How Fragile Basalt Can Be

Columnar Basalt Remains – Yellowstone National Park

I think of basalt as a hard, stable volcanic rock created from lava flows.  Columnar basalt is formed when lava cools slowly.  It forms multi-sided vertical columns as it cools.  These columns are characterized by horizontal fractures.  When the columns are exposed to rushing water, the water carves out these fractures and the columns collapse. This image illustrates the vertical basalt columns as well as the collapsed column residuals.

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