Hydrothermal Explosions Yellowstone's volcanic and hydrothermal history suggests the potential for various kinds of eruptions in the future. The likelihood of a certain type of eruption occurring in the future can be judged by how often eruptions have occurred in the past.
Magma: As already noted, both lava and magma are what results from rock superheated to the point where it becomes viscous and molten. But again, the location is the key. When this molten rock is still located within the Earth, it is known as magma. The name is derived from Greek, which translate to “thick unguent” (a word used to describe a viscous substance used for ointments or lubrication).
What Is A Phreatic Eruption? When magma heats ground or surface water, it results in an explosion of water, steam, rock, and ash, called a phreatic eruption. A scheme of a phreatic eruption: 1: water vapor cloud, 2: magma conduit, 3: layers of lava and ash, 4: stratum, 5: water table, 6: explosion, 7: magma chamber.
Phreatomagmatic eruptions are volcanic eruptions resulting from interaction between magma and water. They differ from exclusively magmatic eruptions and phreatic eruptions. Unlike phreatic eruptions, the products of phreatomagmatic eruptions contain juvenile (magmatic) clasts.
Andesites formed either by mixing of basaltic and rhyolitic magmas, or by contamination of a more mafic parent by the continental crust commonly display textural evidence of disequilibrium (Eichelberger 1975) or field evidence of magma mixing or host rock assimilation (Wiebe et al.
Hawaiian eruptions get their names from the Kilauea Volcano on the Big Island of Hawaii, which is famous for producing spectacular fire fountains. Two excellent examples of these are the 1969-1974 Mauna Ulu eruption on the volcano's flank, and the 1959 eruption of the Kilauea Iki Crater at the summit of Kilauea.
The composition of magma driving most of plinian and subplinian eruptions is intermediate to silicic. From a compilation of 45 Pleistocene and Holocene plinian eruptions (Carey and Sigurdsson, 1989), about 80% have evolved composition (SiO 2 higher than 60 wt%).
Mercalli described vulcanian eruptions as "...Explosions like cannon fire at irregular intervals..." Their explosive nature is due to increased silica content of the magma. Almost all types of magma can be involved, but magma with about 55% or more silica (e.g. basaltic andesite) is most common.