What does this talk have to do with MARGINS (I ask myself)? Ash results from Explosive Volcanism

1. What does this talk have to do with MARGINS (I ask myself)? • Ash results from Explosive Volcanism • Explosions start from Gas escaping magma--CPB magmas carry as much as 10X more gas than other magmas! • Gas escape from CPB explosive eruptions is up to two orders of magnitude more that erupted magma volumes would suggest • Pyroclastic flows, which create most of the fine ash, are much more common in CPB eruptions Kilauea Pu’u O’o

2. The formation, fate and hazards of fine volcanic ash William I Rose Michigan Tech Univ Houghton, MI 49931 USA MARGINS, San José, Costa Rica June 22, 2007

3. Acknowledgments Ash collectors of distal ash samples are few and far between. Here are several vital sources: Sam Bonis for Guatemalan tephra. Andrei Sarna-Wocjicki for Mount St Helens. Game McGimsey for Cook Inlet volcanoes. Nick Varley for Colima. Joop Varekamp and Jim Luhr for El Chichón. Lab facilities access and enthusiastic critical help: Komar Kawatra, Costanza Bonadonna, Clare Horwell, Alain Volentik Students (who really did the work): Adam Durant, Colleen Riley, Jason Evans, Sebastien Dartevelle Helpful participants in data discussions : Raymond Shaw, Steve Self, Gerald Ernst, Jackie Huntoon, Jocelyn McPhie Miscellaneous vital help, mainly technical: Chris R Copeland, Owen Mills, Ken Wohletz, Simon Blott, Steve Boreham and Chris Rolfe Institutional Help:Cambridge University, Univ South Florida, USGS, Michigan Tech Finances: US National Science Foundation

4. Main Points of this talk: Fine ash (<25 µm diameter), as an often dominant part of thin and extensive distal fall is not well preserved in the geologic record. It is much more abundant than before realized. Its formation is greatly enhanced by milling in pyroclastic flows and subsequent elutriation and convective rise. Its distribution is not predictable from patterns of proximal fall deposits, and is largely controlled by meteorological processes. Its transport is well studied from volcanic cloud remote sensing, and it has an atmospheric residence of several hours to 1 day. It is of great hazard concern.

5. Hazards of fine ash particles Effects on jet aircraft -hazards are very serious for many systems on aircraft, engines, hydraulic systems, electric systems, windshields Human health issues for atmospheric particles now center around the measurements of the number of particles which can pass the throat with moving (breathing) air. The critical measurement (PM 2.5) is the number or mass of particles with aerodynamic diameter <2.5µm, because these particles will negotiate the curves of the human throat and be carried all the way to the pulmonary interior. Toxic affects on surface water from soluble chemicals (especially fluorides). Fine particles have high surface areas and sorb higher concentrations of soluble volcanic chemicals. Remobilization of fine ash particles through agricultural practices (plowing; clearing land) may result in high atmospheric particulates for years after deposition. Abrasive and chemical (acid) effects on machinery (including computers!) (abrasion of electrical wires, roofs, other metal surfaces).

6. What this paper reports: New data on size distributions of more than 200 fine-grained volcanic ashfall samples from basaltic, andesitic and dacitic eruptions. Techniques used: Laser Diffraction automated particle grain size distributions using equipment used in a variety of industrial applications. The principle of this method is forward fraunhofer scattering. The manufacturers (instruments) are Malvern Instruments (Mastersizer) and Microtrac (S3500). www.malvern.co.ukwww.microtrac.com New Results: Much improved GSD data, especially for samples with a high proportion of particles finer than sieve ranges (<63 µm diameters). These instruments determine GSDs in the ranges of <0.5 to 2000 µm (-1 to 11 ).

7. New Laser diffraction data on distal/fine ashfall samples from 10 - >1000 km distance, mainly collected as they fell: Volcano Magma Style VEI Dates Fuego Basalt subplinian 2- 4 1973-74 San Miguel Basalt strombolian? 1 1970 Spurr Andesite subplinian 3-4 1992 Colima Andesite peleean/ 1-3? 2000-2006 Redoubt Andesite peleean 2-3 1989-90 Augustine Andesite peleean 2-4 1986 Pinatubo Andesite plinian 6 1991 El Chichón Trachyandesite plinian 5 1982 St Helens Dacite plinian 5 1980 Santiaguito Dacite peleean 1-2 1968-2006 Bruneau-J Rhyolite plinian 8 11 ma

