Craters of the Moon National Monument

Idaho is mountain country:  a seeming unending array of rocky summits with one notable exception — the Snake River Plain part of which we crossed today from Boise to Arco.  The Snake River Plain is a broad flat arc. Like a huge geologic smile, grinning across Idaho, the plains marks the path of a series of volcanic eruptions that took place millions of years ago.

The first eruption on the Snake River Plain, about sixteen million years ago, was in southwestern Idaho.  The cause is believed to have been a "hot spot" that melted deep crystal rock creating a thick magma called rhyolite.  As the rhyolite explosively erupted, it literally blew holes, called calderas in the earth’s crust.

At the same time, the crust of the earth in this area was moving forward to the southwest over the stationary hot spot.  As a result the bottom of the moving plate was repeatedly heated, causing a long sequence of eruptions, which formed a string of calderas across southern Idaho — spaced about two millions years apart.

These explosive rhyolitic eruptions were followed by less violent volcanic activity that produced lave known as basalt which covered the land in thin sheets as the earth’s crust moved away from the hot spot.  These flows also buried the calderas under a frosting of black rock.

At Craters of the Moon, calderas that formed five million years ago are totally buried by basaltic flows.  The most recent eruption to place just two thousand years ago.  Originating from a weakened zone in the earth’s crust known as the Great Rift, fiery fountains shot cinders high into the air forming cinder cones.  Liquid rock poured from the cracks to create dozens of lava flows that can be seen there today.

After driving our RV through the National Monument (there were some tight loops in parking lots), we proceed to Arco ID about 20 miles to the east.  Arco is near the site of the first nuclear power plant in the USA, and is the first city to be totally powered by its power.

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