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If you look at a map of the world, it is easy to see how the shape of Africa's west coast appears to mirror the eastern coast of South America. Studies of the geological structure and rock types on either side of the Atlantic also indicate that the two continents were once joined, e.g. the Appalachians and the Scottish Highlands.

Figure 3.20 Earth's lithosphere is divided into seven large plates and a number of smaller ones which are in continual motion. Volcanoes (red dots) and fold mountains form along plate boundaries. Most of the world's earthquakes also occur where crustal plates meet.

(USGS)

Evidence such as this was used by a German meteorologist, Alfred Wegener (1880–1930), to develop a theory of continental drift. Although his vision of moving land masses was too revolutionary to be accepted at the time, it is now accepted that Earth's lithosphere is divided into a number of huge, jostling plates that are in perpetual motion (Figure 3.20). This is known as the theory of plate tectonics.

According to this theory, the solid lithosphere is “floating” on the denser mantle. The plates are slowly, but continuously jostling against each other. Some are moving apart (diverging), others are colliding (converging) or sliding laterally past each other. This movement, typically a few centimeters per year (comparable to the growth rate of fingernails), is responsible for most earthquakes and volcanoes.

It is also responsible for the gradual growth or shrinkage of the oceans. For example, Europe and North America are creeping further apart as new crust is created on the floor of the Atlantic Ocean. In contrast, the Pacific Ocean is gradually shrinking as oceanic crust is destroyed at the margins.

The lithospheric plates grow at the mid‐ocean ridges, where two slabs of crust move apart and new crust is created. Hot material slowly rising beneath the ridge experiences a reduction in pressure which lowers its melting temperature. This enables rock from the asthenosphere or upper mantle to melt beneath the rift area, forming basaltic lava flows which solidify on or beneath the ocean floor. Over millions of years, as the new oceanic crust piles up to form a huge mountain range, the ocean gradually becomes wider.

An example of a new ocean experiencing its birth pangs can be seen in East Africa, where the crust is pulling apart to form a huge rift valley. The northern end of the Great Rift Valley has already been flooded to form the Red Sea. Large lakes, such as Lake Tanganyika, also occupy parts of the valley.

Since we know that Earth is not increasing in size, it follows that sea‐floor spreading must be balanced by the destruction of crustal material. This is now known to occur at ocean trenches, where a thick, relatively dense oceanic plate is melted as it dives beneath a neighboring plate which is less dense.

The descending slab contains a significant amount of water (Figure 3.23). As the plate descends deeper and deeper, encountering greater temperatures and pressures, this water is released into the overlying wedge of mantle. The addition of water has the effect of lowering the mantle's melting point. Magma produced in this way varies from basalt to andesite in composition.

As it rises to the surface, the magma feeds arcs of volcanic islands, such as Japan, Indonesia, and the Philippines, that lie parallel to the oceanic trench. If the plate sinks beneath a continent, the magma creates a similar belt of volcanoes near the coast, e.g. the Andes of South America, or the Cascades in North America.

These destructive plate margins, often known as subduction zones, are also associated with the deepest (and some of the most severe) earthquakes on the planet, such as the one which caused the Indian Ocean tsunami disaster on December 26, 2004.