Arizona is not known for its active volcanoes, but its landscape is dominated by the products of millions of years of volcanic explosions.  And the plumbing that funnels molten lava to the surface is still intact and waiting for the opportunity to erupt.  The last explosion occurred around the time Normans were invading England.  It could recur at any time.  More ancient activity tore up the landscape and left behind a heritage of destruction and creation.