American Idol (season 8) had Randy Jackson as a judge for the eighth year in a row.
Geothermal gradient is the rate of increasing temperature with respect to increasing depth in the Earth 's interior .. Earth. Earth. Away from tectonic plate boundaries , it is about 25 ° C per km of depth -LRB- 1 ° F per 70 feet of depth -RRB- near the surface in most of the world .. Strictly speaking , geo-thermal necessarily refers to the Earth but the concept may be applied to other planets .. Earth. Earth. A line tracing the gradient through the planetary body is called a geotherm on Earth and other terrestrial planets .. Earth. Earth. On the Moon it is called a selenotherm .. The Earth 's internal heat comes from a combination of residual heat from planetary accretion , heat produced through radioactive decay , and possibly heat from other sources .. Earth. Earth. planetary accretion. planetary accretion. radioactive decay. radioactive decay. The major heat-producing isotopes in the Earth are potassium-40 , uranium-238 , uranium-235 , and thorium-232 .. Earth. Earth. potassium-40. Potassium. uranium-238. Uranium. uranium-235. uranium-235. thorium-232. Thorium. At the center of the planet , the temperature may be up to 7,000 K and the pressure could reach 360 GPa -LRB- 3.6 million atm -RRB- .. GPa. GPa. Because much of the heat is provided by radioactive decay , scientists believe that early in Earth history , before isotopes with short half-lives had been depleted , Earth 's heat production would have been much higher .. Earth. Earth. radioactive decay. radioactive decay. Heat production was twice that of present-day at approximately 3 billion years ago , resulting in larger temperature gradients within the Earth , larger rates of mantle convection and plate tectonics , allowing the production of igneous rocks such as komatiites that are not formed anymore today .. Earth. Earth. mantle convection. mantle convection. plate tectonics. plate tectonics. komatiites. komatiites