Geoscience Reference
In-Depth Information
can get lifted on to land, a process called obduction. Because they are from collision zones,
such rocks are often very distorted, but, by piecing together the evidence from several such
sequences, an overall picture emerges. They are known as ophiolite sequences, from the
Greek for 'snake rock'. The descriptive name is also reflected in the name serpentinite, giv-
en because of the wiggly lines in the green minerals of the forms metamorphosed by hot
water. At the top of an ophiolite sequence are the remains of ocean sediments followed by
pillow lavas and sheets of basalt that may have been injected underground. Then comes
gabbro, the slow-cooled crystalline rock of the same composition as basalt, and at its base
the layered deposits of crystals from the bottom of the magma chamber. Beneath that there
can be traces of the mantle rock from which the basalt was derived.
Lost oceans
Over hundreds of millions of years it is clear that many oceans have both opened and
closed. For a long time, from 1,200 to 750 million years ago, the continents were clustered
into one giant super-continent, surrounded by a single vast ocean spanning two-thirds of
the globe. In the late Pre-Cambrian, the super-continent broke up into separate land masses.
New oceans formed. One of them, the Iapetus Ocean, lasted between about 600 million and
420 million years ago. The join, or suture, where it closed again can today be crossed in
a short drive across northwest Scotland. Half a billion years ago that journey would have
involved a 5,000-kilometre sea crossing. By the Jurassic period of about 200 million years
ago, a great wedge of ocean, the Tethys, had opened up between western Europe and south-
east Asia where it opened into the Pacific. That closed as Africa hinged round into Europe
to form the Alps and India came crashing into Tibet, lifting the Himalayas. Seismic studies
trace remnants of the Tethys ocean floor descending into the mantle.
Over geological time, there have been numerous occasions when new oceans might have
formed but did not. East Africa's Great Rift Valley and the Red Sea and Jordan Valley are
obvious recent examples. The stretching of the North Sea basin, which produced North Sea
oil deposits and Bavarian hot springs, is another. Another few hundred million years and
our ocean charts will be completely out of date again.
