often look like the pages of a book viewed end-on. Watch for them next time you’re out driving. The most common sedimentary rocks are sandstone (made from sand), shale (made from silt and clay), and limestone (from the skeletons of microscopic organisms).

  Metamorphic Rocks

  Igneous and sedimentary rocks do not retain their original form forever. At the surface, they will be broken down by weathering to form new sediments. If they are buried, even more interesting changes can occur as the transforming effects of temperature, pressure, and time do their work. At high temperatures, clays and other common minerals give up water, like a brick baking in a kiln, while at high pressure, atoms in a rock rearrange themselves to form new and denser minerals, just as graphite converts to diamond if buried 100 miles down. Rocks that have been changed since they first formed are called metamorphic rocks.

  These rocks tell incredible stories of Earth’s unrest. Some New England rock outcroppings high on mountaintops contain minerals that could only have formed twenty miles beneath the surface, at temperatures near 1,000°C. Remnant rock structures reveal that those outcrops once lay at the bottom of a deep ocean basin, where their sediments were buried deeper and deeper as more continental material eroded off the land and accumulated in the sea. When an ancient collision of the North American and Eurasian plates crumpled and compressed those ocean basin sediments to form the Appalachian Mountains, deep-buried sediments were subjected to the heat and stress of mountain building. Layered limestone transformed to the marbles of Vermont, while shale turned first to slate and then to schist, a shiny metamorphic rock with big crystals of garnet and other high-pressure minerals. More than 200 million years of erosion and uplift have now brought those ancient rocks back to the surface to tell their tale, to weather and erode, and to begin the whole process anew. Humans contribute to the cycle by quarrying the marble for monuments, gravestones, and other transient reminders of Earth’s constant change.

  Thus the cycle continues. All three forms of rock—igneous, sedimentary, and metamorphic—can weather to form more sediment, and all three can be subducted to melt or metamorphose and start the cycle anew.

  THE WATER CYCLE

  Earth holds only a finite amount of water, and virtually all the water at Earth’s surface now has been there almost since the planet was born, yet it never seems to run out. The “ever-filled purse” that represents our water resource comes about because water, like rock, moves through cycles, constantly being used, constantly being replenished.

  Any water that might have been on Earth’s surface when it first cooled off would probably have blown off by meteorite impacts or the solar wind, the intense stream of particles emitted by the newborn sun. The water that now fills the oceans, as well as the gases that make up the atmosphere, must have waited out this early violent period in our planet’s history safely stored in solid rock. Only later did they come to the surface, through volcanic activity.

  Oceans and atmospheres are not an inevitable consequence of planet formation. Smaller worlds, like Mercury and the moon, are too small to retain any surface fluids. Fast-moving gas molecules like water vapor, nitrogen, or oxygen gradually escape the weak gravitational pull of these bodies. If planet Earth had been much smaller, there would have been no oceans and no life to enjoy them.

  Our planetary reservoir holds almost 500 billion billion gallons of surface water in its oceans, lakes, rivers, ice caps, ground-water, and atmosphere. An unknown (but probably larger) amount is locked up in minerals in the crust and mantle, though this bound water is obviously not readily available for human use. The oceans account for more than 97 percent of the vast surface-water budget. An additional 2 percent is frozen in ice caps and glaciers, leaving less than 1 percent as usable fresh water. These percentages may change slightly, for example during ice ages, but freshwater will never account for more than a small fraction of the total supply.

  The most familiar illustrations of the water cycle depict water evaporating from the oceans, forming clouds that rain on the land, and finally collecting in streams and rivers that return to the sea. This simple water cycle, which takes a few weeks or months to complete, is certainly a part of the story, but the complete cycle is much more complex. It involves many interlocking cyclical processes that occur over times from a few hours to millions of years. We need to understand several key pieces to solve this global jigsaw puzzle.

  The Oceans

  Oceans cover three quarters of Earth’s surface. Averaging about three miles in depth, the oceans are characterized by a thin surface layer (only a few hundred yards deep) that absorbs sunlight. That zone overlies a dark, cold reservoir in which almost all of Earth’s water is stored. The deeper you go into the water, the colder and saltier it gets. The pressure also increases, reaching values of several tons per square inch in the deepest parts of the ocean. Near land, narrow bands of shallow water cover continental shelves—re
gions best thought of as flooded parts of the land rather than as parts of the ocean proper. They usually extend out a few tens of miles, where the bottom drops off abruptly into the abyss of the deep ocean.

