ESS 8 Volcanoes

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ESS 8Volcanoes
From 2/11/99 Nature
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• Volcanoes
• Not in our books
• Ill relate to quakes near end
• Three kinds of volcanism
• Subduction zones
• Mid-ocean ridges
• Hot spots
• Volcano danger mitigation
• Volcanoes on other planets

Basil Wolverton's Apolcalypse
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Lava, magma, and plutons

• Lava - molten rock that comes out of a volcano.
• Magma - molten rock inside the Earth.
• Pluton - cooled, solidified magma. Occurs after
  volcano becomes inactive.

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Paricutin, 1946
Paricutin, Mexico Location 19.5N,
102.2W Elevation 10,400 feet (3,170 m)
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Volcanism
Mt. Fuji

• About 1500 active volcanoes are distributed
  mostly at plate boundaries.
• Ring of Fire around Pacific Ocean is where most
  oceanic plates subduct.
• Volcanism is how the crust grows.
• Continental crust produced by subduction-
  associated volcanism
• Ocean crust produced by mid-ocean ridge spreading

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Locations of volcanoes
Ring of Fire around Pacific Rim
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Why do some volcanoes erupt explosively?

• Gas makes magma rise and erupt
• Erupts either effusively, or explosively.
• If the gas is trapped by the magma,
• it breaks the magma into
• bits and erupts
• explosively.

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Explosivity and Viscosity

• Magma viscosity is key
• Viscosity is the resistance of a fluid to flow
• Honey is more viscous than water
• The mantle is very, very viscous
• General rule The more viscous the magma, the
  more explosive the eruption
• Gas is more easily trapped in viscous magma and
  it expands rapidly causing explosion
• Exception
• If there is not enough gas present, then no
  explosion

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What determines viscosity?

• Chemical composition
• Rocks are mainly made of Si and O, which combined
  is called silica

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Silica gives rocks structure

• Silicon atom surrounded by oxygen forms
  tetrahedron
• Tetrahedra link up to form chains and structures

• Therefore, the more silica (silicon and oxygen)
  there is, the stiffer (more viscous) the fluid

http//volcano.und.nodak.edu/vwdocs/vwlessons/mine
rals.html
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Convecting system
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Relation of volcanism to plate tectonics
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Mantle plume
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Types and sites of volcanism

• Spreading at mid-ocean ridges
• 70 of global magma budget
• Non-explosive eruptions of basalt (low-silica
  lava, i.e. low viscosity)
• Above subducting plates
• 25 of global magma budget
• Explosive eruptions of relatively viscous,
  silica- and gas-rich lavas
• Hot spot, over mantle plumes
• 5 of global magma budget
• Mild eruptions of basaltic lavas

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Vents at divergent vs. convergentplate
boundaries

• Most magma comes from fissures at divergent
  spreading ridges
• Most magma comes from volcanoes (large, roughly
  circular structures) at convergent subduction
  zones
• Mixture at hot spots

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Mid-ocean ridge

• Plate tectonic setting
• Plates pulling apart a few cm / year
• Material upwelling to fill gap
• Magma intrudes and extrudes
• Underwater
• Covers seafloor at ridge
• Also fills in crust below
• Majority of worldwide volcanism

Mid-Atlantic ridge
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Magma chamberat mid-ocean ridge
Gabbro a black coarse-grained rock the
intrusive equivalent of basalt
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How does melt form crust?

• Newly-separated crust
• Melting tends to contain lighter elements in the
  rock
• Mantle is residual left underneath
• After melt has ascended, the rock and sludge left
  behind is heavier

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Pillow lavas

• Basaltic
• Characteristic of underwater volcanic eruptions
• If found on land, indicates earlier period of
  submergence

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2 billion yr-old basalt pillows sea-floor
spreading has occurred for a long time
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Subduction zones

• Subduction pulls down material
• At depths of 100-150 km, slab dewaters, overlying
  asthenosphere melts partially, then magma
  trickles up to surface
• Produces volcanoes located above slab
• Large and numerous
• Very dangerous
• Build-up of melt forms island arcs if under ocean
• Forms mountain belts on continents

