Both plate boundaries and plates move over time. As previously described, plates can change the locations of trenches and subduction zones, as well as the positions of midoceanic ridges and transform faults . For example, subduction at a convergent boundary can stop in one location and begin nearby in another. Plates can become larger or smaller over time depending on the generation rates of new crust at spreading centers and the rates of subduction.
Convection currents. Some geologists favor convection currents in the mantle as the best explanation for plate tectonic movement. It is reasonable to assume that the heat radiated from the core creates convection currents in the mantle, and the mantle rocks begin to move plastically.
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The internal temperature of the earth increases with depth from the surface. Near the surface, the average geothermal gradient is about 25 degrees centigrade (77 degrees Fahrenheit) for every kilometer of depth. Some areas have much higher heat flows because of deep fault zones, rifting, magmatic intrusions, or active tectonic forces. The geothermal gradient can make conditions in deep mines quite uncomfortable and hot enough to explode rocks or bend steel.
The geothermal gradient of 25 degrees centigrade/kilometer is thought to be restricted to the upper part of the crust . If it continued at this rate uniformly from the surface, the internal temperature of the earth would be greater than 2,000 degrees centigrade within the lithosphere a temperature that far exceeds the melting temperatures for all rocks at that depth.
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The internal temperature of the earth increases with depth from the surface. Near the surface, the average geothermal gradient is about 25 degrees centigrade (77 degrees Fahrenheit) for every kilometer of depth. Some areas have much higher heat flows because of deep fault zones, rifting, magmatic intrusions, or active tectonic forces. The geothermal gradient can make conditions in deep mines quite uncomfortable and hot enough to explode rocks or bend steel.
The geothermal gradient of 25 degrees centigrade/kilometer is thought to be restricted to the upper part of the crust . If it continued at this rate uniformly from the surface, the internal temperature of the earth would be greater than 2,000 degrees centigrade within the lithosphere a temperature that far exceeds the melting temperatures for all rocks at that depth.
Read the rest of this entry »