Origin of volcanic ash: Mechanisms for formation

1. Origin of volcanic ash: Mechanisms for formation William I Rose Michigan Technological University Houghton, MI 49931 USA Used materials from Wendy Bohrson, Glen Mattioli and Oleg Melnik Ashfall Graduate Class September 1, 2009

2. This presentation gratefully uses ideas and some materials from other web sources. In particular I acknowledge materials from Wendy Bohrson at Central Washington University, from Glen Mattioli at University of Arkansas and from Oleg Melnik at Moscow State University. Where use of these ideas were made, acknowledgement is included. Educational use of materials in this lecture are encouraged, and this lecture is part of a graduate class in which inter-university cooperation is engendered. Readings that are very important for understanding this material: Sparks, R. S. J., 1978, The Dynamics of Bubble Formation and Growth in Magmas: A Review and Analysis: Journal of Volcanology and Geothermal Research, v. 3, p. 1-37. Dingwell, DB, 2001, Magma degassing and fragmentation, in From Magma to Tephra, ed by A Freundt and M Rosi Elsevier:Amsterdam Parfitt, EA and L Wilson, 2008, Fundamentals of Physical Volcanology, Ch 5 Volatiles, Blackwell, Malden MA USA

3. Aerosols and ash? • “Anaerosolis any relatively stable suspension of particles in a gas, especially in air, including both the continuous (gaseous) and discontinuous (particulate) phases. “ R D Cadle, 1975 • Clouds and precipitation analogy • After initial rapid fallout of large (>1mm) particles, a volcanic cloud remains and it drifts about for days to months. • The volcanic cloud contains a variety of particles: Fine particles of ash, sulfate aerosol particles and hydrometeors (raindrops, snow, hail, graupel..) • The cloud also contains distinct gaseous components, some volcanic and some the result of reactions with atmosphere. • Fall deposits are the result of materials that fall right after emplacement in the eruption column and partly from the drifting volcanic cloud.

4. Why do we care about fine particles? 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. Effects on jet aircraft -hazards are very serious for many systems on aircraft, engines, hydraulic systems, electric systems, windshields 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).

5. cm In the ambient atmosphere, there are many common aerosols, each with a typical size range. S K Friedlander, 2000, Smoke Dust and Haze, Oxford Press

6. 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

7. Gases in magmas are called volatile components, magmatic volatiles, volatile species “Fugitive elements” Volatile species can be dissolved in melt (accommodated in melt structure) Or They can be present as exsolved species (bubbles) Bohrson, Mattioli

8. Basalt Melting Relations & Eruption Temps. Magma typically exists at temperatures between liquidus and solidus temperatures, so some solid phases can occur also, making a three phase system…. Mattioli

9. Magmatic Phases Mattioli

10. Conceptual Models of Silicate Melts Silicon-oxygen bonds form at temperatures above the solidus. This polymerization explains why magma is highly viscous and why silica rich magmas are even more viscous. It also helps us to understand how dissolved decrease viscosity---they inhibit polymerization which lowers frictional resistance. Mattioli From Carmichael et al. 1974

11. Depolymerization of Silicate Melts Mattioli

12. Abundances of Dissolved Volatile Species in Magmas • Estimates based on study of lavas suggest low values of <1% for total volatiles. • Studies based on melt inclusion data (Roggensack, Wade et al) and focussed on convergent plate magmas imply much higher volatile contents (3-9%). • But the magma might be saturated, undersaturated or oversaturated.. • A major point is that sampling of volatiles is problematic (fugitive)..

