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The Earth at Work

The Earth at Work

by Kathy Svitil

Although it feels solid and hard beneath our feet, the outer surface of the Earth is a thin crust of fragile rock, fractured like the cracked shell of an egg. The pieces of the shell are Earth's tectonic plates -- there are 12 major ones -- and they float across a layer of soft rock like rafts in a stream, their motions driven by forces generated deep in the Earth. At their boundaries, the plates spread apart, converge, and slide past one another.

These boundaries, the danger lines described in the SAVAGE EARTH program "Hell's Crust," are the most geologically active regions on Earth. Here, new land is born of the Earth, and old land is consumed. Hot springs spew out mineral-rich waters, volcanoes erupt, and earthquakes tremble -- resulting in devastating tsunamis, floods, and mudslides.

Crustal Plate Boundaries

This map, which shows 20th-century earthquakes in red, illustrates how they cluster on the edges of the major tectonic plates (outlined in yellow).

Through decades of observation and experiment, scientists have developed a picture of Earth's interior and the forces that produce the violent activity we see on the surface.

The outer layer is the crust. At the thinnest spots in the oceans, where new crust is created, it is only a few miles thick; on the continents, the crust averages about 20 miles thick. The crust and, immediately below it, the strong upper part of the mantle (down to a depth of about 60 miles) together make up the Earth's lithosphere. (See The Hot Zones animation, 8K. You will need the free Flash plug-in to view this animation.)

Below the lithosphere is a region of solid, but softer and weaker, rock called the asthenosphere -- the upper portion of Earth's mantle. Below oceanic plates, it extends to depths of at least 217 miles beneath the surface -- and perhaps as much as 434 miles. The oceanic plates slide over this hot, weak layer. (The story is somewhat more complicated for continental crust, which may or may not ride over a layer of asthenosphere. Some scientists, such as David James of the Carnegie Institution of Washington, believe that the continents are anchored into the mantle by deep keels of rock that extend hundred of miles below the surface, and the continental crust and mantle therefore move in concert).
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Link to movie: Eruption on Heimaey
Link to movie: Eruption on Heimaey

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Eruption on Heimaey.

The Earth's entire mantle -- a region of hot rock that, over vast, geological timescales, actually churns like a fluid -- is some 1,800 miles thick. Beneath it lies the 1,300-mile thick outer core, a sea of liquid iron. The motion of that fluid (convection) creates Earth's magnetic fields. Finally, at the center of the Earth, is the solid iron core, a sphere some 1,500 miles across. The inner core is separated from the rest of the planet by the liquid outer core. It may even spin at its own rate, slightly different from that of the rest of the planet.

The core is unusual in other ways. Back in 1995, Ronald Cohen, a geophysicist at the Carnegie Institution, and his colleagues suggested that the inner core -- or at least a good portion of it -- may be a single crystal of iron. Their model of the core was generated after hundreds of hours of processing on a supercomputer. With the supercomputer, Cohen and his colleagues were able to predict the properties of iron at the great temperature and crushingly high pressures (millions of times greater than air pressure at the surface) in the inner core. Those results were then compared with the information that seismologists have obtained about the core from the paths that seismic waves take as they ricochet through the Earth's interior. "To match the seismology you either need a big crystal, or you need smaller crystals that are perfectly lined up with each other," Cohen says. "I'd say that most seismologists don't buy it, but the only other way to explain the data is if the structure of the iron in the inner core is something other than what anybody has ever seen in the laboratory."

Whatever its structure, and despite its great distance, the core plays a vital role in the active geology -- the volcanoes and quakes -- we see on the surface. "The core is really what drives what happens on the rest of the Earth," says Cohen. "Most of the heat for the Earth comes from the core. The Earth is cooling down with time, but the core has cooled down less, and so it is hotter. That heat is transferred up from the core to the mantle, where it is believed to drive things like convection in the mantle, and the plumes that come up to the surface and make islands like Hawaii."

Mantle convection is the process that carries heat from the core and up to the crust. It's an extraordinary amount of heat. In a single year, earthquakes alone release 1026 ergs of energy, or the energy of 100,000 Hiroshima-sized nuclear bombs. And that is just one percent of the total amount of energy that reaches the surface from Earth's innards.

