Ashfall Graduate Class 2009 Lecture #4 William I Rose Michigan Technological University Houghton, MI USA Size of volcanic ash

Tephra is classified on the basis of pyroclast size: ASH -- Very fine-grained fragments (< 2 mm), generally dominated by broken glass shards, but with variable amounts of broken crystal and lithic (rock) fragments. Courtesy of USGS. LAPILLI -- Pea- to walnut-size pyroclasts (2 to 64 mm). They often look like cinders. In water-rich eruptions, the accretion of wet ash may form rounded spheres known as accretionary lapilli (left). Courtesy of USGS. BLOCKS AND BOMBS -- Fragments >64 mm. Bombs are ejected as incandescent lava fragments which were semi-molten when airborne, thus inheriting streamlined, aerodynamic shapes. Blocks (not shown) are ejected as solid fragments with angular shapes. Courtesy of J.P. Lockwood, USGS.

Like many natural and man made materials, volcanic ash is made up of materials of variable size. Some times a sample of ash has a simple size range that can be described in a straightforward way… But there are some methods that have developed to make it easier to compare samples.

Φ phi Φ = - log 2 d (mm) Lognormal size distributions are “expected” and we use a “biased” system to define them ASH LAPILLI BOMBS, BLOCKS

NON-GENETIC CLASSIFICATION OF PYROCLASTIC ROCKS Ash tuff - rock dominated by ash; sometimes simply referred to as tuff. Lapilli tuff - rock dominated by lapilli. Tuff breccia - rock containing 25% to 75% blocks and/or bombs. Pyroclastic breccia - rock containing at least 75% blocks and bombs. Agglomerate - rock containing at least 75% bombs. Agglutinate - rock composed of fused, largely unrecognizable, basalt spatter fragments.

Pyroclastic fall deposits Ashfall -- finer than 2 mm Scoria fall -- strombolian/cinder cone Pumice fall – highly vesiculated silicic pyroclasts from plinian eruptions

d < 1000 μm Φ > 0 d < 30 μm Φ > 5

from S. K. Friedlander, Smoke, Dust and Haze 2000

“Normal” ashfall, with dominant coarse mode Because this fine-grained mode is minor and was grouped in the “pan” portion of sieved samples, it is underappreciated and is quite common. Rose et al., 2007 BV Sieve range Ashfall sample from Fuego Volcano, Guatemala 14 Oct 1974

Forward Fraunhofer Diffraction of laser light How to measure the fine ash

Particle Size Analysis Malvern Mastersizer 2000; Cambridge University Dispersion unit

VF Oct 1974 Fuego. 17 Oct 1974 Fuego. This distal fall has only a very minor coarse mode, with a dominant mode at ~5 . Sieve range Laser diffraction expands the range of precise GSD work to submicron diameters. Rose et al., 2007 BV

distal/fine New Laser diffraction data on distal/fine ashfall samples from 10 - >1000 km distance: VolcanoMagmaStyleVEIDates Fuego Basalt subplinian San Miguel Basalt strombolian?11970 SpurrAndesitesubplinian ColimaAndesitepeleean/1-3? RedoubtAndesitepeleean AugustineAndesitepeleean PinatuboAndesite plinian61991 El ChichónTrachyandesite plinian St HelensDaciteplinian51980 SantiaguitoDacitepeleean Bruneau-JRhyoliteplinian811 ma

Rose et al., 2007 BV

A typical example of the ashes we have studied, this sample has two obvious modes, a coarse mode at 0-1  and a fine mode at 4-5 . These two modes are unlikely partners. Why do they occur together? We suggest that, like raindrops, big pyroclasts capture smaller ones as they fall.

Fine grained fine skewed GSD with Md  of 4-5

Bimodal GSD with two subequal modes.

“Normal” ashfall, with dominant coarse mode Because this fine-grained mode is minor and was grouped in the “pan” portion of sieved samples, it is underappreciated and is quite common.

General pattern of decreasing Md  reflects changes only in the coarse mode

Fu The bimodal GSDs can be unscrambled into lognormal components using mathematical methods. We analyzed the bimodal GSD by looking for lognormal subpopulations, using an SFT software developed by K Wohletz et al., 1989, Jour Geophys Res 94:

stable fine modes at about 4-5  Related ashes such as these from Fuego follow a pattern with stable fine modes at about 4-5  and variable coarse modes which shift to finer medians with distance and then merge with the fine mode. The fine skewed tails appear to contain one or more finer modes which represent no more than a few percent of any fall sample. Rose et al., 2007 BV

Fine grained fine skewed GSD with Md  of 4-5 Rose et al., 2007 BV

GSD patterns of distal fall deposits are bimodal with a coarse population that varies systematically with distance and a persistent fine population at about 4-5 . As the overlap becomes more pronounced the GSD looks skewed.

Fu Simple idea: A bimodal grain size distribution suggests the larger ash particles falling, colliding and capturing smaller, drifting ash particles that are too small to fall as simple particles. But, it is correct?

Fu % % % Finer modes appear to exist, which is why fine skewing occurs.

