When a supervolcano like Yellowstone erupts, residents may have a few hours to escape once the disaster has started, a new study suggests. 
 
 Can you outrun a supervolcano? New evidence from an ancient eruption suggests the answer is a surprising yes. 
 
 "I wouldn't recommend anyone try to outrun a volcano, but there's a few of us that could," said Greg Valentine, a volcanologist at the University at Buffalo in New York. 
 
 By analyzing rocks trapped in volcanic ash, Valentine and his colleagues discovered the lethal ash flow spread at street speeds — about 10 to 45 mph (16 to 72 km/h). It might be hard to sustain this pace on foot, but it's certainly possible by car. [Big Blasts: History's 10 Most Destructive Volcanoes] 
 
 The findings were published today (March 7) in the journal Nature Communications. 
 
 "It's really interesting how you can have such a violent eruption producing such slow-moving flows," said Valentine, co-author of the new study. "They still devastate a huge area, but they're slow and concentrated and dense," he told Live Science. His collaborators include Olivier Roche, of Blaise Pascal University in France and David Buesch, of the U.S. Geological Survey. 
 
 Of course, the safest way to deal with any rumbling volcano is to get as far away as possible. Lots of distance can prevent the most common cause of death associated with volcanoes: being trapped and suffocated by a torrent of ash, rocks and superhot gas that explode out at speeds of up to 300 mph (about 480 km/h). These "pyroclastic flows" are the real volcanic killer, not lava. A pyroclastic flow wiped out the Roman town of Pompeii, and in 1902, Mount Pelée on Martinique unleashed a pyroclastic flow that killed some 29,000 people. [Preserved Pompeii: Photos Reveal City of Ash] 
 
 You should still evacuate 
 
 Volcanologists try to account for such hazards when planning for future disasters. But it's hard to know what will happen when a supervolcano the size of Yellowstone blows its top. The last supereruption on Earth was 74,000 years ago, in Toba, Indonesia. Looking at the rocky remains of past supereruptions can reveal how and why supervolcanoes erupt. 
 
 These rocks were picked up and moved across the Arizona landscape by pyroclastic flows from the Silver Creek caldera, a supervolcano, eruption 18.8 million years ago. Credit: Greg A. Valentine 
 
 When a supervolcano blew in Arizona 18.8 million years ago, the ash spread more than 100 miles (160 km). This single layer, called the Peach Springs Tuff, is more than 450 feet (140 meters) thick in the area close to the volcano and 10 feet (3 m) thick at its edge, 100 miles away. (A tuff is a volcanic rock made of solidified ash.) 
 
 The researchers measured rocks at the bottom of the tuff in Arizona that were carried in the flow. They matched unique rock types back to their source, and found that many of the rocks, whether fist-size or boulders, were carried no farther than a football field. 
 
 Accounting for the size and position of these rocks helped the researchers build a model of how fast and thick the ash flow was as it traveled. It turns out that only a dense, slow-moving pyroclastic flow could suck up the rocks from the surface and trundle them along. A fast, relatively thin flow would have to reach impossible speeds — up to 1,454 mph (2,340 km/h) — to carry the rocks, the researchers found. 
 
 "I think it's plausible but speculative," said Calvin Miller, a volcanologist at Vanderbilt University in Tennessee, who was not involved in the study. "It will be interesting to see how the [scientific] community responds to it. Even if they're right for the Peach Springs Tuff, this is just part of a continuum of eruption styles," Miller told Live Science. 
 
 The origins of the Peach Springs Tuff can be spotted in southwestern Arizona's Black Mountains, near the town of Oatman. The eruption left behind a very large crater called a caldera, though it has been mostly obliterated by erosion and faulting. 
 
 The caldera, called Silver Creek, spewed magma for several days, releasing a volume of about 1,000 times the Mississippi River's daily flow at New Orleans, Valentine said. "If you think about 1,000 Mississippi Rivers coming out of the ground, you can see how [the ash] would have spread out across a huge area," he said. 
 
 However, one expert on the Peach Springs Tuff doesn't buy the scenario. Charles Ferguson, a research geologist with the Arizona Geological Survey, said there are outcrops that suggest the ash moved quickly and energetically, like a typical pyroclastic flow. 
 
 "I think their hypothesis is more problematic than explanatory," Ferguson told Live Science. 
 
