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Volcanoes
Questions for Review
1. Define a fissure eruption, and give an example.
A fissure eruption is an eruption from a long crack in the lithosphere. The Columbia River flood basalts are a continental
example; spreading ridges are an example on the sea floor.
2. The Hawaiian Islands are all shield volcanoes. What are shield volcanoes, and why are they not especially hazardous to
life?
Shield volcanoes are low, broad, flat volcanoes formed from many thin flows of (generally basaltic) lava. Because the lowviscosity lavas that build shield volcanoes tend not to trap gases, eruptions of shield volcanoes are not typically violent and
explosive.
3. The eruptive style of Mount St. Helens is quite different from that of Kilauea in Hawaii. Why?
Unlike Kilauea, Mount St. Helens has a more silicic, viscous, stiff, gas-charged lava in which pressure can build up to cause
an explosive eruption. Mount St. Helens is a volcano with predominantly andesitic magma as is characteristic of subductionzone volcanoes.
4. What are pyroclastics? Identify a kind of volcanic structure that pyroclastics may build.
Pyroclastics are hot fragments of material erupted explosively from a volcano. A cinder cone is a common product of
pyroclastic eruptions.
5. Describe two strategies for protecting an inhabited area from an advancing lava flow.
One strategy would be to slow the flow—either by spraying it with water so that it congeals and dams itself, or by
constructing earthen or other dams in front of it to slow it down long enough to cause it to crystallize, stiffen, and stop.
Another strategy would be to divert the lava, as by using explosives to punch holes in the hardened crust in strategic places to
direct further flow away from populated areas.
6. Describe a way in which a lahar may develop, and a way to avoid its most likely path.
A lahar may form when hot volcanic ash falls on snow or ice on a volcano, melts it, and creates volcanic mud; or, it may form
when heavy rain falls on accumulated ash. Lahars tend to flow down stream valleys, so staying away from such valleys is a
way to avoid lahars.
7. What is a nuée ardente, and why is a volcano known for producing these hot pyroclastic flows a special threat during
periods of volcanic activity?
A nuée ardente is a hot, glowing, denser-than-air cloud of gas and volcanic ash that can flow rapidly down the slopes of a
volcano, charring everything in its path. Nuées ardentes commonly erupt with no immediate prior warning, and they move so
fast that, once one appears, it is impossible to get out of the way quickly enough.
8. Explain the nature of a phreatic eruption, and give an example.
A phreatic eruption is a steam-boiler-style explosion that occurs when abundant subsurface water comes close to a nearsurface magma body, flashes to steam, expands, and blows up the volcano fed by the magma. Examples include Krakatoa,
Tambora, and Thera.
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9. How may volcanic eruptions influence global climate?
Explosive eruptions, particularly, may put large volumes of dust and sulfuric acid aerosols into the atmosphere. These block
sunlight and thus contribute to global cooling, although they precipitate out of the atmosphere in a matter of years.
10. Discuss the distinctions among active, dormant, and extinct volcanoes, and comment on the limitations of this
classification scheme.
Active volcanoes have a recent history of eruption. Dormant volcanoes do not, but neither do they appear very eroded or worn
down, so they are interpreted as merely “sleeping,” with some potential for renewed activity. Those volcanoes classified as
extinct have neither erupted within historic record, nor do they have a very fresh appearance; they are not expected to erupt
again. The problem is that different volcanoes differ widely in the characteristic periodicity of their activity, from hundreds of
years to tens of thousands of years—so just because a volcano has not erupted within historic memory does not necessarily
mean that it is extinct.
11. What is the Volcanic Explosivity Index?
The VEI is a numerical scale used to classify the relative sizes of explosive eruptions, based on how much material is ejected,
how high it rises in the atmosphere (a measure of the force of the explosion), and how long the eruption lasts.
12. Describe two precursor phenomena that may precede volcanic eruptions.
Seismicity increases as rising magma stresses the lithosphere, and may show a characteristic pattern (harmonic tremors) as
eruption approaches. Foci of earthquakes may become shallower as magma rises. The volcano may bulge or show an increase
in tilt as it swells with magma or rising gas pressure. There may be an increase in ground temperature (heat flow) as magma
nears the surface. Also, the composition of gas emitted from the volcano may change as an eruption nears; for example, sulfur
content may increase as more magmatic gas escapes.
13. What is the underlying cause of present and potential future volcanic activity in the Cascade Range of the western United
States?
The Cascade volcanoes exist because of a subduction zone beneath them, and are fed by magma generated there. Subduction
beneath the North American Plate is continuing, so there is an ongoing volcanic hazard.
14. What is the origin of volcanic activity at Yellowstone, and why is it sometimes described as a “supervolcano”?
Yellowstone is powered by a subcontinental “hot spot”. It is sometimes called a “supervolcano” because prehistoric eruptions
there had VEI values up to 8.
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