Answers to Chapter 9 Review Questions

1. Magma and lava both refer to molten rock from which igneous rocks form. However, magma refers to molten rock below Earth's surface, and lava refers to molten rock at Earth's surface.

2. The nature of a volcanic eruption is determined by the (1) composition of the magma, (2) the temperature of the magma, and (3) the quantity of dissolved gases contained in the magma. The composition and temperature of the magma influence the viscosity of this material. The viscosity, in turn, helps determine whether the eruption will be violent or quiet. The viscosity is least for very hot magmas with relatively low silica content. The gases dissolved in the molten material provide the force to propel the liquid rock from the volcano. The quantity of gases present and the ease with which they can escape (dependent on viscosity) determines the nature of the eruption.

3. When magma migrates to a near-surface environment, the gases that were dissolved at great depth begin to rise and expand. The viscosity of the lava determines the ease with which these gases can escape. Highly viscous magma inhibits the escape of gas, which may then accumulate to the point at which the lava is violently ejected from the volcano.

4. Pahoehoe lava forms from fluid basaltic lava and has a smooth or ropy appearance. Aa lava results when more viscous magma cools and has a sharp and jagged surface.

5. Water vapor (70%), carbon dioxide (15%), nitrogen (5%), sulfur compounds (5%), and smaller amounts of chlorine, hydrogen, and argon

6. Water vapor.

7. Both are pebble-sized or larger pyroclastic fragments. Bombs are cooled from ejected magma blobs. They typically have very fine grained, chilled margins, are vesicular, exhibit surface patterns characteristic of solidified liquid, have rounded, twisted shapes produced in flight, and may be flattened and cracked on impact. All bombs are essentially vesicular to a greater or lesser extent. Blocks are lithic clasts broken from preexisting rock. They are typically angular and show none of the morphological features associated with impacts, in-flight movements, and solidification of liquid or partly liquid magma masses. Blocks may or may not be vesicular

8. A volcanic crater is a relatively small depression marking the vent or exit site of erupting lava or pyroclastic material. A crater is excavated by the boring or drilling action of the erupting magma and gases. A caldera is a much larger volcanic depression that forms during or following a large outpouring of lava or pyroclastic debris. Extremely rapid emission of huge quantities of magma, such as occurs during a powerful explosive eruption, evacuates upper portions of the former magma chamber. Thus, the rocks above the chamber fail, and a large, circular to elliptical volcanic depression is formed by collapse and subsidence.

9. Shield volcanoes are among the largest on Earth. These gently sloping domes are associated with relatively quiet eruptions of fluid basaltic lava. They contain very little pyroclastic material. Cinder cones are composed almost exclusively of pyroclastics, are steep-sided, and are the smallest of the volcanoes. Composite cones, as the name suggests, are composed of alternating layers of lava (usually andesitic or rhyolitic in composition) and pyroclastic debris. Their slopes are steeper than those of a shield volcano but gentler than a cinder cone. Composite cones are associated with violent periods of volcanic activity.

10. The volcanoes making up the island of Hawaii serve as excellent examples of shield volcanoes. Paricutin, as well as many small cones on the Colorado Plateau north of Flagstaff, Arizona, are good examples of cinder cones. Mount Fuji in Japan and Mount Shasta, in California, as well as the many volcanoes of the Cascade Range, are examples of composite cones.

11. The volcanoes of Hawaii are the shield variety and were built up over nearly 1 million years by many eruptive cycles. Paricutin, in contrast, is a cinder cone that was created over a period of a few years. It is, in comparison with the volcanoes of Hawaii, tiny.

12. Crater Lake (Oregon) caldera is about 6 miles in diameter. It formed following a major eruption of ash and pyroclastic flows about 7000 years ago. Glacial valleys cutting through the caldera rim and other geologic evidence prove that a complex, composite volcano once existed above the site of the present-day caldera. In contrast, the summit caldera block of Kilauea is about 3 miles in diameter and acts somewhat like a floating cork, rising when magma is accumulating and sinking after an eruption. The rising and sinking movements are gradual, in contrast with the catastrophic collapse that follows large-volume pyroclastic flow eruptions.

13. The largest volcanic structures on Earth are the Yellowstone-type calderas that occur in continental regions. They are not associated with a composite volcano, such as Crater Lake in Oregon. Instead, they occur as very large (tens of miles in diameter) depressions in volcanic terrains dominated by explosive rhyolitic and andesitic magmas. Good examples include Yellowstone National Park in Wyoming, Long Valley Caldera in California, and the Valles Caldera in New Mexico.

