Chapter 7: Volcanoes
New Vocabulary
Lava- magma (or hot, liquid rock)
that reaches the surface
Pyroclasts- hot rock fragments
(from the Greek word “pyro”
meaning fire and “clast” meaning
Pyroclastic flows- mixture of gases
and pyroclastic debris. It is so
dense that it hugs the ground
What is a volcano?
A volcano is a hill or mountain formed when
lava or other molten rock reaches the
surface. However, very fluid lava may
reach the surface and harden into a
horizontal layer.
Volcano 101
Geological Journey
(start at about 28:00 mark)
Why is volcanic activity important to
1. Landforms are created
2. Landforms are destroyed (less common)
3. Provides clues about the interior of the
How does volcanic activity affect
Growth of landforms (ex. Hawaii)
Produces new rock & fertile soil
Tourist attraction
Hawaii is built from a seamount (or
magma plume); taller than Mt. Everest
2. Geothermal energy
3. Effects on weather
• In 1991, Mt. Pinatubo dropped the
average temperature by 0.5°C for
several years because of fine volcanic
• The 1815 eruption of Tambora in
Indonesia caused a very cold summer in
1816 (snow and frost in New England in
the summer)
4. Volcanic catastrophes
• Eruption of Mt. Vesuvius destroyed Pompeii
(much death caused by 5-8 m of hot ash)
• Krakatoa (Indonesia) exploded and caused a
tsunami that killed 34,000
• Mt. St. Helen’s in Washington State, USA
• Volcanoes can kill because of pyroclastic
flows, famine (because of crop destruction),
pyroclastic fragments and ash (roof collapses)
What determines the severity of a
volcanic event?
There are two things that determine the
severity of a volcanic event:
1. Amount of gas in the lava or magma
2. The ease or difficulty with which the gas
escapes. This is determined by the
viscosity (or “thickness”) of the lava.
*Overall, the more viscous the lava and the
greater the volume of gas trying to
escape, the more violent the eruption
What determines the severity of a
volcanic event?
Low viscosity (more like liquid):
• Less silica (ex. basaltic rock)
• Less violent eruptions
• Ex. Hawaii
High viscosity (thick):
• More silica (ex. rhyolite)
• More violent eruptions
• Ex. Mount St. Helens
What kinds of rocks do volcanoes
All volcanic rocks are extrusive and most have fine-grained
crystals because of quick cooling.
Common volcanic rocks include the following:
Obsidian (black volcanic glass)
Pumice (frothy glass; floats in water; good for pedicures)
Rocks with holes (like Swiss cheese)
Volcanic bombs
What are the characteristics of
different types of volcanoes?
First of all, some new volcano lingo:
• Vent: opening through which an eruption
takes place
• Crater: basin-like depression over a vent
at the top of a volcano
• Caldera: a volcanic depression much
larger than the original crater and having a
diameter of at least 1km
Caldera (Crater Lake, USA)
Three major types of volcanoes:
Shield Volcano:
• Broad, gentle slopes
• Lava spreads widely and thinly
• Made of solidified layers of lava flows
• Ex. Hawaiian Islands
Pyroclastic cones:
• Made of pyroclastic fragments (chunks of material)
• Steep slopes
• Less common; tends to wear away quickly
• Less than 500m high
Composite volcanoes:
• Made of alternating layers of pyroclastic fragments and
solidified magma flows
• Steeper than shield volcanoes, but not as steep as
pyroclastic cones
• Built over many years; can be old and very large
• Mostly found around the “Ring of Fire” and the
Mediterranean Belt
• Ex. Mount St. Helens; Mt. Vesuvius; Mt. Etna
Mt. Vesuvius
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