Ten sedimentation experiments (5 hot, 5 slurry) were performed in a 303 cm-long, water-filled, glass flume with a useable width of 19.5 cm (Fig. 1) similar to the set-up used by Freundt (2003) and Allen and Freundt (2006). The main sample used was the fines-rich (<63 µm, ≥30 wt%) ignimbrite from the phonolitic Laacher See volcano (unit T1, Freundt and Schmincke 1986). Clasts >4 mm were removed. Components include white pumice clasts, ash, crystals and lithic clasts (slate, sandstone and basalt). The loose-packed dry bulk density of the tephra sample is 960 kg/m3. The dry air-filled pumice clasts range in density from 500 to ~1300 kg/m3. Most air-filled coarser (>1 mm) pumice clasts are low density (<1000 kg/m3, i.e., buoyant) and accounted for 15 wt% of the sample. T1 is poorly sorted (Inman sorting,  = 3) and bimodal (modes at 0.5-2 mm and 0.064-0.032 mm). Some runs involved modification of T1 by: (1) mixing with the grey, fines-poor (<63 µm, <3 wt%) ignimbrite T3 from Laacher See volcano (Freundt and Schmincke 1986) (H20, S21), (2) adding additional proportions of coarse pumice clasts (H23, H24,S30, S32) and (3) using a layered starting sample (H24) that comprised well sorted ( = 1) and essentially fines-free (<63 µm, < 0.1 wt%) bimodal (1-2 mm and 0.25-0.5 mm) quartz sand at the base, T1 tephra sample with additional pumice in the middle, and fine ash at the top. Two water saturated slurry experiments (S30, S32) involved saturating the starting sample in water for 4 weeks before the run. Experiment H25 involved three consecutive runs using starting samples of similar volume and temperature. Each run was separated by 2 hours to allow for some of the fine ash to settle.

The starting sample was housed in a steel, waterproof box with inner dimensions of 24.8 cm long, 18.1 cm wide and 14.7 cm high, positioned at one end of the flume filled with water. The start of each experiment involved raising a 14.1 cm-wide door allowing the sample to come in contact with the water; most runs involved initial flowage down a ramp (up to 100 cm long) inclined at 26o that rested on the inset floor (Fig. 1). The ramp/floor junction created a break-in-slope (BIS). The floor terminated 20 cm from the far end of the tank to avoid interference from reflected gravity currents, which instead returned underneath the inset floor. The experiments were recorded by video cameras at 25 frames per second: one camera at the side and one above the tank.

Hot runs involved pre-heating the sample in an oven set at 350oC. The aim was to generate hot flows as Freundt (2003) identified that there was a significant change in behavior from cool flows (<~200 oC) to hot flows (>200-250oC). However, actual starting temperatures ranged from 243-310oC. Slurry runs involved mixing the ignimbite with water and stirring to create a suspension prior to opening the door. Initial water to tephra ratios ranged from (0.33-0.46; Fig. 2). Attempts with higher ratios were not possible as stirring was insufficient to counteract clast settling.

Plastic sample collection plates, 7.5 cm long x 18.5 cm wide, lined the floor of the tank. After each run, the water was carefully drained from the tank and the deposit was left overnight. The deposits that collected on each of the plastic plates were weighed, photographed, and sampled for both sedimentary structure (grading, stratification) and grain size analysis. The deposit was relatively firm and competent so that, where thickness and structure allowed, 13-27 grain size samples could be carefully collected from single layers. Structure samples were set in resin, cut and thin-sectioned. Grain size samples were sieved at 1 phi intervals up to 4 phi. For each sieve fraction coarser than 2 phi, components were separated according to whether they sank or floated in water. Samples with more than 10 wt% fine ash were further analyzed by a FRITSCHTM Analysette 22 Laser fine particle analyzer from 5 to 13 phi.

Viscosity measurements

Viscosities of slurries of known bulk densities and water-ash mass ratios were measured by letting a steel ball (diameter 1.561 cm, density 8.144 gcm-3) drop through 32-38 cm deep slurry in a flask of 7 cm diameter. We assume that the ball to flask diameter ratio was sufficiently small that wall effects may be ignored, and that the depth-averaged fall velocity equals the terminal fall velocity. We then apply Stokes’ Law to calculate slurry viscosity as


where u=fall velocity, s=ball diameter, Dr=ball-slurry density difference and g=gravitational acceleration. Stokes’ Law is only applicable to particle Reynolds numbers Re=usrsh-1 <0.4where rs=slurry density.

This condition is satisfied for slurries with water- sample ratios <0.36. For more dilute slurries (S29), Stokes’ Law underestimates viscosity and we use the fall velocity relationship of Eq. (3) in Sparks (1976) for intermediate 0.4<Re<500 to determine viscosity.


In stationary slurries even more dilute than S29 the particles settled too fast to allow for realistic measurements. We thus estimate the viscosities for dilute slurries (S30, S32) by extrapolating to the higher water-ash ratios (Fig. 2).The standard deviations of repetitive fall time measurements indicate errors <20% in the viscosity values determined. We emphasize that the major aspect here is to demonstrate significant viscosity differences between the experimental slurries rather than precise determination of viscosity values.

Viscosity for slurries in the experiments ranged from 0.8-3.5 Pas. More viscous samples (viscosities of 90-83.5 Pas) were attempted (S26 and S28) but these slurries did not flow into the water and did not transform into turbidity currents.


Allen SR, Freundt A (2006) Resedimentation of cold pumiceous ignimbrite into water: facies transformations simulated in flume experiments. Sedimentology 53: 717-734

Freundt A (2003) Entrance of hot pyroclastic flows into the sea: experimental observations. Bull Volcanol 65: 144-164

Freundt A, Schmincke HU (1986) Emplacement of small-colume pyroclastic flows at Laacher See (east-Eifel, Germany). Bull Volcanol 48: 39-59

Sparks RSJ (1976) Grain size variations in ignimbrites and implications for the transport of pyroclastic flows. Sedimentology 23: 147-188

Figure captions

Fig. 1 Flume experiment setup (a) on the inset floor and (b) on the ramp. The sample (dark grey) is housed in box to the left. Plastic plates line the inset floor. V = video, P = pressure sensor, T = thermocouple, measurements in cm (modified from Allen and Freundt 2006).

Fig. 2 Viscosity of tephra/water slurries at various water contents determined by Stokes Law. Experimental slurries are shown and include S28a which was too viscous to transform into a turbulent gravity flow.