Earth's moving and shaking may be caused by a thinner crust
San Francisco — 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.
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.
Directly beneath the plates is a region called the asthenosphere, or sphere of weakness. Its presence is manifest by the fact that it slows and absorbs certain frequencies of sound waves. This is a region where up to 20 percent of the virgin mantle material is molten. Thus, it lacks the mechanical strength of the lithosphere above or of the region called the mesosphere, which lies just below.
When the solid mantle material from the mesosphere rises to a depth of about 90 kilometers, it begins to melt. The melting is the result of a complex interplay among the minerals involved, the heat generated by the decay of radioactive materials in the planet, and the tremendous pressures deep within the Earth.
The plates form a fairly impermeable cover over this layer of partially molten rock. As the plates grow older, however, they also cool and become heavier, until they are significantly more dense than the asthenosphere that supports them. This is an unstable condition, Ward points out: ''They will continue to float, like a paper towel on water, until an edge begins to curl under, and then they begin to sink.''
These are the subduction zones, the great deep-sea trenches where the massive plates dive in ultraslow motion back into the bowels of the planet. In fact, subduction drives plate tectonics, the geophysicist maintains. The fastest forming plates are those with the largest subduction zones.
A common theory has been that heat convection - the process that in the atmosphere gives rise to thunderstorms - drives plate formation. From Ward's new perspective, however, the tensions created by subduction cause the brittle plates to fracture, creating pathways for magma to come to the surface and create new crust.
To explain volcanic chains, such as the Hawaiian Islands, scientists using the prior theory of much thicker plates have been forced to invoke such concepts as mantle plumes and hot spots. While this seems to fit in a few specific instances, there are other cases in which the pattern does not fit, Ward says. Instead, he argues that these volcanoes started along fractures in the plates.
His theory on plate thickness jibes well with the Hawaiian volcanoes. As Mauna Loa has built up its 42,500-cubic-kilometer caldera of black basalt, a hemisphere of depleted mantle with a radius of 45 kilometers has thickened the plate on the bottom until it has reached the bottom of the asthenosphere, Ward says. If this is right, then it should be using up the last of its magma source, the fertile mantle, and shouldn't grow much higher. The fact that island volcanoes build down as well as up explains the wide variety of rock types found in their magma, Ward says. The characteristics of magma change drastically with the depth at which it originates.
There is another aspect to Ward's picture that is startling.
This has to do with the question of what happens to old plates. Once formed, these plates do not remelt nearly as easily as the original mantle material. Earthquakes have been detected as deep as 700 kilometers beneath subduction zones, suggesting that plates keep their integrity to extremely great depths. If this is the case, then a back-of-the-envelope calculation by another scientist shows that the volume of tectonic plates created since the Earth began would be enough to fill up the center of the Earth from the outer boundary of the core - 2,700 kilometers deep - to within 500 kilometers of the surface. As the plates sink toward the center, virgin mantle material has been pushed steadily upward ''like water in a pot when plates are slipped in,'' he explains. The world has another billion years or so before the fertile mantle material is depleted and plate tectonics grinds to a halt.
Only time will tell if this ''thin plate'' theory will be widely accepted. But Ward is optimistic.I have given a number of talks on my ideas, asking for criticism. So far I've managed to find credible answers for all the objections, '' he says.