  The thin upper skin of the sea, called the mixed layer, is special. Heated by the sun and well mixed with atmospheric gases, it teems with life, from microscopic plankton and algae to giant fish and sea mammals. In contrast, the deep ocean is dark and dense, subject to tremendous pressure and chilled to only a few degrees above freezing. This is why shallow waters like those of the continental shelves, the famous fishing grounds of Georges Bank in the North Atlantic, and Chesapeake Bay produce so much seafood. The deep ocean, on the other hand, is a kind of desert, inhospitable and spare of life.

  The deepest ocean waters circulate slowly and can spend many thousands of years in the dark void. Much of the densest, deepest ocean water comes from melting Antarctic ice; that ice water descends to the bottom, spreads out, and slowly rolls across the ocean floor to places as far away as the Bering Strait of the North Pacific.

  Virtually all interactions between the oceans and other parts of the water cycle take place at the ocean surface. Rivers and rain add to the top layer, while surface evaporation returns water to the air. Surface waters also provide coastal areas with a thermal buffer, moderating air temperatures during the coldest and hottest months.

  Ice Caps and Glaciers

  At times in its history Earth has had as much as 5 percent of its water budget tied up in ice caps and glaciers, at other times as little as a fraction of a percent. The amount of ice depends in a complicated way on the positions of the continents, and in a regular and predictable way on variations in Earth’s orbit around the sun and the direction of Earth’s axis of rotation. At present, during a period of moderate temperatures, about 2 percent of the globe’s water (about three-quarters of all freshwater) is frozen.

  The largest concentration of ice on Earth today is in the thick glaciers that cover the south pole. Such large ice caps can accumulate only if a continent covers one of the poles, giving a base of solid ground to support thick ice. Otherwise, we have a situation like the one at the north pole today. As the northern ice cap starts to build up, the weight of new ice pushes old ice deeper into the water, where it melts because of the higher pressure. Thus, without a solid support, an ocean ice cap can never be more than a few hundred yards thick, and can never contain more than a tiny fraction of Earth’s water. For three-quarters of Earth’s history, there were no continents at the poles, and hence no large ice caps. Occasionally there have been continents at both poles, a condition which probably led to more water being locked up in glaciers than at present.

  Scientists have trouble predicting continental wandering, not to mention the effects of those movements on glaciation, but purely astronomical effects on ice caps are easy to predict. The most important effect involves the tilt of Earth’s axis of rotation, which now leans 23 degrees off the axis of Earth’s orbit. Northern and southern hemispheres have opposite seasons at present because when the northern hemisphere is tilted toward the sun (in summer), the southern hemisphere is tilted away. In general, any effect that makes summers cooler contributes to glaciation. The reason is simple: If summers are cooler, ice and snow will stay on the ground longer in Canada and Siberia. This ice and snow will reflect sunlight, further lowering the temperature and allowing still more ice and snow to remain on the ground the next year. The result: over a period of a few thousand years, large sheets of ice spread out from the pole and the high mountains and cover large parts of Europe and North America. When this happens, we say there is an ice age.

  The most recent ice age was in full swing 20,000 years ago, when glaciers extended as far south as Chicago. Today we live in what geologists call an interglacial, a term we find chilling in every sense of the word. On a shorter time scale, chance events also affect the glacial cycle. Major volcanoes can spew dark matter into the atmosphere, blocking sunlight and cooling the planet for a year or two, which creates brief periods of growing glaciers. Increased atmospheric concentrations of carbon dioxide and other greenhouse gases, whether man-made or natural, may have the opposite effect. The best prediction available now is that we are (or should be) heading into another period of glaciation, although the advent of global warming by the greenhouse effect may introduce a temporary glitch in the grand cycle of freezing and melting.

  Earth’s glacial cycles are of more than abstract interest. If more water is tied up in glaciers, less remains to fill the oceans and sea levels will fall. During the last glaciation, for example, the eastern coast of North America was 150 miles farther east than it is today, and on the west coast an ice and land bridge existed between Siberia and Alaska. Anthropologists believe this bridge allowed the ancestors of the Native American Indians to reach this continent. Conversely, geological evidence also points to warm interglacial periods, some within the past hundred thousand years, when oceans were 100 feet higher than today. To see what such a sea level change would mean, imagine the present New York and Los Angeles waterfronts under 100 feet of seawater.

