Volcanic processes
Jan 15, 2016
Volcanic processes
Magma encounters « external » water
Submarine, surtseyan, phreatomagmatic types
Magma containsdissolved gas
Plinian eruptions Magma is viscous
Domes and block-and-ash flows
(Pelean)
Flows and scoria cones(Strombolian, hawaian)
Yes No
Yes No
Yes No
« grey » volcanoesMore explosiveAndesiticSubductions
« red » volcanoesLess explosiveBasalticIntra-plate
Volcanic processesand types
• Dynamic types related to magma/water interactions
• Dynamic types related to dissolved bubbles
• Dynamic types related to domes growth and collapse
• Dynamic types related to lava flows etc.• Destruction of volcanic edifices• Complex edifices
Magma viscosity
Figure 4-1. a. Calculated viscosities of anhydrous silicate liquids at one atmosphere pressure, calculated by the method of Bottinga and Weill (1972) by Hess (1989), Origin of Igneous Rocks. Harvard University Press. b. Variation in the viscosity of basalt as it crystallizes (after Murase and McBirney, 1973), Geol. Soc. Amer. Bull., 84, 3563-3592. c. Variation in the viscosity of rhyolite at 1000oC with increasing H2O content (after
Shaw, 1965, Amer. J. Sci., 263, 120-153).
Viscosity of Anhydrous Magma
Log
Vis
cosi
ty (
poi
se)
10
8
6
4
2
50 60 70Wt. % SiO2
1400o C1200o C1000o C
Oliv
ine
Bas
alt
Bas
alt
And
esite
Dac
ite
Rhy
olite
10
8
6
4
2 4 6 8 10Wt. % H2O
Rhyolite at 1000oC
Log
Vis
cosi
ty (
pois
e)
4
5
3
2
AllLiquid
T (oC)
IncreasingCrystallization
Basalt
Beg
inn
ing
of C
ryst
alliz
atio
n
2
Log
Vis
cosi
ty (
pois
e)
a cb
1400 1300 1200 1100
Mount Saint-Helens:The post-18 May dome
Figure 4-7. Schematic cross section through a lava dome.
Secondary ash fall (+local surges)
Dome growth and collapse
• Cortex of the dome « chills », interior still hot
• Steep slopes, local collapse– Breccias– Pyroclastic flows: « block and ash flow »
• Dense flow, rich in blocks
• Smaller scale than plinian!!
Montagne-Pelée, Martinique(lesser Antilles)
Saint-Pierre, before
Saint-Pierre, after
Magma encounters « external » water
Submarine, surtseyan, phreatomagmatic types
Magma containsdissolved gas
Plinian eruptions Magma is viscous
Domes and block-and-ash flows
(Pelean)
Flows and scoria cones(Strombolian, hawaian)
Yes No
Yes No
Yes No
« grey » volcanoesMore explosiveAndesiticSubductions
« red » volcanoesLess explosiveBasalticIntra-plate
Volcanic processesand types
• Dynamic types related to magma/water interactions
• Dynamic types related to dissolved bubbles
• Dynamic types related to domes growth and collapse
• Dynamic types related to lava flows etc.• Destruction of volcanic edifices• Complex edifices
Lava lakes
Eruptive fissures
Bubbles
Scenic volcanoes!
Stromboli (Italy)
Scoria cones
Piton de la Fournaise (Réunion, Indian Ocean)
Cones groupe in Réunion
Puy de Pariou (France)(note also the pelean dome of Puy de Dome at the back)
Scoria cones
Disrupted cones
Cones and lava flows
Etna (Italy)
Strange things lava flows do…
No, there is nobody here – it is just a weird but completely natural shape…
Multiple lava flows
Piton de la Fournaise (Réunion, Indian Ocean)
Rift zone
Aa and Pahoehoe
Lava caves
Prismation
Scoria base