OUTLINE

1
OUTLINE

• Introduction
• Volcanism
• Types of Volcanoes
• Eruptions of Cascade Range Volcanoes
• Other Types of Eruptions
• Size and Duration of Eruptions
• Predicting Eruptions
• Distribution of Volcanoes
• Plate Tectonics, Volcanoes, and Plutons
• Recap

2
OBJECTIVES
1 In addition to lava, volcanoes erupt large
quantities of gases and solid particles such as
ash. 2 Even though every volcano is unique,
most can be classified as one of only a few basic
types. 3 The shapes of several types of
volcanoes are determined by their eruptive style.
4 Some volcanoes erupt explosively, whereas
others erupt rather quietly and pose little
danger to humans. 5 Eruptions in some areas
yield vast, flat-lying sheets of lava or layers
of ash and other particles rather than volcanoes.
6 Geologists use several methods to monitor
volcanoes in an effort to predict future
eruptions. 7 A semiquantitative scale is used
to express the size of an eruption. 8 Most
volcanism takes place in well defined belts at or
near divergent and convergent plate boundaries.
9 The few active volcanoes far from any plate
boundary probably result from localized melting
of rock at hot spots.
3
Fig. 5-CO, p. 96
4
Fig. 5-1, p. 98
5
Fig. 5-2a, p. 100
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Fig. 5-2b, p. 100
7
Fig. 5-2c, p. 100
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Fig. 5-3, p. 101
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Fig. 5-4a, p. 102
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Fig. 5-4b, p. 102
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Fig. 5-5a, p. 102
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Fig. 5-5b, p. 102
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Fig. 5-6a, p. 103
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Fig. 5-6b, p. 103
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Fig. 5-7a, p. 103
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Fig. 5-7b, p. 103
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Fig. 5-8, p. 104
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Fig. 5-9a, p. 105
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Fig. 5-9b, p. 105
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Fig. 5-10a, p. 106
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Fig. 5-11a, p. 107
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Fig. 5-11b, p. 107
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Fig. 5-12, p. 108
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Fig. 5-13a, p. 109
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Fig. 5-13b, p. 109
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Fig. 5-13c, p. 109
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Figure 1, p. 111
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Figure 2, p. 111
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Fig. 5-14a, p. 112
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Fig. 5-14b, p. 112
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Fig. 5-15, p. 112
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Table 5-2, p. 113
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Fig. 5-16, p. 114
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Fig. 5-16a, p. 114
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Fig. 5-16b, p. 114
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Fig. 5-16c, p. 114
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Fig. 5-17, p. 116
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Fig. 5-18, p. 117
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Fig. 5-19a, p. 118
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Fig. 5-19b, p. 118
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OBJECTIVES
1 In addition to lava, volcanoes erupt large
quantities of gases and solid particles such as
ash. 2 Even though every volcano is unique,
most can be classified as one of only a few basic
types. 3 The shapes of several types of
volcanoes are determined by their eruptive style.
4 Some volcanoes erupt explosively, whereas
others erupt rather quietly and pose little
danger to humans. 5 Eruptions in some areas
yield vast, flat-lying sheets of lava or layers
of ash and other particles rather than volcanoes.
6 Geologists use several methods to monitor
volcanoes in an effort to predict future
eruptions. 7 A semiquantitative scale is used
to express the size of an eruption. 8 Most
volcanism takes place in well defined belts at or
near divergent and convergent plate boundaries.
9 The few active volcanoes far from any plate
boundary probably result from localized melting
of rock at hot spots.
56
SUMMARY

• Volcanism is the process by which magma and its
  associated gases erupt at the surface. Some magma
  erupts as lava flows, and some is ejected
  explosively as pyroclastic materials.
• Only a few percent by weight of a magma consists
  of gases, most of which is water vapor. Sulfur
  gases emitted during large eruptions can have
  far-reaching climatic effects.
• The surface of an aa lava flow consists of rough,
  angular blocks, whereas a pahoehoe flow has a
  smoothly wrinkled surface.
• Columnar joints form in some lava flows when they
  cool. Pillow lavas form under water and consist
  of interconnected bulbous masses.
• Volcanoes are mountains built up around a vent
  where lava flows and/or pyroclastic materials are
  erupted.
• The summits of most volcanoes have a circular or
  oval crater or a much larger caldera. Many
  calderas form by summit collapse when an
  underlying magma chamber is partly drained.

57
SUMMARY

• Shield volcanoes have low, rounded profiles and
  are composed mostly of mafic flows that cooled to
  form basalt. Small, steep-sided cinder cones form
  where pyroclastic materials that resemble cinders
  are erupted and accumulate. Composite volcanoes
  are composed of lava flows of intermediate
  composition, layers of pyroclastic materials, and
  volcanic mudflows known as lahars.
• Viscous masses of lava, generally of felsic
  composition, are forced up through the conduits
  of some volcanoes and form bulbous lava domes.
  Volcanoes with lava domes are dangerous because
  they erupt explosively and frequently eject nuée
  ardentes.
• Fluid mafic lava erupted from long fissures
  (fissure eruptions) spreads over large areas to
  form basalt plateaus.
• Pyroclastic flows erupted from fissures formed
  during the origin of calderas cover vast areas.
  Such eruptions of pyroclastic materials form
  pyroclastic sheet deposits.
• Geologists are learning about predicting the size
  and timing of volcanic eruptions, but all
  predictions are still an inexact science.
• Most active volcanoes are distributed in linear
  belts. The circum-Pacific belt and Mediterranean
  belt contain more than 80 of all active
  volcanoes.

58
SUMMARY

• Volcanism and plutonism take place at speading
  ridges where plates diverge and at convergent
  plate margins where subduction occurs. Partial
  melting of a subducted plate generates
  intermediate and felsic magmas.
• Magma derived by partial melting of the upper
  mantle beneath spreading ridges accounts for the
  mafic plutons and lavas of ocean basins.
• The two active volcanoes on the island of Hawaii
  and one just to the south are thought to lie
  above a hot mantle plume. The Hawaiian Islands
  developed as a series of volcanoes formed on the
  Pacific plate as it moved over the mantle plume.