Fine mode is located to WNW, while the bulk of the coarse ash is W to WSW HYSPLIT shows that 4 km winds are to WNW while winds at 8-12 km are W and WSW

Origin of particles in volcanic clouds Explosive vesiculation-- As pressure drops in ascending magma--overpressured bubbles burst Hydrothermal explosions-- rock fractured by thermal shock from contact between magma and water Milling-- abrasion and grinding of particles can occur in pyroclastic flows and in the vent Chemical and meteorological processes-- condensation, sublimation, surface chemical reactions forming acids, salts, hydrometeors and aggregates of mixed origin Others : Breakage of glass at crystal faces, Breakage of crystals from melt inclusion overpressures, etc

Ashfall of elutriated fine ash above a block and ash flow at Santiaguito Volcano on 1 Dec We have analyzed many co-PF ashes and they generally show this type of fine (Md  = ~5), fine-skewed GSD pattern.

A distal ashfall sample fro the August 1992 eruption of Crater Peak/Spurr also shows a fine (Md  = 5) and fine skewed GSD. Thus this type of GSD can result from eruptions that have no pyroclastic flows.

Voronoi method of weighting data Bonadonna & Houghton, 2005

Overall the Fuego subplinian eruptions have only a few percent of the fine mode, which appears to come from the pyroclastic flow milling. Larger plinian eruptions likely have a much higher proportion of co-PF ash. Rose et al., 2007 BV, in press

Total Grain-size distribution weighted by mass and by isopach volume, compared to Carey and Sigurdsson [1982]. 18 May 1980 Mount St Helens Fall deposit

Crater Peak/Spurr, Sept 1992 Andesite, Sub-plinian Total GSD

hydrometeor hydrometeor — Any product of condensation or deposition of atmospheric water vapor, whether formed in the free atmosphere or at the earth's surface; also, any water particle blown by the wind from the earth's surface. condensation deposition water vapor free atmosphere wind 1)liquid or solid water particles formed and remaining suspended in the air, for example, damp (high relative humidity) haze, cloud, fog, ice fog, and mist;relative humidityhazecloudfogice fog mist 2) liquid precipitation, for example, drizzle and rain;precipitationdrizzlerain 3)freezing precipitation, for example, freezing drizzle and freezing rain;freezing precipitationfreezing drizzlefreezing rain 4)solid (frozen) precipitation, for example, snow, hail, ice pellets, snow pellets (soft hail, graupel), snow grains, and ice crystals;snowhailice pelletssnow pelletssoft hailgraupelsnow grainsice crystals 5) falling particles that evaporate before reaching the ground, for example, virga;particles virga 6) liquid or solid water particles lifted by the wind from the earth's surface, for example, drifting snow, blowing snow, and blowing spray.drifting snowblowing snowblowing spray These are probably more abundant than volcanic particles in many or most volcanic clouds! From AMS glossary…

Conclusions: All ash falls contain significant pyroclasts that are finer than 50µm in diameter. A very common feature of distal ashfalls is a mode at 4-5  (31-63 µm diameter) which may be paired with a coarse mode, or alone with a skewed shoulder toward finer particles. Bimodal ashes seem to represent cases where larger pyroclasts sweep up smaller particles as they fall. Most ash falls from the finer modes much more quickly (< 24 hrs) than simple fall would suggest, even when the particles are not being swept. This suggests either aggregation or a role for hydrometeors. The fine ash mode may surely be generated by pyroclastic flows and elutriated upward where this ash may either join the plinian column or be dispersed by lower level winds. In some cases, however, milling by pyroclastic flows does not occur and fine ash is still present. This may be evidence for milling within the vent.

The sizes of ash particles sensed optimally by these methods are about 1-25 µm (~5-9  ).

Stage 1: First hour--rapid fallout of large( >500 µm diam) near the volcano (<25 km), affecting a small area (<~300 km 2 ) and forming a primary fallout blanket Stage 2: During first hours: Volcanic cloud expansion by advection and diffusion by winds; aggregate fallout of 90% of fine ash (<25 µm diam) forming secondary fallout region of ~ 5 x 10 4 km 2 Stage 3 : During several days: movement with winds and decreasing area, and slowly decreasing ash and SO 2 masses Rose et al, 2002, J Geology, 109: Based on the 1992 Crater Peak/Mt Spurr eruptions

Eruption style and the size of ash produced. The “total grain size distribution” of an explosive eruption can be estimated from the deposit, if enough is known about all parts of the fall deposit. Experiments suggest that energetic eruptions produce more fine ash by vesiculation alone. Several examples show clearly that pyroclastic flows contribute large fine fractions to the overall GSD, presumably by milling (Soufrière Hills--Bonadonna et al, 2002; Pinatubo--Dartevelle et al., 2002; Fuego, 1974; Rose et al., 2007; Colima ; Evans et al., Geology, in press)

Fallout of fine ash during stage 2 Aggregation observed in laboratory and the field Accumulation by collision/coalescence-- analogy with meteorological clouds Areas of secondary maxima marked by bimodal SD fallout of particles of expected size occurs WITH much smaller particles Highest masses of fine ash may occur on dispersal axis or on the side Evidence for fallout with ice noted Observations of mammatus clouds Remote sensing shows that a vast majority of fine ash falls out in stage 2