Animation: Mid-Ocean Ridge
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Flash animation, 11K.
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Researchers aren't sure exactly how mantle convection happens -- some say that the entire mantle convects as a whole; others say that the upper and lower parts of the mantle convect separately from one another; still others think the real story lies somewhere in between. In any case, the result is the same: in certain regions, upwelling hot magma can break through the crust and reach the surface. In the oceans, magma reaches the surface at the boundaries between plates called spreading centers, like the Mid-Atlantic Ridge, and there new oceanic crust forms. (See Mid-Ocean Ridge animation, left.) The birth of new crust is accompanied by earthquakes. Also created are volcanic islands (like Surtsey and Heimaey, off the coast of Iceland, featured in the program "Hell's Crust") and hydrothermal vents (see Sidebar Two: "Black Smokers.")

The creation of crust at mid-ocean ridges "accounts for about ninety-five percent of the volcanic activity on Earth," says geophysicist Don Forsyth of Brown University, a principal investigator on the MELT project, which is conducting a detailed study of how melted rock flows from the mantle and into the spreading centers. "But it is under the oceans, and so it is not as obvious as what goes on at Mount St. Helens or in Hawaii."

The new crust eventually cools, and over time it is pushed to the side by still more melted rock rising up from within the Earth, in a continuous process. The plates move apart, away from the mid-ocean ridges. Eventually, after more than 150 million years, the cold crust is carried to subduction zones (places where one plate sinks beneath another), where it sinks back into the Earth -- dipping beneath another oceanic plate, or beneath a continent -- and melts once more. (See Subduction animation, right.) Researchers are still debating what force drives the motion of a plate across the ocean basin; one popular theory holds that the force of gravity alone -- from the weight of a plate sinking into a subduction trench -- is enough to pull the rest of it along.

Subduction zones, like the so-called Ring of Fire (see Sidebar Three) that surrounds the basin of the Pacific Ocean, are among the most violent of Earth's danger lines. The scraping of one plate on another generates powerful earthquakes; the heating of the plate within the depths of the mantle releases fluids which melt the rock over it, producing blobs of molten rock, or magma, that surface as volcanoes.

Animation: Subduction
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Animation, 16K. Flash required.

If the magma rises below the ocean, the result is volcanic islands (like the islands of Japan); if the magma rises under land, it forms chains of volcanoes (like the Cascades Range, home to Mount St. Helens, in the Pacific Northwest). Volcanic explosions of lava, hot gas, dust, and ash (a threat in their own right) can trigger landslides and mudflows, while strong offshore quakes and volcanic explosions can create dangerous tsunamis. The explosion of Indonesia's Krakatoa volcano in August of 1883, for example, generated a tsunami that killed a staggering 36,000 people in neighboring coastal towns.

Even if you live away from plate boundaries, you aren't necessarily safe from Earth's displays of power. The Hawaiian Islands -- located thousands of miles from any plate boundary -- are a prime example. There, a hot plume of magma has risen from within the mantle and broken through the crust. As the Pacific plate moved over the top of the plume -- called a hot spot -- over tens of millions of years, each of the volcanic islands was created. Researchers have identified at least forty other active hot spots on the Earth, which are responsible for the birth of numerous other volcanic island chains (as well as the volcanoes, hot springs, and geysers of Yellowstone).

Earthquakes, too, can occur outside of the plate boundaries. Within the interior of a plate, stresses -- from buckling, stretching, or compression of the rock -- can build up, until the rock finally breaks. Back in the winter of 1811-1812, a series of three of these intraplate quakes devastated the city of New Madrid, Missouri; other big quakes have been recorded in Charleston, South Carolina, and Boston, Massachusetts. In 1976, a quake in the interior of China, which may have been an intraplate quake, killed an estimated 240,000 people in the Chinese province of Tangshan.

Seismic map courtesy of the National Geophysical Data Center.

Article: The Earth at Work | Sidebar One: Probing the Depths | Sidebar Two: "Black Smokers" | Sidebar Three: Ring of Fire | ANIMATION
Hell's Crust: Our Everchanging Planet  | 
The Restless Planet: Earthquakes
Out of the Inferno: Volcanoes  |  Waves of Destruction: Tsunamis


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