Science Matters (28 page)

Earth’s magnetic field owes its origin to the rotation of the liquid outer core, but beyond that we know little about why our planet behaves like a giant magnet. Since the core is electrically neutral, its rotation does not produce an electric current and cannot, in and of itself, produce a magnetic field. There are, however, somewhat more complex ways in which a rotating neutral conductor can create a field, so that’s not the real quandary scientists face.

The real problem is that Earth’s north and south poles have not always been where they are now. The magnetic pole wanders, usually remaining near the north pole, but changing position by about twenty-five miles every year. Today the pole lies near Ellesmere Island in northern Canada at a latitude of about 83 degrees north, and it is moving steadily northward.

In addition, at various times in the past, Earth’s magnetic field has reversed, a process that may take a few hundred or thousand years while the “north” pole shifts to Antarctica. We can see over 300 of these reversals in the geological record, and we really don’t have a very good idea of why they happen. Geologists are faced with the unenviable task of producing a theory that predicts a steady magnetic field that at seemingly random times flips directions.

N
EXT TIME YOU’RE AT
the beach, pick up a handful of sand and look at it—really examine it. You’ll notice that each grain differs from its neighbors. Some may be black, others shiny; still others may be green or white or various shades of brown. If you look at the grains under a microscope, more differences appear. Some look smooth and rounded, some sharp and angular. All of these differences arise because the grains of sand you are holding, despite their differences, share one important property: all are part of one of the great cycles that operate on our planet.

The grains of sand are different colors because each comes from a different rock inland from the beach. The grains have different shapes because they may have been washed to the beach, buried, incorporated into new rocks, uplifted, and washed to new beaches many times. The great cycles of weathering and erosion of rocks, sedimentation, and creation of new rocks has gone on since Earth’s beginning, and will continue until the sun
burns out and the planet dies. Because of this cycle, it is possible that you hold in your hand the very first grain of sand that formed on the very first beach when Earth was young.

As scientists examine nature in operation, they recognize many ongoing processes—natural actions that constantly change the surface of the globe. Rain falls, gradually washing away rocks and soils and creating sand and silt. Rivers flow, carrying those sediments from hills and mountains to the valleys below. Ocean and lake waters evaporate, creating new rain clouds. Rocks, water, and atmosphere—the matter that forms the outer layers of our planet—are forever being shifted from place to place.

Water evaporates from the oceans and flows back, sometimes on the surface, sometimes underground, and sometimes stopping for a while in an inland lake. When the climate turns cold, water is taken up in huge ice sheets that spread out from the poles, and sea levels fall around the world. With warmer weather the ice sheets retreat and the water flows back into the sea. Like rocks, water moves in cycles.

Even the air moves in stately cycles, from the prevailing winds that bring us our daily weather to the long-term effects that constantly change the climate.

In fact,

Today scientists recognize that all of Earth’s cycles are connected, each influencing the others. We are beginning to see our planet as a kind of marvelous machine, full of turning gears and moving parts. And most wonderful of all, we are beginning to understand how that machine works and how all the parts fit together.

No feature on Earth is permanent. Mountains weather away, continents break apart, oceans disappear, glaciers form and melt. Change is the hallmark of our planet. Yet amidst all this change, there is constancy. For all practical purposes Earth has a fixed budget of atoms. For an atom to be used in one structure, it must be taken away from another. Like a child in a room filled with wonderful building blocks, Earth has a large but finite number of pieces to play with.

Earth’s surface displays a remarkable variety of rocks. But despite this variety, geologists classify rocks into only three basic types: igneous, sedimentary, or metamorphic. Cataloging rocks is not just an academic exercise. Each type of rock records a different complex past—a past revealed by mineral textures and form. Each type of rock can be changed from one form to another and then back again. Geologists call these transformations the rock cycle.

The crust of our planet began as molten rock; from space that early Earth must have appeared as a spectacular incandescent ball. The rock cycle could not begin until that glowing outer layer began to solidify. In the beginning, all rocks on Earth were igneous—fire-formed.

Volcanism is the most spectacular process that produces new igneous rocks today. Volcanic rocks arrive at the surface, either in
the air or underwater, as magma—the molten form of rock. The most obvious and destructive volcanoes occur on land, where huge fountains of incandescent molten rock light the night sky and rivers of lava destroy life and property while reshaping the landscape. Most of those volcanoes produce dark basalt lavas, which possess a sticky fluid consistency before they harden. Occasionally, as in the Mount St. Helens eruption in 1980, lavas are thick and viscous like tar so that little flow can occur. An epic explosion may be required to relieve the pent-up pressure.

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.

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).

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.

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.

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—regions 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.