13. Glasses and Melt Inclusions • Glassy volcanic lava samples contain some gas trapped when cooling happened, but substantial gas escaped to the atmosphere • We can measure abundances in submarine glasses because little to no degassing invoked; magma cools on contact with seawater • We can study melt inclusions, which are blobs of melt (glass) surrounded by crystal. • Interpretation is that these blobs of melt do not lose volatiles because “armored” by solid crystal. • Melt inclusions preserve the liquid with dissolved gases, trapped at the time the host crystal enclosed it. Bohrson

14. Solubility of Volatiles in Magmas • Concentration of a volatile species that can be dissolved in a melt (accommodated in melt structure) • For a particular P, T, X, there is a maximum amount of H2O that can be dissolved in a melt • Solubility is a function of P, T, X--most important are composition and pressure

15. Water Solubility vs. Pressure “Cold-Seal” bomb Pressure medium Platinum capsule: contains melt + dissolved volatiles To pump to increase pressure From Moore et al., 1998

16. Pressure Effects on Volatile-rich Systems

17. If they represent such a small fraction of the mass--why do we pay attention to volatile species? • Play a fundamental role in forcing magma to ascend, and erupt • For example, typical percentage by mass might be 0.1%; equivalent to 90% bubbles in magma!

18. Enormous Volume increase with decreasing Pressure Bohrson

19. Common Magmatic Volatile Species • Volatiles are defined as those chemical species that at near atmospheric P and high T appropriate for magmas, exist in a gas or vapor phase. • Common chemical species include: H2O (steam), CO2, H2, HCl, HF, F, Cl, SO2, H2S, CO, CH4, O2, NH3, S2, and noble gases He and Ar. H2O and CO2 dominate! • Most volatile species consist of only six low-atomic weight elements: H, C, O, S, Cl, and F. Small but measurable amounts of these elements can be dissolved in both the coexisting melt and crystalline phases. • Oxygen is the major ion in all three phases in magmatic systems: solid, liquid, and volatile.

20. Direct volcanic gas sampling requires a lucky occurrence in an often dangerous spot, like this 980 C fumarole within the active crater at Poas in Costa Rica in 1989. Most gas sampling sites suffer from atmospheric or meteoric contamination, and they can only be reached during inactive periods. So direct sampling is a very limited data source about magmatic volatiles.

21. Masaya, Nicaragua; 1972

22. White Island, NZ measured Rose, Chuan, Giggenbach, Kyle & Symonds, 1986, Bull Volcanol 48: 181-188

23. Selected Direct samples of volcanic gases from rift volcanoes.

24. Phase Diagram of H2O • Critical Point: for a volatile species is the T, P at which there is no physical distinction between liquid and gas • Exsolved volatiles are above the critical point. Called supercritical fluids. Bohrson

25. Supercritical Fluids……. • Density more like liquid • Solubility like those of liquid • Diffusivities like those of gas • Viscosity like those of gas • Supercritical Fluids in Magmas • Density very LOW • specific volume (volume/mass) very LARGE --10-10,000 cm3/gm

26. Magmatic Volatile Reservoirs We often assume, especially for numerical models, that all volatiles in magma are H2O PH2O < Pf Px = Py = Pz Isostatic = Lithostatic pressure Mattioli

27. Interpreting Solubility Diagrams Definitions Undersaturated Saturated Oversaturated Bohrson

28. Solubility with More than One Volatile Bohrson

29. Summary of ascent issues Cooling of magma Saturation and supersaturation Formation of bubble nuclei Bubble growth Formation of solid phase nuclei Growth of crystals Interpretation of textures Bohrson The changes that occur in magma as it rises are fundamental to volcanology and the whole eruption process. All these things happen together, and the different rates of change they follow lead to a variety of outcomes, including intrusions that cool in place, domes that rise passively and violent, explosive eruptions. Accurate understanding of each could lead to numerical models that could forecast outcomes….

