NAME ________________________________ Date ______________ Class _____________ Ch. 18 Study Guide (Earth Science) 18-1 Multiple Choice Identify the choice that best completes the statement or answers the question. ____ 1. A bowl-shaped depression that forms around the vent of a volcano is a a. magma chamber. c. vent. b. crater. d. sill. ____ 2. Broad, gently sloping volcanoes with quiet eruptions are called a. cinder cones. c. shield volcanoes. b. hot spots. d. composite volcanoes. ____ 3. Lava erupts through an opening in Earth’s crust called a a. caldera. c. vent. b. volcano. d. crater. ____ 4. Which of the following forms when the top or side of a volcano collapses into the magma chamber? a. vent c. pyroclastic flow b. dike d. caldera ____ 5. Most volcanoes form a. at hot spots. c. in the center of ocean plates. b. in the middle of continents. d. at plate boundaries. ____ 6. The most explosive volcanoes are a. hot spots. c. cinder cones. b. composite volcanoes. d. shield volcanoes. ____ 7. When magma reaches Earth’s surface, it is called a. lava. c. calderas. b. a vent. d. a pyroclastic flow. ____ 8. ________ form(s) where plates move apart. a. Hot spots. c. Subduction zones. b. Divergent volcanism. d. Convergent volcanism. ____ 9. Volcanoes that form far from plate boundaries are associated with a. subduction zones. c. ocean ridges. b. divergent boundaries. d. hot spots. 18-1 Continued 10. Label the diagrams as composite, cinder-cone, or shield volcano. Identify the type or types of volcano being described as Shield, Cinder, or Composite Volcano. 11. ____________________ Forms when small pieces of magma are ejected into the air then fall back to Earth and pile up around a vent. 12. ____________________ Has broad, gently sloping sides and a nearly circular base. 13. ____________________ Mauna Kea in Hawaii is an example. 14. ____________________ Forms when layers of basaltic lava accumulate during a nonexplosive eruption. 15. ____________________ Forms when layers of hardened lava chunks alternate with lava. 16. ____________________ Small volcano with steep sides. 17. ____________________ Forms from lava that contains relatively small amounts of gases and silica. 18. ____________________ Forms from lava that is higher in water and silica content than lava that forms shield volcanoes 19. ____________________ Fueled by magma that contains large amounts of silica, water, and gases. 20. ____________________ Potentially the most dangerous to humans and most destructive to the environment. 21. ____________________ Mount St. Helens and Mount Rainier are examples. 18-1 Continued 22. Use each of the terms below just once to complete the passage. Hawaiian Islands crust divergent hot spots Iceland mantle volcanoes ocean ridges Circum-Pacific Belt western convergent Most of the world’s volcanoes form along (a) ______________________ plate boundaries. Slabs of oceanic crust descend into the (b) ______________________ and release water, causing portions of the mantle to melt. The magma that forms is forced upward through the overlying plate and forms (c) ______________________ when it reaches Earth’s surface. The (d) ______________________ marks the locations of most convergent boundary volcanoes. It stretches along the (e) ______________________ coasts of North and South America and down the eastern coast of Asia. At (f) ______________________ plate boundaries, magma is forced upward into fractures and faults that form as plates separate or spread apart. Most of the volcanoes that form along divergent boundaries are located underwater along (g) ______________________. This type of volcanic activity can be observed above sea level in (h) ______________________, which sits atop the Mid-Atlantic Ridge. Some volcanoes that form far from plate boundaries form over (i) ______________________, which are unusually hot regions of Earth’s mantle. At hot spots, high-temperature plumes melt rock. The magma that forms moves upward toward the (j) ______________________ and melts the crust to form a volcano. As a tectonic plate moves over a hot spot, a string of volcanoes forms. The (k) ______________________ are the result of a hot spot. 18-2 Continued Indicate whether the statement is true or false. ____ 23. Magma is a mixture of molten rock, suspended minerals, and gases. ____ 24. Pressure decreases with depth below Earth’s surface. ____ 25. Magma forms when rocks begin to melt. ____ 26. Wet minerals and rocks melt at lower temperatures than do dry minerals and rocks. ____ 27. As pressure increases, the temperature at which a dry substance melts increases. 18-2 Continued Answer the following questions: 28. How does the viscosity of magma change as magma cools? _______________________________________________________________________ 29. Does cooler magma flow more or less quickly than hotter magma? _______________________________________________________________________ 30. Is the viscosity of magma that is high in silica higher or lower than magma that is low in silica? _______________________________________________________________________ 31. Which type of lava—basaltic lava or rhyolitic lava—flows faster? Explain. _______________________________________________________________________ _______________________________________________________________________ 18-3 Matching a. stock d. pluton b. sill e. batholith c. laccolith f. dike ____ 32. Largest pluton ____ 33. Mushroom-shaped pluton ____ 34. Instrusive rock body. ____ 35. Irregularly shaped pluton that is similar to a batholith, but smaller in size. ____ 36. Pluton that is parallel to the rocks it intrudes. ____ 37. Pluton that cuts across preexisting rocks. .