A volcanic eruption is one of the most awe-inspiring sights of nature. Molten rock, or magma, flows down the volcano’s slopes while boulders are hurled miles into the sky. Yet the most destructive, most lethal part of an eruption is the pyroclastic flow.
From a distance, a pyroclastic flow looks like an avalanche. Grey clouds of ash and rock race downhill at speeds of 100 miles per hour or more, annihilating everything in their path. Though the observer can’t tell, a pyroclastic flow has two parts: a river of rocks heated to hundreds of degrees that follows the contours of the ground, while above it, suspended by the heat below, a cloud of ash and dust seems to boil as it storms down the slopes. (A more detailed explanation is provided on the website of the Volcano Information Center run by the University of California at Santa Barbara.)
Not every eruption involves a pyroclastic flow, but they are a feature of many of the most destructive. Here’s a list of some of the most significant:
1883: Off the coast of Sumatra, Krakatoa explodes releasing massive pyroclastic flows. As summarized by San Diego State University, these crossed open water to devastate the island of Sebesi, home to some 3,000 people, and reached parts of the mainland.
1902: On the Caribbean island of Martinique, Mount Pelée erupts, sending a pyroclastic flow down its flanks to destroy the city of Saint-Pierre. Close to 30,000 people died, suffocated and burned alive by the cloud of superheated ash and dust. More details on the U.S. Geology Service website.
1912: Some 290 miles south of Anchorage, in the biggest eruption of the 20th century, Novarupta spews forth the most massive pyroclastic flows ever recorded. Thankfully, there were no fatalities, but as described on Geology.com, the ash and rock swept more than 15 miles downhill, completely filling a river valley and transforming it into a broad flat plain.
1980: Mount St Helens, part of the Cascades in Washington State, erupts. The USGS reports that pyroclastic flows engulfed much of the surrounding lowlands, in places to a depth of 50 meters.
1995: Montserrat's Soufrière Hills volcano rained ash down on the town of Plymouth. There were no fatalities, as the population of some 4,000 had already been evacuated. The town has since been abandoned.
Finally, no listing of historic pyroclastic flows would be complete without the most famous of all, those thrown out by Vesuvius in AD 79. The eruption of this volcano, located on the beautiful Bay of Naples in what is now Italy, destroyed the thriving Roman cities of Pompeii and Herculaneum. It is believed that some 2,000 people perished in Pompeii, buried in ash and lava. The death toll in Herculaneum is unknown although the city is thought to have been hit hard by the superheated ash.
Volcanic eruptions are some of the most awe-inspiring demonstrations of natures power. Molten lava is spectacular, but for sheer destructive power the pyroclastic flows are what to watch out for.