The PST 
 
 The PST contains lithic blocks entrained from the local substrates by the pyroclastic currents and deposited within the ignimbrite19. We have identified 20 key outcrops at extension-corrected distances of ~30–150 km to the east and west of the Silver Creek caldera (Fig. 1, Supplementary Table 1 and Figs 1, 2, 3). Closer outcrops are lacking due to post-eruptive faulting and burial in subsiding basins on either side of the caldera. Locally derived lithic block types correspond to rock types in the local substrates. To the west of the caldera these include various volcanic, granitoid and metamorphic rocks that were present essentially in and on alluvial fans and also on local highlands at the time of eruption19, 22. East of the caldera, the lithics are mainly basaltic and granitic, corresponding to ignimbrite emplacement onto Precambrian granite locally covered by Cenozoic basaltic lavas and scoria and with broad fluvial channels containing those clast types12, 15. 
 
 Figure 1: Map of the Peach Spring Tuff and locally derived lithic clasts. (a) The map shows locations with stratigraphic, paleomagnetic and geochronologic data used to correlate the Peach Spring Tuff (PST). Labelled are critical locations from this paper (Supplementary Table 1), Silver Creek caldera and structural-tectonic domains14, 18, 35. Cities—B, Barstow; K, Kingman; N, Needles; V, Valentine. Structural domains—ECSZ, Eastern California Shear Zone; BR, Basin and Range; CREC, Colorado River Extensional Corridor; CPTZ, Colorado Plateau Transition Zone; CP, Colorado Plateau. Mountains—AlM, Alvord Mountain; AqM, Aquarius Mountains; BM, Bullion Mountains; CM, Cottonwood Mountains; DR, Dagett Ridge; HM, Hualapai Mountains; NM, Newberry Mountains; OWM, Old Woman Mountains; PM, Peacock Mountains; SM, Ship Mountains. (b) Photograph at PST0695 in Kane Wash, Newberry Mountains, California (tape is 50 cm). The PST lies on a sandstone substrate. (c) Photograph at 85-PST-50 near town of Valentine, Arizona (tape is 1 m). The PST ignimbrite lies on ash layers that record initial phases of the PST eruption and covers fluvial sediments. Full size image (208 KB) Figures index 
 
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 Figure 2: Characteristics of substrate-derived blocks in the Peach Spring Tuff. (a) Mean size of the largest blocks (the arrow indicates a large block at PST0695). (b) Corresponding velocity of the parent pyroclastic flows calculated from equation (1) as function of the corrected distance from the Silver Creek caldera. Error bars represent the range of velocities calculated. Full size image (57 KB) Previous 
 
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 Figure 3: Geologic map of the Kane Wash area in the Newberry Mountains in California. It shows the westward-draining Kane Spring palleovalley (updated from unpublished map by Brett Cox, USGS). Locations of large substrate-derived lithic clasts at base of Peach Spring Tuff at PST0695 and PST1318, and a substrate conglomerate at PST2102 are indicated. Map unit Tkbc is the most likely source area of the clasts. Dashed line patterns represent streams. Full size image (230 KB) Previous 
 
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 Substrate-derived blocks larger than 10 cm are found at almost all of the studied outcrops, with mean size up to 70–90 cm for blocks at sites ~140–150 km (corrected) west of the vent with one outlier of 139 cm at ~140 km (corrected) (Fig. 2). Entrainment of the typical 10 cm blocks from a subhorizontal substrate into a dilute, turbulent current would require flow speeds >100 m s−1 at heights of a few hundred metres (see Fig. 5a of ref. 13) that are maintained over many tens of kilometres of flow distance. This conclusion holds even if the maximum speed considered is at about one fifth the current height as assumed for more realistic velocity profiles typical of natural currents. The largest blocks, in particular, could not have been entrained by dilute pyroclastic currents because the required speed would have been >200–650 m s−1 (at one fifth the current height), which is unrealistic at this distance from the source13 (Supplementary Fig. 4 and Supplementary Discussion; compare, for example, maximum current speeds of ~170 m s−1 at 4–6 km from vent for the lateral blast eruption at Mount St Helens, 1980 (ref. 23)). 
 
 Figure 4: Size of the blocks in the Peach Spring Tuff and their source area at three locations in Kane Wash in California. (a) Photograph at location PST2102 of the large boulder conglomerate (Tkbc in Fig. 3). Blocks L01-22 were measured (for scale L22, top left, is 62 × 110 × 110 cm). Note the base of the Peach Spring Tuff (PST) on the top left is covered by <2 m of talus. Boulders up to 1.5 m diameter on the ground surface (possibly not in place, noted NIP?) are either from the pre-PST or post-PST conglomerate, and indicate the large size of clasts in these conglomerates. (b,c) Size of the five largest blocks in the Peach Spring Tuff at PST0695 and PST1308 (see Fig. 3), and at top of the pre-PST conglomerate at PST2102. (b) D is the equivalent diameter of the blocks. (c) C is the short length of the blocks. Full size image (130 KB) Previous 
 