30. Types of volcanic eruptions • Extrusive • Lava flows or domes Explosive Gas-particle dispersion flows out of the vent

31. Vulcanian Plinian Strombolian Explosive volcanic eruptions

32. Cooling of magma As magma rises from mantle levels to the surface, the wallrock changes from ductile to brittle. Diapirs are thought to exist in ductile regions, and above there are dikes and cylindrical conduits. With rise of magma, there is a cooling of the wallrock temperature and an increase in the temp contrast between the magma and the wallrock. Geometry of feeder system influences rate of heat loss. The strong relationship of viscosity with T is important in causing magma viscosity to increase as the magma approaches the surface. Bohrson

33. Saturation and supersaturation The availability of H2O and other volatiles in the source region and the degree of partial melting gives rise to an unsaturated magma with perhaps <1-7 % volatiles. The effect of dissolving volatiles in silicate magma is depolymerization--resulting in a dramatic viscosity decrease. With rise, the solubility of volatiles decreases and eventually the magma is saturated. This threshold is academic and barely noticable, because volatiles do not begin to escape. Why? If the magma continues to rise and pressure decreases, the magma enters a state of supersaturation, which does lead eventually to bubbles. Bohrson

34. Vesiculation Stages Bubble Nucleation Froth Saturation New Nucleii and Growth Fragmentation From: Sparks (1978)

35. Formation of bubble nuclei • Bubbles nucleate and grow when magma reaches super-saturation. • Equivalent to when vapor pressure equals or more typically exceeds confining P. This allows critical fluid to separate (equal to formation of bubble) • Super-saturation can occur via: • Decompression = first boiling • Crystallization = second boiling The most important activation energy barrier for bubble formation is the formation of a nucleus---the accumulation of many molecules of gas that can sustain enough pressure to avoid being resorbed by the magma that surrounds them. There is probably a critical minimum size of nuclei that can survive, perhaps around one micron in diameter. In a probabilistic sense the evolution of a nucleus of critical size, one that has a >50% chance of growing rather than shrinking, is quite unlikely so most nuclei get resorbed. Only when a nuclei reach critical sizes can volatiles escape--this is why supersaturation is needed. An analogous concept is undercooling. Bohrson

36. Growth of bubbles • Once bubbles successfully nucleate, they grow! • Rate of growth function of a number of variables: • concentration of volatiles • rate of diffusion (diffusivity) • density and viscosity of magma • surface tension of bubble • Size of bubble in part result of competition between nucleation and growth If the magma is low in viscosity, bubbles will rise in the liquid after growing to some critical size. Thus they may reach the top of a magma chamber or even burst in an open vent, releasing gas passively. In a viscous magma bubbles never grow large enough to rise by gravity, because resistance of the magma to flow is too great. Diffusion of gas through a viscous liquid is slow--slowest in very viscous ones. Each growing bubble has a gradient around it. Overpressure develops in bubbles as bubbles grow, but as the bubbles “feel” each other’s presence, their growth is inhibited. Thus a particular vesicle size is reached when a foam is formed. Bohrson

37. Early Bubble Growth • Early in bubble growth history, diffusion efficient. • Bubbles grow by addition of volatile(s) by diffusion. • Bubbles can commonly maintain equilibrium between volatile(s) in bubble and volatile(s) in melt. • Growth is viscosity-limited. That is, although volatiles are diffusing into bubble, bubble still has to expend energy to grow against surrounding melt. • Higher viscosity makes bubble growth more difficult. Bohrson

38. Bubble Growth Stages • A referred to as exponential growth stage. • B referred to as parabolic growth stage. • In Stage C neighbor bubbles can limit bubble growth by competing for volatile “resources.” Bohrson

39. Bubble Growth during Decompression • Rate of decompression also very important: • At depth in conduit, bubble grows initially by diffusion. • As magma accelerates upward in conduit, bubbles grow by expansion. • That is critical fluid/gas phase is expanding against melt. • Expansion limited by viscous resistance of melt and neighboring bubbles. • Thus “excess” pressure develops, which can lead to fragmentation. Bohrson

