Geoscience Reference
In-Depth Information
Chapter 9
The oceanic lithosphere: ridges,
transforms, trenches and oceanic islands
9.1 Introduction
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. Under-
standing the structure of the oceanic lithosphere and the mid-ocean ridges is
particularly important because it provides a key to understanding the mantle.
9.1.1 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 aver-
age 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.
Mid-ocean ridges
The mid-ocean ridges are a chain of undersea mountains with a total length of over
60 000 km. The ridges typically rise to heights of more than 3 km above the ocean
basins and are many hundreds of kilometres in width. The Mid-Atlantic Ridge was
discovered while the first trans-Atlantic telegraph cable was being laid. The East
Pacific Rise was discovered by H. M. S. Challenger in 1875. As was discussed in
Chapter 2, the mid-ocean ridges mark the constructive boundaries of the plates.
Hot material rises from the asthenosphere along the axes of the mid-ocean ridges
and fills the space left by the separating plates; as the material cools, it becomes
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