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Plutonformation
Future pluton
Pluton - cooled, solidified magma chamber. Seen
only after volcano becomes inactive.
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Exposed plutons in Yosemite have cooled and been
uplifted, and the volcanoes above them eroded off.
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Sill
Dike
Batholith
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West coast volcano map
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Cascades Range

• Continental arc volcanoes related to subduction
  of Juan de Fuca plate under North American plate
• From Northern CA to Southern B.C.
• Shasta
• Mt. Hood
• Mt.Rainier
• Mt. Saint Helens
• Mt. Baker

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Cascades activity
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Oregon-Washington cross-section
Cascade Mts
Olympic Mts
Ridge
Keller, 7-5
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Mt. St. Helenshistory

• Previous active in 1857
• Has erupted similarly or at least 40,000 years
• 1980 eruptions
• A week of increasing seismicity
• Then 6 weeks of small eruptions
• Then big eruption of May 18, 1980

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Mt. Saint HelensBefore After Eruption
May 18, 1980
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Mt. St. Helens lateral blast

• Magma intruded causing swelling of NE flank
• Earthquake triggered massive landslide
• Gas steam under high pressure released in
  lateral blast, pyroclastic flow
• Vertical columnar eruption blanketed region with
  ash
• Melted glaciers produced lahars

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Bulge develops
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Dangerous Post
Keller, 5-3
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Mt. St.Helensinaction
Keller, 5-23d
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Closer view
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Damage map
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Hot spot volcanism

• Overlies upwelling plumes in the mantle
• Cylindrical, not sheet-like as with ridges
• Big volcanoes, like on Hawaii
• Even bigger volcanoes where plumes punch through
  continents
• Small part of world-wide volcanism
• Not necessarily near plate boundaries

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Plume
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Young and Old
www.punaridge.org/doc/factoids/ Hawaii
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Island ages
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Island age progression
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Hawaii-Emperor Chain
Keller, 2-18a
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General Volcano Hazards

• Explosion
• Pyroclastic flows
• Lahars (mudflows)
• Ash with toxic gases
• Lava
• Tsunamis
• Earthquakes

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More terminology eruptive products

• Pyroclast - fragment of volcanic material ejected
  into air during eruption, can take many forms
• Pyroclastic flow - a hot, glowing cloud of
  volcanic ash, fragments of rock, and gases that
  move rapidly downhill away from eruption site,
  can travel up to 200 km/hr

Mt. Unzen, Japan
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Pyroclasticflow
1902 at Mont Pelée Martinique where 29,000 people
died
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Trees
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Blasted trees
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Even more trees
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Mud flow (Lahar)
scientist
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Ash fall, actually Pinatubo, Indonesia
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Erebus spindle, Antarctica
Antarctica
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Bomb at Yasur, New Hebrides
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2 types of Hawaiian lava have different
viscosities and gas content
Pahoehoe
Aa
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Destruction by lava in Hawaii
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Blocky flow on Iceland
Keller, 5-18
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Famine

• Laki, Iceland 1783-1784 eruption
• Killed most of the livestock
• Fluorine contaminated grass
• Crop failure
• Acid rain
• Resulted in 9,000 deaths (1/4 of Icelands
  population at the time)

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Mechanics of a volcano

• Magma rises from deep in Earth
• Lava flows out of central vent
• Volcanic structure forms by repeated lava flows

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Types of volcanic structures

• Shield volcano
• Large, broad, gently sloping, built by successive
  flows of lava from central vent
• Cinder cone
• Small, built from pyroclastic fragments
• Stratovolcano/composite
• Medium sized, built from alternating lava flows
  and pyroclastic deposits

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Shield volcano - fluid lava flows easily
spreads widely
Example Island of Hawaii
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Shield volcano
Numerous layers
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Lava Lake
Mihara-yama on November 16, 1986
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Fissure eruption
Izu-Oshima Volcano on November 21, 1986
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Hawaii

• 5 eruptive centers
• Highest is Mauna Loa
• Most active recently is Kilauea

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Cinder cone
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Cinder cone

• Profile determined by maximum angle at which
  debris remains stable
• Larger fragments fall near summit can form
  steeper slopes
• Finer particles carried farther form gentle
  slopes

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SP Crater, AZ - cinder cone
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Stratovolcano
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Stratovolcano