8. Volcanic Clouds- the home of fine ash and a domain of both volcanology and meteorology Rapid fallout of coarse particles (30min) Clouds and precipitation Volcanic Clouds and ashfall Hydrometeors important in both clouds and VC CCN abundant in VC, rare in clouds Cloud glaciation early (-18 C) VC have many small icy particles that form during cloud ascent and sublimate. Clouds, especially vertically developed ones, develop larger hydrometeors. VC can become quite heavy with a load of fine particles--sinks as a whole

9. VF 74-103 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.

10. The aerodynamic size of a pyroclast strongly influences its life in the atmosphere. Large pyroclasts (>0.5 mm diameter or < 1 ) fall out in stage 1 (<30 mins) and form exponentially thinning ash blanket. Most of the volume of pyroclasts is in this group and they are found in thick fall deposits close to the volcano. Medium size pyroclasts (0.5 mm to 50 µm; ~1-4 ) fall out as simple particles in stage 2 (<1 day) and bring smaller pyroclasts out with them. Fine pyroclasts (<50 µm; > 4  ) fall with larger pyroclasts, may nucleate precipitation and fall, especially during stage 1 and 2, and may aggregate due to atmospheric turbulence and stick due to static forces or moisture.

11. Most fall deposit work has been done on the proximal region where large, rapidly falling pyroclasts dominate. So medium and small pyroclasts are understudied and needed to understand and predict the fate of fines. Figure from Cas & Wright, Volcanic Successions

12. 18 May 1980 fall deposits of MSH are very fine-grained (>50% < 30 µm) and have nearly identical GSD at distances >300 km. This suggests fallout of all particles together… AJ Durant et al., 2007, in prep.

13. Pinatubo’s July 15, 1991 eruption column was fed by a co-ignimbrite cloud and the ashfall was marked by a much more prominent fine ash proportion. Such clouds likely entrain more water vapor also. Dartevelle et al, 2002, Geology 30: 663-666.

14. Colima Volcano has produced ashfalls from both vertical explosions and from co-PF events during 2005-2007. We have compared 20 examples of each with results that show clearly--although the bulk compositions are similar, the GSDs are systematically different, with the Co-PF samples being poorly sorted and with high content of fine grained particles. Evans, Varley, Huntoon & Rose, 2007 Co-PF Vertical Explosion

15. Ash-rich and ash-poor volcanic clouds---Separation observed in many remote sensing examples Manam 27 Jan 2005 0355 UTC MODIS 1-4-3 True

16. 14 Oct 1974 Fuego has had >60 brief subplinian VEI 2-3 eruptions which produce mainly thin fall units that are unpreserved in the geologic record.

18. Bimodal GSD with two subequal modes.

22. “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.

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

24. 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

25. Fully glaciated volcanic cloud with abundant CCN • Density of cloud increases as a whole • Latent heat effects significant in rise and possibly in fall • Bright band effects during descent (thawing) • Overall sublimation/evaporation • Quite different from a thunderstorm Above Ephrata on 18 May Photo by Douglas Miller

26. Mammatus Simulation thunderstorm cirrus outflow anvil snowflake aggregation induced entire layer descends Simulation time: 20 minutes! 10 µm snow aggregate diameter contours dry sub-cloud layer Kanak and Straka, Atmos. Sci. Let. 7: 2–8 (2006) ~6000 m

27. Conceptual Model: Distal Fallout

28. Few IN Bergeron Large Ice HM Precipitation Many IN Small ice HM Little Precip Sublimation Meteorological Cloud Volcanic Cloud

29. Individual eruption analyses underway/completed: Pinatubo fall deposits: Dartevelle et al 2002 Geology 30: 663-666. Bruneau-Jarbridge 11 my rhyolitic fall deposit Rose et al 2003 J Geol 131: 115-124. Fuego 14 Oct 1974 Rose et al, Bull Volcanol in review. Fuego Feb-Mar 1973 eruptions Rose et al, in prep. Mount St Helens, 18 May 1980 Durant et al., in prep. Colima, 2005-2006 co-PF and vertical explosion ashfalls: J Evans MS thesis MTU, in prep. Santiaguito 1968-2005 co-PF and vertical explosion ashfalls: Rose et al., in prep. Crater Peak/Spurr 1992 eruptions: Durant et al., in prep. El Chichón 1982 Rose and Durant, in prep

30. Conclusions • Fine ash is especially generated in PFs; PF-rich eruptions have much more fine ash and this fine ash falls out “prematurely” in hours to a day. • Hydrometeors are a big part of the story of fine ashfall. • Volcanic Clouds generate IN overseeding which inhibits precipitation. • GSD does not change with distances >~100-300 km--fallout of whole cloud mass. • Separation of volcanic clouds into higher, fine ash poor, gas rich and lower, fine ash rich portions which may disperse in different wind fields. • Modeling of fallout cannot be derived from proximal fallout patterns