Stratovolcanoes form through explosive eruptions depositing material near a central vent. Strombolian eruptions are relatively minor eruptions of lava and tephra lasting a short time. Plinian eruptions are the largest eruptions at stratovolcanoes. They produce massive ash clouds rising high into the atmosphere. The magma forming stratovolcanoes rises when water trapped both in hydrated minerals and in the porous basalt rock of the upper oceanic crust is released into mantle rock of the asthenosphere above the sinking oceanic slab. More-viscous (less-fluid) lava both impedes the escape of gases and can clog the vent or “throat” of a volcano, in both cases ramping up the pressure and leading to more explosive eruptions that may surge forth at velocities exceeding 1, 000 miles per hour. Usually constructed over a period of tens to hundreds of thousands of years, stratovolcanoes may erupt a variety of magma types, including basalt, andesite, dacite, and rhyolite. All but basalt commonly generate highly explosive eruptions.
Examples of strato volcanoes include Mt. St. Helens, Mt. Rainier, Pinatubo, Mt.
Volcanic eruptions do not occur just anywhere. Sixty percent of all active volcanoes are found at crustal plate boundaries such as the Pacific Plate, which has become known as the Ring of Fire because of the active volcanoes on its perimeter.
This lava plugs up the plumbing in stratovolcanoes, allowing them to build up tremendous amounts of pressure. Of all the volcanoes on Earth, stratovolcanoes are the most dangerous. They can erupt with little warning, releasing enormous amounts of material. And they don't always erupt nicely from their tops.
There are three main types of volcano - composite or strato, shield and dome. Composite volcanoes, sometimes known as strato volcanoes, are steep sided cones formed from layers of ash and [lava] flows. Eruptions are explosive due to the thick, highly viscous lava that is produced by composite cone volcanoes.
A pyroclastic flow (also known as a pyroclastic density current or a pyroclastic cloud) is a fast-moving current of hot gas and volcanic matter (collectively known as tephra) that moves away from a volcano about 100 km/h (62 mph) on average but is capable of reaching speeds up to 700 km/h (430 mph).
The time period between eruptions depends on how fast the rock melts, which is influenced by the speed of the sinking plate. The Earth has several subduction zones and the subducting plates generally move at a constant speed of up to 10 centimetres per year.
Composite Volcanoes. Composite volcanoes, sometimes called stratovolcanoes, are typically deep-sided, symmetrical cones of large dimension built of alternating layers of lava flows, volcanic ash, cinders, blocks, and bombs and may rise as much as 8, 000 ft above their bases.
Not surprisingly, supervolcanoes are the most dangerous type of volcano. Supervolcanoes are a fairly new idea in volcanology. The exact cause of supervolcano eruptions is still debated, however, scientists think that a very large magma chamber erupts entirely in one catastrophic explosion.
The two main factors that influence how a volcano will erupt are viscosity and gas content. The magma is very sticky and resists the expansion of the gas bubbles. Ultimately, enough bubbles grow and expand to blow the magma into ash size fragments and eject them violently into the atmosphere.
Different types of volcanoes include stratovolcanoes, shield, fissure vents, spatter cones and calderas.
Rhyolite caldera complexes
Mount St. Helens, Washington, is the most active volcano in the Cascade Range. Its most recent series of eruptions began in 1980 when a large landslide and powerful explosive eruption created a large crater, and ended 6 years later after more than a dozen extrusions of lava built a dome in the crater.
A supervolcano is a large volcano that has had an eruption with a Volcanic Explosivity Index (VEI) of 8, the largest recorded value on the index. This means the volume of deposits for that eruption is greater than 1, 000 cubic kilometers (240 cubic miles).