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Chapter 12-2 Study Guide – Types of Volcanoes
Ms. Grady
What Controls Eruptions (explosions)?
 Two important factors influence whether an eruption will be explosive
or quiet.
1. The amount of water vapor and trapped gases in the magma.
2. How much silica is present in the magma.
Review vocabulary:
Magma = hot, melted rock material beneath the Earth’s surface.
Molten rock is found in pockets called Magma Chambers.
Trapped (blocked or confined) Gas
 Gases such as water vapor and carbon dioxide are trapped in magma
by the pressure of the surrounding magma and rock.
 Quiet eruptions =Gas can escape easily.
As magma reaches the surface, it is under less pressure and gas can
leak out.
 Explosive eruptions = If gas builds up to high pressures, it will cause
explosive eruptions.
 Example: Mount St. Helens in 1980.
Water Vapor
 At some convergent plate boundaries, the magma can contain a lot of
water vapor.
 The reason for this is because oceanic plate material and some of its
water slide under other plate material at convergent plate
boundaries.
 Trapped water vapor in the magma can cause explosive eruptions.
Composition (what is made up of) of Magma
 The second reason that affects the type of eruption (quiet or explosive)
is what the magma is made up of or in other words, its composition.
 Magma can be split into two major types: Silica Rich/Silica Poor
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 The greater the silica content = the more viscous (higher viscosity/more
explosive) or thick the magma.

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Silica Rich Magma/Explosive Eruptions
Magma that is high in silica is called
granitic magma.
It is light colored lava which causes
explosive eruptions (blasts).
Silica hardens in the vents and forms
rocks.
 Streams and new lava build up under
the rocks.
 When the pressure becomes too
much and too great, a violent
explosion happens.
 When this type of lava cools on the
surface it forms the igneous rock,
rhyolite, which looks like granite.
Granitic magma is found at ocean to ocean
convergent boundaries.

Silica Poor Magma/Quiet Eruptions
Magma that is low in silica is called basaltic
magma.
It is fluid and produces quiet flows or nonexplosive eruptions.
Ex. Kilauea, Hawaii
Basaltic magma is found at hot spots and
along the mid-ocean ridges.
Pahoehoe (pa-HOY-hoy) lava = lava that
forms a ropelike structure.
 It is hot, fast-moving, and thin.
AA (Ah-ah) lava = Pahoehoe lava which
flows at a lower temperature.
 It is slow moving, thicker and stiff.
Basaltic lava that flows under water produce
pillow lava formations.
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Andesitic Magma
 It is the amount of silica between the basaltic and granitic magma.
 Andesitic magma is rich in silica than basaltic magma. However, it has
less silica than granitic magma.
 It is more explosive than basaltic magma.
 It is found when an ocean plate subducts under a continental plate.
 The word “andesitic” comes from the Andes Mountains.
 Example of an andesitic eruption was Krakatau (near Indonesia).
Gaseous Lava
 When lava has large amounts of gases (such as steam and carbon
dioxide) hardens, then it forms rocks with many holes in them because
of the gas bubbles.
 Pumice and Scoria are igneous rocks formed from this type of lava.
Forms of Volcanoes (types of volcanoes)
 The three basic types of volcanoes are: Cinder cone Volcanoes, Shield
Volcanoes, and Composite Volcanoes
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Shield Volcanoes
 Quiet eruptions made up of basaltic lava spread out in flat layers.
 Because it is runny, the lava flows over a large area and forms a wide
(broad) volcano with gently sloping sides.
 After several eruptions (explosions), a dome-shaped mountain is
formed.
 Example: Mauna Loa (4 km over sea level) in the Hawaiian Islands.
Fissures = Basaltic lava can also flow into Earth’s surface through large
cracks.
This type of eruption creates flood basalts (where lava flows through
fissures and spread out over the large ground).
Example, Columbia Plateau in Northwestern USA was formed as the lava
spread out over the ground.
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Cinder Cone Volcano
 Volcanoes made mostly of cinders and other rock particles that have
been blown into the air.
 Cinder cones form from explosive eruptions.
 The cones are not high because the material is loosely arranged.
 Cinder cones have a narrow (thin) base (bottom) and steep sides.
Example: Paricutin in Mexico
Explosive eruptions throw lava and rock high in the air.
Tephra = bits of rock or solidified lava that are dropped from the air.
Tephra can change in size from volcanic ash, to cinders, to larger rocks called
“bombs and blocks”.
 Volcanic Ash (0.25 – 5 mm) falls to the Earth and forms small rocks.
 Cinder (5 mm – a few cm)
 Volcanic bombs (a few cm to several meters) are molten and harden as
they travel through the air.
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Composite Volcanoes
 Volcano built of alternating layers of rock particles and lava
 First a violent eruption→ hurling volcanic bombs→ cinder and ash out of
vent
 Then a quiet eruption→ produces lava flow →lava flow covers the rock
particles.
 Cone-shaped mountain forms after alternating eruptions.
 Example Mount Vesuvius, Mount Rainier, and Soufriere Hills
 Mount Vesuvius most victims died as a result of suffocation
Krakatau (island in Sunda Straits near Indonesia)
 One of the most violent eruptions in recent times
 36,000 killed in 1883
 The island fell into the magma chamber and disappeared
 Anak Krakatau growing (Child of Krakatau) formed in early 1900s