Date post: | 13-Jul-2015 |
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Importance of Volcanic Activity
1) Destructive – predict eruptions2) Landforms created3) Clues to Earth’s interior
(temp, pressure, composition)
Viscosity of Magma
Viscosity = liquid’s resistance to flow
Low viscosity = low resistance to flow (flows __________)
High viscosity = high resistance to flow (flows ____________)
Viscosity depends on:
1) Temperature of magmaHotter magma = _____ viscosityCooler magma = _____ viscosity
Viscosity of Magma
2) CompositionMafic magma = ___________
- _______ viscosity magmaFelsic magma = ___________
- _______viscosity magma- doesn’t occur as lava flows
Viscosity of Magma
3) Dissolved gas contentMore dissolved gas = lower
viscosityAs gases escape, lava becomes
_______ viscous
Materials extruded in eruptions
1) Pyroclastic debris (solid)“pyro” = fire; “clastic” = piecesDefined by particle sizea) Dust-sized = volcanic ashb) Gravel-sized = cinders
Pyroclastic Debris
c) Boulder-sized = two types:i) angular = volcanic blocks- pre-existing rx blown apart by eruption
Materials extruded in eruptions
2) Lava – most common is mafica) Pahoehoe lava- smooth, wrinkled surface- low viscosity (very hot lava)
Mafic lava features
b) A’a lava- sharp jagged surface- cooler lava, so higher viscosity- thicker lava flows
Materials extruded in eruptions
3) Gases- gases expand as magma
approaches surface - causes eruptions
Gases
Most common gas = _________ - source of water for oceans and
atmosphere (“outgassing”)Also carbon dioxide, sulfur,
chlorine
Types of Volcanoes
1) Shield volcano- largest- gently sloping cone b/c mafic
magma flows easily from vent - cones grow wider than tall
Types of Volcanoes
2) Composite Cone - layers of lava and pyroclastic
debris - steep symmetrical cone
Composite Cone
Located in:- Circum-Pacific belt (“Ring of
Fire”)- Mediterranean Belt- Caribbean Isl.
Associated features:
b) Nuee ardente (“glowing avalanche”)
- dense steam cloud of toxic gases and red-hot ash
- travels down the cone at very high wind speeds
Types of Volcanoes
3) Cinder cones- smallest volcano- built of loose cinders- steep-sided - end-stage of eruption cycle
Cinder cones
Short eruption lifespan- few weeks up to a yearEx: Paricutin, Mexico (1943) Sunset Crater, AZ
Types of Volcanoes
4) Other volcanic features a) Caldera - summit collapses into emptied
magma chamberEx: Crater Lake, OR
Other volcanic features
c) Lava Plateau (Flood Basalt)- mafic lava erupts out of fissures
(cracks in crust)- magma covers hundreds of
miles
Intrusive Structures
Most magma cools at depth- exposed only after uplift &
erosion- also called ___________
Pluton Classification
2) Relationship to pre-existing rock (“country rock”)
Discordant – cuts acrossConcordant –parallel to/between
Intrusive Structures
1) Dike - tabular discordant pluton- magma travels in fractures- often associated with volcanic
activity (Ex: Ship Rock, NM)
Intrusive Structures
2) Sill- concordant tabular pluton- magma squeezes between
layers- shallow intrusion
Question:
How can you tell if a layer of basalt in sedimentary rocks is a sill or a buried lava flow?
CLUES:
Intrusive Structures
3) Laccolith (“laccos” = lens)- concordant massive pluton- magma thickens as it cools - arches overlying rock into
dome Ex: Henry Mtns., Utah
Intrusive Structures
4) Batholith (“bathos” = deep)- discordant massive pluton- 100’s of mi.2 (largest pluton) - graniteEx: Sierra Nevada Mtns., CA