I’m sure you’ve heard, ad nauseum, about the plate tectonics underlying the earthquake and tsunami in Japan. Indeed, it seems that plate tectonics, which produces earthquakes, volcanoes and tsunamis with devastating consequences is a force of destruction, pure and simple. But does it have a positive side as well?
The theory of plate tectonics, which at this point does the best job of explaining the earth’s geology, is based on the idea that the earth’s surface is made up of a number of semi-rigid plates which slide around over the earth’s surface. They interact primarily at their edges, where they may be pulling apart (as in the mid-Atlantic and the African rift valleys) sliding past each other (as in the San Andreas fault of California) or colliding.
Plates are made up of ocean crust, sometimes with relatively light continental crust on top. Ocean crust is dense enough to slide under other plates; the lighter continental rock above it resists being pulled under, and buckles or folds if it is on top of two colliding plates. Thus collisions of two plates with continents on top generally leads to mountain ranges such as the Himalayas.
Collisions between ocean plates and plates with light continental rock atop generally lead to subduction zones, such as the one off the west coast of South America, where the oceanic crust is pulled under the lighter continental crust. The sediments pulled down with the ocean crust are gradually heated and melted, reappearing as volcanic magma. Thus the volcanic spine of the Andes.
If two oceanic plates collide one is normally pulled under the other, but it is less obvious which will be subducted, and in fact this may change over time. The same melting of sediments occurs, and a line of volcanoes, such as the Aleutian Islands, normally develops next to the subduction zone.
Plates don’t slide past each other smoothly. They stick and then break loose, producing earthquakes. If they are just sliding past each other they may produce earthquakes but there is generally not much vertical movement. If one plate is being pulled under another, however, the sticking normally results in a bowing up of one plate, and when that sticking is released, there may be considerable vertical movement. If that movement is under water, a tsunami is created. This is what happened with the great Alaska earthquake, and has now happened off the coast of Japan.
But what would happen if the plates all just stopped? If there were no more plate tectonics? More, if there had never been any plate tectonics?
First, the earth would be flat and completely covered with water, if there were any water on the face of the earth. Mountains are constantly being eroded by the forces of weather. Given far less geologic time than has actually passed, any initial irregularities in the surface of the earth would have been smoothed out. Plate tectonics is and has been the main mountain builder on our planet.
Second, there is some question as to whether we would have an atmosphere. Certainly we’d have a hard time breathing the mixture of carbon dioxide, water vapor and other compounds put out by volcanoes, but then we’d have a hard time breathing the atmosphere prevailing when life evolved. Plants convert the gasses produced by volcanoes into an atmosphere we can breathe.
Third, plate tectonics is part of the way radioactive heating in the earth’s core is transferred to the surface. It’s one of the reasons we don’t have the radical resurfacing we think we see on Venus.
Plate tectonics can certainly produce devastation, but like weather, it’s something we have to live with. Japan has actually done a superb job of preparation, but there are prices we must pay for living on a dynamic planet, one which can support life. One of those prices has just come due.