volcanology - anaspides.net · 2010-05-24 · volcanology author: jocelyn mcphie subject:...
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VOLCANOLOGY
1. Eruption Processes & Products
2. Volcanoes
3. Volcanic Hazards and Resources
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1. EruptionProcesses and
Products
Mayon Volcano, Philippines, 1984
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Volcano…..….structure where magma is erupted….includes the erupted products
ERUPTION STYLE
EXPLOSIVE EFFUSIVE
pyroclastic deposits lavas
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Explosive• Magma with high gas content• Magma with high viscosity
- gas forms bubbles- bubbles are trapped- P build-up
explosive eruption
- rapidly expanding cloud ofgas + particles (buoyant)- jet (driven by decompression)- magma is torn apart byexplosions
magma is torn apartby explosions
rapidly expandingcloud of gas +particles
jet
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particles“PYROCLASTS”
crystals &broken crystals
pumice or scoria
glassshards
(walls ofbubbles)
wallrock fragments(“lithic clasts”)
1 mm
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EXPLOSIVEERUPTION
pyroclasticdeposits
Pyroclastic fall
pyroclastic fall
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Pyroclastic flow• hot, laterally moving, gas-particle dispersion• high particle concentration
pyroclastic flow
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Pyroclastic surge• warm-hot, laterally moving, gas-particle dispersion• low particle concentration• turbulent
pyroclastic surge
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Fall deposits• good layering, often graded (coarse to fine)• mantle uneven surfaces (uniform thickness)• smaller grain size & thinner with distance from source
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Flow deposits• massive• infill topography, flat top surface• poorly sorted• may be very thick (100’s m)• aka “ignimbrite”
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Ignimbrite….. pumice-rich pyroclastic flow deposit….. may be welded• particles still v.hot (= 500°C) when deposited• load pressure causes hot, sticky particles to weld together• pore spaces between particles, and vesicles in pumice areeliminated
“instant” hard, solid rock called “welded ignimbrite”e.g. Battleship Rock ignimbrite, New Mexico
pumice
lithic clast
flattened pumice
lithic clast still the same
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Surge deposits• good layering, cross bedding• partly mantle, partly infill topography• usually only small volume & close to source
P’c fall, flow and surge depositsmay be lithified to form pyroclastic rock
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GrainsizePYROCLASTS
> 64 mm block (angular), bomb (fluidal)2 - 64 mm lapilli< 2 mm ash
PYROCLASTIC ROCKS> 64 mm pyroclastic breccia, agglomerate2 - 64 mm lapillistone, lapilli tuff< 2 mm tuff
these terms should be reserved for provenpyroclasts and pyroclastic rocks
i.e. generated by EXPLOSIVE ERUPTIONS
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Effusive• Magma with low gas content• Magma with low viscosity
- gas forms few bubbles- these grow and rise- bubbles break easily- magma continues to rise andeventually erupts
effusive eruption
(generally only minor, mildaccompanying explosive activity)
Lavas: coherent + autoclastic
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Lavas….. surface flows of molten rock….. behaviour & features controlled by
• composition, especially SiO2
• temperature• volatile content• shape of vent• gradient of substrate• discharge rate
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LavasLow SiO2 lava e.g. basalt• two common types: pahoehoe - fluidal surface
a’a - lots of fragments, “clinker”• sheets, tubes, channels; thin but extensive
High SiO2 lava e.g. rhyolite• thick, short flows; domes• abundant loose blocks of pasty, congealed lava
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Characteristic features of lavas….. once cooled ∅ “instant”, solid rock….. may be flow banded….. may contain euhedral crystals (“phenocrysts”)….. may be vesicular….. may contain volcanic glass if rapidly cooled….. may have columnar jointing….. foreign rock fragments are uncommon….. typically include both coherent & clastic textures
(cf. clastic texture of pyroclastic rocks)
lava
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Autobreccia• broken fragments of lava• formed during active flow of hot lava when cooler brittleparts of the lava are torn apart and break up• fragments may be flow banded and vesicular• typically tabular, slabby and twisted shapes
autobreccia
coherent lava