Lava, or molten rock that erupts from volcanoes and fissures, is so hot because rock only melts at very high heat. Typically, only a small portion of any rock melts, even at depths of around 93 miles inside the Earth.
The inner core of rock does not melt because its chemical composition differs from the outer layer. At extreme depths, the entire outer core of rock can melt. Lava is typically around 750 degrees Celsius in temperature. It can become as hot as 1,250 degrees Celsius. Once the molten rock is on the surface of the Earth, it is known as lava. Magma is the term for molten rock that has not yet erupted from a volcano or fissure.