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Volcanoes
Volcanoes
• A volcano is an opening where molten
rock reaches the Earth’s surface. The
rock may come form the crust or from the
mantle. Volcanoes release internal
pressure under the crust.
Types of Volcanoes
• 1. shield volcano: this is a short dome
shaped volcano. It usually produces mafic
lava, which is dark, hot, thin, and erupts
gradually. Shield volcanoes are usually in
the ocean (Hawaii).
• 2. Cinder cone volcano: a cinder cone is
tall and cone shaped. It produces felsic
lava, which is cool, thick, and erupts
violently.
– Cinder cones also erupt solid material called
tephra. Cinder cones are usually on
continents
• 3. Combination- Combination volcanoes
contain both felsic and mafic lava.
– They are found throughout the world
Types of Lava
• 1. Pillow lava: this forms when a volcano
erupts under water. It cools instantly,
forming pillow shaped rocks.
• 2. Pahoehoe: long ropy strands of rock.
Forms when hot (mafic) runny lava cools.
•
3. Aa (ah-ah): lumpy jagged rocks.
Forms when thick (felsic) lava cools.
Types of tephra:
1. Volcanic ash (less than 2mm)
2. Volcanic dust (2-10mm)
3. Volcanic lapilli (10-100mm)
4. Volcanic bombs (largest). Bombs are
molten when they erupt, and harden on
the way to the ground.
Plutons
• Plutons: when magma cools under the
surface, it forms masses called intrusions,
or plutons.
Batholiths: large (over 1000km) plutons
Sills: horizontal plutons
Dikes: vertical plutons
Laccoliths: dome shaped
• Plutons usually push up on the crust and
create hills or mounds. Some batholiths
form the base of mountains.
Caldera
• Caldera: when a volcano erupts with
incredible force, the sides are weakened.
In some cases, the volcano caves in after
an eruption. This forms a caldera
(cauldron). The caldera fills with rain
water and can create a lake (Crater Lake
in Oregon)