ARTICLE Received 6 May 2015 | Accepted 29 Jan 2016 | Published 7 Mar 2016 DOI: 10.1038/ncomms10890 OPEN Slow-moving and far-travelled dense pyroclastic flows during the Peach Spring super-eruption O. Roche1, D.C. Buesch2 & G.A. Valentine3 Explosive volcanic super-eruptions of several hundred cubic kilometres or more generate long run-out pyroclastic density currents the dynamics of which are poorly understood and controversial. Deposits of one such event in the southwestern USA, the 18.8 Ma Peach Spring Tuff, were formed by pyroclastic flows that travelled 4170 km from the eruptive centre and entrained blocks up to B70–90 cm diameter from the substrates along the flow paths. Here we combine these data with new experimental results to show that the flow’s base had high- particle concentration and relatively modest speeds of B5–20 m s À 1, fed by an eruption discharging magma at rates up to B107–108 m3 s À 1 for a minimum of 2.5–10 h. We conclude that sustained high-eruption discharge and long-lived high-pore pressure in dense granular dispersion can be more important than large initial velocity and turbulent transport with dilute suspension in promoting long pyroclastic flow distance. 1 Laboratoire Magmas et Volcans, Universite´ Blaise Pascal-CNRS-IRD, OPGC, 6 Avenue Blaise Pascal, TSA 60026-CS 60026, F-63178 Aubie`re, France. 2 United States Geological Survey, 345 Middlefield Road, MS 973, Menlo Park, California 94025, USA. 3 Department of Geology and Center for Geohazards Studies, University at Buffalo, Buffalo, New York 14260, USA. Correspondence and requests for materials should be addressed to O.R. (email: [email protected]). NATURE COMMUNICATIONS | 7:10890 | DOI: 10.1038/ncomms10890 | www.nature.com/naturecommunications 1 ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms10890 xplosive volcanic super-eruptions expel magma volumes of thickness to the diameter of a circle that has the same area as that several hundred cubic kilometres or more and generate of the deposit), with a.r.B10 À 4 and average thickness of B30 m Eparticle-gas flows called pyroclastic density currents1,2 comparable to similar deposits4,5,7,20. The parent pyroclastic (note we also use the term pyroclastic flow synonymously with flows propagated mainly over regional subhorizontal or gently pyroclastic density current, with no implications related to sloping terrains that included alluvial fans with abundant loose dynamics). There are many locations around the world where clastic surface material, and varied local low to moderate relief the deposits of these pumice-rich currents (ignimbrites) extend including scoria cones and hills up to B200-m high. High 4100 km from their source vents3–6. These surprisingly long mountains blocked the flow15,16,19,21. Post-depositional erosion run-out distances raise fundamental questions about the flow and regional extension broke up the original sheet-like deposit so velocity and propagation mechanism. Specifically, does long run it now occurs in scattered outcrops as far as 240 km from the out require high initial flow speeds and mass fluxes near the source caldera. eruptive source? Also, are the density currents fully turbulent and Here we combine field data on entrained lithic blocks with dilute or is much of their mass carried in concentrated, basal experimental results to show quantitatively, for the first time, that granular dispersions? It is commonly assumed that the long run the basal portions of the parent flows of the PST super-eruption out of large-volume pyroclastic density currents requires high had high-particle concentration and relatively modest and initial (proximal) flow speeds of 100–300 m s À 1, which might uniform speed of B5–20 m s À 1 even though the distance result from collapse of explosive eruption columns from heights travelled exceeded 170 km. We report distances from the Silver of a few kilometres3,7,8. There are different views on the physics Creek caldera that have been corrected for regional extension and that control transport and deposition once a pyroclastic current strike-slip translation up to B80 km across three structural starts. One end-member mechanism is that the currents must be domains in California and up to B12 km across the Colorado fast and highly turbulent, carrying pyroclastic particles mainly as Plateau Transition Zone (CPTZ; see Methods). The results a dilute (o1 vol.%) suspended load9. The other end-member further suggest the currents were fed by an eruption mechanism has flows propagating as dense (430–40 vol.