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GOPH375 > Ch 6 > Flashcards

Ch 6 Flashcards

1
Q

Most volcanism is associated with plate boundaries:

A
  • 73% at spreading centers
  • 15% at subduction zones
  • 12% at hot spots
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2
Q

No volcanism is associated with:

A

transform faults or continent-continent collisions

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3
Q

Subduction-zone volcanoes are explosive and dangerous

A
  • Subduction zones last 10s of mils of yrs
  • Volcanoes may be active any time even after centuries of quiet
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4
Q

Eruption of Mt. Vesuvius, 79 C.E

A
  • Cities of Pompeii and Herculaneum buried by massive eruption, blew out ~half of Mt Vesuvius
  • Clouds of hot gas (500C), ash and pumice enveloped cities
  • Many tried to escape near sea, but buried by pyroclastic flows
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5
Q

Vesuvius was inactive for 700yrs before 79CE eruption

A

People lost fear and moved closer to volcano
After 79CE, there was another eruption every 10-250ish yrs
- 500 yrs of quiet then 1631 eruption killed 4k ppl
- 18 cycles of activity between 1631-1944, nothing since
- 3mil ppl live within danger of Vesuvius today; 1mil on slopes of volcano

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6
Q

Of 92 naturally occurring elements:

A
  • 8 make up >98% of Earth’s crust
  • 12 make up 99.23% of crust
  • O and Si most abundant (typically as SiO4 tetrahedron that ties up positively charged atoms to form minerals)
  • Excluding O, 11 most abundant elements are all positively charged and form oxides
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7
Q

Plutonic rocks

A

Intrusive, magma cools slowly and solidifies beneath surface

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8
Q

Volcanic rocks

A

Extrusive, magma erupts and cools quickly at surface

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9
Q

Order of crystallization in magma

A

First: Fe or Mg w/ SiO4: olivine, pyroxene, amphibole, and biotite
Then plagioclase/K feldspar
Lastly: Quartz (SiO4)

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10
Q

Types of Magma: based on SiO2 composition and plutonic vs volcanic

A

1) <55% SiO2 = gabbro/basalt
2) 55-65% SiO2 = diorite/andesite
3) >65% = granite/rhyolite

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11
Q

What is the most abundant dissolved gas in magma?

A

Water

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12
Q

As magma rises, pressure decreases, water becomes steam

A
  • Basaltic magma has lower H2O content = peaceful, safe eruptions
  • Rhyolitic magma has higher H2O content and high viscosity = violent, dangerous eruptions
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13
Q

Spreading centers are ideal for volcanism because:

A
  • Sit above hot asthenosphere
  • Asthenosphere has low SiO2
  • Plates pull apart = asthenosphere rises and melts under low pressure, changing to high temp, low SiO2, low volatile, low viscosity basaltic magma that allows easy escape of gas
  • All factors promoting peaceful eruption
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14
Q

Subduction zones have violent eruptions because:

A
  • Basaltic rock of subducting plate with water in it dehydrates
  • Water added to upper mantle rock promotes melting
  • Hydrous basalt magma rises and crystallizes, remaining magma has altered composition
  • Hunks of crust contaminate magma further (magma mixing)
  • Magma temp decreases while SiO2, H2O content, and viscosity increase = violent
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15
Q

Three things will cause rock to melt:

A

1) Decreasing pressure
2) Increasing temp
3) Increasing H2O

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16
Q

Beginning of how a volcano erupts

A

Begins with heat at depth
- Rock that is superheated = rises
- As hot rock rises, pressure decreases, so some melts
- Volume expansion leads to eventual eruption

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17
Q

Decompression melting

A

Occurs when pressure decreases, v common way to melt rock
- Magma source: plastic flow of asthenosphere (heats during subduction and rises)
- Melt as pressure lowers (host rock incorporated, H2O remains dissolved)
- Rise continues, bubbles form adding buoyant propulsion

18
Q

How a volcano erupts

A
  • Nearly molten asthenosphere rock is hot enough to flow as a solid, which is the magma reservoir
  • When pressure is lowered, rock melts and increases in volume, fracturing nearby rock which then melts
  • Magma at depth is under too much pressure for gas bubbles to form (gases stay dissolved in magma)
  • As magma rises toward the surface, pressure decreases and gas bubbles form and expand, propelling the magma further up
  • Eventually, gas bubble volume may overwhelm the magma, fragmenting it into pieces that explode out as a gas jet
19
Q

What is the role of water content in magma?

A

H2O concentration determines if an eruption is peaceful or explosive
- Basaltic magma can erupt violently with enough water
- Rhyolitic magma usually erupts violently b/c of high water content, high viscosity
Water that enters magma during an eruption can enhance the explosiveness

20
Q

Nonexplosive Eruption Types

A

Pahoehoe: smooth ropy rock from highly liquid lava
Aa: rough blocky rock from more viscous lava

21
Q

Explosive eruptions

A

Pyroclastic debris: broken up fragments of magma and rock from violent gaseous explosions, classified by size
- Largest debris settles closest to eruption site
May be deposited as:
- Air-fall layers settled from ash cloud, layers of fining upwards
- High-speed, gas-charged pyroclastic flow does not sort into particle size, unsorted debris

22
Q

Very quick cooling:

A

Obsidian: volcanic glass that forms when magma cools v fast
Pumice: porous rock from cooled froth of magma and bubbles
Scoria: rough crusts or chunks of basalt full of holes from expanding gases

23
Q

Volcanism at Divergent Plate Boundaries

A
  • Asthenosphere daylights
  • Low SiO2, high T, low volatile
  • Low-viscosity basaltic magma
  • Easy escape of gases (peaceful eruptions)
24
Q

