Deadly and Yet Necessary, Quakes Renew the Planet
By WILLIAM J. BROAD
Published: January 11, 2005
They approach the topic gingerly, wary of sounding callous, aware that the geology they admire has just caused a staggering loss of life. Even so, scientists argue that in the very long view, the global process behind great earthquakes is quite advantageous for life on earth - especially human life.
Powerful jolts like the one that sent killer waves racing across the Indian Ocean on Dec. 26 are inevitable side effects of the constant recycling of planetary crust, which produces a lush, habitable planet. Some experts refer to the regular blows - hundreds a day - as the planet's heartbeat.
The advantages began billions of years ago, when this crustal recycling made the oceans and atmosphere and formed the continents. Today, it builds mountains, enriches soils, regulates the planet's temperature, concentrates gold and other rare metals and maintains the sea's chemical balance.
Plate tectonics (after the Greek word "tekton," or builder) describes the geology. The tragic downside is that waves of quakes and volcanic eruptions along plate boundaries can devastate human populations.
"It's hard to find something uplifting about 150,000 lives being lost," said Dr. Donald J. DePaolo, a geochemist at the University of California, Berkeley. "But the type of geological process that caused the earthquake and the tsunami is an essential characteristic of the earth. As far as we know, it doesn't occur on any other planetary body and has something very directly to do with the fact that the earth is a habitable planet."
Many biologists believe that the process may have even given birth to life itself.
The main benefits of plate tectonics accumulate slowly and globally over the ages. In contrast, its local upheavals can produce regional catastrophes, as the recent Indian Ocean quake made clear.
Even so, scientists say, the Dec. 26 tsunamis may prove to be an ecological boon over the decades for coastal areas hardest hit by the giant waves.
Dr. Jelle Zeilinga de Boer, a geologist at Wesleyan University who grew up in Indonesia and has studied the archipelago, says historical evidence from earlier tsunamis suggests that the huge waves can distribute rich sediments from river systems across coastal plains, making the soil richer.
"It brings fertile soils into the lowlands," he said. "In time, a more fertile jungle will develop."
Dr. de Boer, author of recent books on earthquakes and volcanoes in human history, added that great suffering from tectonic violence was usually followed by great benefits as well. "Nature is reborn with these kinds of terrible events," he said. "There are a lot of positive aspects even when we don't see them."
Plate tectonics holds that the earth's surface is made up of a dozen or so big crustal slabs that float on a sea of melted rock. Over ages, this churning sea moves the plates as well as their superimposed continents and ocean basins, tearing them apart and rearranging them like pieces of a puzzle.
The process starts as volcanic gashes spew hot rock that spreads out across the seabed. Eventually, hundreds or thousands of miles away, the cooling slab collides with other plates and sinks beneath them, plunging back into the hot earth.
The colliding plates grind past one another about as fast as fingernails grow and over time produce mountains and swarms of earthquakes as frictional stresses build and release. Meanwhile, parts of the descending plate melt and rise to form volcanoes on land.
The recent cataclysm began in a similar manner as volcanic gashes in the western depths of the Indian Ocean belched molten rock to form the India plate. Its collision with the Burma plate created the volcanoes of Sumatra as well thousands of earthquakes, including the magnitude 9.0 killer.
But despite such staggering losses of life, said Robert S. Detrick Jr., a geophysicist at the Woods Hole Oceanographic Institution, "there's no question that plate tectonics rejuvenates the planet."
Moreover, geologists say, it demonstrates the earth's uniqueness. In the decades after the discovery of plate tectonics, space probes among the 70 or so planets and moons that make up the solar system found that the process existed only on earth - as revealed by its unique mountain ranges.
In the book "Rare Earth" (Copernicus, 2000), which explored the likelihood that advanced civilizations dot the cosmos, Dr. Peter D. Ward and Dr. Donald Brownlee of the University of Washington argued in a long chapter on plate tectonics that the slow recycling of planetary crust was uncommon in the universe yet essential for the evolution of complex life.
"It maintains not just habitability but high habitability," said Dr. Ward, a paleontologist. (Dr. Brownlee is an astronomer.) Most geologists believe that the process yielded the earth's primordial ocean and atmosphere, as volcanoes spewed vast amounts of water vapor, nitrogen, carbon dioxide and other gases. Plants eventually added oxygen. Meanwhile, many biologists say, the earth's first organisms probably arose in the deep sea, along the volcanic gashes.
"On balance, it's possible that life on earth would not have originated without plate tectonics, or the atmosphere, or the oceans," said Dr. Frank Press, the lead author of "Understanding Earth" (Freeman, 2004) and a past president of the National Academy of Sciences.
The volcanoes of the recycling process make rich soil ideal for producing coffee, sugar, rubber, coconuts, palm oil, tobacco, pepper, tea and cocoa. Water streaming through gashes in the seabed concentrates copper, silver, gold and other metals into rich deposits that are often mined after plate tectonics nudges them onto dry land.
Experts say the world ocean passes through the rocky pores of the tectonic system once every million years or so, increasing nutrients in the biosphere and regulating a host of elements and compounds, including boron and calcium.
Dr. William H. Schlesinger, dean of the Nicholas School of the Environment and Earth Sciences at Duke, says one vital cycle keeps adequate amounts of carbon dioxide in the atmosphere. Though carbon dioxide is thought to cause excessive greenhouse-gas warming of the planet, an appreciable level is needed to keep the planet warm enough to support life.
"Having plate tectonics complete the cycle is absolutely essential to maintaining stable climate conditions on earth," Dr. Schlesinger said. "Otherwise, all the carbon dioxide would disappear and the planet would turn into a frozen ball."
Dr. Press, who was President Jimmy Carter's science adviser, said the challenge in the coming decades would be to keep enjoying the benefits of plate tectonics while improving our ability to curb its deadly byproducts.
"We're making progress," Dr. Press said. "We can predict volcanic explosions and erect warning systems for tsunamis. We're beginning to limit the downside effects."
http://www.nytimes.com/2005/01/11/science/11plat.html?pagewanted=1&th
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