The deeper a rock is within the Earth, the hotter and denser it is. Both temperature and pressure increase with depth. With every kilometer in depth the temperature increases by about 25°C (45°F), and the pressure increases by about 250 atmospheres. (One atmosphere = 14.7 lb/sq in, the average pressure of the atmosphere at sea level.

Heat
Most heat is generated by the decay of radioactive elements within minerals. This sends subatomic particles speeding through the surrounding mineral structure. There, the particles collide with atoms—breaking bonds and producing heat.

Pressure
The weight of all the rocks above squeezes those below—just as pressure from overlying water crushed this plastic foam cup and pressure from Earth’s atmosphere makes your ears pop when you change elevation. This foam cup was hung outside the Japanese research submersible Shinkai 2000, which descended 1.4 km (0.9 mi) into the ocean. The 140 atmospheres of water pressure squeezed the air out of the cup, shrinking it to a third of its original size.