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

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

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

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

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

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

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