Earth & Space Science: Session 7
A Closer Look: Anorthosite
Anorthosite is a kind of igneous rock that is composed mostly of the mineral plagioclase. Plagioclase is made of relatively lightweight elements like silicon, calcium, and aluminum. As the Moon was forming, it was heated not only from within, but by the continual bombardment of asteroids and planetary bodies. The outer several hundred kilometers of the lunar mantle was molten, yielding a magma ocean that began to cool and separate. When the magma started to crystallize, the light plagioclase crystals floated up toward the surface, while denser minerals sank to form the dense interior of the moon. Anorthosite formed the early lunar crust, of which the highland regions are made.
The Earth has much less anorthosite than the Moon, although it is found in some of the Earth’s oldest rock formations. When the Earth was first forming, anorthosite was probably produced the same way it was on the Moon. The Earth, however, is much bigger, and its crust was more dynamic. Because of plate tectonics, which continually destroy and create the Earth’s surface, most of the anorthosite that existed has probably been processed into the Earth ’s interior.
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