Chapter 8 Plate Tectonics

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210 CHAPTER 8: PLATE TECTONICS Types of Plate Boundaries Figure 8-11 is a world map from the Earth Science Reference Tables. This map shows the boundaries of Earth’s lithospheric plates. Plate boundaries are some of the most active zones of earthquakes and volcanic eruptions. These boundaries can be classified into four types based on the relative plate motions. DIVERGENT BOUNDARIES The double lines on the map indicate rift boundaries. These are places such as the Mid-Atlantic Ridge where heated material rises toward the surface. At the Figure 8-11 Plate boundaries are places where Earth’s rigid lithospheric plates meet. At these boundaries, new lithosphere is created, destroyed, or the plate moves parallel to the boundary.
WHAT IS EARTH’S INTERNAL STRUCTURE? 211 same time, the lithosphere is spreading away from the ridge allowing magma to come to the surface, cool, and make new lithosphere. At the mid-ocean ridges new ocean floor spreads away from the plate boundary. For this reason these areas are called divergent plate boundaries. Recall that this is also a region of many small and shallow earthquakes. Ocean ridges are where heat energy is escaping from Earth’s interior most rapidly. CONVERGENT BOUNDARIES The lines with black rectangles indicate subduction zones. These are the places where old, cool lithosphere sinks into Earth. Because the motion of the lithosphere is toward the plate boundary, zones of subduction are also known as convergent plate boundaries. It is common to find continental and oceanic lithosphere meeting near subduction zones. You may recall that the rock of the continents is granitic and therefore resists subduction. A good example of a convergent plate boundary is the Peru-Chile Trench along the western coast of South America. The Nazca Plate is ocean floor and mafic in composition. It is therefore easily drawn into Earth. As it descends, the edge of the plate bends and shifts causing earthquakes. Descending plates are cooler than their surroundings so they stay rigid until the have absorbed heat. Seismologists therefore observe many earthquakes in these regions. Subduction zone earthquakes can be large because the brittle rock can absorb a great deal of energy before it breaks. The earthquakes can also have deep foci because the descending slab of lithosphere is cooler and less bendable than other rocks at the same depth. Observation of deep-focus earthquakes has given geologists an important tool that allows them to locate the plates as they move into Earth at subduction zones. Descending plates are the only place where earthquakes can originate from deep within Earth. On the eastern side of the Peru-Chile Trench, most of the continental, felsic rocks are too light to be drawn down by subduction. They therefore bend, break, and pile up to make intensely folded and faulted mountains. In fact, the Andes Mountains along the western side of South America are second only to mountains of Asia in height. Some of the felsic
Page 1 and 2: Chapter 8 Plate Tectonics DO CONTIN
Page 3 and 4: Figure 8-1 If the continents were m
Page 5 and 6: Figure 8-2 Earth’s surface is com
Page 7 and 8: Figure 8-4 As new crust is created
Page 9 and 10: DO CONTINENTS MOVE? 201 Plates are
Page 11 and 12: Figure 8-7 Inferred properties of E
Page 13 and 14: Pressure WHAT IS EARTH’S INTERNAL
Page 15 and 16: Figure 8-8 The progression of ages
Page 17: Figure 8-9 Dots on this map indicat
Page 21 and 22: DOES EARTH’S GEOGRAPHY CHANGE? 21
Page 23 and 24: TERMS TO KNOW asthenosphere meteoro
Page 25 and 26: Data Table Age of Volcanic Features
Page 27 and 28: 10. At point A, the East Pacific Ri

Chapter 8 Plate Tectonics

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