by: BRAD LEWIS
S OME 4.6 BILLION YEARS AGO A CLOUD OF DUST CON-DENSED INTO PLANET EARTH, which soon turned molten from meteorite impacts and radioactive decay. As it cooled, heavier materials sank, forming a layered globe 8,000 miles in diameter . Still cooling, the earth roils from its core , bringing heat to the surface for release. This convection creates earth-quakes and volcanoes—such as Hawaii’s Kilauea —that shape our life-bearing lands and seas. Scientists have long pondered what lies beneath the earth’s surface. New technology yields some answers.
Eons of earthly turmoil have fueled
centuries of scientific debate.
S EISMIC ENERGY EXPLODED in all directions as the earth moved deep beneath Bolivia in June 1994 . As shock waves rippled through the crust, high-rise offices swayed in Iowa. Houseplants shook in Minnesota. Even eastern Canada trembled. At magnitude 8.3, it was the biggest earthquake in decades.
More than a million quakes jolt the earth each year. Most don’t make much of a stir. Perhaps a few are devastating. Yet even those are small stuff in the big geologic picture. “We humans are self-centered, so we worry about what will affect us,” says Raymond Jeanloz, a geophysicist at the University of California at Berkeley. “But hazards like volcanoes and earthquakes are very superficial results of the grand motion of this whole planet.”
Jeanloz and his colleagues have no hope of exploring inner earth in person. There are no secret volcanic passages to the core like the one that guided Jules Verne’s characters on their fictional Journey to the Center of the Earth. The real world’s deepest borehole reaches only seven and a half miles beneath Russia’s Kola Peninsula.
Lack of access hasn’t hindered conjecture, however. In 1665 German scholar Athanasius Kircher drew an early cross section of the earth. In his view, winds fan subterranean reservoirs of fire, inflaming volcanoes around the globe.
Three decades later English scientist Edmond Halley suggested that the earth held concentric spheres inside. Glowing gas that illuminated these inner worlds escaped from the North Pole to form the aurora borealis. Isaac Newton’s explanation of gravity soon helped prove his friend Halley wrong. It enabled scientists to calculate the earth’s density, which increases with depth.
By the late 19th century, physicists realized that the earth was radiating heat into space. Lord Kelvin estimated its rate of cooling and calculated that the earth was between 20 million and 100 million years old. Naturalist Charles Darwin disagreed. His theory of evolution required a much longer time span.
Lord Kelvin and other Victorian scientists figured that the earth was as rigid as steel. Not so, argued German meteorologist Alfred Wegener in the early 1900s: The continents moved around the planet. His idea stirred rancorous debate for decades. Rebounding in the 1960s as the central concept of plate tectonics, continental drift revolutionized geology. Scientists now know the earth is dynamic and enormously complex. At the surface ride more than a dozen huge, stiff fragments, or plates. They move at a slower-than-snail’s pace—only inches a year—but cover thousands of miles over millions of years. As they collide and separate, they change the face of the globe by deforming and rearranging its features.
The engine that propels tectonic plates lies below: hot inner layers that churn like thick soup simmering in very slow motion. The details are nowhere near that simple, though, and researchers are only beginning to close in on them. Working in fields such as seismology, geodynamics, geochemistry, and mineral physics, they ponder where this planet has been and what will become of it.
Interpretations vary greatly. Disagreements are rife, and tempers flare. “It’s amaz-ing how emotional people get about the inside of the earth,” says Jeanloz. “I can think of a few individuals who won’t even speak to me because of my conclusions.
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