Science Matters (26 page)

Relativity, despite its flashy aura, is actually a staid and settled part of physics. It has become a tool that cosmologists and particle physicists use to understand the origins of the universe and the basic structure of matter, rather than a field of study in and of itself. In this respect, at least, it resembles Newtonian science.

The only area that might be called a frontier is the experimental testing of general relativity. This field is active right now because advances in electronics have finally given experimenters the ability to measure many of the extremely small differences that are supposed to exist between general relativity and Newtonian physics.

One of the most elaborate tests of general relativity, called Gravity Probe B, was launched into orbit by NASA in 2004. Over thirty years in development and planning, this experiment consists of carefully machined quartz spheres set into rotation. Relativity predicts that the presence of the rotating Earth will cause
the axis of rotation of the spheres to wobble a little, and this small deviation from normality can be measured. This experiment required amazing precision. The quartz spheres must be so perfectly round that were they blown up to the size of planet Earth, their highest “mountains” would be no more than a foot high!

In 2008 the results of Gravity Probe B were announced, and as expected, general relativity passed another experimental test. In coming decades, we can expect more high-precision tests of relativity.

I
MAGINE LYING ON A
sun-drenched Malaysian beach on the coast of the Indian Ocean. It’s the day after Christmas 2004, and you’re enjoying a much-deserved vacation. Suddenly the tranquility is broken. Frantic local officials start to shout warnings. “Get off the beach! Run to high ground!” You don’t hesitate and join the other beachgoers dashing inland. Just in time you escape to the upper floors of your hotel as a churning wall of water crashes across the beach and through the streets.

More than 225,000 people in Sri Lanka, Thailand, Indonesia, Malaysia, and other countries bordering the Indian Ocean perished in the tsunami that followed the Great Sumatra-Andaman earthquake of 2004—one of the strongest ever recorded. Off the west coast of Sumatra, Indonesia, rock layers on the ocean floor, which had been strained by centuries of slow crustal movement, suddenly snapped, releasing energy like a coiled spring. More powerful than a thousand nuclear bombs, shock waves traveled through Earth at supersonic speed. The sudden faulting of the
ocean floor also set into motion immense waves of water—the tsunamis that would cause such tragic coastal devastation.

Earthquakes and volcanoes offer dramatic testimony that our planet is not at rest. For centuries humans viewed these destructive natural phenomena as purely chance events, governed by the whims of gods and unpredictable in their savagery. But earthquakes and eruptions do not occur randomly. You can live a long life as a resident of New York City, where earthquakes are very rare, and never feel the slightest tremor, much less worry about a volcano erupting in Central Park. But if you live along the California coast for just a few years the chances are that you will feel the earth shake, and if you reside on the big island of Hawaii for just a few months you will probably witness the eruption of a nearby volcano.

Even if scientists can’t predict exactly when a “big one” will hit, they can tell you why earthquakes and volcanoes occur in certain parts of the globe. Earthquakes, volcanoes, mineral deposits, and even the oceans and the continents themselves are the surface manifestations of tremendous forces at work deep within Earth. Only within the past thirty years have scientists finally begun to understand the forces that operate to change the face of our planet—sometimes, as in San Francisco, in violent ways. The central fact that governs their new insights into the workings of our planet can be summed up as follows:

The forces that drive this constant change are generated deep inside the Earth, where nuclei of radioactive elements constantly decay. The energy of decay is converted to heat, and this heat slowly seeps up toward the surface. Over hundreds of millions of
years, rock warmed by the radioactive decay rises slowly to the surface, cools, and then sinks to be warmed again. The earth beneath us, viewed over a long time, is really no different from a pot of boiling water on your stove.

Encasing this roiling interior, called the mantle, is a layer of rock usually less than thirty miles thick, floating on the churning material like an oil slick on boiling water. In response to this churning, the thin outer skin of our planet breaks up, moves around, and reassembles in unceasing movement. And on top of this thin layer of restless rocks, like scum riding on an oil slick, are the continents—the part of the globe that we, in our arrogance, call “solid earth.”

The “boiling” of the interior rocks causes Earth’s continents to float around, collide, tear apart, and link together again. In the process, ocean basins open and close, mountain ranges are thrown up and weathered down and the surface constantly changes. Alone among the planets of the solar system, Earth is restless. It is the only planet still in the process of forming itself—still being born.

The roiling surface of the Earth is very like the surface of a boiling pot of water, except that instead of a fluid moving around, as in the pot, it is the solid rocks of Earth’s interior that “boil.” To be sure, this boiling process is very slow.

Rocks rarely travel faster than about an inch a year, but give a convecting rock a million years and it can travel miles. In a hundred million years it can move the length of a continent.

