Amateur geologists know that the Earth's crust is a puzzle of tectonic plates and that most of the planet's earthquakes and volcanic activity occur along their margins. But they often assume that the shape of the plates corresponds closely to the contours of our Continents and Oceans. In fact, the seven large plates have both continental and oceanic regions; it is important to realize that the plates are 50 miles thick and that the surface features that we see are but a thin veneer on their upper side.
Africa, Australia and Antarctica all sit near the center of their plates, which extend into the ocean on all sides. On the other hand, some plates are far more complex and their shape is less predictable from the surface features. The North American plate extends from the West Coast of the U.S. to the mid Atlantic Ridge and from southern Mexico to the Arctic Ocean; however, it also extends from the center of Iceland to eastern Siberia, including the Bering Sea, the northeast flank of Asia and the northern islands of Japan. It is interesting to note that eastern Siberia and northern Japan are on the North American plate while southern California is not; the latter lies on the eastern edge of the Pacific plate and is inching northwestward along the San Andreas Fault.
In addition to the seven large plates, there are numerous smaller ones. Among the more well known are the Indian Plate, which is crunching into southern Asia and lifting the Himalayas, and the Nazca Plate, which is subducting beneath the west edge of South America and creating the volcanic range of the Andes.