14. Ship Rock, a well-known landmark in northwestern New Mexico, marks the subsurface plumbing system of a former volcano. The igneous rock is much harder than surrounding sedimentary strata. As erosion gradually cut into the bedrock, spires and sharp ridges of igneous rock were left towering above the more

easily eroded sedimentary rocks. Ship Rock itself is the central magma pipe that once fed magma upward to the volcano. The sharp ridges extending outward from the central spire are dikes representing radial cracks filled with magma injected outward from the central pipe.

15. Large, voluminous volcanic edifices such as Mounts Rainier, Washington, and Shasta, California, are composite cones (stratovolcanoes). They are built by repeated, central-vent eruptions over time spans ranging up to a million years or more, interspersed with eruptions from flank fissures and satellite centers. Higher- viscosity magmas (andesite to rhyolite) erupt explosively or form thick, stubby, lava flows that, unless the lava is unusually hot, move only short distances from the vent.

The Columbia Plateau is an eroded, uplifted flood basalt province of mid-Tertiary age. Over a million years or more, basaltic lava flows are erupted repeatedly from fissure vents. The lavas collect as pools in topographically low areas and solidify to sheets of basalt. At first only low areas are buried; eventually, the lava stack thickens, and higher parts of the former land surface are buried. Later flows rest exclusively on earlier ones, and the lava pile attains a relatively flat upper surface.

16. Dikes and sills are tabular masses (thin in one dimension). They differ because sills are concordant, whereas dikes are discordant. Massive intrusions include laccoliths, which are concordant, and batholiths, which are discordant. Batholiths are by far the largest of all intrusive features.

17. Laccoliths are known to be emplaced at shallow depths. Domed strata above a laccolith may be exposed at the surface before erosion cuts down far enough to expose the igneous rock. Thus the domed strata may suggest that the top of a laccolith lies a short distance below the surface.

18. The largest of all intrusive igneous rock bodies are batholiths. They are massive, possibly teardrop shaped, and discordant. By definition, their surface exposure exceeds 100 square kilometers.

19. Partial melting is believed to produce most, if not all, magmas. As rock is heated the minerals having the lowest melting points liquefy first. If this melted portion becomes segregated from the remaining material, it will have a composition different from that of the original rock.

20. Basalt. Divergent boundaries lie above slowly rising, largely solid mantle plumes that turn laterally as they near the surface, carrying the diverging plates in opposite directions. Melting temperatures of rock-forming minerals increase with higher pressure and decrease with lower pressure. As the plume rises, pressures and melting temperatures are lowered, but the plume loses very little of its heat; thus rock temperatures stay constant. Eventually, temperatures exceed the melting range and partial melting occurs.

21. The Ring of Fire refers to the volcanic mountains ranges and islands that surround much of the Pacific Ocean. Many of the active volcanoes on Earth today are located on the Ring of Fire.

22. Very large composite volcanoes (stratovolcanoes), like those on the Ring of Fire, typically erupt explosively. The 1991 eruption of Pinatubo in the Philippines was the second most powerful eruption of the twentieth century, being surpassed only by the 1902 eruption of Santa Maria in Guatemala. The 1980 eruption of Mount Saint Helens is another good example.

23. Intraplate volcanism.

24. Basalt.

25. Short-term climatic changes produced by volcanic eruptions are caused by the release of large quantities of sulfur dioxide gas, which combines with water to produce tiny droplets of sulfuric acid. These droplets, called aerosols, reflect solar radiation back into space while remaining in the atmosphere for several years.

Answers to Earth System Questions

1. A great and prolonged increase in volcanic activity will add substantial amounts of volcanic dust to the atmosphere, block sunlight, cause a global lowering of temperatures, and alter the general pattern of atmospheric and oceanic circulation. The reduction in both sunlight and temperature will have a substantial impact on the biosphere, perhaps resulting in mass extinctions. Lowering temperatures will result in increased cloud cover and precipitation. Consequently, erosion will be more pronounced, and additional sediment and water will be added to the oceans.

2. Volcanic regions, especially those such as the Hawaiian Islands and the Philippines that are located in tropical regions, often offer rich, fertile, and productive soils along with majestic scenery.