Pyroclastic Flow | Universe Today

Written by Fraser Cain

Think about the dangers of a volcano: lava flows, explosions of rock and ash raining down. Dangerous stuff, but the big risk is a pyroclastic flow. This is a devastating cloud of hot gas and rock that speeds down the side of a volcano faster than 700 km/hour, with temperatures higher than 1000 degrees C. More people have died to pyroclastic flows than any other volcanic event.

The biggest pyroclastic flows can contain more than a thousand cubic kilometers of rock and gas, and travel for hundreds of kilometers; although, those haven't been seen for hundreds of thousands of years. In recent history, flows have contained up to 10 cubic kilometers of material, and traveled for several kilometers. They can leave deposits of ash and rock hundreds of meters thick.

When a pyroclastic flow is released from a volcano, it travels rapidly downhill, hugging the ground, destroying everything in its path. It was pyroclastic flows that destroyed the ancient towns of Pompeii and Herculaneum in 79 AD. And 19 people were killed in 1997 by a pyroclastic flow on the island of Montserrat. Flows can even move across water. In the 1883 eruption of Krakatoa, one flow reached the coast of Sumatra 48 kilometers away.

Pyroclastic flows are caused by a sudden event in the volcano. For example, during the eruption of Mount Saint Helens in 1980, an explosion in the mountain caused part of it to collapse, and material moved swiftly downhill. Flows can also happen when an eruption column collapses.

In the event of a pyroclastic flow, the strategy is to head for higher ground, as the flow will follow the contours of the landscape. Of course, the best course of action is to be as far away from an erupting volcano as possible.

We have written many articles about volcanoes for Universe Today. Here's an article about different types of volcanoes. And here's an article about different types of lava.

Want more resources on the Earth? Here's a link to NASA's Human Spaceflight page, and here's NASA's Visible Earth.

We have also recorded an episode of Astronomy Cast about Earth, as part of our tour through the Solar System – Episode 51: Earth.

Filed under: Astronomy