EOSC 114 Volcanoes
Types of pyroclasts
Ash <2mm Lapilli 2-64mm Bombs >64mm
Magma temperature
600-1200ºC
Magma begins with ___% gas content
<10
Types of lava
Pahoehoe and Aa
Volcanic Explosivity Index (VEI)
0-8, total volume erupted, each increase = 10x more volume
Monitoring and prediction steps
1) Hazard maps 2) Monitoring
Types of volcanoes
1. Cinder cones 2. Shield volcanoes 3. Stratovolcanoes 4. Calderas
4 regions of volcanic system
1. Eruption region 2. Storage region 3. Transport region 4. Source region
Types of monitoring
1. Seismology* - GPS, Tiltimeters 2. Gas emission 3. Thermal imaging 4. Lahar flow detection 5. Satellite observation (InSAR - detects elevation changes)
Formation of volcanic arcs
1. Volatiles + water released into mantle, causes partial melting 2. Rising magma encounters felsic crust which remelts, creates intermediate magma 3. Creates felsic and intermediate magmas/lavas
Formation of oceanic volcanic arcs
1. Volatiles and water released into mantle, causes partial melting 2. Mafic and intermediate magmas/lavas reach surface
Key characteristics of VEI
1. Volume of ash 2. Height of eruption cloud above vent 3. Duration of eruption
Magma viscosity
10-10^15 pascals
Magma density
2.5-3.3 g/cm^3
Vulcanian eruption
Andesitic/rhylotic, viscous, very explosive, sustained explosions of ash
Explosivity depends on
Amount of bubbles, rate of rise, bubble retention
How are magma bubbles created
As magma rises, lower pressure, less solubility, bubbles created
4 rock types darkest to lightest
Basalt, andesite, dacite, rhyolite
Mafic rock examples
Basalt, gabbro
Strombolian eruption
Basaltic/andesitic, mildly explosive
Type of lava at continental crust
Felsic, magma rises in a dike
Explosive eruption volcanic processes
Column of ash Pyroclastic ashfall Pyroclastic flows Blocks and bombs close to vent
Two factors that determine type of volcanism
Crust composition, melt origin
Lava composition
Crystals, bubbles
Mafic color
Dark
How fast does extrusive rock cool
Days to months
2 elements of transport region
Dike, sill
What lavas do continental hot spots produce?
Felsic, intermediate
Lava
Extruded magma out of surface
Caldera
Felsic, explosive, rare
Felsic rock examples
Granite, rhylote
Types of eruptions
Hawaiian, Strombolian, Vulcanian, Plinean
Cold magma viscosity
High (felsic)
Felsic silica content
High - 65-75%
Plinain eruption
Highest viscosity, highest gas content, dacitic/rhylotic ash
Sill
Horizontal intrusion of magma
Pyroclastic flow
Hot gas and ash rushes downslope, channeled in valleys, velocity (40-400km/h), temp. - 100-600ºC
Hot magma viscosity
Low (mafic)
Mafic silica content
Low - 45-55%
Hawaiian eruption
Low viscosity, low gas, low explosivity
Formation of stratovolcano
Interbedded lava flows, pyroclastic flows, lahars Stays active for 100,000 years
Stratovolcano
Intermediate/felsic, frequent, explosive
Mt Baker hazards
Pyroclastic flows, lava flows, lahars, pyroclastics
Formation of shield volcano
Lava erupts from fissures, runs down gentle slopes, forms layers of rock
Volcanic hazards
Lava flow, pyroclastic flow, ash fallout, lahar, gases, sector collapse
Effusive eruption volcanic processes
Lava flows (low viscosity, low gas) Lava dome (high viscosity, low gas)
Formation of cinder cone
Layers of ejecta settle at angle of repose (30º-40º) Erupt for a few years then never again
Felsic color
Light
What lavas do oceanic hot spots produce?
Mafic
Primary magma
Mafic magma + volatiles = primary magma
Type of lava at mid ocean ridge
Mafic, basalt
Shield volcano
Mafic, effusive, frequent/continuous eruptions
Cinder cone
Mafic, explosive, frequent
Extrusive rock
Magma cools on Earth's surface, quick cooling, tiny crystals
Intrusive rock
Magma hardens beneath Earth surface, slow cooling, large crystals
Mafic composition
Magnesium, iron
Composition of magma
Melt, crystals, bubbles
Magma
Melted rock in the Earth (below surface)
Divergent plate boundaries where volcanoes occur
Mid ocean ridge, continental rift
Is there volcanism at transform plate boundaries
No
Location of volcanoes
Plate boundaries and hotspots
Felsic composition
Potassium, Sodium, Aluminium, Silica
A'a lava
Ragged, high viscosity, fast
What is the most important factor in determining igneous rock texture (grain size)
Rate of cooling
Formation of caldera
Roof of magma chamber collapses, produces devastating eruptions
Pahoehoe lava
Smooth, ropy, slow, low viscosity
Mantle plumes are ___ and ___
Stationary and pulsatory (according to magnetism)
Mt Baker
Stratovolcano nearest to Vancouver
Convergent plate boundaries where volcanoes occur
Subduction (oceanic-oceanic, oceanic-continental)
Flow and eruption style depends on
Temperature Gas Crystal content
How fast does intrusive rock cool
Tens to hundreds of thousands of years
Phreatomagmatic eruption
Term used to describe volcanic eruptions that involve magma and water coming in contact; the resulting steam caused extremely explosive activity - creates compressed ash and compressionary lapilli
Dike
Vertical intrusion of magma
Sector collapse
When a volcano becomes structurally unable to support its own mass, a large portion of the volcano can fail catastrophically. - can result in lateral blast (pressure reduced more explosions occur)
Pumice
a very light and porous volcanic rock formed when a gas-rich froth of glassy lava solidifies rapidly.
