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).
A pyroclastic flow from Mayon in the Philippines, seen on September 23, 1984. I've labeled the main parts of a pyroclastic, with the leading surge, main body and accompanying ash cloud. Image: Chris Newhall / USGS - Wikimedia Commons. 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.
Pyroclastic flows from the May 9 eruption of Santiaguito. Image: CONRED - Guatemala So, when faced with a pyroclastic flow, why would you try to get up close to one? Julio Cornejo, an INSIVUMEH observer from the Santiaguito volcano observatory (OVSAN), did just that to capture some video (see top and below) that has surprised everyone that has seen it. Cornejo was able to film the very far end of a pyroclastic flow generated by the dome collapse at Santiaguito (see above), when it had lost most of its energy but was still moving. At this point a flow is likely still capable of engulfing and suffocating someone in hot ash and gasses, as happened to many people in the 1902 eruption of Mont Pelee. So, Cornejo is very lucky to have made it out alive with this footage, but what he got was remarkable. Rudiger Escobar Wolf, a volcanology post-doctoral researcher at Michigan Tech who studies volcanoes in Guatemala, posted a sequence of videos from Cornejo and INSIVUMEH and annotated some of the video to understand what we're seeing. I've also watched the video closely and have two here that show some likely never-before-filmed examples of how pyroclastic flows can be destructive even after they slowed to a snail's pace relative to their usual speed. The video at the top of this post captures the best action, where the pyroclastic flows, moving at a remarkably slow speed, reach the area near Cornejo. You start seeing a debris-filled channel, like with material from pyroclastic flows from earlier in that same day. Ash starts billowing in from the left and small rocks are seen rolling in at ~0:28. You can notice that many of the trees are stripped of leaves, likely from the earlier flows. Not long after the first small boulder, you hear rumbling and cracking from two likely sources: (1) larger boulders in the pyroclastic flow itself and (2) trees coming down. If you need more evidence of the flow knocking down trees, just wait ... multiple trees come down as if being run over by a bulldozer. These trees are likely being pushed over by rolling boulders in the flow as the forest is slowly consumed, almost more like a slow-moving a'a lava flow than what we picture with pyroclastic flows. You can check out the full annotation from Rudiger Escobar Wolf for this video with all the details. So, how do we know that this is a pyroclastic flow and not a lahar (mudflow?). The main evidence, to me at least, is the accompanying ash clouds which are both whitish and grey/brown, betraying that ash is present. Lahars don't usually have (if ever) accompanying ash clouds and don't usually leave steaming tephra deposits (see video above) because the mixing with meteoric water keeps lahars cool. Some lahars can generate steam if they are close to the source, as this video from June 7 shows near the volcano observatory at Santiaguito, but any deposits of this lahar will likely be relatively cool. In construct, hot ash flows remain hot for some time after being deposited, and persistent steam vents are common. Now, all this volcanic material could become lahars later on, when rainfall can wash this loose ash down the stream channel, so even after the eruption is over, the hazards remain from this volcanic debris. Currently, lahars like these are even threatening to take out the volcano observatory at Santa Maria. Santiaguito (Santa Maria) has numerous explosive eruptions, some of which you can catch on the INSIVUMEH webcam. However, this great on-the-ground video by Julio Cornejo should provide plenty for volcanologists to discuss about how pyroclastic flows behave as they grind to a halt. Cornejo said afterwards that he was glad to have gotten the video but doesn't plan to get that close to a pyroclastic flow ever again -- advice we should all heed! Videos courtesy of Julio Cornejo/OVSAN-INSIVUMEH, used by permission.
