In this case, this is a collapsed
crater caused by a massive volcanic eruption. So much material is ejected from the
Earth's crust that the upper layers fall into the void below, leaving a somewhat circular
depression. Crater Lake in Oregon is one classic example.
In central Arizona, three massive calderas (on the order of 10 - 15 miles in
diameter each) are superimposed upon each other in the Superstition Mountains, east of
Phoenix. Their surface expression is barely visible, however. The ash and
debris from these explosions comprise most of the rock in the Superstitions, and was
deposited around 25 million years ago.
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