Ch 6 Flashcards
Most volcanism is associated with plate boundaries:
- 73% at spreading centers
- 15% at subduction zones
- 12% at hot spots
No volcanism is associated with:
transform faults or continent-continent collisions
Subduction-zone volcanoes are explosive and dangerous
- Subduction zones last 10s of mils of yrs
- Volcanoes may be active any time even after centuries of quiet
Eruption of Mt. Vesuvius, 79 C.E
- 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
Vesuvius was inactive for 700yrs before 79CE eruption
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
Of 92 naturally occurring elements:
- 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
Plutonic rocks
Intrusive, magma cools slowly and solidifies beneath surface
Volcanic rocks
Extrusive, magma erupts and cools quickly at surface
Order of crystallization in magma
First: Fe or Mg w/ SiO4: olivine, pyroxene, amphibole, and biotite
Then plagioclase/K feldspar
Lastly: Quartz (SiO4)
Types of Magma: based on SiO2 composition and plutonic vs volcanic
1) <55% SiO2 = gabbro/basalt
2) 55-65% SiO2 = diorite/andesite
3) >65% = granite/rhyolite
What is the most abundant dissolved gas in magma?
Water
As magma rises, pressure decreases, water becomes steam
- Basaltic magma has lower H2O content = peaceful, safe eruptions
- Rhyolitic magma has higher H2O content and high viscosity = violent, dangerous eruptions
Spreading centers are ideal for volcanism because:
- 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
Subduction zones have violent eruptions because:
- 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
Three things will cause rock to melt:
1) Decreasing pressure
2) Increasing temp
3) Increasing H2O
Beginning of how a volcano erupts
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
Decompression melting
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
How a volcano erupts
- 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
What is the role of water content in magma?
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
Nonexplosive Eruption Types
Pahoehoe: smooth ropy rock from highly liquid lava
Aa: rough blocky rock from more viscous lava
Explosive eruptions
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
Very quick cooling:
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
Volcanism at Divergent Plate Boundaries
- Asthenosphere daylights
- Low SiO2, high T, low volatile
- Low-viscosity basaltic magma
- Easy escape of gases (peaceful eruptions)
Volcanism at Subducting Plate Boundaries
- 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)
Three V’s of Volcanology
Viscosity: controls whether magma flows easily or piles up
Volatiles: may escape harmlessly or explode
Volume: greater vol = more intense eruption
Shield Volcanoes
- 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
Hawaiian-Type Eruptions
Floods of lava spill out and flow as rivers
- Lasts days or years
- Usually not life-threatening
- Destroys infrastructure
Volcanic Explosivity Index (VEI)
Evaluation of eruptions according to volume erupted, the height of eruption column, and duration. Scale from 0-8.
Flood Basalts
- 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
Scoria Cones
- 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
Stratovolcanoes
- 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
Vulcanian-type Eruptions
- Alternate between highly viscous lava flows and pyroclastic eruptions
- Common in early phase of eruptive sequence before larger eruptions (“throat clearing”)
Plinian-type Eruptions
- 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
Vesuvius, 79 CE
- 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
Volcanic Mudflow
- 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
Lava Domes
- 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
Calderas
- 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
Krakatau, Indonesia, 1883
- 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
Santorini, Greece
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
Eruptive Sequence of a Resurgent Caldera
- 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
