Plate tectonics

The theory of plate tectonics was formulated in the late 1960s and brings together the theories of sea floor spreading and continental drift. The continents and ocean basins are believed to have evolved over the past 200 million years or so and plate tectonics provides an explanation for the way in which this may have happened.

According to current theories of plate tectonics and sea floor spreading, the outer crust of the earth (the lithosphere) is made up of about 20 separate lithospheric plates which cover the molten mantle rather like a cracked shell. Most plates carry continental masses plus adjacent ocean floor whilst some carry only ocean floor. The rigid, cool plates are not fixed in position, but ride on top of the hot partly molten asthenosphere, moving continents and ocean basins with them. These movements over the mantle are not yet fully understood but the main driving force is thermal energy. A complex convective flow deep within the mantle is probably involved (Whitmarsh et al., 1996).

Submarine ridges, including the mid-oceanic ridges, are believed to mark the

Constructive margin Constructive margin Destructive margin

Continental Abyssal Abyssal Continental Continental Mountain shelf plain plain slope shelf range

Continental Abyssal Abyssal Continental Continental Mountain shelf plain plain slope shelf range

Destructive Margins
Figure 1.4 Diagram to illustrate the theory of sea floor spreading. Submarine ridges mark lines of tectonic plate separation, where new crust is created as mantle material moves to the surface. Ocean trenches are formed and crust is lost where continental and oceanic plates collide.

lines where lithospheric plates are moving apart. To fill the gaps between separating plates, molten basalt wells up from the interior to the surface, forming submarine ridges which gradually subside laterally to become new ocean floor. As the new crust is formed, the earlier crust spreads away. The spreading rate has been calculated as ranging from less than 1 cm per year from the mid-Atlantic ridge to 16 cm per year from the East Pacific Rise between the Pacific and Nazca plates. Along the centre line of each ridge there is a depression which marks the actual line of division from which lateral spreading is taking place (Figure 1.4).

Where the edges of moving plates collide one plate is forced below the other to form a deep oceanic trench with adjacent volcanic islands. This is known as subduction, and the edge of the plate is forced down into the mantle and resorbed. Where an oceanic plate collides with a continental plate, mountain ranges may be thrust up consisting of volcanic material and folded sediments. Where two oceanic plates collide, a volcanic island arc develops. Subduction is the basis of the so-called 'ring-of-fire' in the Pacific Ocean which is an almost continuous chain of volcanoes surrounding it.

Submarine ridges and trenches are therefore associated with areas of volcanic activity resulting from these movements of the earth's crust. The ridges are essentially different from mountain ranges on land because they are formed entirely of extrusions of igneous rock into the sea floor, whereas mountains on land consist mainly of folded upthrusts of sedimentary rock.

On this theory, the Atlantic and part of the Indian Oceans are thought to be younger than the Pacific, and to have originated in Triassic times when a splitting of the lithosphere was followed by a break-up and separation of continental blocks. The Atlantic and Indian Oceans may still be enlarging at the expense of the Pacific by a westward drift of North and South America and by a combination of northward and eastward drifts of the crustal plates bearing Africa, Eurasia and Australia (Dietz and Holden, 1970; Anderson, 1992).

The margins and relative movements of the major plates are shown in Figure 1.5. Comparison with Figure 1.3 will reveal the coincidence of ridges and trenches with lines of separation and subduction between lithosphere plates.

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