What Comes Up Must Go Down

Harry Hess's theory of seafloor spreading described how new ocean crust was created at the mid-ocean ridges and was consumed in the seafloor trenches or along the edges of continents. While this seemed like a logical idea, there was little data to support it when it was first proposed. But as geologists gathered additional information about the crust from volcanoes and earthquakes, Hess's ideas began to look better and better.

High concentrations of earthquakes and volcanoes along the mid-ocean ridges clearly supported the idea that magma created new ocean crust as it flowed out of the Earth along these features. The question was, did earthquakes and volcanoes offer any evidence to suggest that subduction was also happening? The answer was a resounding yes.

Almost as soon as seismologists started recording data from earthquakes, they discovered that most of them had a shallow focus. This meant that the point of motion was not very far below the surface, usually less than 6 miles (10 km). Every so often they would record an earthquake that had a focus that was deeper than 62 miles (100 km).

When seismologists plotted these "deep focus" earthquakes on a map, they discovered that they only occurred under ocean trenches or along the edges of continents where tall mountains and active volcanoes were located. In other words, deep focus earthquakes only happened in areas that Hess had said were subduction zones. The most logical explanation was that deep focus earthquakes were triggered by pieces of old ocean crust that were sinking back down into the Earth to be recycled. If this were truly the case, it would also explain the mountains and volcanoes. As one piece of crust slid under the other, the surface would be bent up to create the mountains. The enormous friction caused by pieces of the crust rubbing against each other would melt some of the rock to create the magma that would then flow to the surface to form volcanoes.


By the late 1960s, the plate tectonic puzzle was almost complete. Almost all the pieces were in place to support a bold new theory of how the surface of Earth constantly changes. Only one last question remained to be answered: How could pieces of the crust move about if the rocks of the mantle were solid? The answer was obvious—maybe the mantle isn't so solid after all.

Seismologists conducting research on the speed of seismic waves moving through Earth generally found that the deeper the waves went, the faster they moved. For some reason, though, there was a zone between 65 and 210 miles (100 and 350 km) below the surface where both P- and S-waves slowed down. This "low-velocity zone" did not make sense because the density and composition of the rocks in this area appeared to be similar to those found above and below it. The only other explanation for this drop in wave speed was that the rocks in this zone were not as solid as those surrounding it. In fact, seismic velocities in this zone were so low that in some places the rock seemed to behave like a super-thick fluid.

Scientists named this area the asthenosphere to distinguish it from the area of the mantle above and below it. They now believe that the asthenosphere behaves like a fluid because of pockets of basaltic magma spread throughout the solid rock. This magma is thought to be the source of the new crust that forms in the mid-ocean ridges. It is also thought to be the layer on which the continents "float" as they move around the Earth. With the discovery of the asthenosphere, all geologists had to do was fit all the pieces together. Once this was done, the theory of plate tectonics was finally born.

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