Ch 06 - Volcanic Activity

Answers to Review Questions and Critical Thinking Questions

Review Questions

1. Magma is molten rock beneath the Earth’s surface.
2. Viscosity is resistance to flow, and is determined by magma composition and temperature.
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 interiors.
6. Lava tubes help move magma by insulating the magma and keeping it hot and fluid.
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 four.
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 events.