]> Moderate to violent ejection of solid or very viscous hot fragments of new lava in short-lived, cannon-like bursts. Ash and fine ash are emitted with gases and ascend to form a cauliflower-like eruption cloud. In geology, geothermal refers to heat sources within the planet. Strictly speaking, geo-thermal necessarily refers to the Earth but the concept may be applied to other planets. Geothermal is technically an adjective (e.g., geothermal energy) but in U.S. English the word has attained frequent use as a noun (otherwise expressed as g. heat, g. source, or geotherm). The planet's internal heat was originally generated during its accretion, due to gravitational binding energy, and since then additional heat has continued to be generated by the radioactive decay of elements such as uranium, thorium, and potassium. The heat flow from the interior to the surface is only 1/20,000 as great as the energy received from the Sun. Peleean eruption is characterized by explosions of moderate to extreme violence in which solid or viscous hot fragments of new lava are ejected -- commonly as pyroclastic fallout and pyroclastic flows. These eruptions are usually associated with silicic magmas. Phreatic eruption is an explosion that follows the transformation of groundwater into steam. No incandescent or juvenile material is erupted. Plinian eruptions are characterized by paroxysmal ejection of large volume of ash and pumice as a well-defined eruption column or "jet;" often precedes caldera collapse. The resulting tephra fallout covers an area of more than 500 square kilometers. Weak-to-violent, sporadic, ballistic eruptions of tephra, generally of mederately fluid basaltic or andesitic magma. A hydrothermal vent is a fissure in a planet's surface from which geothermally heated water issues. Hydrothermal vents are commonly found near volcanically active places, areas where tectonic plates are moving apart, ocean basins, and hotspots.