Introduction and the ophiolite model

Abstract

The islands of the ocean basins were familiar to early geologists. They were well acquainted with the volcanoes of Iceland, the Azores, the Canaries and the Mediterranean islands. When Darwin landed on St Paul’s Rocks in the equatorial Atlantic from the Beagle in 1831, he could recognize the anomaly of the peridotite mylonites that he found there. These mylonites are the only subaerial outcrop of an active oceanic transform fault zone, although that phrase could only have been used after 1965. However, early nineteenth century geologists were much less clear about what lay below the surface of the sea. This is not surprising as the first reliable deep-sea sounding was not made by Sir James Clark Ross until 1842. Even now it is difficult to convey to land-bound lay people the great depth of the oceans, the fundamental differences between continents and oceans, and the constant renewal of the ocean floor by seafloor spreading and subduction, when their concepts are bounded by a wrinkled sea surface viewed from the air. In the early nineteenth century such ideas lay beyond everyone’s grasp.