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 Figure 5: Schematic of the experimental device used in this study. The flow of fine (80 μm) particles, generated from a reservoir (dashed rectangle) by release of a fluidized granular column with high interstitial air pore pressure, entrains 1.6 mm diameter steel beads (dark grey dots) that form initially a granular substrate at distance between x 1 and x 2 (see Supplementary Table 2). The rigid substrate (light grey) before and beyond the granular substrate is either smooth or made rough by gluing a layer of glass beads of diameter of 0.7 or 1.5 mm. Distance of entrainment of substrate beads is not to scale. Horizontal arrows indicate relative velocities at given height above the substrate; the large arrows below the top of the substrate represent the front velocity of the flow (black) and of the advancing aggrading basal deposit (red). Full size image (48 KB) Previous 
 
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 The transport distance of the blocks by the pyroclastic currents is estimated to be up to several hundreds of metres, based on detailed field mapping from earlier studies16, 19 and our recent field work (Fig. 3 and Supplementary Discussion). In the Kane Wash area, California, the northern flanks of the Kane Spring paleovalley reveal a conglomerate with subrounded basaltic-andesite boulders that are up to 1–1.2 m in diameter (identified in Fig. 3 by unit Tkbc) exposed just below the PST (Fig. 4a). This boulder conglomerate is the most likely source of the lithic clasts found in the PST about 650–800 m downstream to the west-southwest at locations PST0695 and PST1308 where the nature and shape of the blocks are similar to those in the conglomerate (Supplementary Figs 1 and 2). Field evidence suggests that the locally derived lithic clasts were incorporated into and redistributed within independently moving, relatively small pyroclastic flows or within a single, main pyroclastic flow to form lithic-rich horizons19. Our new field work at location PST0695, in particular, shows that the lowest 2 m of the PST contains a concentration of numerous large, subrounded basaltic-andesite lithic clasts of mean size up to >60–70 cm and whose bottoms are about 50 cm above the base of the PST (Fig. 1b, Supplementary Fig. 1 and Supplementary Table 1). 
 
 To address the entrainment mechanism of the substrate-derived lithic clasts found in the PST, and considering that the parent pyroclastic density currents could not be fully dilute turbulent mixtures (as stated above) and rather had a dense basal granular dispersion, we conducted a series of laboratory experiments on dense gas-particle flows propagating on a granular substrate. 
 
 Experiments 
 
 We performed experiments on dense granular flows of fine (<80 μm) particles with high pore gas pressure propagating on a loose granular substrate of coarse (~1.6 mm) beads inserted into a rigid substrate, as analogues to the concentrated basal parts of pyroclastic currents over local erodible substrates (Fig. 5 and Methods, see Supplementary Movies 1–6). Such flows have a fluid-like behaviour and propagate as (almost) inviscid mixtures until either pore pressure diffuses out or material supply is exhausted10. When propagating on a granular substrate, the sliding head of the flow generates both shear and a short-lived upward pore pressure gradient at the flow-substrate interface12. Shear promotes extraction of the substrate particles, which are first dragged slowly over a distance of a few bead diameters just above the top of the substrate before being uplifted at a given distance behind the flow front (or leading edge) because of the pressure gradient. Laboratory experiments11, 12 demonstrate that the pressure gradient initially increases with time after passage of the flow front, and the onset of uplift occurs at a critical upward pressure gradient whose associated uplift force counterbalances the weight of individual beads (see Fig. 4 of ref. 12). This shows that the pressure gradient, which is proportional to the square of the flow front velocity, is the main cause of the onset of uplift of the substrate beads dragged at the top of the substrate. Experiments involving beads with different densities but the same size (that is, different weight) reveal different critical pressure gradients and further confirm that the model of ref. 12 we adopt hereafter is robust with respect to the clear relationship between the onset of particle uplift and the square of the front velocity. Other mechanisms, including those similar to that in single-phase fluid flows (for example, Basset and Magnus forces)24, as well as kinetic sieving known for dry granular flows25 might occur but appear to be minor influences in the experiments in promoting onset of uplift. Reference12 points out that though particle uplift by granular flows shares similarities with that of single-phase fluid flows, the shear stress and vertical forces over the substrate particles are of different natures. Nevertheless, kinematic sieving and squeeze expulsion caused by particle interactions can contribute to controlling the rise height of the beads once the pore pressure gradient has caused onset of uplift. As discussed by ref. 13, however, large beads whose density is larger than the bulk density of the fluidized mixture of fines with high pore fluid pressure should ultimately sink because of buoyancy effects, which actually occurred in our experiments as described below. 
 
