7. Look at Figure 4 on the following page. What type of volcanic arc is present on the west coast of South America? How do you know? Figure 4: Earthquake depth in South America. Depth increases as vou go from Doint A to point B. following the path of the subducting The distribution of earthquakes also provides clues to plate activity. Most earthquakes around the world occur in association with plate boundaries. Shallow-focus earthquakes, within about 70 km of the surface, occur at all plate boundaries. However, in subduction zones, bands of progressively deeper earthquakes are observed, produced when an oceanic plate is thrust down into the mantle. The shallowest quakes are produced at the surface boundary between plates, while the deepest quakes occur deeper into the upper mantle (see Figure 4). 8. Using Figure 4, describe the distribution of earthquakes along the west coast of South America as you travel from west to east. Mantle plumes, also known as hot spots, are sources of magmas in the upper mantle that can produce volcanic activity far away from plate boundaries. Mantle plumes apparently remain in fixed positions for millions of years, while the plate above continues to move. The result of this can be a chain of volcanoes, such as the Hawaiian Islands. The active volcanoes in Hawaii are believed to be directly over the mantle plume, while ongoing movement of the Pacific Plate has carried older volcanoes off the hot spot, causing them to become extinct. 9. What is an extinct volcano? 81 Depth (km) -10 -20 -30 -80 -40* -80* O 100 200 300 400 500 600 700 800 A -74 A -72 South America -70 60 -70* 00 O -70 -68 -66 -64 Longitude (degrees) -62 -60 B -60 B 100 km -50* -10 -20 -30 -40 -50* -60 1 10 100 1000 #of Earthquakes