 Southwestern supervolcanoes 
 
 Kodachrome slides, held by geologist Greg Valentine of the University of Buffalo, show images of geologic formations associated with the supereruption of the Silver Creek caldera. Credit: Douglas Levere 
 
 The Peach Springs Tuff covers parts of Arizona, Nevada and California, from Barstow, California, to Peach Springs, Arizona. Geologists use the creamy white and pink rock as a unique marker in the region. 
 
 The western United States suffered at least 100 of these huge eruptions starting about 40 million years ago (a consequence of shifting tectonic plates). It's not clear whether every one of these supervolcanic blasts sent out slowly moving ash flows, but Valentine said he sees similar evidence in other areas. 
 
 The powerful Peach Springs eruption ejected 72 cubic miles (300 cubic km) of pulverized rock into the air. For comparison, the 1980 eruption of Mount St. Helens in Washington blasted out 0.24 cubic miles (1 cubic km) of material. And the 1991 eruption of Mount Pinatubo in the Philippines spewed 2.4 cubic miles (10 cubic km) of material. 
 
 Any supereruption will likely come with a fair amount of warning, similar to the bulge that foreshadowed the Mount St. Helens eruption. The new findings suggest that people living near a supervolcano might have a few hours to evacuate once the disaster starts, the researchers said. 
 
 Follow us @livescience, Facebook & Google+. Original article on Live Science. ||||| Supervolcanoes capable of unleashing hundreds of times the amount of magma that was expelled during the Mount St. Helens eruption of 1980 are found in populated areas around the world, including the western United States. 
 
 A new study is providing insight into what may happen when one of these colossal entities explodes. 
 
 The research focuses on the Silver Creek caldera, which sits at the intersection of California, Nevada and Arizona. When this supervolcano erupted 18.8 million years ago, it flooded parts of all three states with river-like currents of hot ash and gas called pyroclastic flows. These tides of volcanic material traveled for huge distances -- more than 100 miles. 
 
 The new study suggests that pyroclastic flows from the ancient eruption took the form of slow, dense currents -- and not fast-moving jets as some experts previously thought. 
 
 The research combines recent laboratory experiments with field data from the 1980s -- some of it captured in colorful Kodachrome slides -- to show that the rivers of ash and gas emanating from the Silver Creek caldera likely traveled at modest speeds of about 10 to 45 miles per hour. 
 
 "Intuitively, most of us would think that for the pyroclastic flow to go such an extreme distance, it would have to start off with a very high speed," says study co-author Olivier Roche. "But this isn't consistent with what we found." 
 
 The research was conducted by Roche at Blaise Pascal University in France, David C. Buesch at the United States Geological Survey and Greg A. Valentine at the University at Buffalo. It will be published on Monday, March 7 in Nature Communications. 
 
 Research on pyroclastic flows is important because it can help inform disaster preparedness efforts, says Valentine, a UB professor of geology and director of the Center for GeoHazards Studies in the UB College of Arts and Sciences. 
 
 "We want to understand these pyroclastic flows so we can do a good job of forecasting the behavior of these flows when a volcano erupts," he says. "The character and speed of the flows will affect how much time you might have to get out of the way, although the only truly safe thing to do is to evacuate before a flow starts." 
 
 New and vintage data come together to tell the story of a supervolcano 
 
 The new study favors one of two competing theories about how pyroclastic flows are able to cover long distances. One school of thought says the flows should resemble turbulent, hot, fast-moving sandstorms, made up mostly of gas, with few particles. The other theory states that the flows should be dense and fluid-like, with pressurized gas between ash particles. The new research supports this latter model, which requires sustained emissions from volcanoes, for many pyroclastic flows. 
 
 The findings were based on two sets of data: results from recent experiments that Roche ran to simulate the behavior of pyroclastic flows, and information that Buesch and Valentine gathered at the Silver Creek Caldera eruption site in the 1980s when they were PhD students at the University of California, Santa Barbara, supplemented by some more recent fieldwork. 
 
 "I always tell students that they should take good notes while they're working in the field, because you never know when it could be useful," says Valentine, who has a fat binder full of Kodachrome slides showing images he snapped around the Silver Creek caldera. 
 
 The data that he and Buesch collected included photographs and notes documenting the size, type and location of rocks that were lifted off the ground and moved short distances by pyroclastic flows during the ancient eruption. 
 
