Geoscience Reference
In-Depth Information
ocean crust right across the north Atlantic between Greenland and Scotland. Seismic sur-
veys reveal that there are about 10 million cubic kilometres of additional basalt there, sev-
eral times the volume of the Alps, or enough to cover the entire USA with a layer one kilo-
metre thick. A lot of it didn't erupt on to the surface but was injected beneath the crust,
under-plating it. The Hatton bank off the coast of Greenland is a bulge caused by such in-
jections of basalt. The mantle plume that is now under Iceland may have been what caused
the north Atlantic to begin to open about 57 million years ago. The volcanic activity ap-
pears to have started with a series of volcanoes, some of which are still preserved in the
Inner Hebrides and Faeroe Islands northwest of Scotland.
Where oceans go to die
Ocean crust is constantly forming. As a result, it is difficult to find any truly ancient ocean
floor. The oldest dates back to the Jurassic, about 200 million years ago, and is in the west-
ern Pacific. A segment about 145 million years old was recently discovered near New Zea-
land. But such ages are rare; most of the ocean floor is less than 100 million years old. So
where have all the ancient oceans gone?
The answer lies in a process called subduction. As the Atlantic widens, the Americas on
one side and Africa and Europe on the other are slowly moving apart. But the Earth is not
getting bigger overall, so something must be taking up the slack. It appears to be the Pacif-
ic. The Pacific seems to be ringed by great trenches, up to 11,000 metres deep. Behind them
is a ring of volcanoes on islands or on continents, the so-called Pacific ring of fire. Seismic
profiling shows how the ocean plate - the thin ocean crust and as much as 100 kilometres
of mantle lithosphere beneath - is plunging back down into the Earth. In the 100 million
years of its existence, the rocks of the lithosphere have steadily cooled and contracted, be-
coming more and more dense so that they can no longer float on the asthenosphere. It is
this process of subduction that is one of the driving forces of plate tectonics: a pull rather
than a push.
