At high temperatures metamorphic rocks may melt to become magmas . Magmas rise to the surface, crystallize to become igneous rocks and the processes starts over. External because erosion operates at the Earth's surface. The energy source for this process is solar and gravitational. Internal Processes Processes that produce magmas, volcanoes, earthquakes and build mountain ranges.
Energy comes from the interior of the Earth, Most from radioactive decay - nuclear energy.
Principle of Uniformitarianism Processes that are operating during the present are the same processes that have operated in the past. If we look at processes that occur today, we can infer that the same processes operated in the past. Mt. St. Helens, Size of earthquakes. Perhaps a better way of stating the Principle of Uniformitarianism is that the laws of nature have not changed through time. Thus, if we understand the physical and chemical laws of nature, these should govern all processes that have taken place in the past, are taking place in the present, and will take place in the future.