Three-fifths of the surface of the solid Earth is oceanic lithosphere, all of which has been formed during the last 160 Ma or so along the mid-ocean ridges. Understanding the structure of the oceanic lithosphere and the mid-ocean ridges is particularly important because it provides a key to understanding the mantle.
Beneath the waves
Bathymetric profiles across the oceans reveal the rugged nature of some of the seabed and something of the scale of its topography (Figs. 9.1 and 9.2). The deepest point on the surface of the Earth was discovered during the voyage of H. M. S. Challenger (1872–6). The bottom of the Challenger Deep in the Mariana Trench (western Pacific Ocean) is 10.92 km below sea level, and Mauna Kea on the island of Hawaii rises to 4.2 km above sea level from an ocean basin more than 5 km deep. Such features dwarf even Mount Everest (8.84 km above sea level). The average global land elevation is 0.84 km, whereas the average depth of the oceans is 3.8 km. Although the seabed is hidden from us by the oceans, the imprint of its shape is revealed by the sea surface and gravity (see Fig. 5.4 and Plates 7 and 8).
The seafloor can be classified into four main divisions: mid-ocean ridges, ocean basins, continental margins and oceanic trenches.