 Many of the rocks the pair observed were relatively large -- too large to have been shifted by sandstorm-like pyroclastic flows, which do not pick up heavy objects easily. Denser flows, which can move sizable rocks more readily, likely accounted for the rock patterns Buesch and Valentine observed. 
 
 To figure out how fast these dense flows may have been moving when the Silver Creek caldera erupted 18.8 million years ago, the team relied on a model developed by Roche through experiments. 
 
 In his tests, Roche studied what happened when a gas and particle mixture resembling a dense pyroclastic flow traveled across a substrate of beads. He found that faster flows were able to lift and move heavier beads, and that there was a relationship between the velocity of a flow and the weight of the bead it was capable of lifting. 
 
 Based on Roche's model, the scientists determined that the ancient pyroclastic flows from the supervolcano would have had to travel at speeds of about 5 to 20 meters per second (10 to 45 miles per hour) to pick up rocks as heavy as the ones that Buesch and Valentine saw. It's unlikely that the flows were going much faster than that because larger rocks on the landscape remained undisturbed, Valentine says. 
 
 The findings could have widespread applicability when it comes to supereruptions, says Valentine, who notes that patterns of rock deposits around some other supervolcanoes heavily resemble those around the Silver Creek caldera. ||||| Imagine being near a volcano when it unleashes a gigantic eruption. I’m not talking something fairly piddling like the 1980 eruption of Mount St. Helens or even the 1991 eruption of Pinatubo in the Philippines. I’m talking one of these eruptions that the tabloids and conspiracy websites say will destroy civilization, like Yellowstone or Toba. The common response is that everyone within hundreds of kilometers of the volcano would be killed almost instantly thanks to the fast moving pyroclastic flows that can rush outward from the caldera volcano for more than 150 kilometers (~100 miles). That idea is based on what we can see from these flows at smaller eruptions, where they race down the sides of the volcano at speeds over 500 kilometers per hour (300 mph). Cities like Pompeii and St. Pierre were wiped out mere moments after an eruption thanks to these avalanches of hot volcanic debris and ash. 
 
 However, we’ve never been able to examine first-hand the results of a really giant eruption that puts Vesuvius and Pelée to shame. So, we need to look at the deposits left by such gargantuan events to figure out how they might be similar or different than their smaller brethren. Do the pyroclastic flows race out at the same speeds and are these flows the same mix of hot gasses and ash? The answer to those questions can help us better prepare for such an eruption and interpret the deposits left by these monsters in the past. 
 
 A new study in Nature Communications by Olivier Roche (Université Blaise Pascal), D.C. Buesch (USGS) and Greg Valentine (University at Buffalo) has taken a stab at quantifying the speed of one of these massive eruptions and the results surprised me: Maybe we wouldn’t be so doomed? 
 
 Roche and others looked at the Peach Springs Tuff, a massive volcanic deposit that erupted from the Silver Creek Caldera in Arizona about 18.8 million years ago. Now, the Peach Springs Tuff dwarfs most eruptions in the past few thousand years, with at least 1,300 cubic kilometers (or enough to cover all of Manhattan with almost 22 kilometers—~13.6 miles!—of volcanic debris). Deposits of the Peach Springs Tuff can be found over 170 kilometers (~105 miles) from the caldera and in those places, the deposits are still 10 meters (~30 feet) thick! This was an enormous eruption in an area where we don’t tend to imagine super-eruptions occurring. 
 
 The Peach Springs Tuff was big enough that the pyroclastic flows moved large pieces of rock (see above) that were lying on the ground before the eruption happened—kind of like how a stream picks up rocks and trees during a flood. By looking at the size and weight of these chunks of rocks, you can estimate the speed that the flow had to be moving if you make some assumptions about the flow itself. 
 
 If it is mostly made of hot gases and tiny ash particles, then it can’t move big rocks without moving really fast. If it has a lot of heavier grains of volcanic debris, then it can move big rocks at slower speeds because it has more strength. Additionally, the longer you apply that force, the greater your ability to move the rock. You can picture it this way: Try moving a bowling ball with just a fan for a minute, then try with a firefighter’s hose for 10 minutes. The added density of the water from the hose means you can move that bowling ball easier at slower velocity of flow, especially if you have more time. 
 
 So, Roche and others looked at the sizes of blocks picked up by and incorporated into the Peach Springs Tuff (see above). Now, they didn’t move the full distance—that is, the 70 centimeter boulder found 150 kilometers from the caldera didn’t move 150 kilometers. It might have only been moved 100 meters, but it was moved by the flow of volcanic material during the eruption. 
 
