June 12, 2014

GUATEMALA: Amazing Video of a Pyroclastic Flow at Santiaguito Volcano

Wired Science
written by Erik Klemetti
Wednesday June 11, 2014

Santa Maria (which is known also as Santiaguito) in Guatemala put on quite a show, with impressive explosive eruptions that produced numerous pyroclastic flows (also known as pyroclastic density currents, or PDCs) during May 2014. As many of you know, pyroclastic flows are some of the most deadly of the volcanic hazards. However, an intrepid geologist from the INSIVUMEH (the geological survey of Guatemala which monitors the country’s copious volcanic activity), got close enough to capture some amazing footage of a pyroclastic flow in action.

Pyroclastic flows are more or less avalanches made of hot volcanic debris. They can be formed from the collapse of an ash column when the force pushing the volcanic debris (tephra) can no longer push that material up against gravity, and it falls back down to earth as a flow. They can also be formed from the disintegration of a volcanic dome, where lava is slowly extruding until it gets too steep, and gravity causes the dome to collapse. The crumbling dome material flows down the side of the volcano in the form of a pyroclastic flow.

Why are pyroclastic flows so dangerous? Two reasons: speed and temperature. The “glowing avalanche” (as they have been colloquially called) move down the slopes of the volcano at speeds of hundreds of kilometers per hour, so if you’re in the path of these flows, you aren’t going to be able to move fast enough to get out of the way. The flows themselves are made of two components: gas (well, volcanic gases mixed with air) and debris (ranging in size from ash to giant boulders). So, although each flow is considered a single event, it is actually a sequence of events that start with a surge of fine ash, then the main body of the flow made up of all the massive debris (along with more ash) and finally after the flow has passed, ash filtered from the accompanying ash cloud that comes with the flow (see right).

These flows can move out away from the volcano distances of a few kilometers to over 50 kilometers in very large eruptions (like the 186 AD Taupo eruption). And unlike lahars (volcanic mudflows), pyroclastic flows have enough energy to jump over barriers like ridges, so they don’t have to stay confined to stream channels. They have the strength to knock down full forests of mature trees with this energy and mass, as we saw during the 1980 eruption of Mount St. Helens.

Now, as I said, these flows are made of volcanic gases and debris, both of which can be hot. We’re talking 500ºC (~930ºF) or more, so most biological material in the path of the flow doesn’t stand much of a chance, because even if you don’t get taken out by the boulders and rocks, you’ll suffocate and burn in the flow. This is why victims of pyroclastic flows are usually found with profound burns and contorted limbs, like the victims at Pompeii. The heat of the flow can also kill and burn trees, even if the flow didn’t knock them down — and this is one way volcanologists have been mapping the extent of pyroclastic flows activity during some eruptions.

Pyroclastic flows are highly dangerous and unpredictable events on a volcano — that is, they can sometimes behave in ways that volcanologists don’t expect. This is why sometimes even the biggest volcanic experts can be swept away and killed by pyroclastic flows or why people who travel into evacuation zones might be killed when new pyroclastic flows occur unexpectedly.

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