Volcanism

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Volcanism
Gulp!
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Tectonic Settings of Igneous Activity
Figure 5.11
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The Worlds Active Volcanoes
Fig. 5.28
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Volcanism Associated with Plate Tectonics
Fig. 6.19
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Material ejectedfrom volcanoes

• Lava Magma that has flowed on the surface of the
  Earth.
• Tephra Fragments that solidified in the air
  during eruption.

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Looking at lava .
Figure 5.4
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Looking at lava .
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Looking at lava .

• What controls lava viscosity?
• Temperature Higher temperature less
  viscous.
• SiO2 content Higher SiO2 more viscous.

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Two main types of lava

• Mafic or basaltic
• Lower viscosity
• Faster lava flows
• Calmer eruptions
• Felsic or rhyolitic
• Higher viscosity
• Slower lava flows
• More violent eruptions

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Basaltic lava

• Erupts at 1000 to 1200C
• Can flow as fast as 100 km/hr (but usually a
  few km/hour)
• Can travel as much as 50 km from volcano
• Flood basalts Very fluid basaltic flows that
  spread out in sheets over the landscape.
  
  
  Layered.

Columbia River flood basalts in Washington and
Oregon.
Figure 6.2
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Basaltic lava

• Basaltic lava flowing downhill forms pahoehoe
  or aa.
• Pahoehoe (Hawaiian ropy). Thin sheet of
  lava cools on the surface. Skin is twisted and
  dragged downhill to form ropes.
• Aa (Hawaiian ouch). Lost gas, is more
  viscous than pahoehoe. Cools to form thick
  skin. Skin breaks into jagged blocks.

Figure 6.3
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Basaltic lava

• Pillow lava Pillow-like blocks of basalt. 1 m
  wide. Formed underwater. Blob of basalt
  extruded underwater (like toothpaste), skin cools
  quickly (quenches) to form glassy rind.

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Eruptive stylesand landforms

• Fissure eruptions
• Central eruptions
• Shield volcanoes
• Domes
• Cones
• Stratovolcanoes (composite)

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Fissure eruptions

• Not all lava flows have volcanoes.
• When low-viscosity lava erupts from cracks in the
  Earth tens of kilometers long.
• Make flood basalt provinces.

Laki fissure (Iceland) erupted in 1783 extruding
the largest lava flow in human history (Fig.
6.13).
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1971 Fissure Eruption, Kilauea, Hawaii
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Fissure Eruptions Form Lava Plateaus
Figure 6.13
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Central Eruptions
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Shield volcanoes

• Low-viscosity lava flows (low silica, mafic).
• Successive lava flows.
• Gently sloping flanks (between 2 and 10 degrees)
• Tend to be very large (many 10s of km in
  circumference)

Fig. 6.9
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Mauna Loa -- worlds largest structure -- 10 km
above ocean base -- base diameter of 120 km --
took 1 million years to grow from successive lava
flows
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?
Is Mauna Loa about to erupt again?
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Volcanic domes

• Forms above a volcanic vent
• Viscous lava usually silica-rich (or cooler
  magma)
• Associated with violent eruptions

Fig. 6.9
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Mt. St. Helens
Lava Dome
Lyn Topinka/USGS
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Cinder Cone

• Formed of pyroclastics only
• Steep sides 30 degrees
• Relatively small
• Short duration of activity

Fig. 6.9
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Cerro Negro Cinder Cone, near Managua, Nicaragua
in 1968 (erupted again in 1995 and 1999)
Mark Hurd Aerial Surveys
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Pyroclast?
A volcanic rock fragment ejected into the air
during an eruption. Loss of gases due to
pressure drop results in explosive eruption.
Classified according to size.
Volcanic ash lt2 mm in diameter.
Volcanic bombs Blobs of lava that cool as they
fly trough the air. Can be as big as houses.
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Pyroclasic Eruption at Arenal Volcano, Costa Rica
Gregory G. Dimijian/Photo Researchers
Fig. 6.5
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Volcanic Bomb
Fig. 6.5
Science Source/Photo Researchers
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Lithification of pyroclasts
Volcanic tuffs Rocks created from smaller
fragments. Volcanic breccias Rocks formed from
larger fragments.
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Volcanic Breccia
Fig. 6.7
Fig. 5.8
Doug Sokell/Visuals Unlimited
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Welded Tuff California
1 foot
Gerals and Buff Corsi/Visuals Unlimited
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Ash-flow Sheets Draping Topography, Japan
S. Aramaki
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Composite volcano

