earthquakes chpt 8 study guide

True false 37 gas bubbles decrease in number and

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True False 37. Gas bubbles decrease in number and volume as magma keeps rising upward to lower pressures, helping to propel magma upward through fractures or pipes toward an eruption. True False 38. Although there are 92 naturally occurring elements, a mere eight make up more than 98 percent of the Earth's crust. True False 39. Oxygen and silicon are so abundant in the Earth's crust that their percentages dwarf those of all other elements. True False 40. The process of mineral formation in a cooling magma is called crystallization. True False 41. While there are many hundreds of different minerals, the overwhelming majority of the Earth's crust is composed of just eight common rock-forming minerals. True False 42. No magmas contain dissolved gases. True False 43. The higher the viscosity of a magma, the more fluid is its behavior. True False 44. The highest temperatures and highest SiO 2 contents are in basaltic magma, giving it the lowest viscosity and easiest fluid flow. True False
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45. The lowest temperatures and highest SiO 2 contents occur in rhyolitic magma, material so viscous that it commonly does not flow. True False 46. Over 90 percent of volcanism is associated with the edges of tectonic plates, and most of the rest is caused by hot spots. True False 47. About 80 percent of the magma reaching the Earth's surface is basaltic, with only about 10 percent andesitic and 10 percent rhyolitic. True False 48. Over 80 percent of Earth's magma extruded through volcanism occurs at oceanic spreading centers. True False 49. Basaltic magma has the lowest viscosity, so more of it reaches the surface; the more viscous rhyolitic magmas are so sluggish that they tend to be trapped deep below the surface where they cool, solidify, and grow into the larger mineral crystals of plutonic rocks. True False 50. At oceanic spreading centers, magmas are rhyolitic in composition. True False 51. Basaltic magmas have low contents of SiO 2 , low temperature, low viscosity, and allow easy escape of gases, producing peaceful eruptions. True False 52. Beneath continents, rising basaltic magmas are contaminated by sediments and continental-crust rocks, altering magma compositions; the resultant andesitic-to-rhyolitic magmas have high contents of SiO 2 , relatively low temperatures, and high viscosity, which make it difficult for gases to escape, causing explosive eruptions. True False 53. The primary reason magma forms at subduction zones is that the subducting plate carries a cover of sediments, water, and hydrated minerals down with it, which lowers the temperature required for the adjacent overlying mantle lithospheric rock to melt. True False 54. Magma compositions are independent of the amount of crustal rock melted and incorporated into the rising magma. True False 55. Magma rising in a subduction zone tends to get more "contaminated" with high-silica minerals than does magma rising in a spreading ridge. True False 56. If the magma cools and solidifies below the surface, it crystallizes as volcanic rocks.
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