Tectonics and Landforms

Modern Theory of Plate Movement
and Continental Drift

The movement of lithospheric plates referred to as continental drift, is believed to be caused by the radioactive decay of elements in the core and mantle that produces heat. The heat in turn creates convection currents in the mantle which "drive" the plates along their path of movement. When plates collide, heavier, more dense plates dive beneath lighter, less dense crustal plates along subduction zones. As the heavier plate moves downward into the mantle, the increase in temperature and pressure drive water and other volatile fluids ("dewatering") from the oceanic crust. At a depth of 100 kilometers (60 miles), the water-rich fluids decrease the melting point of mantle rock causing it to melt. The magma that slowly moves upwards and may be extruded onto the surface as lava (Figure 15.13). Some plates slip past one another, creating earthquakes, (Figure 15.13) like what happens along the San Andreas fault in California. In many places the crust is separating and moving away in opposite directions, or diverging as happened to create the Great Rift Valley of Africa.

Figure 15.13 Sea floor movement and plate tectonics 
Courtesy of USGS

Figure 15.14 Global distribution of earthquakes and volcanoes
Courtesy of USGS

Though many earthquakes seemingly occur along plate boundaries, they can occur far away from the edges of plates too. One of the most well-known seismic regions is the New Madrid Seismic Zone located in the Mississippi Valley of the central United States. Investigate the New Madrid Seismic Zone by reading The Mississippi Valley - "Whole Lotta Shakin' Goin' On" . Then return here to continue.

Figure 15.15 Plate tectonics: Evidence of plate movement

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