• Alternating layers of pyroclastic material and
  lava flows
• Lava also solidifies in fissures that form
  riblike dikes that strengthen cone
• Fujiyama, Mt. Etna, Mt. Vesuvius, Mt. St.
  Helens

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Mt Rainier, Washington
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Stratovolcano in action
Lascar, South America, 1993
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Volcanic domes
Mt. St. Helens
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Shiprock, NM - volcanic pipe
Radial dikes
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Craters and calderas

• Crater
• Bowl-shaped pit at summit of most volcanoes
  centered on vent, may fill with water
• Caldera
• Larger basin-shaped depression much larger than
  crater, resulting from collapse of mountain into
  empty magma chamber

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Crater Lake, OR
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Two large westernUS calderas

• Long Valley, CA
• 700,000 yrs ago
• Recent earthquake swarms possibly indicating
  movement of magma underground
• Yellowstone, WY
• 600,000 yrs ago
• Ash layers covered West Coast

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Long ValleyModerateeruption(lt 1
km3ejected)smallerthanMt. St. Helens
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Distributionof Bishop Tuff

Valle Grande
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Schematic history ofLong Valley Caldera
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Calderas
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Long Valley Caldera Structure
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Anak Krakatoa
6-km caldera from the 1883 Krakatoa eruption
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Rocks from volcanoes
Intrusive
Extrusive
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Basalt
Vesicular
Columnar
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Vesicular basalt
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Obsidian
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Pumice
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Relation to earthquakes

• Volcanoes and quakes occur near plate boundaries
  (convergent and divergent)
• Volcanic eruptions involve deforming crust and
  lead to earthquakes
• Earthquakes can change stress in region and lead
  to volcanic eruptions

Mallet
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Predicting Eruptions

• Surface deformation of volcano
• as it fills up with magma and bulges
• Acceleration and shallowing of seismicity
• Need a local network
• Minor eruptions usually precede major eruptions
• More predictably than foreshocks precede big
  quakes

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Volcanic earthquakes

• Volcano-tectonic earthquake (VT)
• Normal event
• Long-period event (LP)
• Slower (longer-period) motions
• Tremor-like episode of closely-spaced long-period
  events.
• Harmonic tremor
• Explosive eruption

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Types of quakes
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A definition

• Harmonic Tremor A continuous release of seismic
  energy typically associated with the underground
  movement of magma. It contrasts distinctly with
  the sudden release and rapid decrease of seismic
  energy associated with the more common type of
  earthquake caused by slippage along a fault.

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Build-up of activity
Two days before
Just before
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Accelerating activity
Bang!!
Little eruptions
Seismicity
Plumes
Tremor
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Pinatuboeruption
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Steps to reduce danger

• Zoning laws
• Dont live under active volcano
• Warning systems can work
• Volcano monitoring can be cheap
• 5 to 10 seismic stations works ok
• Evacuation plans
• Government has to plan in advance
• Dont ask scientists to manage crises

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Merapi Volcano

• The island of Java has more than 600 people per
  square km, one of the highest population
  densities in the world.
• Yogyakarta city, with a population of 3 million,
  is 15 miles (25 km) south of Merapi.

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Volcanohazardzoning
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Volcanoes on other planets

• Moon
• Mars
• Venus
• Io (a moon of Jupiter)

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Ponds of basalton near side of Moon
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Vintage of planetary volcanism

• Was active 3.8 to 3.1 billion years ago
• Dated by rock sample recovered and counting
  crater density
• But volcanism on Mars and Venus seems to still be
  active
• At least it is less than 1 billion years old.
• Mars and the Moon have a frozen lid
• Venus might have plate tectonics.
• Io has activity even as we watch it

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Olympus Mons on Mars
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Planetary volcanosize comparison
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Sif Mons on Venus
2 km high and nearly 300 km across, exaggerated
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Pancake domes on Venus
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Io, a moon of Jupiter
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Sudden appearence
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Summary

• Three kinds of volcanism
• Plate subduction, plate divergence, and mid-plate
  hot spots
• Three types of volcanic structures
• Shield volcanoes, cinder cones, and
  stratovolcanoes/ composite volcanoes