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.