The heat that drives this motion of the mantle comes from two sources: radioactive decay of materials in the mantle rocks and heat left over from Earth’s formation. Scientists sometimes argue over how much heat comes from each source, with most favoring radioactive decay as the main source. From the point of view of the mantle, though, the source of heat is irrelevant.
However the heat got there, it has to be moved to the surface by convection and then radiated into space.

Earth scientists of all persuasions have embraced an elegant planetary model that identifies a simple underlying cause for Earth’s violent moods. This model, discovered in the 1960s and dubbed plate tectonics, describes the interaction between plates, which form Earth’s thin, brittle outer layer, and the vast mobile portions of the mantle that lie beneath the crust and make up four-fifths of the solid Earth. “Tectonics” comes from the Greek root “to build,” so plate tectonics refers to a theory of how Earth’s surface is built from plates.

Our current picture of Earth’s outer layers is simple. Most of the mantle is completely covered by a crust of basalt, a brittle, dense, dark volcanic rock. If you’ve ever enjoyed Hawaii’s black sand beaches or passed by the rusty brown cliffs and columns of the Hudson River Palisades in New York, you’ve seen basalt. As the mantle rolls and boils, Earth’s outer cover breaks into many thin, brittle plates, each hundreds or even thousands of miles across but generally only a few tens of miles thick. These plates then move about and interact with other plates. Geologists recognize three main types of plate boundaries: divergent, convergent, and neutral boundaries.

New crust forms at
divergent
plate boundaries, places where mantle convection moves plates apart and brings new material to the surface. A great mountain chain in the middle of the Atlantic Ocean (the Mid-Atlantic Ridge) marks one such zone of upward mantle convection. The island of Iceland lies along this ridge and consists entirely of volcanic rock. Other diverging boundaries occur beneath continents. If you visit the Great Rift Valley in East Africa you can actually stand on a place where the mighty African continent is just beginning to be torn apart.

The great plates that make up Earth’s surface can interact in several ways. (A) They can diverge along a volcanic ridge where new crust is constantly being formed; (B) they can converge so that one plate must be subducted beneath the other; or (C) they can scrape against each other to form a fault zone, which may produce violent earthquakes. In each case, the plate motions result from movement of the convecting mantle below
.

If new plate materials are forming at diverging boundaries, then old material must be destroyed somewhere else, since Earth itself is not getting any larger. This destruction occurs at
convergent
boundaries where the plates are pushed together, driving one beneath the other, or subducting it. The subducted plate goes back into the mantle, where it can melt and mix with mantle rocks to join the reservoir of material ready to start the whole cycle again.

A number of major plates, along with many smaller pieces, form Earth’s surface. Arrows at the plate boundaries of this map indicate the relative direction of plate motions
.

The surface manifestations of a subduction zone depend on whether the colliding plates are carrying continents or not. If neither has a continental passenger, the result is a deep ocean trench like the Marianas Trench near the Philippines. If only one plate is carrying a continent, the continent crumples up as it approaches the region of contact, forming chains of mountains like the Andes in South America. And if both plates carry continents, then those continents crash together and form a high mountain range in the middle of the new, combined continent. The Himalayas are the scar that resulted when the Indian subcontinent crashed into Asia, and the Alps remind us of the event that joined Italy to Europe.

In some places plates may scrape against each other at a
neutral
plate boundary, scouring out long, destructive earthquake zones. The San Andreas fault, which runs the length of California and periodically disrupts West Coast cities, follows the boundary between the North American plate to the east and the Pacific plate to the west. The 1989 San Francisco earthquake was just one of countless shocks triggered by the inexorable movement of these two plates.

These examples of plates in motion reveal that continents and plates are not the same things. The continents ride on top of the plates, but there is only enough continental material to cover about one-quarter of Earth’s surface. Consequently, Earth is (and always has been) about three-quarters ocean. At present Earth has only six continents, but we recognize at least a dozen major plates and there are probably a host of poorly defined smaller plates as well. The South American, Australian, and Antarctic continents lie within the boundaries of much larger single plates. The North American plate contains almost all of the continent of North America and about half of the Atlantic Ocean. The Eurasian and African plates, on the other hand, are composites of several continental masses. Continental blocks that were once separate, such as India and Asia, have fused. Ancient continents, fancifully named Gondwanaland, Laurasia, and Pangaea by geologists, have been pulled apart.

Plate tectonics reveal to us that nothing on Earth’s surface—no river or valley, no ocean or plain, not even the tallest mountain on the largest continent—is permanent.

Scientists of the 1960s were not the first to suspect that continents move. In 1912 the German meteorologist Alfred Wegener constructed a theory he called continental drift to explain in part why the west coasts of Africa and Europe so closely match the east coasts of the Americas. Wegener explained the fit by theorizing that the continents were once joined and somehow drifted apart to adopt their present positions. This explanation precisely fits plate tectonics today, but Wegener’s continental drift hypothesis did not really anticipate the modern theory. His model just happened to have one thing—the motion of continents—in common with the modern view.