The rocks above, which includes the crust, is called the lithosphere. The lithosphere is made up of comparatively rigid plates that are moving, floating in the upper mantle like giant tabular ice sheets floating in the ocean. Since molten rock is a liquid S-waves cannot travel through molten rock. The core makes up about 15 percent of the earth's total volume and about one-third of its mass. This data, plus information from the nature of meteorites indicates that the earth's core is mostly made of iron or a nickel and iron alloy.
Divergent boundaries occur between two plates moving away from each other.
Magma forms as the plates separate, decreasing pressure on the mantle below. This molten material from the asthenosphere rises, cools, and adds new crust to the edges of the separating plates. The new crust tends to move horizontally from both sides of the divergent boundary, usually known as an oceanic ridge. A divergent boundary is thus a new crust zone. Transform boundaries occur between two plates sliding by each other.