Subduction
If new oceanic lithosphere is created at mid-ocean ridges, where
does it go? Geologists had the answer to this question before Vine and
Matthews presented their hypothesis. In 1935, K. Wadati, a Japanese
seismologist, showed that earthquakes occurred at greater depths towards
the interior of the Asian continent. Earthquakes beneath the Pacific
Ocean occurred at shallow depths. Earthquakes beneath Siberia and China
occurred at greater depths. After World War II, H. Benioff observed the
same distribution of earthquakes but could not offer a plausible explanation.
The movement of oceanic lithosphere away from mid-ocean ridges
provides an explanation. Convection cells in the mantle help carry the
lithosphere away from the ridge. The lithosphere arrives at the edge of
a continent, where it is subducted or sinks into the asthenosphere.
Thus, oceanic lithosphere is created at mid-ocean ridges and consumed at
subduction zones, areas where the lithosphere sinks into the
asthenosphere. Earthquakes are generated in the rigid plate as it is
subducted into the mantle. The dip of the plate under the continent
accounts for the distribution of the earthquakes. Magma generated along
the top of the sinking slab rises to the surface to form stratovolcanoes.
