Download Volcanoes - Sonoma Valley High School

Volcanoes
Types of Eruptions
• Violent and explosive
• Quiet and flowing
– Depends on trapped gases and
magma composition
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Trapped Gases
• Water vapor and carbon dioxide
are trapped in magma
• At low pressure, they escape
quietly when they reach the
surface
• At high pressure, they escape
violently when they reach the
surface
Magma Composition
• Two types:
– Basaltic – less silica and very
fluid; produces quiet eruptions
– Granitic – lots of silica, high
water vapor content, and very
thick; produces very violent
eruptions
Forms of Volcanoes
• Three forms:
– Shield
– Cinder Cone
– Composite
Shield Volcano
• Formed by quiet eruptions
• Basaltic lava builds up in flat
layers
• Gently sloping sides
• Ex: Hawaiian Islands
Cinder Cone Volcano
• Caused by explosive eruptions
• Granitic lava thrown high into
the air
• Lava cools into different sizes
of volcanic material called
tephra
• Steep-sided, loose slopes
Composite Volcano
• A mix of the other two types
• Quiet or violent
• Basaltic or granitic
• Steep or gentle slopes
• Layered or tephra
Plate Tectonics and Igneous Activity
• Most of the volcanoes on Earth occur
along the margins of the ocean basins,
within the circum-Pacific, known as the
Ring of Fire.
• 2nd group is found in the deep-ocean
basins, Hawaii and Iceland.
• 3rd group is composed of volcanic
structures that are irregularly distributed in
the interior of the continents.
Convergent Plate Boundaries
• The basic connection between plate tectonics
and volcanism is that plate motions provide the
mechanisms by which mantle rocks melt to
generate magma.
Convergent Plate Boundaries
• Ocean-Ocean
• Volcanism at a convergent plate where
one oceanic slabs descends beneath
another results in the formation of a chain
of volcanoes on the ocean floor.
Convergent Plate Boundaries
• Ocean-Continent
• Develop where slabs of oceanic lithosphere
are subducted under continental lithosphere
to produce a continental volcanic arc.
Divergent Plate Boundaries
• Most magma is produced along the
oceanic ridges during seafloor spreading.
Intraplate Igneous Activity
• Occurs within a plate, not at a plate boundary.
• Yellowstone National Park
• Most intraplate volcanism occurs where a mass
of hotter than normal mantle material called
mantle plume rises toward the surface.
• Caused by hot mantle plumes rising up from the
core mantle-boundary, causing decompression
melting and forming small areas of volcanic
activity on the surface.
Review
• How are the locations of volcanoes related
to the plate boundaries?
• What causes intraplate volcanism?
• Where is most of the magma produced on
Earth on a yearly basis?
• What is the ring of fire?