Why do volcanoes (only sometimes) erupt explosively?

Transcription

1 Why do volcanoes (only sometimes) erupt explosively? , effusive 1980, explosive Michael Manga Gonnermann and Manga, Annual Reviews of Fluids Mechanics, 2007

2 Why do volcanoes erupt explosively? (textbook version) Effusive eruption: No fragmentation Water, CO 2, SO 2

3 Why do volcanoes erupt explosively? Open questions: When, where and how does fragmentation occur? Why so much diversity in eruption style?

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5 Volatile exsolution and bubble growth

6 Three key processes 2. Loss of gases, called outgassing, supresses eruption outgassing

7 Vesicular magma is permeable Klug et al. (2002) Connections between bubbles allow gases to escape from magma Permeability depends on vesicularity and bubble size

8 Three key processes 3. Fragmentation P out If stresses in film surrounding bubbles too large P in If P in - P out > critical value then film ruptures bubble melt film

9 A second way to break magmas... Deformation rate η mr /G Condition: strain rate > CG/η mr with C ~ 0.01

10 Are deformation rates high enough to fragment ascending magma? we will refer to this brecciation

11 Three key processes 1) Nucleation (forming new) and growth of bubbles 2) Outgassing (loss of gas from the magma) 3) Fragmentation and brecciation (breaking magma into pieces) Approach 1. Lab experiments and theoretical models to study individual processes and properties 2. Computer simulations 3. Test models with measurements made on rocks

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13 Conduit flow conservation of mass, momentum, energy (include viscous dissipation; density, rheology from subgrid model) non-turbulent, no fragmentation, single phase magma (melt + bubbles) cylindrical conduit, radial velocity is zero steady flow

14 Conduit flow conservation of mass, momentum, energy (include viscous dissipation; density, rheology from subgrid model) non-turbulent, no fragmentation, cylindrical conduit single phase magma (melt + bubbles) radial velocity is zero steady flow

15 Subgrid model: Volatile exsolution and bubble growth Proussevitch and Sahagian (1998) Solubility of H 2 0, CO 2 from Liu et al. (2005) Diffusivity of H 2 0, CO 2 from Zhang and Behrens (2000)

16 Subgrid model: Volatile exsolution and bubble growth Conservation of mass, momentum and energy, coupled with solubility model and modified Redlich-Kwong equation of state for water-co 2 mixtures Lensky et al. (2001) Bird et al. (1960)

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18 S-R N d

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20 Melt viscosity depends on amount of dissolved water and temperature (and composition) Hess and Dingwell (1996) Melt viscosity depends on deformation rate Magma viscosity affected by presence and properties of bubbles and crystals

21 Strain-rate dependent viscosity of melt phase from Simmons et al Silicic magmas are similar (Webb and Dingwell)

22 Strain-rate dependent viscosity of bubbly suspension Pal (2003) fit to data from Rust and Manga (2002) increasing shear rate

23 Vesicular magma is permeable Klug et al. (2002) Connections between bubbles allow gases to escape from magma Permeability depends on vesicularity and bubble size Outgassing efficient when - exceeds rate of gas exsolution

24 Fragmentation criteria: thresholds determined experimentally P out P in bubble melt film If P in - P out > critical value then film ruptures Condition: strain rate > CG/η mr with C ~ 0.01 e.g., Webb and Dingwell (1990), Webb (1997), Papale (1998)

25 P out P in bubble If P in - P out > 1 Mpa/φ then film ruptures melt film

26 Example: Mount St Helens 1980 conditions viscosity limits expansion fragmentation

27 Why do volcanoes erupt explosively? Open questions: When, where and how does fragmentation occur? Why so much diversity in eruption style?

28 Change in eruption style with changing ascent rate outgassing possible St Helens, 2005 ascending magma

29 Change in eruption style with changing ascent rate outgassing brecciation Little Glass Mountain, CA, 500 AD

30 We predict that flowinduced fragmentation (brecciation) occurs at the sides of conduits Is there any evidence that this occurs?

31 Obsidian is banded at all scales Do these bands (in some cases) record fragmentation?

32 Power spectrum: Scale invariant banding Band widths are scale invarient over 4 orders of magnitude

33 Brecciation, rewelding and deformation 10 cm 10 cm

34 Simple shear rotation and stretching

35 A representative model Cantor model fragment, change color reorient, reweld stretch Bands consistent with repeated brecciation, reorientation of fragments, welding (stick back together) and stretching (reproduce power law and multifractal characteristic of bands)

36 Change in eruption style with changing ascent rate brecciation fragmentation outgassing effusive Mono craters, CA 1350 AD pumice + obsidian

37 Mono Crater, CA 2

38 2

39 model (non equilibrium) closed open

40 Does brecciation always happen? Not if the magma rises fast enough

41 Does brecciation always happen? Not if the magma rises fast enough

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43 Change in eruption style with changing ascent rate outgassing brecciation St Helens 1980 heating fragmentation

44 Change in eruption style with changing ascent rate outgassing brecciation Pinatubo 1991 heating fragmentation

45 Basaltic (low viscosity) eruptions Increasing bubble/melt speed and volume fraction of bubbles

46 Basaltic eruption styles

47 Basaltic eruption styles

48 Basaltic eruption styles

49 Basaltic eruption styles

50 Basaltic eruption styles

51 << 1 <10 3 d

52 Why do volcanoes (only sometimes) erupt explosively? Interplay between bubble growth, brecciation, outgassing, and fragmentation governs eruption style

53 How do bubbly fluids respond to rapid decompression? Experimental model