What Are Stratovolcanoes?

Stratovolcanoes are also referred to as composite cones. The base of their slope is gentle but they rise steeply at the summit. They appear to be concave in an upward direction. Stratovolcanoes tend to be the most beautiful in appearance but they are also the most deadly.

How stratovolcanoes form:

Stratovolcanoes will typically form where plates converge and one plate descends below an adjacent plate. Stratovolcanoes are built from layers of alternating lava flow. These layers can be pyroclastic flows, airfall tephra, volcanic mudflows or debris flows. This layering is what makes them a composite cone. Stratovolcano eruptions can differ with basaltic flows, andesitic lava, dactic lava or combinations of these flows.

Stratovolcanoes will usually have a small center crater but when extremely large eruptions occur the resulting crater can be larger. They will also usually have a lava dome in the center due to thick lava that has been pushed upward but was unable to flow.

Type of eruption:

The eruptions of most stratovolcanoes are Plinian eruptions and highly explosive. They have deadly pyroclastic flows of hot fragments and toxic gases. The lava advances at great speed down the slopes of the volcano. The lava can reach a speed of over 100 miles per hour and the lava can be over 1000 F. Stratovolcanoes differ from shield volcanoes as their eruptions are infrequent and they may go several hundred years between eruptions.

The explosive nature of stratovolcanoes comes from the amount of silica in the magma. The more silica the magma has, the thicker it becomes and the magma traps more gases. As the magma rises, the pressure lowers and the gases expand. Once the gas can not expand any further its internal pressure increases. When the pressure finally releases the gases and magma explode outward.

Examples of stratovolcanoes:

Subduction related stratovolcanoes are found in many areas but are prevalent along the Pacific Rim. These regions are known as the Ring of Fire. Mt. Mayon in the Philippines is a stratovolcano. Mt. Fuji in Japan, Mt. St. Helens in Washington state and Mt. Agua in Guatemala are a well.

Stratovolcanoes also commonly have smaller cinder cones along their flanks and may have multiple vents. The extreme pressure and consequent eruption of stratovolcanoes has actually been known to completely blow up the volcanic mountain leaving just the root remaining. Eruptions of stratovolcanoes can also create tsunamis from the tremendous release of energy. This effect occurred in Krakatoa in 1883 and 36,000 people on the nearby islands of Sumatra and Java lost their lives.

References:

http://www.msstate.edu/dept/GeoSciences/CT/TIG/WEBSITES/LOCAL/Summer2003/Nelson_Angela/msh.html

http://www.geology.sdsu.edu/how_volcanoes_work/stratovolc_page.html

http://www.extremescience.com/stratovolcanoes.htm