 Our experiments involved a substrate of steel beads of diameter ~1.6 mm (Supplementary Table 2). They were carried out at flow front velocities >0.97 m s−1 that caused a pressure difference >82 Pa required for uplift of the steel beads12. They show how during flow propagation, substrate particles are entrained within a basal zone whose upper surface migrates first rapidly upwards to a height of ~5–8 mm, as beads are uplifted, and then slowly downwards, as beads settle (Fig. 6, Supplementary Fig. 5). High-speed videos reveal that the particle velocity increases upward, similar to the local internal flow velocity, and that at any given time some beads have ascending (uplift) trajectories while others have descending (settling) trajectories. The transport distance of uplifted beads cannot be determined accurately since it exceeds the field of observation and entrained particles are hidden intermittently by the matrix of fines, but it can be relatively large (up to ~1–1.5 m) once the flow propagates onto a rigid substrate downstream. Another important observation is that entrained particles are overtaken from below by the advancing front of the aggrading deposit that forms at flow base (Fig. 6d). This front begins a few centimetres behind the flow front and advances at a similar speed. Therefore, substrate-derived beads are deposited downstream near the base of deposits that form either on granular or rigid substrates. The final resting height of a given entrained particle is determined by a competition between the particle’s uplift and descent history, and the upward-advancing aggradation surface at a given location. 
 
 Figure 6: Laboratory experiment of air-particle flow on a granular substrate. (a) Snapshots from high-speed videos at sequential times after passage of the flow front at middle of images (vertical black line) in experiment C3 (repeated six times, Supplementary Table 2). The horizontal black line shows the top of the substrate of steel beads. Blue and red dashed lines indicate the upper surface of the zone of the entrained substrate beads and of the basal deposit, respectively, and arrows show relative motion. (b) Detailed views of (top) the flow at 0.048 s and (bottom) the final deposit (note white flow fines penetrating into the substrate interstices). Arrows indicate the direction and velocity of the entrained beads (circled), and white contours delimit air bubbles. Note that beads have either ascending (uplift) or descending (settling) trajectories. (c) Height of the upper surface of the zone of the entrained beads and of the basal deposit above the substrate (h) and velocity of the uppermost beads (U) as a function of time. The particle velocity increases upward, similar to the local internal flow velocity. (d) Schematic successive views showing (left) beads (black) entrained from the substrate (grey) by the sliding flow head, and (right) the advancing aggrading basal deposit (red) that freezes beads entrained downstream and that finally settle back towards the substrate. Horizontal arrows represent the internal flow and beads’ velocity as well as the flow front (U f ) and deposit advancing front (U d ) velocities ~2.5 m s−1, higher than the maximum entrained beads velocity (~1.6 m s−1 in c). Full size image (144 KB) Previous 
 
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 Pyroclastic flow speeds and eruption rates 
 
 According to experimental findings and theory12, 13, a flow of front velocity U f entrains blocks whose short (subvertical) length is up to a critical value C. Using C based on our field observations (Supplementary Table 1), we can calculate the front velocity from 
 
 where ξ is a shape factor (equal to 2/3 for an ellipsoid and 1 for a parallelepiped13), ρ p is the block density, ρ f ~1 kg m−3 is the gas density, g is the gravitational acceleration, γ≈0.06 is an empirical factor12 and ρ=875–1,400 kg m−3 is the bulk flow density12, 13 (see Methods). From equation (1) (with ξ=1), the largest blocks at the PST key outcrops give flow speeds of ~5–20 m s−1 across large flow distances with different substrates (Fig. 2b). The relative uniformity of the speed estimates suggests that the 5–20 m s−1 range is realistic. Furthermore, the fact that many of the largest blocks in the ignimbrite are just smaller than the largest blocks remaining on the substrate in their source areas shows that the PST parent flows sampled blocks up to a critical size and that our estimates are not simply minimum speeds related to the lack of sufficiently large blocks in the original substrates (Fig. 4 and Supplementary Discussion). For instance, in the Kane Wash area (Fig. 3), the five largest substrate-derived blocks at sites PST0695 and PST1308 in the PST are just smaller (with the exception of the outlier 80 × 120 × 150 cm at PST0695) than the blocks located at top of the pre-PST conglomerate unit (PST2102) from which the boulders are inferred to have been entrained (Fig. 4), which suggests that our calculated flow velocity up to ~15–20 m s−1 at these sites is realistic. 
 
 Flow speeds of 5–20 m s−1 and the run-out distance of ~170 km correspond to a minimum flow duration of ~2.5–10 h, which does not account for the time needed to aggrade the final deposit or to stack flow units if the PST emplacement involved pulses (we note, however, that field evidence at many PST sites suggest one single flow unit15, 19). These durations are reasonable if internal gas pore pressure, which greatly reduces internal friction in the currents, is long-lived and if a sufficient pressure head provided by the drop height from sustained fountaining at the vent(s), enhanced by gentle slopes away from the source area, is maintained along the current’s path10, 20. The pore pressure diffusion timescale is estimated from ||||| 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. ||||| 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. ||||| 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.)