Earth's moving and shaking may be caused by a thinner crust
Don't look down, but the solid ground beneath your feet may be thinner than you think. This is the somewhat radical contention of Peter L. Ward, a senior scientist at the US Geological Survey.Skip to next paragraph
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Since returning to research from managing the Survey's earthquake prediction program, Dr. Ward has reviewed some 4,000 scientific papers dealing with the nature of the lithosphere, Earth's rocky, outermost shell. From this broad study he has concluded that it must be half the thickness geologists have assumed for the past 15 years. He presented his arguments last week at the annual meeting of the American Geophysical Union.
Early scientists such as Descartes and Leibnitz in the 16th century realized that the Earth was not a simple ball of rock, but must be made up of various layers. The earliest conception was that the planet consisted of a molten interior surrounded by a thin, rocky crust. By the 18th century this view had been refined to conceive of a strong core with a mushy or liquid layer underlying a hard crust several thousand miles thick.
But it wasn't until the 1960s that Earth scientists discovered a simple, all-embracing idea that allowed them to interpret the changes continually being wrought in the earth's surface. It's called plate tectonics. According to this concept, the earth's surface is covered by a small number of rigid ''plates,'' and major geologic change occurs primarily at their boundaries. These plates fit closely together but are in constant, if generally undiscernable, motion. This movement takes place at rates ranging from millimeters to centimeters a year.
Plate tectonics has done much to explain the nature and location of earthquakes, volcanoes, and other features on the face of the earth. But the precise nature of the processes that create, drive, and ultimately destroy crustal plates over millions of years has remained a matter of considerable mystery.
Plate boundaries have been mapped by plotting the epicenters of earthquakes. Deep-sea submersibles have surveyed the mid-ocean ridges where new crust is manufactured by underwater volcanic activity. The deep-sea trenches where old plates plunge back into the depths of the Earth have been mapped with sonar. But trying to peer through miles of solid rock is an iffy proposition, even with all the instruments of modern science. And many of the manifestations of plate tectonics remain confusing, even conflicting.
Now Dr. Ward feels he has made a major conceptual breakthrough in deciphering the mechanism behind tectonics. But to do so he has had to challenge entrenched views in a number of disciplines.
''Recognition that the plates forming the outer, mobile layer of the earth are only half as thick as commonly assumed provides an answer to many problems that have arisen,'' he says.
To back this assertion he invokes a wide range of geophysical measurements of the undersea plates, which scientists primarly study because they're much simpler than those that carry the world's continents. These include several types of seismic data, measurements of the amount of heat flowing through undersea plates, their electrical conductivity, and details of petrology (the study of different types of rocks).
Tectonic plates, Ward says, have two layers. The top layer is 6 to 7 kilometers thick. It is formed from the magma, or molten rock, which seeps up through cracks in the crust. The second and generally thicker layer is made up of ''depleted mantle,'' which separates from the magma when the latter is formed. This layer averages about 33 kilometers in thickness. The two layers are termed the lithosphere.