Pyroclastic flows are spinning mixtures of pyroclasts (small pieces of obsidian, ash, pumice, and cinders) and very hot gases. They flow down the side of the volcano at speeds up to 100 miles per hour and at temperatures sometimes over 700 degrees Fahrenheit!! With temperatures that high pyroclastic flows kill everything it their path.

There were two pyroclastic flows from Mt. St. Helens main eruption in 1980. The first flow was called the "stone wind" and it annihilated everything in its path. Huge trees over one hundred feet tall were snappped and splintered like twigs. Temperatures of over 700 degrees ate up all the oxygen in the area. All animal life in its path was destroyed in seconds including 57 humans. Later in the day another pyroclastic flow piled pumice and ash in thick deposits for many miles around the mountain.

The photo on the left is a pyroclastic flow down the north flank of Mt. St. Helens.

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