Volcanism at Subducting Plate Boundaries

A
  • Subducting basaltic plate carries sea water
  • Water + melted upper mantle lower melt T, decreases viscosity
  • Magma rises opening existing fractures
  • Saturated host rock mixed into melt (melt T decreases, SiO2 and H2O content increase)
  • Viscosity increases plus volatiles (violent eruptions)
25
Q

Three V’s of Volcanology

A

Viscosity: controls whether magma flows easily or piles up
Volatiles: may escape harmlessly or explode
Volume: greater vol = more intense eruption

26
Q

Shield Volcanoes

A
  • Low viscosity, low volatiles, large volume
  • Basaltic
  • Gently dipping, thin layers of volcanic rock
  • Thousands of layers form broad, gentling slope volcano
  • Great width compared to height
27
Q

Hawaiian-Type Eruptions

A

Floods of lava spill out and flow as rivers
- Lasts days or years
- Usually not life-threatening
- Destroys infrastructure

28
Q

Volcanic Explosivity Index (VEI)

A

Evaluation of eruptions according to volume erupted, the height of eruption column, and duration. Scale from 0-8.

29
Q

Flood Basalts

A
  • Low viscosity, low volatiles, very large volume
  • Largest volcanic events known on Earth
  • Characterized by immense amounts of magma, gas, and heat, and short geologic duration
  • Global effects: CO2 and SO2 released into atmosphere
    Some flood basalts coincide with mass extinctions
30
Q

Scoria Cones

A
  • Medium viscosity, medium volatiles, small volume
  • Low conical hills
  • Basaltic to andesitic pyroclastic debris
  • Can have summit crater with lava lake
  • Form during single eruption lasting hours to several years
31
Q

Stratovolcanoes

A
  • High viscosity, high volatiles, large volume
  • Steep-sided, symmetrical volcanic peaks
  • Composed of alternating layers of pyroclastic debris and andesitic to rhyolitic lava flows
  • Magma composition can vary from eruption to eruption, altering eruptive styles from Vulcanian to Plinian
32
Q

Vulcanian-type Eruptions

A
  • Alternate between highly viscous lava flows and pyroclastic eruptions
  • Common in early phase of eruptive sequence before larger eruptions (“throat clearing”)
33
Q

Plinian-type Eruptions

A
  • Occur after “throat clearing”
  • Gas-powered vertical columns of pyroclastic debris up to 50km into atmosphere
  • Commonly final phase in eruptive sequence 2-3 such eruptions each century
34
Q

Vesuvius, 79 CE

A
  • Subduction of Mediterranean seafloor beneath Europe
  • Most of 4k ppl killed by 3m layer of pyroclastic flows Vulcanian-type eruption
  • Followed by Plinian-type eruptions with pyroclasts reaching 32km heights
  • Seismic waves define 400km^2 magma body 8km under Vesuvius today
  • Millions of ppl live around Bay of Naples area
35
Q

Volcanic Mudflow

A
  • Steam blown up into atmosphere during eruption can cool, condense, and fall as rain that picks up loose volcanic ash then flows downslope as lahars
  • Town of Herculaneum survived the 79 CE eruption only to be buried 20m deep by lahars
36
Q

Lava Domes

A
  • High viscosity, low volatiles, small volume
  • Form when high-viscosity magma at vent of volcano cools quickly into hardened plug
  • Gases accumulated at top of magma chamber power Vulcanian and Plinian blasts until most volatiles have escaped
  • Remaining magma is low-volatile, high viscosity paste
  • Oozes to vent and cools quickly in place, forming plug
  • Forms in hours to decades
  • Can fracture allowing new eruption of break and fall as landslide
37
Q

Calderas

A
  • High viscosity, high volatiles, very large volume
  • Large (2-75km) volcanic depressions formed by roof collapse into partially emptied magma reservoirs
    Form at different settings:
  • Summit of shield volcanoes, such as Mauna Loa or Kilauea
  • Summit of stratovolcanoes, such as Crater Lake or Krakatau
  • Giant continental caldera, such as Yellowstone or Long Valley
38
Q

Krakatau, Indonesia, 1883

A
  • Part of volcanic arc above the subduction zone between Sumatra and Java
  • After earlier collapse, Krakatau built up during 17th century
  • Quiet for 2 centuries then active in 1883
  • Moderate Vulcanian eruptions from dozen vents
  • Led up to enormous Plinian blasts and eruptions 80km high and audible 5k km away
  • Triggered tsunami 35m high killing 36k ppl
  • Caused skies in Europe to turn blood red
  • Has been building new cone Anak Krakatau since 1927
39
Q

Santorini, Greece

A

Mediterranean plate subducting beneath Europe -> many volcanoes including strato Santorini in Aegean Sea

Series of eruptions around 1628BCE:
- Buried city of Akrotiri in 70m of debris
- Huge caldera to depths of 390m below sea lvl now exists where once was large island
- Took less than 100yrs to build up of 40-60km^3 of magma that erupted

40
Q

Eruptive Sequence of a Resurgent Caldera

A
  • V large volume of rhyolitic magma bows ground upward
  • Accumulates cap rich in volatiles and low-density material
  • Circular fractures form around edges -> Plinian eruptions then pyroclastic flows as more magma is released than can vent upwards
  • Lower pressure causes gas to come out of solution in great volumes
  • Magma erupts out, most volume as pyroclastic flow
  • As magma body shrinks, land surface sinks into void
  • Isostatic imbalance due to huge loss of mass leads to inflow of new mass magma creating resurgent dome -> next eruption

GOPH375 (10 decks)

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