Ch 06 - Volcanic Activity
Answers to Review Questions and Critical Thinking Questions
1. Magma is molten rock beneath the Earth’s surface.
2. Viscosity is resistance to flow, and is determined by magma composition
3. Shield volcano—basalt; composite volcanoes—andesite; volcanic
domes—rhyolite; cinder cones—basalt
4. Shield volcano—erupts “quietly,” producing lava flows;
composite volcanoes—erupts both quietly and explosively, producing
lava flows and pyroclastic deposits; volcanic domes—explosive; cinder
cones—mildly explosive. Eruptive style is governed largely by viscosity
and gas content.
5. Most volcanoes are associated with plate tectonic boundaries, particularly
divergent margins and subduction zones. Hot spot volcanoes form in plate
6. Lava tubes help move magma by insulating the magma and keeping it hot
7. The Hawaiian Islands are a series of volcanoes created as the Pacific
plate has passed over the hot spot, which now lies beneath the big island.
8. A geyser originates where groundwater comes in contact with hot rock
and is converted to steam.
9. Caldera eruptions are dangerous because of the volume of pyroclastic
debris erupted and spread across the landscape, and because of the violence
of the eruptions.
10. Primary effects: lava flows, pyroclastic activity, and gas release;
secondary effects: lahars, floods, fires, global cooling.
11. Lava flow control methods have included bombing, hydraulic cooling,
and wall construction.
12. Ash falls are the widespread deposition of volcanic ash propelled
high into the atmosphere. Lateral blasts are explosive ejections of pyroclastic
material from the side of a volcano. Ash flows are hot, fast, gas-rich
flows of ash and other pyroclastic debris.
13. Major gases include water vapor, carbon dioxide, carbon monoxide,
sulfur dioxide and hydrogen sulfide. The most hazardous are the latter
14. Mudflows are produced by melting of snow and ice, collapse of volcanic
edifices, and heavy rain falling on thick ash.
15. Methods of eruption prediction include monitoring of seismic activity,
of thermal, magnetic, and hydrologic changes, of topographic changes,
and of gas emissions. The geologic history of a volcano is also important.
Critical Thinking Questions
1. An answer to this question should focus on the origins, settings, and
processes associated with andesite. The andesite probably came from a
composite volcano above a subduction zone (e.g., the Cascade Range), and
was produced by quiet, lava flow-producing activity that may have been
associated with explosive eruptions.
2. The public relations program should focus on the potential hazards
associated with the volcano, the probability of those potential hazards
becoming real, and the actions that will need to be taken by residents
in the event of an eruption. The emphasis should be on the actual likelihood
of an eruption and its attendant hazards, rather than on low probability