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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.
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References

1. file://localhost/vwdocs/vwlessons/plate_tectonics/part9.html
2. file://localhost/vwdocs/vwlessons/atg.html
3. file://localhost/vw.html
4. file://localhost/vwdocs/vwlessons/plate_tectonics/part11.html