Geoscience Reference
In-Depth Information
12. How ocean lithosphere subducts beneath a continent, accreting sediments along
the margin and producing volcanic activity inland.
The cold, dense rock which sinks in a subduction zone has been under the sea, so it is wet.
There is water in the pore spaces and also bound chemically in minerals. As the slab sinks
and the pressure and temperature increase, the presence of water lubricates its flow but also
lowers the melting point of some of its components, which rise through the surrounding
crust to feed the ring of fiery volcanoes. As we saw in the last chapter, the rest of the slab
of lithosphere continues down into the mantle, at least to the 670-kilometre boundary with
the lower mantle, but eventually sinks perhaps as far as the base of the mantle. Seismic
tomography can help trace its billion-year journey.
There are several different types of boundary between the slabs of continent and ocean
lithosphere that make up the tectonic plates of the Earth. In the ocean there are the con-
structive boundaries of ocean ridges and the destructive ones where subduction occurs.
This can take place where ocean lithosphere dives down beneath a continent, as in the case
of the west coast of South America, forming the volcanic peaks of the Andes. Or ocean
can dive beneath ocean, as with the deep trenches of the western Pacific, where the ring
of fire comprises volcanic island arcs. There are boundaries where one plate grinds its way
alongside another, such as along the coast of California. And there are also plate boundar-
ies where continent runs into continent, but we will discuss them in the next chapter.
What's left on land
Not everything vanishes with a lost ocean. Where ocean lithosphere dives beneath the con-
tinents or where an entire ocean is squeezed out between two land masses, much of the
sediments get scooped up and added to the continents. That is one of the reasons why so
many marine fossils are to be found on land. Occasionally, entire masses of ocean crust
