GC: n

CT: 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.

S: http://www.cliffsnotes.com/sciences/geology/inside-the-earth/geothermal-gradients (last access: 21 December 2014)

N: 1. geothermal (adj): 1875, from geo- (word-forming element meaning “earth,” ultimately from Greek geo-, comb. form of ge “earth”) + thermal (1756, “having to do with hot springs,” from French thermal (Buffon), from Greek therme “heat, feverish heat,” from PIE gwher- “to heat, warm” (cognates: Latin fornax “an oven, kiln,” formus “warm,” Old English wearm). Sense of “having to do with heat” is first recorded 1837. The noun meaning “rising current of relatively warm air” is recorded from 1933).
gradient (n): “steep slope of a road or railroad,” 1835, principally in American English, from grade (n.) by analogy of quotient, etc. It was used 17c. as an adjective, of animals, “characterized by walking;” in that case probably from Latin gradientem, present participle of gradi “to walk.”
2. The increase in the temperature of the earth with the increasing depth from the surface …
3. As the depth of a well increases, so does the temperature; this phenomenon is known as the temperature gradient.
4. … expressed either in degrees per unit depth or in units of depth per degree.
5. … averages about 1°F per 60 ft, but may be considerably greater.
6. When applied to the Earth, the term “geothermal gradient” may be used (as a synonym for “thermal gradient”).
7. Not to be confused with “geothermal degree.”
8. thermal gradient; geothermal gradient; temperature gradient: terms used at Natural Resources Canada – Earth Sciences Sector.

S: 1. OED – http://www.etymonline.com/index.php?allowed_in_frame=0&search=geothermal&searchmode=none; http://www.etymonline.com/index.php?allowed_in_frame=0&search=gradient&searchmode=none (last access: 21 December 2014). 2 to 8. TERMIUM PLUS – https://bit.ly/38EIF8y (last access: 21 December 2014).

SYN: thermal gradient, temperature gradient, heat flow value, heat gradient.

S: TERMIUM PLUS – https://bit.ly/38EIF8y (last access: 21 December 2014)

CR: geothermal energy, geothermics, gradient, ocean’s thermal gradient.