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Chapter 7, section 2
Why study volcanic
eruptions?
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Kinds of lava →
Volcanic Eruptions
 Lava allows scientists to study the material the mantle is made of, and
helps scientists to better understand Earth’s interior temperatures
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Viscous lava →
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All lava is not the same; the viscosity of lava varies
Viscosity- the inability for a liquid to flow
↑ viscosity=↓ ability to flow/move
Lava that has more silica is more viscous, lava that has less silica
is less viscous
The ingredients (composition) of the lava determines if a volcano will
have a violent eruption or a quiet eruption
Viscous lava has lots of silica and is thick. It traps large amounts of
gases and causes violent, explosive eruptions
o Example- Mount St. Helens
Violent eruptions usually eject pyroclastic material (rock fragments)
from the vent. The pyroclastic material can range in size from ash to
volcanic blocks the size of houses
Non-viscous lava →
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Non-viscous lava has less silica and is thin. It allows the gases to
bubble out slowly and normally has quiet, non-explosive eruptions
o Example- Hawaiian volcanoes
Volcanic features →
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Volcanic cone- ejected material that builds up around the volcano
Crater- large pit formed at the top of the volcanic vent when material
is blown out during an eruption (usually becomes wider with erosion)
Caldera- large depression formed with the volcanic cone collapses
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Types of volcanoes →
1) Shield volcanoes Large base (& large in size, although not normally the tallest volcanoes)
 Gentle slopes (not steep)
 Low viscosity lava
 Non-explosive eruptions
 Layers build up slowly over time
o Example- Mauna Loa in Hawaii (the largest volcano on Earth)
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2) Cinder cones Small base (& small in overall size)
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 Very steep slopes with a large crater (opening)
 Low viscosity lava that is high in gas content
 Explosive eruptions, but only erupt a few times
o Example- Paricutín in México (grew 300 ft. in 5 days)
3) Composite volcano (also called Stratovolcano) Large base and tall (often has snow on the peaks)
 Steep slopes
 Medium-high viscosity lava
 Explosive eruptions that occur suddenly after years of being inactive
 Made of alternate layers of pyroclastic material and lava
o Examples- Mount St. Helens (deadliest and most economically
destructive volcanic event in U.S. history)
Predicting eruptions →
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Volcanic eruptions are hard to predict because volcanoes remain
dormant (inactive) for long periods of time but there are some
instruments that allow volcanoes to be monitored
Seismographs are used to detect volcanic eruptions. As pressure builds
inside the volcano small earthquakes are produced. The number of
earthquakes increase before the volcano erupts
GPS is used to monitor a volcano. The surface tends to bulge upwards
before a volcano erupts, slightly changing its elevation