The process that allows magma to flow to surface of earth Geothermal gradient
- increase in T with depth
- T of asthenosphere reach 1100 to 1200º C
- sufficiently hot for rocks to melt
Magma less dense than surrounding rocks Rises because of buoyancy forces
- may travel through pre-existing cracks
- may melt rocks above it Eventually reaches surfaces
- forms volcano: a hill of material that has erupted
Lava important because it is sample of material at depth
- not perfect sample
- losses gasses to ocean and atmosphere
- gain or lose solid components on way up Ancient lava deposits provide information about eruptions in the past
Type of lava controls type of eruption
- high gas content, more violent eruption
- high silica content, more viscous lava, more violent eruption
- cooler temperatures, more viscous, more violent
Basaltic lavas
- high T eruptions: 1000 to 1200ºC
- low silica
- low viscosity
- fast flows, 100 km/hour possible, commonly few kilometers per hour
Deposit types depend on terrain:
- flood basalts, highly fluid basalts cover wide area (e.g. Columbia Plateau, W. Washington State)
- Pahoehoe (Hawaiian for ropy). Fluid lava that forms skin that deforms to look like rope
- Aa, basaltic lava that has lost gases, more viscous, breaks into jagged blocks
- Pillow lavas, lumps of lava resembling pillows, they form underwater
Rhyolitic Lavas
- cooler T than basalt, 800 to 1000ºC
- moves more slowly than basalt
- thick bulbous deposits Andesitic lavas
- intermediate viscosity between basalt and rhyolite
- intermediate properties
Lava textures
- vesicles from gases
- extreme example, pumice Pyroclastic deposits
- Lavas with high gas and water content under high pressure
- when they move near the surface, explosive eruptions may occur
- most common in rhyolitic and andesitic lavas
Volcanic ejecta
- Pyroclasts: any fragment of volcanic material ejected into air.
- Ash: the smallest material ejected, < 2 mm in diameter
- ejecta may be very large (10s of m)
- dust may travel long distances
Pyroclastic flows
- mixture of ash, dust, gases flow downhill
- speeds up to 200 km/hr
- T ~800ºC
- Mont Pelee, Martinique
Volcanoes shapes vary
- commonly cone shaped, Mt. Fuji, Japan, Mauna Loa, Hawaii, Mt. Rainier Washington- Not always cone shaped: Flood basalts of W. Washington flatlying
Shape of volcano depends on eruptive style
Shield volcanoes mafic (basalt)
- broad base from low viscosity basalt
- many successive eruptions
- Mauna Loa: 120 by 10 km
- shaped like shield
Volcanic domes felsic (rhyolite)
- smaller than shield volcanoes
- rounded domes, steep sided
- often plug vents, trap gases, explosive
Cinder cones
- cones made of pyroclasts
- large particle near vents, small far away
- steep sided, small, few hundred meters Composite volcanoes
- alternating pyroclastics and lava flows
- dikes within volcano add strength
- common type, include Mt. St. Helens
Crater- pit at summit of volcano
- alternately filled and drained of lava
- sides steep, may erode to become larger
Calderas- if large amount of magma eject, space left below
- roof collapses, leaves depression
- range from few km to 50 km (Yellowstone)
- as more magma moves in, caldera floor may move up.
Phreatic explosions
- when water flows to magma
- flash converted to steam
- explosive eruptions
- Mt. St. Helens example
Diatremes
- vent and feeder pipes filled with breccia
- some diatremes from deep
- kimberlites in S. Africa
flow of basaltic lava from long cracks in earths crust mid-ocean ridges are example
Flood basalts
- common deposits of fissures
- flow over large planes
- Columbia Plateau, 200,000 km2
- flows up to 100 m thick
Ash-flow deposits- similar to flood basalts, but Pyroclastic
- also cover large areas
Lahars- volcanic mud flows
- occur when pyroclastic flow hits glacier or stream
- consistency of cement
- flow rapidly
- can be large deposits
Edifice failure- tops of volcanoes slide off
- Mt. St. Helens example
- Hawaii, big waves, although not cause of coral deposits
Volcanic gases- largely water vapor
- also CO2, SO2, N, H, CO, S, Cl Volcanism and climate change
- apparent link
- gas release affects atmospheric chemistry
- pressure change from sealevel change
Fumaroles, Hot springs, Geysers- fumeroles: continuous release of volcanic gases
- hot springs & geysers: heated ground water ejected into air
volcanism follows plate boundaries
composition varies depending on boundary Divergent boundaries
- magma generated several 100 km from ridge, to depths of ~150 km
- form 1 to 2% melt
- also location of hydrothermal vents
Convergent zone volcanism
- subduction carries cold material into mantle
- partial melting because of water create magma other than basalt
Intraplate volcanims
- often associated with long volcano chains
- no earthquakes: aseismic ridges
- form from hot spots
- because they are deeply rooted (core/mantle boundary ?), they dont move
- plate moves over them forming scar
- fissure eruptions may be super-plumes very large hot spots