MAGMATISM/VOLCANOES

Question 1

o Extrusive (volcanic

Answer 1

– quickly cooling, small crystals, rhyolite

Question 2

o Intrusive (plutonic

Answer 2

), slowly cooling, large crystals, granite

Question 3

the volcanic system

Answer 3

 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

Question 4

• Understand ow the volcanic system model is used to explain how magma is generated, to how it reached the surface

Answer 4

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

Question 5

volanoes may vary by

Answer 5

morphology
volcanic products
explosivity

Question 6

volcanic products

Answer 6

 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

Question 7

explosivity

Answer 7

 Mafic magma
* Low gas, low viscosity: effusive eruptions
 Felsic
* High gas content, high viscosity: explosive

Question 8

5 properties of magma

Answer 8

density
viscosity
silica content
temp
volatiles

Question 9

density

Answer 9

 2.5-3.3g/cm3
 (water: 1g/cm3)
 Crust: 2.7-3.3g/cm3
 Mantle 3.3-5.7g/cm3

Question 10

viscosity

Answer 10

 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

Question 11

o Silica content

Answer 11

 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)

Question 12

o Temperature

Answer 12

 600-1200C
 Hot magma
* Low viscosity, low silica (mafic)
* Non-explosive (lava)
 Cool magma
* High viscosity, high silica (felsic)
* Explosive (ash)

Question 13

o Volatiles (gas content)

Answer 13

 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

Question 14

where volcanism takes places

Answer 14

plate boundaries and hot spots

Question 15

o Mid-oceanic ridges

Answer 15

 Most voluminous volcanism on earth
 Not exposed at earths surface (Iceland is the exception)

Question 16

o Continental volcanic arcs- subduction volcanoes

Answer 16

 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

Question 17

o Oceanic volcanic arcs

Answer 17

 Magma below
 Intermediate afic magma/lavas higher up

Question 18

o Hotspots

Answer 18

o Hotspots
 Mafic magma from a plume of hot mantle
 Plumes are stationary and pulsatory

Question 19

• Explain how viscosity and gas content control explosivity of eruptions.

Answer 19

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

Question 20

• Know how to classify a volcano based on morphography and how the

Answer 20

cinder cones, shield volcanoes
stratovolcanoes
calderas

Question 21

cinder cone

Answer 21

 Frequent eruptions
 Layers of pyroclastic ejecta
 Mafic
 Small volcanoes that never grow up, usually erup for a few years then never again

Question 22

shield volcanoes

Answer 22

 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

Question 23

o Stratovolcanoes

Answer 23

 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

Question 24

o Calderas

Answer 24

 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

Question 25

o Explosivity

Answer 25

 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

Question 26

types of explosive eruptions

Answer 26

effusive, explosive, Hawaiian, strombolian, vulcanian, plinian, phreatomagmatic
pyroclastic

Question 27

• Pyroclastic

Answer 27

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

Question 28

o Phreatomagmatic

Answer 28

 Contact between water and magma
 Water flashes to steam
 Violently explosive
 Surtseyan and phreatoplinian
* (water vapour cloud, compresses ash, crater, water

Question 29

o Plinian

Answer 29

 Andesitic/ rhyolitic ash
 Violently explosive
 Sustained columm or ash
 Pyroclastic
* (ash plume, volcanic ash rain)

Question 30

o Vulcanian

Answer 30

 Viscous andesitic/ rhyolitic magma
 Very explosive
 Sustained explosions of ash
* (Ash plume, lapilli, volcanic ash rain, Volcanic bomb)

Question 31

o Stombolian

Answer 31

 Basaltic/andesitic magma
 Idly explosive
 Bombs, lavas
* (lava fountain, Volcanic bomb, Lava flow)

Question 32

o Hawaiian

Answer 32

 Low viscosity basaltic magma
 Low explosivity (effusive)
* Lava flows
* Fire fountain
 (lava fountain, lava lake, lava flow)

Question 33

o Explosive

Answer 33

 Gas Driven violent eruptions  pyroclastic deposits
 Bouyant eruption column of ash
 Proclastic airfall
 Proclastic flows (column collapse)
blacks and bombs proximal to vent

Question 34

o Effusive

Answer 34

 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

Question 35

• Understand how the VEI scale works

Answer 35

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