cific rim; and mountain building, as in the Himalayas, the Caucasus, and the Alps. At transform boundaries, plates slide past one another, as along the San Andreas fault in California, commonly producing large earthquakes but little volcanic activity.
A key component of the plate tectonics model is the nearly rigid moving plate. The plates are nearly rigid because the rocks near Earth’s surface are cool and therefore strong and difficult to deform, even on geological timescales. At greater depths, temperatures rise and the rocks become soft and deformable (Questions 4 and 6). As a consequence, most plates extend to a depth of only about 50 to 200 km below the surface. The relative strength of the plates allows them to move without significant internal deformation. The motion of all points on any rigid plate can be fully described by only two pieces of information: the location of a “pole” about which the plate rotates and the rate of rotation about the pole; this property of rigid plates gives plate tectonics its simplicity and mathematical elegance.
Complexities in the plate model arise from differences in the types of crust that comprise the plates. The two types of rocky crust, oceanic and continental, are distinguished by thickness, composition, and age (see also Question 2). Oceanic crust is thin (5 to 9 km), young (less than 200 million years old), and for the most part fairly uniform in chemical composition, consisting of basalt, which is a volcanic rock with silica (SiO2) content of about 50 percent by weight. In contrast, continental crust is thick (30 to 70 km), varies in age from young to very old (4 billion years), and also varies greatly in composition. The average composition is andesitic, which is a volcanic rock with about 58 percent SiO2, but locally the composition varies from less than 40 percent to greater than 70 percent SiO2, with the upper crust being much more silica rich than the lower crust. In general, rock with higher SiO2 content is less dense, melts at a lower temperature, and is more deformable than rock with lower SiO2 content. Thirty to forty percent of the radioactive heat-producing elements are concentrated in the continental crust, and as a consequence the deep parts of the continental crust