GEOL 1003 Ch5.ppt
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Transcript of GEOL 1003 Ch5.ppt
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Volcanoes
Chapter 5
Mt. St. Helens,
Washington
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Magma Sources and Types
Magma sources tend to be 50 to 250 kmdeep into the crust and upper mantle
Temperatures increase as depth increases
Some of the internal heat is left over from
the earths formation; more heat isgenerated by the decay of radioactiveelements in the earth
Volcanoes are generated at: Divergent Plate Boundaries
Convergent Plate Boundaries
Hot Spots
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Figure 5.2
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Magma Sources and Types
Magma compositions vary in SiO2, iron,magnesium, and volatile gases
Mafic magmalow in SiO2(45-50 %) buthigh in iron, and magnesium
Felsic magmahigh in SiO2(up to 75 %)but low in iron, and magnesium
Intermediate magmaintermediate range
of SiO2 (50-65 %), iron, and magnesium
Amount of volatile gases will affectexplosive characteristics of eruptions
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Magma Sources and Types
Mafic magmas produce basaltlavas
Intrusive equivalent is gabbro
Intermediate magmas produceandesitelavas
Intrusive equivalent is diorite
Felsic magmas produce rhyolitelavas Intrusive equivalent is granite
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Figure 5.3
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Magma at Divergent Plate Boundaries
Magma produced at a Divergent Plate Boundary
is typically melted asthenosphere material
Asthenosphere is extremely rich in
ferromagnesian (ultramafic) and a melt from it is
mafic (or ultramafic) Basalt is emplaced as new seafloor at the
spreading ridge or a rift
Rift systems in continental crust may melt
granitic crust and produce andesite or rhyolitelavas
A bimodal suite of extrusive igneous rocks
characterize rift volcanoes
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Magma at Convergent Plate Boundaries
Magmatic activity at convergentboundaries is complex
The composition of the subducted plate
determines the composition of the lava Subducted continental crust may melt andproduce rhyolite lava
Subducted oceanic crust may melt and
produce basalt or andesite lava Subduction of sediments derived from the top
of the subducted slab may produce a varietyof lavas
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Magma at Hot Spots
Magmas associated with a hot spot
volcano in an ocean basin will produce a
basalt lava
Magmas associated with a hot spot
volcano under continental crust generally
will produce a felsic lava (and often an
explosive one)
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Figure 5.4
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Types and Locations of Volcanoes
Seafloor Spreading Ridges
Most voluminous volcanic activity
About 50,000 km of ridges around the world
Mostly under the oceans - except at Iceland
Generally, harmless mafic fissure eruptions Continental fissure eruptions
Pour out of cracks in lithosphere
Result in large volume of flood basalts Columbia Plateau (over 150,000 km2and 1
km thick)
Other locations include India and Brazil
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Figure 5.6
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Figures 5.7 a and b
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Types and Locations of Volcanoes
Shield volcanoes Very large, flat, with abundant thin basalt flows Basalt is less viscous than andesite or rhyolite
Shield like shape - larger area relative to height
Examples: Hawaiian Island chain Volcanic Domes
Composed of more viscous andesite or rhyolite these lavas do not flow
Ooze out onto surface from a tube and pile up closeto the vent
Compact, small, and steep sided
Various locations around Pacific Ring of Fire
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Figures 5.8 a, b , and c
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Figures 5.9 a and b
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Figure 5.10 a and b
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Types and Locations of Volcanoes
Cinder Cones
Minor explosive volcano
Batches of lava shot into the air as pyroclastics
Size of pyroclastics range from ash (very fine),
cinders, bombs, or blocks (very coarse)
Pyroclastics fall close to the vent creating a cone
shaped volcano Example: Particutin, Mexico
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Figures 5.12 a and b
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Figures 5. 11 a, b, c, and d
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Types and Locations of Volcanoes
Composite Volcanoes(Stratovolcanoes)
are built up of layers of lava and
pyroclastics
Mix of lavas and pyroclastic layers allows for a
tall volcano to form
Usually associated with subduction zones
These tend to be violent and explosive Example: Mount St. Helens, Cascade Range,
Northwest U.S.A.
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Figures 5.13 a and b
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Hazards Related to Volcanoes
Lava, the principal hazard? But not life-
threatening generally
Pyroclastics, more dangerous than lava flows
Lahars, a volcanic ash and water mudflow Pyroclastic Flows- Nues Ardentes
Toxic Gases
Steam Explosions Secondary Effects; Climate and Atmospheric
Chemistry
1
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Figures 5.1 a and b
Fi 5 14
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Figure 5.14
Fi 5 15 d b
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Figures 5.15 a and b
Fi 5 16
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Figure 5.16
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Figure 5.17
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Mt. Mazama, Oregon today
Crater Lake,
Oregon; imageis from NASA
Figure 5 18
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Figure 5.18
Figure 5 19
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Figure 5.19
Fi 5 22
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Figure 5.22
Figure 5 23a
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Figure 5.23a
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Figure 5 26
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Figure 5.26
Figure 5 27
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Figure 5.27
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Predicting Volcanic Eruptions
Classification by activity
Active:erupted in recent history
Dormant:no historic erupts but not badly
eroded Extinct:no historic eruptions and badly
eroded
Volcanic Precursors
Seismic activity
Bulging, tilting or uplift
Monitoring gas emissions around volcano
Figure 5 28
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Figure 5.28
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Present and Future Volcanic
Hazards in the United States
Hawaii: active or dormant volcanoes
Cascade Range: a series of volcanoes in
the western United States and
southwestern Canada resides above the
Pacific Northwest subduction zone
The Aleutians: South-central Alaska and
the Aleutian island chain sit above asubduction zone
Long Valley and Yellowstone Calderas
Figure 5 31
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Figure 5.31
Figure 5 29
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Figure 5.29
Figure 5 32
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Figure 5.32
Figures 5 33 a and b
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Figures 5.33 a and b
Figure 5 34
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Figure 5.34
Figures 5 35 a and b
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Figures 5.35 a and b
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Fig. 5.36 Track of North America