一The Earth comprises three principal layers: the dense, iron-rich core, the mantle made of silicate (硅酸盐) that are semi-molten at depth, and the thin, solid-surface crust There are two kinds of crust, a lower and denser oceanic crust and an upper，lighter continental crust found over only about 40 percent of the Earths surface. The rocks of the crust are of very different ages. Some continental rocks are over 3,000 million years old, while those of the ocean floor are less than 200 million yea
