If the ages of Earth and Moon are nearly identical...why
are most rocks
found on the Moon so much older than Earth rocks?
The reason is that the moon stopped being geologically active soon after it
formed, but the earth is still active. By "active," I mean that the earth
is constantly recycling its minerals: rocks that have been formed erode,
wash out to sea, and are carried into the earth's mantle with subducting
tectonic plates, to melt and perhaps come to the surface again as lava. The
age of a rock is the time since the rock solidified.
Richard E. Barrans Jr., Ph.D.
PG Research Foundation, Darien, Illinois
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Update: June 2012