Volcanism Flashcards
what is magma
-molten rock
-the partial melting of the asthenosphere
What are the three mechanisms of magma
- decompression
- addition of volatiles
- addition of heat
explain decompression
-the temperature in the upper mantle is hot enough to melt rock (~1200°C), the high pressure at keeps it solid
-so a decrease in pressure w/o decrease in temp = melting
*occur at divergent boundaries, continental rifts and hot spots
explain the addition of volatiles
-the addition of volatiles lowers the temperature of the mantle (breaks chem. bonds within silicate minerals)
-volatiles are incorporated, and then arent released until the subduction zones
-interaction with “dry” asthenosphere = melting
explain the addition of heat
-melting; if the temperature of the rock is greater than the temperature of the temperature of melting at depth
-rising of magma can melt the surrounding rocks; assimilation/ mixing of magmas
what is magma with a high silica content like?
-high viscosity (slow flow)
-restricted to the vent region
-steep volcanoes
-volatiles (explosive and pyroclastic flows)
what is magma with a low silica content like?
-low viscosity (fast flow)
-travels far (100s of kms)
-gently sloping volcanoes
-little volatiles (effusive eruptions)
explain basaltic magma
SiO2 content: low (-50%)
Viscosity: low
Dissolved gases content: low
Style of eruption: non-explosive
Tendency to form lavas: high
Tendency to form
pyroclastic: low
Temperature melting: 1200
explain andesitic magma
SiO2 content: intermediate (-60%)
Viscosity: intermediate
Dissolved gases content: intermediate
Style of eruption: intermediate
Tendency to form lavas: intermediate
Tendency to form
pyroclastic: intermediate
Temperature melting: intermediate
explain rhyolitic magma
SiO2 content: high (-70%)
Viscosity: high
Dissolved gases content: high
Style of eruption: explosive
Tendency to form lavas: low
Tendency to form
pyroclastic: high
Temperature melting: 800-900
What are the four types of volcanoes
-volcanic domes
-stratovolcanoes
-shield volcanoes
-cinder cones
describe volcanic domes
-rhyolitic
-high viscosity (prevents lava from flowing) causes dome shaped steep sided mounds
-explosive eruptions + pyroclastic flows (if dome collapses)
*monocraters
describe stratovolcanoes
-rhyolitic/ andesitic magmas (high viscosity- prevents from flowing far)
-alternating layers (strata) of hardened lava + pyroclastic deposits = steep sided cones
-explosive/ pyroclastic
*mt fuji, mt st Helens
describe shield volcanoes
-basaltic (low viscosity)
-broad domes w/ gentle slopes
-effusive eruptions
describe cinder cone volcanoes
-basaltic (low viscosity)
-volatile rich; explodes and is blown out into the air as fragments which cool and fall back as scoria
-fallout forms cones w/ slopes 30-40 degrees
-towards end of eruption lava may drain from base
where does most magmatism occur
-divergent + convergent plate boundaries
-interior of tectonic plates (interpolate magmatism/ hot spots)
describe distribution of magmatism at divergent boundaries
1) midocean ridge
-basaltic
-new ocean crust
-pillow lavas
-when 2 newly formed oceanic plates move away from each other
2)continental rift
-continent is pulled apart and thinned by faults
-rising basaltic leads to partial melting of CC
-magma = basaltic-rhyolitic
-can lead to all types of volcanoes
describe distribution of magmatism at convergent boundaries
1) ocean to ocean subduction/ island arc
-andesitic magma
-mainly explosive
2) ocean to continent subduction/ continental arc
-rising magma = partial melting of crust (rhyolitic- andesetic)
-generally higher silica content
-mainly explosive
describe distribution of magmatism at hot spots
1) oceanic hot spots
-interior oceanic plate above mantle plume
-hot rock is moved from base of mantle up to the base of the lithosphere, it is decompressed and experiences partial melting
-basaltic
-effusive
2)continental hot spots
-interior of continental plate above mantle plume
-basaltic magma causes partial melting of crust
–rhyolitic
-very explosive
explain the special cases of the distribution of magmatism
1)iceland
-midocean ridge + hot spot
-basaltic
-effusive eruptions (fissure eruptions, shield volcanoes, cinder cones)
2)flood basalts
-continental rift + hot spot
-basaltic
-partial melting @ rift from plume
-low viscosity lava spread over vast areas (large igneous provinces)
-traps (stairs)
what are the rock products of magmatism
1)underground
-intrusive igneous rock
-slow cooling
-big crystals= phaneritic
2)on the surface
-extrusive igneous rock
-rapid cooling
-small crystals= aphanitic
