What are lahars and pyroclastic flows?
The most destructive aspect of volcanoes are lahars and pyroclastic flows.
Lahars are mudflows created when water (from rain or meltwater from glaciers) and volcanic ash mix. This deadly combination can have devastating results on the surrounding area. When lahars settle they can be metres thick and as hard as cement. Lahars can occur long after a volcanic eruption.
Pyroclastic flows are avalanches containing hot volcanic gases, ash and rock. They are the most deadly event to happen at a volcano. Pyroclastic flows have killed over 90,000 people since 1600AD. 30,000 people were killed by a pyroclastic flow from Mount Pelee in 1902.
Pyroclastic flows are incredibly dangerous for several reasons. They are fast moving. On steep volcanoes, pyroclastic flows can reach speeds of 450 miles per hour. Pyroclastic flows are incredibly hot. They come from explosive eruptions or from the collapse of a lava dome. Temperatures can be up to 1000°C. The clouds mask dense avalanches carrying rock. The force of a pyroclastic flow can destroy buildings and flatten trees.
Possibly one of the most famous pyroclastic flows was the one caused by the eruption of Mount Vesuvius in 79 AD which covered the Roman city of Pompeii with ash and volcanic debris. 1.5 million tons of rock, pumice and ash we released every second.
The video below shows a more recent pyroclastic flow at Mount Ontake, Japan.
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