 Now, even at distances as far as 140 kilometers (~88 miles), the Peach Springs Tuff was happily moving rocks that are 70 to 90 centimeters across (a few feet). That is an impressive feat! So, were they being moved by something thin and fast briefly or something thicker and slower for a longer duration? 
 
 By modeling the force needed, Roche and others found that the blocks that far away couldn’t have been moved by something thin and fast because it would have required speeds over 720 to 2,340 kilometers per hour (447-1454 mph), which is wildly unrealistic based on any known volcanic process. Even some of the fastest known pyroclastic flows observed, such as the blast at Mount St. Helens, was moving around ~600 kilometers per hour (370 mph). 
 
 So, then what if the flow was dense instead? Roche and others ran experiments looking at miniature pyroclastic flows made of beads and sand to see how such flows could move larger particles. What they found is that such denser flows could move these large blocks at speeds closer to 18 to 72 kilometers per hour (11 to 44 mph). That is much slower than what we see at smaller eruptions, but for those smaller flows, we see what is happening within a few kilometers of the volcano. If the flow moves out 150+ kilometers, then maybe it can slow down but still have enough oomph to move block. 
 
 What that would require is a constant push from the eruption itself. If the Silver Creek Caldera erupted for 2.5 to 10 hours at a sustained rate of 38 to 150 million cubic meters per second, then these flows could move blocks even moving at only a few tens of kilometers per hour. Now, that eruption rate is huge, tens to hundreds of times more than Pinatubo, Tambora or Novarupta, some of the biggest eruptions of the last few centuries. 
 
 This means that the eruption of the Peach Springs Tuff was at least as large if not larger than the super-eruptions like Toba or Taupo. Yet, if you were 150 kilometers from the eruption, you might have upwards of 10 hours to get out of harm’s way (well, at least out of the way of the massive pyroclastic flows—the resulting ash fall and climate cooling is a little trickier to handle). 
 
 What does this all mean? Well, it means that cities near(ish) to large volcanoes like Yellowstone or the Campei Flegrei might have a fighting chance** to survive in the face of such a catastrophic eruption. Rapid and organized evacuation of cities might allow for people to leave in time, much like people can evacuate before a hurricane. However, that should be seen as a last resort. It is really careful volcano monitoring that can save lives most effectively, letting people know when they need to leave before they have to worry about a pyroclastic flow barreling towards them … but it is nice to know that it might not be bearing down on you as fast as we thought. 
 
 **Addendum (4:00 PM EST March 7): I wanted to clarify a few things after an email exchange with Dr. Valentine (from this study). First, it is clear that this study does not imply that evacuations can be effective in places like Naples near the Campei Flegrei after an eruption has started. Naples is far too close to have the finding of this study play any role. Remember, volcano monitoring and evacuation before the eruption is the best solution. Additionally, this study focussed on a single eruption from the Silver Creek caldera, so applying it to all very large eruptions is untested at this point.
– Think supervolcanoes that devastate entire regions are terrifying? Well sure, but you might be able to outrun them—according to a study that says one prehistoric supervolcano churned out lava at only 10 to 45 miles per hour, Live Science reports. "It's really interesting how you can have such a violent eruption producing such slow-moving flows," says study co-author Greg Valentine. "They still devastate a huge area, but they're slow and concentrated and dense." Published in Nature Communications, the study analyzed an Arizona supervolcano known as Silver Creek Caldera that erupted 18.8 million years ago. It churned out lethal tides of gas and ash known as pyroclastic flows for over 100 miles, inundating parts of Nevada and California, ScienceDaily reports. By analyzing the vast volcanic deposit left by Silver Creek Caldera, researchers found that large rocks had been moved considerable distances. Their conclusion: Only a constant, heavy flow would have moved rocks that were already miles from the eruption. Not everyone buys it, and the researchers warn against taking their study as an excuse for last-minute evacuations. What's more, smaller eruptions (like the one that wiped out Pompeii) are known to spew pyroclastic flows at hundreds of miles per hour, Wired reports. But it seems anyone 90 miles away from Silver Creek Caldera would have had about 10 hours to escape. "I wouldn't recommend anyone try to outrun a volcano, but there's a few of us that could," says Valentine. (Parts of Washington and Oregon were covered in mysterious ash last year.)