My wife and I went to Devil's Postpile National Monument. It is an
example of columnar jointing. It has the typical six-sided columns that one can
easily recognize. This is caused by rapid cooling from the outside, and this
causes shrinkage cracks that produce a distinctive look. The igneous rock is
classified as mafic, which you can tell by its color and grain. It is basalt
that is fine-grained and has a high content of magnesium and iron. The rock
gets its color mainly from plagioclase feldspar and pyroxene. The rock looks
like a typical example of a fine-grained basalt. This particular rock is unique
in that it is extrusive, not intrusive. Columnar jointing is typically caused
beneath the surface. This particular site was formed above the surface when a 400ft
deep lava lake was formed after running through the valley. The formation was
deep in the former lava lake, but a glacier exposed them.
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