The volcanos of the Eastern Mohave desert are largely extinct due to the way in which they were formed. Each cinder cone and lava dome in this region had its origin deep within the Earth’s upper mantle. This geologic story begins roughly 100 miles north-east of the intersection of the North American and Pacific crustal plates–which form the San Andreas fault–where this fault crosses the San Bernardino Mountains at Cajon Pass. Though these plates are running laterally against one another, there is sufficient subduction of the Pacific plate to draw water-rich oceanic crust deep into the mantle below the North American plate. The introduction of water into the mantle at these depths lowers pressure such that large bubbles of hot magma form which begin to rise up and into the Earth’s solid crust. It’s essentially the same process which causes bubbles to form in a bottle of pop after the cap is opened. These “bubbles” of molten rock burn their way slowly through miles of solid rock as they rise towards the surface., cooling, changing, and growing smaller as the go. Tens of thousands of years later, the molten bubbles of magma arrive at the surface to “pop” as small, short-lived volcanos, or simply to bulge through the surface–without popping–to form nicely symmetrical lava domes.
While driving through the desert on Interstates 15, 40 or old Route 66, it’s possible to see dozens of these extinct volcanos and lava domes frozen at the moment of their death, when the last of their molten magma cooled upon reaching the surface of the Earth. We can even gauge how long their journey was by the color and texture of their rocks; with dark volcanos rising more quickly than light-colored volcanoes which enjoyed a longer, or slower, trip from the liquid mantle to the surface of the Earth.
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