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Volcanoes
Volcano Definition
- A vent at which lava, pyroclastic debris (ash
and fragments of previously solidified rock),
and gas erupt.
- Eruption may build a mountain around the
vent. (Mountain is also called a volcano).
- Anatomy of a volcano:
- magma chamber at depth
- a vent to the surface
Word bank: conduit, vent, crater, side
vent, ash cloud, crust, magma
chamber, lava
Where do Volcanoes Occur?
Divergent volcanism:
1. Mid-ocean ridges
• Where plates separate under water
• Creates new sea floor
2) Rifts
Where plates
separate on land
•
•
e.g., East African rift
(next slide)
Iceland (this slide)
Great African Rift Valley
• Bbb
Mid Ocean Ridge
It is thought that the Great Rift
Valley will eventually become an
ocean.
Where do Volcanoes Occur?
Convergent Boundaries
- the majority of major
volcanoes
- e.g., the Andes,
Japan, Aleutians (ring
of fire)
Where do Volcanoes Occur?
Hot spots
- e.g., Hawaii;
Yellowstone Park
Nature of
lava
• Characteristics of lava
depend on its temperature
and viscosity.
– viscosity is a measure of
the ability of a fluid to
resist flow
• high viscosity means thick
• low viscosity means watery
and fast
• The higher the temperature
of a lava, the lower its
viscosity (i.e., the easier it
flows)
• The greater the gas
content, the lower the
viscosity. Frothy lava flows
more easily.
Types of Eruptions
• Lava dominated eruptions:
– Characteristic of low viscosity eruptions.
– If lava has low viscosity, it fountains out easily. You will see lava fountains,
lakes and rivers, that flow long distances.
Low Viscosity flow  fountains, lakes & rivers of lava
Types of Eruptions
• Explosive eruptions:
– Sticky, very viscous lava clogs up the vent. Gas (CO2 and
H2O) that come out of solution and make bubbles. Gas
pressure builds up and the volcano explodes.
– Result is a cloud of ash that blankets countryside or an
ash flow that rushes down the volcano side at speeds up
to 100 km/h, incinerating everything in its path.
• Explosions can also occur when water gets into
magma chamber and turns to steam or when part
of the mountain slips away and pressure is released.
Lava Vocab
• Tephra – materials that explode along with
lava
– Cooled chunks of lava
– Rocks, ash, parts of crust, etc.
• Pyroclastic flow
– Rapidly moving clouds of tephra mixed with hot,
suffocating gases
– http://www.youtube.com/watch?v=WbkPo0Hws
0M
Types of Volcanoes
• The fissure volcano: is a long crack in the earth's surface through which
magma erupts. These cracks may form as two tectonic plates pull apart.
• The shield volcano: is a broad, shallow volcanic cone, which arises
because the running lava, which is fluid and hot, cools slowly.
Types of Volcanoes
• The dome volcano: has a steep, convex slope from thick, fast-cooling lava.
• The ash-cinder volcano: throws out (in addition to lava) much ash into the
air. Through this the volcanic cone is built up from alternate layers of ash and
cinder.
Types of Volcanoes
• The composite volcano: are also built up from alternate layers of lava and
ash but, besides its main crater, it has many little craters on its slope.
• The caldera volcano: is an older volcano with a large crater which can be 62
miles (100km) wide. In this crater many little new craters are formed.
Intrusive Activity
• Plutons –
– Intrusive igneous rock bodies formed by cooled
magma that forced its way into the crust and cooled
before hitting the surface
• 4 types
– Batholiths and stocks
– Laccoliths
– Sills
– Dikes
Batholiths
• Large, irregularly shaped
• Coarse-grained igneous rock
• Common in interior of many mountain chains
Laccoliths
• Lens-shaped
• Smaller than batholith
• Stone Mountain, GA
Sills
• Magma
intrudes
parallel to
layers of rock
Dike
• Cuts across
preexisting rocks
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