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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 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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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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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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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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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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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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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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Figure 5 34

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Figure 5.34

Figures 5 35 a and b

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Fig. 5.36 Track of North America

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GEOL 1003 Ch5.ppt