Answer to Question 1
The principle of uniformitarianism is based on the premise that present-day processes have operated throughout geologic time.The physical and chemical laws of nature have remained the same over time. Thus, even though the rates and intensities of geologic processes have varied during the past, the physical and chemical laws of nature have remained the same.
Answer to Question 2
Temperature increase with depth, or geothermal gradient, is approximately 25C/km near the surface. In areas of active or recently active volcanism, the geothermal gradient is greater than in adjacent nonvolcanic areas, and temperature rises faster beneath spreading ridges than elsewhere beneath the seafloor. Much of Earth's internal heat is generated by radioactive decay. When isotopes decay, they emit energetic particles and gamma rays that heat surrounding rocks. Because rock is such a poor conductor of heat, it takes little radioactive decay to build up considerable heat, given enough time. Unfortunately, the geothermal gradient is not useful for estimating temperatures at great depth. But if various studies and hypotheses are reasonably accurate, the geothermal gradient in the mantle is only about 1C/km.
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