%) discharging magma at rates up to B107–108 m3 s À 1 for a granular dispersions (with an accompanying more dilute upper minimum of 2.5–10 h. These rates are 2–3 orders of magnitude part), in which high pore gas pressure reduces internal friction7 larger than well-documented pre-historical or historical and allows nearly inviscid flow while a deposit aggrades upward eruptions. We conclude that the long travel distances of such from the base10,11. Arguments for and against each of these pyroclastic flows are related mainly to sustained high mass flux of mechanisms have been mainly qualitative with a few exceptions9, material erupting from the volcano along with slow gas pore because of the difficulty in relating deposit characteristics to pressure diffusion within the flows due large amounts of ash, quantitative fluid dynamics theory. This debate is not just rather than from high initial flow velocities and/or transport via academic; an understanding of the mechanisms by which dilute turbulent suspension. pyroclastic flows propagate is essential to accurately forecast related hazards at active volcanoes. Recent works suggest that the size (that is, weight) of lithic Results blocks entrained from the substrate by the pyroclastic flows and The PST. The PST contains lithic blocks entrained from the preserved within the resulting ignimbrites can be used to test local substrates by the pyroclastic currents and deposited within emplacement models and to estimate parent flow densities the ignimbrite19. We have identified 20 key outcrops at (related to particle concentration) and speeds. Experiments12 extension-corrected distances of B30–150 km to the east and demonstrated that lithic block size could be used to estimate the west of the Silver Creek caldera (Fig. 1, Supplementary Table 1 speed of a dense flow whose deposition forms an ignimbrite unit. and Figs 1–3). Closer outcrops are lacking due to post-eruptive Theoretical considerations13 show that dilute turbulent currents faulting and burial in subsiding basins on either side of the with realistic speeds (generally o100 m s À 1 outside the proximal caldera. Locally derived lithic block types correspond to rock area) are capable of entraining blocks with diameters up to types in the local substrates. To the west of the caldera these B10 cm from subhorizontal substrates, while dense granular include various volcanic, granitoid and metamorphic rocks that flows can entrain blocks of a few metres size at lower speeds. were present essentially in and on alluvial fans and also on local The Peach Spring Tuff (PST, southwestern USA) is an example highlands at the time of eruption19,22. East of the caldera, the of an ignimbrite that contains large (410 cm) substrate-derived lithics are mainly basaltic and granitic, corresponding to lithic blocks. It was deposited from pyroclastic currents that ignimbrite emplacement onto Precambrian granite locally travelled more than 170 km from their source during the covered by Cenozoic basaltic lavas and scoria and with broad super-eruption at the Silver Creek caldera B18.8 Myr ago14. fluvial channels containing those clast types12,15. The dense-rock equivalent volume of erupted material was Substrate-derived blocks larger than 10 cm are found at almost 41,300 km3; at least half was deposited regionally by pyroclastic all of the studied outcrops, with mean size up to 70–90 cm for density currents15–18, and an estimated equal amount was blocks at sites B140–150 km (corrected) west of the vent trapped in the subsiding caldera14. The extra-caldera ignimbrite with one outlier of 139 cm at B140 km (corrected) (Fig. 2). is poorly sorted and massive with an ash matrix that comprises Entrainment of the typical 10 cm blocks from a subhorizontal 60–80% by volume of the deposit, based on field and thin section substrate into a dilute, turbulent current would require flow analysis17,19. The ignimbrite ranges from non-welded to densely speeds 4100 m s À 1 at heights of a few hundred metres (see welded, and vitric to crystallized (feldspar and cristobalite) as a Fig. 5a of ref. 13) that are maintained over many tens of result of hot emplacement, which suggests little mixing with kilometres of flow distance. This conclusion holds even if the ambient air during flow. It originally covered a broad area of maximum speed considered is at about one fifth the current B32,000 km2 over what is now western Arizona, southeastern height as assumed for more realistic velocity profiles typical of California and southern Nevada. The local deposit thickness is natural currents. The largest blocks, in particular, could not have 5–40 m at most sites, with local increase to B220 m in been entrained by dilute pyroclastic currents because the required topographic depressions where the material accumulated, and speed would have been 4200–650 m s À 1 (at one fifth the current the most distal deposits are 410-m thick15,16. The ignimbrite height), which is unrealistic at this distance from the source13 qualifies as a low aspect ratio deposit