MAGMATISM/VOLCANOES Flashcards
o Extrusive (volcanic
– quickly cooling, small crystals, rhyolite
o Intrusive (plutonic
), slowly cooling, large crystals, granite
the volcanic system
Source
* Primarily, melting of the mantle
* Secondary, melting of the crust
Transport
* Along dikes and sills
Storage region
* Magma pods below the surface in large magma chambers
Eruption
- Understand ow the volcanic system model is used to explain how magma is generated, to how it reached the surface
o Volcanoes generated where magma and gas leak out from the earth’s crust
o Magma is creates by melting pre-existing rock below the earth surface, if it reaches the surface is extrudes as lava or it explodes as pyroclastic material
o Extrusive (volcanic) – quickly cooling, small crystals, rhyolite
o Intrusive (plutonic), slowly cooling, large crystals, granite
volanoes may vary by
morphology
volcanic products
explosivity
volcanic products
Lava
* Molten rock on the surface of the earth
* A mix of element
o Si, O, Fe, Mg, Al, Ca, Na, K
Pyroclastic material
* Material ejected form the volcao
o Tiny ash particles up to house sized blacks of rck
Gas
* Dissolved in the magma
* Low pressure near the surface allows escape
* Mostly H2o, and Cos
explosivity
Mafic magma
* Low gas, low viscosity: effusive eruptions
Felsic
* High gas content, high viscosity: explosive
5 properties of magma
density
viscosity
silica content
temp
volatiles
density
2.5-3.3g/cm3
(water: 1g/cm3)
Crust: 2.7-3.3g/cm3
Mantle 3.3-5.7g/cm3
viscosity
10 -10’4 Pa s (water 10’-3 Pa s)
Resistance to FLOW
Control flow, eruption style
Depends on temp, gas content and crystal content
High viscosity = more thick
o Silica content
High silica content 65-75% SiO2
* Felsic igneous rocks
o Light in colour
o Rich in K, Na, Al, Si
Low silica content 45-55% SiO2
* Mafic igneous rocks
o Dark colour
o Rich in Mg, Fe
o Low viscosity (runny)
o Temperature
600-1200C
Hot magma
* Low viscosity, low silica (mafic)
* Non-explosive (lava)
Cool magma
* High viscosity, high silica (felsic)
* Explosive (ash)
o Volatiles (gas content)
Dissolved gas: H2O, CO2, SO2
Magma begins with <10% gas dissolved H2O, CO2, SO2, Cl
Felsic
* 4-6 WT % dissolved volatiles
Mafic
* 01-1 wt% dissolved volatiles
Magma rises lower P less soluble less soluble bubbles
where volcanism takes places
plate boundaries and hot spots
o Mid-oceanic ridges
Most voluminous volcanism on earth
Not exposed at earths surface (Iceland is the exception)
o Continental volcanic arcs- subduction volcanoes
Mafic magma
* Near the subduction line
Felsic magma
* In the middle
Felsic and intermediate magmas/lava
* Higher up
Ex- juan de fuca plate and north American plate
o Oceanic volcanic arcs
Magma below
Intermediate afic magma/lavas higher up
o Hotspots
o Hotspots
Mafic magma from a plume of hot mantle
Plumes are stationary and pulsatory
- Explain how viscosity and gas content control explosivity of eruptions.
o Magmas produc bbbles (gas exsolves) during ascent
o Bubbles expand as the magma rises (lower P)
o Foaming explosivity
o Depends on
Amount of bubbles
Rate of rise
Bubble retention
- Know how to classify a volcano based on morphography and how the
cinder cones, shield volcanoes
stratovolcanoes
calderas
cinder cone
Frequent eruptions
Layers of pyroclastic ejecta
Mafic
Small volcanoes that never grow up, usually erup for a few years then never again
shield volcanoes
Frequent- continuous eruptions
Lava erupts from fissure, runs down gentle slopes cooling
Erupts often
Mafic lava flows
Flows of pahoehoe followed by A’a
Generally not very explosive
o Stratovolcanoes
Frequent eruptions
Interbedded lava flows, pyroclastic flows, Ihara
Usually intermediate or felsic
Frequently explosive, often viscous magma
Erupt many time and stays active for 100.000 years
o Calderas
Rare eruptions
Created from lage explosive eruption of felsic pyroclastic material. The caldera is created when the roof of the magma chamber collapse
Up to 10 km across
Different from crater
o Explosivity
Viscosity and gas content are very important in determining the eruption style
As magma rises pressure decreases bubbles begin to form
* But viscosity fights bubble growth
Pressure rises inside the bubbles until the strength of the liquid magma is overcome fragmentation occurs
Pyroclastic material (tephra) is produced
types of explosive eruptions
effusive, explosive, Hawaiian, strombolian, vulcanian, plinian, phreatomagmatic
pyroclastic
- Pyroclastic
o Fall
Eruption columns: 10s of km
Widespread distribution of ash n the downwind direction
Ash blankets topography
o Flows
Gas- pyroclast mixtures
Gravity-driven flows
Flow down slope, channelled in valleys
Velocity = 40 to >400km/h
Temp= 100-600C
o Phreatomagmatic
Contact between water and magma
Water flashes to steam
Violently explosive
Surtseyan and phreatoplinian
* (water vapour cloud, compresses ash, crater, water
o Plinian
Andesitic/ rhyolitic ash
Violently explosive
Sustained columm or ash
Pyroclastic
* (ash plume, volcanic ash rain)
o Vulcanian
Viscous andesitic/ rhyolitic magma
Very explosive
Sustained explosions of ash
* (Ash plume, lapilli, volcanic ash rain, Volcanic bomb)
o Stombolian
Basaltic/andesitic magma
Idly explosive
Bombs, lavas
* (lava fountain, Volcanic bomb, Lava flow)
o Hawaiian
Low viscosity basaltic magma
Low explosivity (effusive)
* Lava flows
* Fire fountain
(lava fountain, lava lake, lava flow)
o Explosive
Gas Driven violent eruptions pyroclastic deposits
Bouyant eruption column of ash
Proclastic airfall
Proclastic flows (column collapse)
blacks and bombs proximal to vent
o Effusive
Outpouring of molten magma from the vent lavas
Passive eruption of magma
* Lava flows (mafic- intermediate)
* Lava domes (felsic intermidate)
* Gravitational collapse of lave flows/domes- pyroclastic flows
- Understand how the VEI scale works
o Running from 0-8
o Dependent ton how much volcanic material is thrown out, to what height and how ling the eruption lasts
o An increase of index indicates an eruption that is 10 times as powerful
