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A Closer Look: Anorthosite
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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