caldera

   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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