40. What drives upward ascent and causes fragmentation? Acceleration • Acceleration due to onset of vesiculation and bubble growth • Bubbles form and grow, thus leading to development of a mixture with lower density. Provides upward acceleration in conduit. • As bubbles grow, they merge and eventually form a foam that disintegrates due to high strain rates in bubble + melt mixture. (Tensile strength of of melt exceeded). • High strain rates lead to development of very thin bubble walls, which rupture as magma accelerates up conduit. Bohrson

41. What drives upward ascent and causes fragmentation? Decompression • Rapid decompression of an already vesiculated magma • Bubbles + melt suddenly exposed to lower pressure. • Develops a fragmentation surface, which is surface where bubble disruption occurs • As bubbles burst, pressure in adjacent (lower) row of bubbles decreases. Yields a fragmentation wave Bohrson

42. Schematic view of the system Flow regimes and boundaries. Homogeneous from magma chamber until pressure > saturation pressure. Constant density, viscosity and velocity, laminar. Vesiculated magma from homogeneous till magma fragmentation. Bubbles grow due to exsolution of the gas and decompression. Velocity and viscosity increases. Flow is laminar with sharp gradients before fragmentation due to viscous friction. Fragmentation zone or surface (?). Fragmentation criteria. Gas-particle dispersion from fragmentation till the vent. Turbulent, high, nonequilibrium velocities. subsonic in steady case, supersonic in transient. Conduit flow during explosive eruptionOleg Melnik ¶ ¶ t x

43. Products of fragmentation • Deposit characteristics may reveal information about fragmentation mechanism • For example, vulcanian eruptions typically have narrow range of size distribution for pyroclasts • Suggests fragmentation mechanism is decompression (fragmentation wave) Bohrson

44. Fragmentation Bohrson

45. Bubble Growth: Basaltic vs. Silicic Systems • In general, silicic magmas form smaller bubbles (0.001-0.1 cm) compared to basaltic (0.1-5 cm). • WHY? • Diffusivity is slower in silicic magmas • Viscosity is higher, so more resistance to bubble growth. Bohrson

46. Explosive vesiculation Magma holds potential energy in overpressured bubbles which burst, forming pyroclasts. Magma viscosity inhibits: 1.diffusion of gas in magma 2. bubble growth and 3. bubble movement (coalescence). Thus silicic magmas have smaller and more highly pressurized bubbles and they may generate finer ash with higher surface area. Bubbles must have a critical size to survive--a scale of ~1 micron diameter has been suggested. If ture, this may limit the amounts of fine ash generated by this process. Sparks, RSJ 1978 Dynamics of bubble formation and growth in magmas, JVGR 3 1-37.

47. Many pyroclasts are obviously broken bubble wall shards. Clearly for these, explosive vesiculation is a dominant fragmentation mechanism. W I Rose, 1987, in Clastic Particles ed by J R Marshall, Van Norstrand

48. Models of fragmentationOleg Melnik 4 FP - fragmentation at fixed porosity. 4 OP- critical & R - = p p m + 4 g m overpressure in a R growing bubble p s æ ö 3 & & & + r + 2 ç ÷ 2 R R R g p 2 R è ø m small 4 SR - critical elongation strain- rate

49. Formation of solid phase nuclei Crystallization is also driven by cooling and volatile loss from the magma. Undercooling, or cooling at a rate faster that crystallization can keep pace with, creates impetus for overcoming activation energy barriers. In the same way as vesicle formation happens, critical crystal nuclei must attain a size which ensures their survival from reabsorption by the magma. At some critical undercooling there is a peak nucleation rate for each mineral in each magma. If the magma is appropriately cooled it will nucleate some phase readily (and others perhaps not).

50. Growth of Crystals Phenocrysts form mostly long before eruption. They can record tidal effects, mixing effects and magma movements of other sorts. Rapid growth features include skeletal or bow-tie crystals, elongate spinifex crystals and spherulites. Textures in the groundmass of many lavas record events that occur near the surface before eruption. Examination of these requires high magnification--back scattered xray images using SEM or microprobe.