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▲ ▲ Juan de Fuca plate ▲ ▲ ▲ Pacific plate Rocky Mountains Basin and Range San Andreas Fault East Pacific Rise North American plate ▲ ▲ ▲ ▲ ▲ ▲ Cocos plate Nazca plate Appalachian Mts. ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ Caribbean plate Andes Mountains ▲ ▲ ▲ South American plate Antarctic plate Scotia plate Mid-Atlantic Ridge ▲ ▲ ▲ ▲ ▲ Chapter 7 Plate Tectonics 201 ▲ ▲ ▲ African plate Eurasian plate A. Divergent boundary B. Convergent boundary C. Transform fault boundary Figure 7.8 Continued Galapagos Ridge Chile Ridge Canadian Shield Antilles Arc Iceland Alps ▲ ▲ ▲ ▲
202 Unit Three Forces Within of such a continental collision is a majestic mountain range such as the Himalayas. In the following sections we will briefly summarize the nature of the three types of plate boundaries. Divergent Plate Boundaries EEA R T H S C I E N C E Forces Within Plate Tectonics Most divergent ( di = apart, vergere = to move) plate boundaries are located along the crests of oceanic ridges and can be thought of as constructive plate margins since this is where new oceanic lithosphere is generated (Figure 7.9). Here, as the plates move away from the ridge axis, the fractures created are filled with molten rock that wells up from the hot mantle below. Gradually, this magma cools to produce new silvers of seafloor. In a continuous manner, spreading and upwelling of magma adds oceanic lithosphere between the diverging plates. As we shall see later, divergent boundaries can also form on the continents (Figure 7.10). North America Lithosphere Asthenosphere Mid-Atlantic Ridge Magma chamber Figure 7.9 Most divergent plate boundaries are situated along the crests of oceanic ridges. Oceanic Ridges and Seafloor Spreading Along well-developed divergent plate boundaries, the seafloor is elevated forming the oceanic ridge. The interconnected ocean ridge system is the longest topographic feature on Earth’s surface, exceeding 70,000 kilometers (43,000 miles) in length with crests commonly 2 to 3 kilometers higher than the adjacent ocean basins. Representing 20 percent of Earth’s surface, the ocean ridge system winds through all major ocean basins in a manner similar to the seam on a baseball. The term “ridge” may be misleading, as these features are not narrow but have widths from 1000 to 4000 kilometers. Further, along the axis of some segments are deep down-faulted structures called rift valleys. The mechanism that operates along the oceanic ridge system to create new seafloor is appropriately called seafloor spreading. Typical rates of spreading average around 5 centimeters (2 inches) per year. Comparatively slow spreading rates of 2 centimeters per year are found in the North Atlantic, whereas spreading rates exceeding 15 centimeters (6 inches) have been measured along Rift valleys Europe Africa Lithosphere Asthenosphere
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