Dust Storms mudflows Processes such as these are constantly acting upon and within the Earth to change it. Many of these processes are cyclical in nature. Rain comes from clouds - falls on surface, picks up sand, silt and clay, carries particles to river and into ocean. Water then evaporates to become clouds, which move over continents to rain again. Most surface rocks started out as igneous rocks- rocks produced by crystallization from a liquid.
Erosion moves particles into rivers and oceans where they are deposited to become sedimentary rocks.
Sedimentary rocks can be buried or pushed to deeper levels in the Earth, where changes in pressure and temperature cause them to become metamorphic rocks. 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.