An explosion is a rapid expansion of the volume associated with a very strong external release of energy, usually accompanied by the generation of high temperatures and the release of high pressure gases. Supersonic explosions caused by high explosives are known as blasts and are transmitted by shock waves. Subsonic explosions are caused by low explosives through a slower combustion process known as combustion. Explosions can occur in nature due to the large flow of energy. Most natural eruptions arise from volcanic or stellar processes of various types. [Explosive volcanic eruptions occur when magma rises from below, and has very dissolved gas in it. The pressure decreases as the magma rises and causes gas to exit the solution, which leads to a rapid increase in volume.] Explosions also occur as a result of impact events and in phenomena such as hydrothermal eruptions (also due to volcanic processes). Explosions can also occur outside of Earth in the universe in events such as supernovae. Explosions occur frequently during wildfires in eucalyptus forests as the volatile oils on the treetops suddenly burn.