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Chapter 18: Volcanoes
18.1 Volcanoes
Main Idea: The locations of volcanoes are mostly determined by plate tectonics.
Volcanism
Volcanism: describes all the processes associated with the discharge of magma, hot fluids, and gases.
 There are type main types of volcanism: convergent, divergent, and hot spots.
Convergent Volcanism
 In an oceanic-continental subduction zone, the denser oceanic
plate slides under the continental plate into the hot mantle.
Parts of the plate melt, magma rises, and a volcano forms.
o
 Most land volcanoes result from oceanic-continental subduction.
o
These volcanoes are characterized by explosive eruptions.
o
These volcanoes form 2 major belts.
Two Major Belts
1) Circum-Pacific Belt (Pacific Ring of Fire):
corresponds to the outline of the Pacific Plate.
2) Mediterranean Belt: correspond to the
boundaries between the Eurasion, African, and
Arabian plates.
Divergent Volcanism
 Occur at divergent plate boundaries
 Eruptions at divergent boundaries tend to
be nonexplosive. At ocean ridges they
often form huge piles of lava called pillow
lava.
Hot Spots
 Some volcanoes form far from plate boundaries over hot spots.
Hot Spot: an unusually hot area in Earth’s mantle where high-temperature plumes of mantle
material rise toward the surface; they are stationary (do not move).
Flood basalt: where lava flows out of long cracks in Earth’s crust
Fissure: cracks in the Earth’s surface

A The Hawaiian islands are located over a plume of magma. The hot spot formed by the magma plume
remained stationary while the Pacific Plate slowly moved northwest.

The volcanoes on the oldest Hawaiian island, Kauai, are inactive because the island no longer sits above the
stationary hot spot.

The world’s most active volcano, Kilauea, on the Big Island of Hawaii, is currently located over the hot spot.

Chains of volcanoes that form over stationary hot spots provide information about plate motions.

The rate and direction of plate motion can be calculated from the positions of these volcanoes.
Parts of a Volcano
Magma Chamber: contains the magma inside the volcano
Conduit: tubelike structure that magma travels through to
reach vent (connects magma chamber to vent/crater)
Vent: opening in Earth’s crust through which volcanic
material reaches the surface
Crater: bowl shaped depression that forms around the
central vent at the summit of a volcano
Caldera: large crater, up to 50 km in diameter, that can
form when the summit or side of volcano collapses into the
emptied magma chamber during or after an eruption.
Types of Volcanoes
Shield
Cinder Cone
Composite
Largest
Long, gentle slopes
Quiet eruptions
Smallest
Steep-sloped, cone-shaped
Explosive eruptions
Intermediate
Cycles through periods of explosive
and quiet eruptions
18.2 Eruptions
Main Idea: The composition of magma determines the characteristics of a volcanic eruption.
Composition of Magma

The composition of magma determines a volcano’s explosivity, which is how it erupts and how its lava flows.
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Temperature, pressure, dissolved gases, and viscosity all affect the composition of magma.
Viscosity: materials resistance to flow, directly related to amount of silica in magma

Magma’s interaction with overlying crust, its temperature, pressure, and amount of dissolved gases greatly
affect the amount of silica the magma contains, and thus, its viscosity and potential explosivity.

In general, as the amount of gases in magma increases, the magma’s explosivity also increases.

Gases in magma include water vapor, carbon dioxide, sulfur dioxide, and hydrogen sulfide.
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Factors that determine magma behavior aid scientists in predicting the explosivity of an eruption.
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Explosive Eruptions = Low Temp + High Silica (High Viscosity)
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Quiet Eruptions = High Temp + Low Silica (Low Viscosity)
Types of Magma
Tephra: erupted materials
Pyroclastic flows: rapidly moving clouds of
tephra mixed with hot, suffocating gases
Basaltic
Andesitic
Rhyolitic
Low Silica, Quiet
Eruptions
Intermediate Silica and
Eruptions
High Silica, Explosive
Eruptions