ENV 151 Study Guide - Quiz Guide: Volcanic Bomb, Eruption Column, Lava
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1. Explain how the viscosity of magma influences the explosiveness of a volcano.
More viscous magmas tend to generate more explosive eruptions.
2. List these three magmas in order from most silica rich to least silica rich, based on their
compositions: mafic (basaltic), felsic (rhyolitic), intermediate (andesitic).
Felsic, intermediate, mafic.
3. The eruption of what type of magma may produce an eruption column?
4. Why is a volcano that is fed by highly viscous magma likely to be a greater threat to life
and property than a volcano supplied with very fluid magma?
Volcanoes with viscous magma tend to produce violent eruptions that include clouds of
hot gas and ash. The blast from the eruption can also cause collapse of part of the
5. Describe pahoehoe and aa lava flows.
Pahoehoe lava flows smoothly and sometimes generates smooth surfaces that look ropy
or braided. Aa flows are rough with jagged blocks and spines of volcanic ejecta.
6. How do lava tubes form?
Lava tubes can form in pahoehoe flows when the inner part of the flow remains at a
high temperature, allowing lava to flow through, while the outer portion cools and
7. List the main gases released during a volcanic eruption. What role do gases play in
Water vapor, carbon dioxide, nitrogen, and sulfur dioxide. These gases escape as the
confining pressure over a magma is released, releasing the gases into the atmosphere.
8. How do volcanic bombs differ from blocks of pyroclastic debris?
Volcanic bombs are emitted as molten rock and cool after being ejected. Pyroclastic
debris blocks are ejected as solid materials.
9. What is scoria? How does scoria differ from pumice?
Scoria is a basaltic pyroclastic rock that is dark and riddled with vesicles from magma
gas. It is darker and denser than pumice and does not float as pumice might.
10. How is a crater different from a caldera?
A crater is a small depression at the top of a volcano. A caldera is a crater that has a
diameter greater than 1 kilometer (0.6 miles).
11. Distinguish among conduit, vent, and crater.
A conduit is the tube through which magma flows from the magma chamber to the vent.
The vent is where magma is emitted from the volcano and a crater is a bowl-like
depression at the top of the volcano.
12. What is a parasitic cone, and where does it form?
Parasitic cones form on the flanks of volcanoes and tend to form in mature volcanoes
where magma erupts from fissures along the flank or base of the volcano.
13. What is emitted from a fumarole?
14. Describe the composition and viscosity of the lava associated with shield volcanoes.
The lava will be basaltic and have low viscosity.
15. Are pyroclastic materials a significant component of shield volcanoes?
16. Relate lava tubes to the extent of lava flows associated with shield volcanoes.
Since shield volcanoes are not very steep, their lava flows fast and far from the vent by
way of lava tubes. Therefore shield volcanoes tend to have lava tubes associated with
17. Where are the best-known shield volcanoes in the United States? Name some examples
in other parts of the world.
Hawaii. Canary Islands, Galapagos, and Easter Island.
18. Describe the composition of cinder cones.
Basaltic; generally loose scoria.
19. How do the size and steepness of slopes of a cinder cone compare with those of a shield
Cinder cones are much smaller and steeper than shield volcanoes.
20. Over what time span does a typical cinder cone form?
Less than one year for most, less than a month for half.
21. What zone on Earth has the greatest concentration of composite volcanoes?
Ring of Fire that circles the Pacific Ocean rim.