Volcanoes
There are two main types of volcanoes there are the violet explosive strato-volcano (acid cone) and the less violent shield (fissure) volcano.
A volcano is an opening, or rupture, in a planet's surface or crust, which allows hot magma, ash and gases to escape from below the surface.
Volcanoes are generally found where tectonic plates are diverging or converging. A mid-oceanic ridge, for example the Mid-Atlantic Ridge, has examples of volcanoes caused by divergent tectonic plates pulling apart; the Pacific Ring of Fire has examples of volcanoes caused by convergent tectonic plates coming together.
By contrast, volcanoes are usually not created where two tectonic plates slide past one another. Volcanoes can also form where there is stretching and thinning of the Earth's crust (called "non-hotspot intraplate volcanism"), such as in the East African Rift, the Wells Gray-Clearwater volcanic field and the Rio Grande Rift in North America.
Volcanoes can be caused by mantle plumes.
These so-called hotspots, for example at Hawaii, can occur far from plate boundaries. Hotspot volcanoes are also found elsewhere in the solar system, especially on rocky planets and moons.
Shield Volcanoes:
Shield volcanoes, so named for their broad, shield-like profiles, are formed by the eruption of low-viscosity lava that can flow a great distance from a vent, but not generally explode catastrophically. Since low-viscosity magma is typically low in silica, shield volcanoes are more common in oceanic than continental settings. The Hawaiian volcanic chain is a series of shield cones, and they are common in Iceland, as well. The shield volcanoes are often much hotter than the stratovolcanoes with an average tempreture of 1,200 degrees C.
Stratovolcanoes:
Stratovolcanoes or composite volcanoes are tall mountains composed of lava flows and other ejecta in alternate layers, the strata that give rise to the name. Stratovolcanoes are also known as composite volcanoes, created from several structures during different kinds of eruptions. Strato/composite volcanoes are made of cinders, ash and lava. Cinders and ash pile on top of each other, lava flows on top of the ash, where it cools and hardens, and then the process begins again. Classic examples include Mt. Fuji in Japan, Mayon Volcano in the Philippines, and Mount Vesuvius and Stromboli in Italy.
In recorded history, explosive eruptions by stratovolcanoes have posed the greatest hazard to civilizations, as ash is produced by an explosive eruption. No supervolcano erupted in recorded history. Shield volcanoes have not an enormous pressure build up from the lava flow. Fissure vents and monogenetic volcanic fields (volcanic cones) have not powerful explosive eruptions, as they are many times under extension. Stratovolcanoes (30-35°) are steeper than shield volcanoes (generally 5-10°), their lose tephra are material for dangerous lahars. The average tempreture of a stratovolcano is 800 degrees C.
Volcano Prediction:
Volcanoes give off many warnings, such as animal and bird movement, melting snowcaps, and crate lakes drying up. However, more efficient and effective detection methods related to the inner workings of an active volcano have been developed. The most common of these instruments are tiltmeters and seismographs.
Case Study: Mount St Helens
Mount St Helens Washington State, NW USA is located in the Cascade mountain range and prior to its eruption in 1980 it had been active for over 100 years. The volcano sits on a destructive boundary where the Juan de Fuca plate meets the North American plate.
Mount St Helens erupted on May 18th 1980 following a period of activity which began in March 1980 with an earthquake measuring 4.0 on the richter scale. What followed was 3 months of seismic activity as magma rose within the mountain. As the magma rose, a large bulge grew on the north flank of the volcano, this was due to a blockage in the main vent resulting in the growth of a cryptodome (mound of viscous lava) in the side of the volcano.
On May 18th, an earthquake measuring 5.1 on the richter scale caused a landslide on the northern flank of the volcano, which in turn exposed the cryptodome below, resulting in a sudden release of pressure and a cataclysmic eruption in the form of a lateral (sideways) blast. The blast zone consisted of 230 square miles with the eruption leaving a 'lunor' landscape in its wake.
