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.
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The earth is surrounded by a magnetic field . Magnetic lines of force originate from north and south magnetic poles, which are about 11 .5 degrees away from the geographic North and South Poles. The magnetic field is strongest at the magnetic poles. The positions of the magnetic poles have changed over time and appear to be rotating around the geographic poles on
an axis tilted from the geographic axis by 11 .5 degrees . The magnetic field traps high-energy particles created by the sun’s ultraviolet radiation, thus protecting our environment on Earth.
The magnetic field is thought to be generated by the liquid outer core. If this liquid material is metallic, as geophysical studies suggest, its flow as a result of heat convection would create an electric current Electric currents induce magnetic fields.
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The upper and lower mantle. Seismic data suggest that most of the mantle is composed of solid rock. P waves travel at an average of about 8 kilometers per second through the mantle, suggesting it is composed of ultramafic rocks such as peridotite. The behavior of P waves indicates the mantle can be divided into two parts : the upper and lower mantle. The upper mantle begins at a depth of from 5 to 50 kilometers (3–30 miles) and extends to a depth of approximately 67 0
kilometers (400 miles) from the surface ; the lower mantle extends from a depth of about 670 kilometers (400 miles) to about 2,900 kilometers (1,740 miles) .
The lithosphere. Changes in P wave velocities have identified other boundaries in the mantle . Together the crust and the uppermost mantle form the lithosphere. This brittle exterior shell of the earth ranges in thickness from about 75 kilometers beneath oceans to about 175 kilometers beneath continental masses (45–105 miles). The maximum depth of the lithosphere from the surface is thought to be no more than 200 kilometers (120 miles).
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