  Freshwater

  The underlying principle of the science of hydrology, which concerns itself with the study of the water cycle, is that water is a mobile resource. It flows by gravity from high to low, thus shaping the land and playing a central role in the rock cycle. It evaporates from land and sea into air, forming clouds and playing a major part in the weather cycle. It falls as rain on the land, filling reservoirs with fresh water and providing the chemical medium for life.

  Only a small fraction of Earth’s liquid resources are available to land plants and animals as freshwater. Rainwater that falls to land can follow many different paths. Some water penetrates the soil, some enters lakes, ponds, and streams, some evaporates quickly and returns to the clouds, but most (as much as 99 percent of the total) becomes part of the vast underground reservoir of groundwater. Groundwater accumulates in aquifers—porous rocks such as sandstone—where continuous networks of tiny spaces between mineral grains form huge reservoirs. Water soaks in wherever the aquifer is exposed at the surface, and the reservoir fills by the force of gravity. Aquifers, bounded top and bottom by impervious rock layers, can be tapped by deep water wells. It can take many thousands of years for an aquifer to fill up with water, so tapping that water is analogous to mining a mineral deposit. Throughout the American West, wells are going dry as the supply of stored water is drained. The fact that they will be replenished in a few thousand years is of scant comfort to ranchers and farmers.

  Humans, who depend on the natural water cycle for their survival, affect the cycle in many ways. We divert streams for crop irrigation, the largest single use of water. We dam rivers for hydropower. We create artificial lakes for water storage and recreation. We use flowing water to purge unwanted chemicals from our factories and sewage from our homes.

  Until fairly recently, humans regarded freshwater as an inexhaustible resource, one that could be used with little regard for long-term consequences. To some extent this attitude was justifiable, since some aspects of the cycle are resilient. Evaporation can purify water in a relatively short time. We can clean up polluted rivers, streams, ponds, and coastlines in a few years, as newly evaporated water replaces the old, polluted fluid. But other problems are less amenable to short-term solutions. Contaminated groundwater may remain polluted for decades, thus diminishing our stock of potable fresh water at the same time that growing populations demand more. Politicians, as well as scientists, now realize that humans are an important part of Earth’s water cycle.

  THE ATMOSPHERIC CYCLE

  People instinctively distinguish three principal atmospheric cycles. Weather is the short-term cycle, somewhat unpredictable and almost always different from the “average” weather for a particular day or month. The longer-term cycle related to Earth’s movement around the sun we call the seasons, and we use them to number human lives and accomplishments. On a much longer time scale we recog
nize the cycle of climate. The climate of a region changes much more slowly, though many scientists think we are entering a period of relatively rapid climate change as a result of human activities (see Chapter 19). But even under normal circumstances, the passage of several generations may be sufficient to alter the severity of winters, to turn productive farmland into desert, or to transform swamps into firm ground.

  Weather, seasons, and climate all involve Earth’s atmosphere, an envelope of gas surrounding our planet that is in its way as complex as the oceans and the crustal rocks. To understand the weather you must understand how the atmosphere is constructed. The atmosphere behaves in many ways like the solid Earth and its oceans. Like hot rocks in the mantle or currents in the sea, air circulates. And like the solid Earth and the oceans, the atmospheric system has layers that differ in temperature and pressure.

  The troposphere is the warm layer of air next to Earth’s surface. It extends up about 40,000 feet and provides the expressway for commercial jet travel. The troposphere, high enough to cover Mount Everest, contains most of the cloud systems we see from Earth, but large thunderstorms often produce clouds that stick out above it. Above the troposphere lie successive layers called the stratosphere (to 150,000 feet), mesosphere (to 260,000 feet), and the ionosphere, each of which plays a role in the overall behavior of the ocean of air in which we live.

  Convection and the Weather

  Convection, the same mechanism that drives plate tectonics, causes weather in the near surface layers. The lower atmosphere is a giant convecting system, powered by solar energy and wrapped around the turning planet. Most of the sun’s energy arrives near the equator, where warm air expands and rises. On a nonrotating planet, rising air would circulate from equator to pole in high atmospheric currents, eventually to cool and descend near the poles. The surface flow of air would travel from pole to equator. In the northern hemisphere of such a world, wind and weather would usually come from the north.