Download Ch. 7.2 Volcanic Eruptions

Ch. 7.2 Volcanic
Eruptions
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Two general types of lava.
Mafic lava—dark when hardened;
rich in iron and magnesium; usually
thin and flowing; commonly forms
ocean floor.
Felsic lava—rich in silica…less iron
and magnesium; forms continental
crust; lighter colored when
hardened.
Kinds of Eruptions
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Lava flow eruptions—usually quiet;
produce red-hot rivers of flowing
lava. Thin mafic lava. Gases escape
easily.
Explosive eruptions—thicker felsic
lava with lots of trapped gases,
produce ash, and various sizes of
molten and solid particles shooting
into the air.
Volcanic Rock Fragments
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Pyroclastic material results from
exploded felsic lava. The type of
pyroclastic material depends on particle
size.
Volcanic ash—particles less than 2mm in
diameter.
Volcanic dust—particles less than .25mm.
Lapilli—larger particles less than 64mm.
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Volcanic bombs—large red-hot lava
blobs thrown out of the volcano.
Volcanic blocks—solid rock blasted
away from the volcano. May be as
big as houses.
Volcanic Features
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Volcanic cone—the buildup of lava
and pyroclastic materials around the
vent. Three main types…
Shield Volcanoes
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Broad at the base with gently sloping
sides.
Result from eruptions of mafic lava.
Cinder Cones
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Only a few hundred meters high at
most; very steep sides.
Result from explosive eruptions of
solid fragments.
Stratovolcanoes (Composite
Volcanoes)
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Formed when a volcano experiences
both quiet lava flows and explosive
eruptions.
Alternating layers of different
materials build up into very high
volcanic mountains.
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Crater—the funnel-shaped pit at the
top of a volcanic vent.
Caldera—large basin-shaped
depression formed when a volcanic
cone collapses because the magma
chamber below empties (Ex:
Yellowstone Park).
Predicting Eruptions
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Small earthquakes often increase in
frequency and strength just before
eruptions.
There may be slight bulging of the
volcano’s surface prior to an
eruption.
There may be changes in gases
coming out of the volcano.
Past history is important.