The conduction of heat out of a section of lithosphere causes the rock to thicken and become more insulated to heat flowing in from the mantle; as this thicker section is buried by the descending column of lithosphere, it descends into surrounding rock layers with a higher relative geothermal gradient.
22.
A variation in surface temperature induced by climate changes and the Milankovitch cycle can penetrate below the Earth's surface and produce an oscillation in the geothermal gradient with periods varying from daily to tens of thousands of years and an amplitude which decreases with depth and having a scale depth of several kilometers.
23.
Along continental margins the average thickness of the HSZ is about 500m . 1 The upper limit in oceanic sediments occurs when bottom water temperatures are at, or near 0?celsius, and at a water depth of approximately 300 meters . 1 The lower limit of the HSZ is bounded by the geothermal gradient.
24.
The Geothermal gradient article says : " Very deep mines, like some gold mines in South Africa, need the air inside to be cooled and circulated to allow miners to work at such great depth . " The Geothermal energy article says : " the geothermal gradient of temperatures through the crust is 25 30 �C ( 77 86 �F ) per kilometer of depth in most of the world ."
25.
The Geothermal gradient article says : " Very deep mines, like some gold mines in South Africa, need the air inside to be cooled and circulated to allow miners to work at such great depth . " The Geothermal energy article says : " the geothermal gradient of temperatures through the crust is 25 30 �C ( 77 86 �F ) per kilometer of depth in most of the world ."
