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 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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Both extrusive and intrusive igneous rocks are derived from magmas. The temperature and pressure conditions in the crust and upper mantle influence the melting temperatures of the minerals in the rocks .
Temperature and pressure increase with depth from the surface and eventually reach a point at which rocks melt . The geothermal gradient is the rate at which temperature increases with depth. In the upper crust, the geothermal gradient is about 2 .5 degrees centigrade for every 100 meters (330 feet).
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