• Alternating pyroclastic layers and lava flows
• Slopes intermediate in steepness
• Intermittent eruptions over long time span
• Mostly andesite
• Circum-Pacific Belt (Ring of Fire),
  Mediterranean Belt

Fig. 6.9
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Mt Fujiyama, Japan
Fig. 5.15
Raga/The Stock Market
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Caldera
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Crater Lake, Oregon
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Phreatic Eruption
An extremely explosive eruption that occurs when
hot lava encounters cool seawater. Huge
quantities of steam are released.
Phreatic eruption on a Pacific island south of
Tokyo.
Fig. 6.11
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Hot Spots
Fig. 6.19
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Hot Spots
What is a hot spot? A hot spot is the surface
expression of a mantle plume. What is a mantle
plume? A narrow, cylindrical jet of hot material,
rising from deep within the Earth (perhaps the
core-mantle boundary) that gives rise to surface
volcanism.
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Hot Spots
Fig. 6.22
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Hot Spots
Fig. 6.20
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Hot Spots
Fig. 6.20
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Hot Spots
Fig. 6.20
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Large Igneous Provinces
Fig. 6.21
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Types of Volcanic Hazards

• Lava Flows e.g. Hawaii, 1998
• Gas e.g. Lake Nyos (Cameroon), 1984
• 1700 people killed
• Ash fall e.g. Mt. Pinatubo, 1991
• Pyroclastic flows e.g. Mt. Pelee, 1902
• 28,000 killed
• Lahars (mudflows) e.g. Nevado del Ruiz, 1985
• 23,000 killed
• Tsunami e.g. Krakatoa, 1883
• 36,417 killed

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May 1990 Eruption of Kilauea, Hawaii
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San Juan, Mexico. Buried by Paricutin Lava Flows.
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U.S. Active Volcanoes
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Before May, 1980
Emil Muench/Photo Researchers
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After May, 1980
David Weintraub/Photo Researchers
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(No Transcript)
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Japan
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Mt. Pinatubo
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Mudflow or Lahar

• A mixture of water and pyroclastic material and
  sand, gravel, and boulders, in a concrete-like
  slurry capable of moving up to 100 km/hour
• Flow is supported by collisions between clasts

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Mudflow
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23,000 killed in 1985 by volcanic mudflows,
Nevada del Ruiz
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Columbia
The only remaining buildings in Armero, Colombia,
72 km dowstream from Nevado del Ruiz volcano,
destroyed and partially buried by lahars on
November 13, 1985. Lahars reached Armero about
2.5 hours after an explosive eruption sent hot
pyroclastic flows across the volcano's broad ice-
and snow-covered summit area. Although flow
depths in Armero ranged only from 2 to 5 m, three
quarters of its 28,700 inhabitants perished.
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Mt. Rainier
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Escaping a Pyroclastic Flow at Mount Unzen,
Japan, 1991(Fig. 6.8)
Pyroclastic flow (nueé ardente)

• Mixture of hot gases, ash, and rocks forming a
  super-heated and dense current capable of moving
  150 km/hr.
• Buoyancy due to heated gas, density due to ash-
  turbulence keeps particles suspended in flow

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Pyroclastic flows erupted by Mount Pinatubo on
June 15, 1991, buried the Marella River valley
(SW of Pinatubo) with pumice, ash, and other
volcanic rocks to depths of between 50 and 200 m.
This eruption was one of the largest in the 20th
century, depositing about 5.5 km3 of rock debris
over nearly 400 km2.