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magma

melted rock in earth

lava

melted rock on earth’s surface

plutonic rock

magma solidifed below surface

volcanic rock

lava solidifed above surface

what does magma contain?

dissolved gases, aka volatiles, with gas solubility increasing with pressure

what is the most abundant dissvoled gas?

water (steam)

viscosity

measure of internal resistance to flow

low viscosity

flows easily like water

high viscosity

flows poorly like toothpaste

what does magma viscosity depend on?

• temperature

• mineral crystal content

• silica content

temperature — magma viscosity

higher temperature lowers viscosity

mineral crystal content — magma viscosity

increases viscosity

silica content — magma viscosity

silica provides strong 3D chemical bonds, increasing viscosity

what crust is 60% silica?

continental crust

what crust is 48% silica?

oceanic crust

what does eruption explosiveness depend on?

• magma viscosity

• amount and ease of release of dissolved gas in magma

what causes peaceful eruptions?

low viscosity so theres easy gas escape

what causes explosive eruptions?

high viscosity so theres difficult escape

what are the three volcanic rock types?

• basalt

• andesite

• rhyolite

basalt

• black

• 80% volume at earth’s surface

• 45-55% silica

• 1000-3000 C magma temp

• low viscosity

• 0.1-1% water dissolved in magma

• easy gas escape (peaceful)

andesite

• dark grey

• 10% volume at earth surface

• 55-65% silica

• 800-1000 C magma temp

• mid viscosity

• 2-3% water dissolved in magma

• mid gas escape

rhyolite

• light colour

• 10% volume at earth surface

• 65-75% silica

• 600-900 C magma temp

• high viscosity

• 4-6% water dissolved in magma

• difficult gas escape (explosive)

how does eruption begin?

through heat at depth, where heated rock rises, and reduced pressure creates decompression melting, where bubble volume may overwhelm magma, exploding out as a jet

decompression melting

reduced pressure allows dissolved gas to form bubbles, propelling magma upwards

tectonic setting

volcano type depends on this, some can be safe and others not. the difference depends on the magma type, which also depends on tectonic setting

where does 90% of volcanism occur?

divergent and convergent plate boundaries

where does 10% of volcanism occur?

at hotspots

voluminous magma production at MORs

as plate diverge, the hot asthenosphere rises to fill gap, reducing pressure and increasing melt

MOR viscosity

low viscosity magma, with easy gas escape. MORs have peaceful eruptions with smooth lava as pillow basalts and sheet flows

pillow basalt

bulbous stacked shapes due to lava hitting cold water, cooling instantly into solid crust while the inside inflates with molten rock

sheet flows

extensive blankets of fluid lava that spread across ocean floor

continental subduction zones

subducting oceanic plate carries water sediments into asthenosphere. by 100km, heated plate releases water, lowering melting point of mantle. hot magma rises through continental crust, so magma has more silica

what volcanoes do the continental subduction zones produce?

andesitic-rhyolitic volcanism with viscous magma, explosive, high silica ash and steam

oceanic subduction zones

less explosive than continental subduction zones, as magma rises through oceanic crust which is thinner and lower silica content

collision zones

no volcanism because crust is not subducted into asthenosphere

transform faults

no volcanism

what do hotspot locations have?

mantle plumes of rising hot, molten material

oceanic hotspots

hot basaltic magma upwells through oceanic crust, resulting in smooth low viscosity magma and peaceful eruptions

continental hotspots

magma rises through thick, silica rich continental crust, partially melting it. causes explosive volcanism

what is eruption type determined by?

magma type, which is determined by tectonic setting

what are the 5 basic types of eruption?

• icelandic

• hawaiian

• strombolian

• vulcanian

• plinian

icelandic type

most peaceful eruption, with low viscosity basaltic lava from linear vents. easy gas release causes ‘curtain of fire’. creates low wide volcanic plateaus

hawaiian type

peaceful lava outpouring, easy gas release, forming small lava fountains. forms “shield” volcanoes with broad, gentle slopes.

strombolian type

increasing magma viscosity, moderately explosive eruptions and fountaining

vulcanian type

alternates between viscous lava flows and explosions of rock and ash. higher silica. characterized by strato or composite volcanoes

strato/composite volcanoes

steep symmetric peaks built of alternating ash/rock and lava flows

plinian type

powerful vertical eruption of gas, ashes, rock to great heights. most violent eruption type. typically 2 or 3 a century

what was plinian type named after?

pliny the younger who described vesuvius eruption in AD 79

what is the eruption intensity based on?

• volume of material erupted

• height of eruption column

• duration of eruption