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Chapter 9: Volcanoes Study Guide
TEST
1. Define a volcano.
A vent or fissure in the Earth’s surface through which magma and gases are expelled
2. Define magma.
Molten rock
3. Define lava.
Magma that has been expelled by a volcano during an eruption
4. List two differences between explosive and nonexplosive eruptions.
Nonexplosive eruptions are common and they feature relatively calm flows of lava. Explosive
eruptions are less common and produce large, explosive clouds of ash and dust.
5. How do silica levels affect an eruption??
Silica rich magma traps explosive gases.
6. Describe the four types of lava.
Aa- lava pours out quickly and forms a brittle crust.
Pahoehoe -lava flows slowly like wax, its glassy surface has rounded wrinkles.
Pillow lava- forms when lava erupts under water. The lava forms pillow like shapes.
Blocky lava- is cool, stiff lava that does not travel far from the erupting vent. Blocky lava usually
oozes from a volcano and forms jumbled heaps of sharp edged chunks.
7. Describe the four types of pyroclastic material.
Volcanic bombs – are large blobs of magma that harden in the air. The shape of this bomb is caused by
spinning magma
Lapilli- “little stones” are pebble like bits of magma that hardened before they hit the ground
Volcanic Ash- forms when the gases in stiff magma expand rapidly and the walls of the gas bubbles
explode into tiny, glasslike silvers. Ash makes up most of pyroclastic material
Volcanic blocks- the largest pieces of pyroclastic material, are pieces solid rock erupted from a
volcano.
8. How does a volcanic eruption affect climate? As volcanic ash and gases spread throughiout the
atmosphere they can cause global temperatures to drop. A small shift in temperatures can
disrupt climates all over the world.
9. Complete the following table:
Type of Volcano
Shield
Cinder cone
Composite/stratovolcanoes
Formation
Built of layers of lava
Made of pyroclastic
material
Form from pyroclastic and
lava flows
Example
Hawaii
Paricutin, Mexico
Mt. Shasta, Mt. Rainier, Mt.
St. Helens
10. Define a crater.
A funnel shaped pit near the top of the central vent of a volcano
11. Define a caldera.
A large semicircular depression that forms when the magma chamber below a volcano partially
empties or collapses
12. Define a lava plateau.
A wide flat landform that is a result of repeated nonexplosive lava eruptions
13. What conditions are needed for rock to melt?
When temperature increases or pressure on the rock decreases rock will melt and magma will form
14. Where are 75% of the world’s most active volcanoes found?
75% of the world’s most active volcanos are found along the ring of fire. This is where the North
American plate and the pacific plate interact.
15. Define a rift zone.
An area of deep cracks that form between two plates that are pulling away from each other.
16. How does magma form at a divergent boundary?
Mantel material rises to fill the gap of the divergent boundary
17. How does magma form at a convergent boundary?
Magma forms at subduction zones (type of convergent boundary), when the less dense rock sinks the
rock is melted.
18. Define subduction.
The movement of one tectonic plate beneath another
19. Define a hot spot.
A volcanically active place far from a tectonic boundary where magma is released.
20. Define mantle plume.
Mantel plumes are areas in the mantle where hot rocks are rising towards the surface and melting, hot
spots are the surface appearance of a plume
21. Describe two theories that explain the existence of hot spots.
Some scientist think hot spots are the direct result of the rising column of magma from a mantle plume.
Other scientists believe they are a result of cracks in the mantle
22. Give four methods that scientists use to predict volcanic eruptions.
Scientists study volcanos by tracking seismic waves, measuring the volume of gases released by
volcanos and by measuring the slope of volcanos. Changes in these areas can indicate that a volcano is
getting ready to start.
23. Describe the three categories of volcanoes.
Three categories of volcanos are Extinct, have not erupted in recorded history and never will.
Dormant volcanos are currently erupting but the recors of past eruptions suggests they may erupt in
the future. Active volcanos are currently erupting.