Geology, the study of our planet Earth, is essentially a science of keen observation of all the natural processes of the planet that go on around us all the time. Consequently, a great deal is known about the earth, although, of course, there is still a great deal to be learned. Let’s know detail about what are crustal plates.
You have already seen, for example, what makes the rain fall, how rivers form, how volcanoes work, even that whole continents are able to move, and so on. What has been needed, however, is an idea, a theory, which would knit together all that is known about the separate processes into one unified whole.
This would be a theory to explain how continents drift, why earthquakes occur only in certain parts of the Earth, why there are deep ocean trenches in some areas or oceanic ridges in others.
Today we seem to have such a theory explaining all, or most of what has been puzzling the experts for many years. This is the all-embracing theory of plate tectonics. The word ‘tectonics’ comes from the Greek tecton, a builder. In other words, tectonics refers to building and structure, in this case of the Earth itself. Thus, the idea of plate tectonics suggests that the surface of the Earth is made up of a number of rigid plates comprising the thickness of the crust and the upper part of the mantle.
The plates are free to move, and indeed are doing so today, floating on the plastic substance of the deeper layer of the Earth’s mantle, called the asthenosphere. They may well be kept in motion by the convection currents that have been mentioned before.
The important point to remember is that the plates are solid and rigid so that almost all the activities that we have mentioned such as volcanoes and earthquakes, are to be found associated with the edges of these plates, or the plate margins, as they are more usually called.
There are three main types of margin. The plates may just slide past each other, such as the San Andreas Fault in California, and this is called a conservative margin. The mid-oceanic ridges, where mantle material is welling up from the crust, are known as constructive margins.
In other places, such as the west coast of South America, where the island arcs occur that you have already learned about, one crustal plate is slipping beneath the other at a destructive margin.
The deep ocean trenches, such as the Marianas Trench, are to be found here, where the sea floor is pulled down. Earthquake activity is often concentrated in a zone known as the Benioff zone in these areas.
You can see, then, that in one unifying idea, the Earth’s activities can be viewed as a whole, rather than as a number of isolated processes.
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