Plate Tectonics

The continents of Earth (Source: The Great Globe Gallery)

The drifting of continents is now understood as one manifestation of large-scale convection in the mantle of the Earth. This was the missing link that geologists failed to envisage in the early 20th century, except for Arthur Holmes, whose suggestion was ignored. This larger framework of convection includes the changing distribution of land and sea, the making of continental material in mountain chains, the distribution of earthquakes and volcanism in space and time and the chemistry of the ocean and the atmosphere. We now truly have an appreciation of the "origin of continents and ocean basins", thanks to plate tectonic theory. The basic thesis of Wegener (that continents move across the face of Earth) is abundantly confirmed.

Convection in the Earth's mantle. Hot rock (yellow) rises while cooler rock (blue) sinks. (Source: The Dynamic Earth)

How do they move? Continents do not plow through the mantle (which is impossible, given the strength of the mantle material) but ride passively on 100-km thick "plates", which are in turn carried along by convection in the mantle. Convection, of course, is a familiar process to all, it is what happens in a pot with soup when it is heated from below. (It is most evident if the heating unit is has a hot center). The broth rises in the hot places and sinks in the (relatively) cooler ones, mainly along the walls. The less dense ingredients (e.g., fat) accumulate on top of the sinking soup, its greater buoyancy keeping it there. Similarly, since continents are made of materials too light to be swallowed by the mantle in the subduction zones (the sinking limbs of the convection),they stay on top of the boiling mantle, banging into each other in one place and separating in another. Right now, India and Tibet are in collision, a process that generates the Tibetan highlands and the Himalayas. Recent and ongoing separation between landmasses once joined can be observed around the Red Sea rift and the Gulf of California. In the East African Rift Valley we can see how a continent breaks apart.

Why all or most of the different landmasses were united into one big continent at the end of the Paleozoic (Wegener's "Pangea") is not clear. For this to happen, the older Palezoic continents had to converge and join up (making such mountains as the Appalachians and the Urals in the process) and convection patterns had to change subsequently to tear the "supercontinent" apart again.

Earth's Tectonic Plates (Source: NGDC)

"Plate tectonics" has two elements, and of these two "tectonics" is by far the older. James Hutton (1726-1797), who invented what we now call the "rock cycle" (in his book "Theory of the Earth", published in 1795), guessed that ordinary soft sediments accumulating on the surface of Earth were somehow brought into conditions of great heat and pressure. As a result, they were cooked into crystalline rocks, and then re-exposed in mountains on the surface, after being tilted and jumbled and folded. The cycle would then begin anew with erosion of the mountains and the making of sediments. The hidden portion of the rock cycle, the one involved in making crystalline rocks like schist and gneiss and granite, and folding and shearing them while building mountains, is the field of study called "tectonics". Everywhere we look, mountains are being vigorously eroded and the resulting sediment is transported into the ocean basins. Yet, after billions of years of Earth history, we still have mountains, and we still have deep ocean basins. The reason is that the ocean floor is renewing itself by a process called sea floor spreading, and the sediment accumulating on it is removed into zones where mountains are made, by a process called subduction. The two processes occur at the edge of plates, well-defined regions covering the Earth like large pieces of armor. Where the plates meet, they move apart (seafloor spreading) or they converge (subduction) or they move past each other (fault zone). The main zones of seafloor spreading are more or less in the center of the ocean basins. Here hot mantle material comes up and makes new sea floor, and that is why the sea floor is high in the center (rather than being the deepest part of the basin). The main zones of subduction are all around the rim of the Pacific. These zones are marked by trenches (where the crust buckles downward) and by mountain ranges and volcanoes landward of the trenches (where heated materials return, in part, to the surface).

A hydrothermal vent.

Among the many new concepts arising from the theory of plate tectonics (which includes seafloor spreading and continental drift) are entirely new ideas about the chemistry of the ocean. In trying to understand why the sea has the salt it does, we must worry not just about weathering on land, input by rivers, precipitation, and reactions of seawater with sediment. We must also consider the exhalations of the newly ascending magma and the reactions of seawater with the newly exposed basaltic seafloor, and we have to take into account the materials disappearing into the trenches. Furthermore, we must worry about the enormous salt deposits which can accumulate within new ocean basins, created by rifting and as yet semi-isolated from the world ocean. These new concepts are having an important impact on the thinking about seawater composition. Two discoveries were especially important in this development. One was the discovery of hot vents on the East Pacific Rise, and their direct sampling by the ALVIN submarine of Woods Hole Oceanographic Institute. The analysis of vent waters shows that its composition differs from that of seawater, and represents a control on seawater composition, through geologic time. The other was the discovery of vast amounts of salt at the bottom of the Mediterranean basin, which drew attention to the loss of salt from seawater through evaporation. The Mediterranean desiccation event is but one example of the fact that ocean basins and seas with restricted access to the world ocean can accumulate large amounts of salt. Such basins developed many times in geologic history. There are large salt deposits of Permian age in many parts of the globe (about 200 million years old). This was the time of initial breakup of Pangaea. Subsequently salt basins developed in the Caribbean (Triassic), in a deep narrow North Atlantic Basin (Jurassic) and in a deep narrow South Atlantic (Cretaceous). We do not know the amounts of salt extracted from the ocean at these various times. They were considerable, however and markedly reduced the salinity of seawater. Also, such extraction of salt tend to alter seawater composition itself, because calcium and sulfur are removed preferentially in the build-up of the salt deposits.