Main Types of Volcanoes

Volcanoes are mounds on the surface of the earth with a conduit that connect them to the inside of the earth and through which accumulations of magma, vapor and other gases flow up from underneath the earth´s crust. Once magma reaches the surface through the volcano conduit it becomes solid and gives shape to a volcano.

The magma flows through cracks in the earth´s crust at the boundaries of tectonic plates; therefore, volcanoes usually form where tectonic plates diverge or converge. Not all volcanoes are formed the same way, as the lava viscosity and the power of the explosion determines their shape.

Volcanoes exist all over the surface of the earth, but the majority of them lie along the boundaries of the largest tectonic plate (Pacific Plate, with over 40 million square miles (100 million square km). More than 70% (nearly 450) of the earth´s active volcanos are situated here and the largest eruptions have taken place in this region too.

There are four distinct types of volcanoes; they have been classified based on the form they take after thousands of eruptions, which is what finally gives them their shape. Some volcanoes are hundreds of miles in diameter and very tall, while others are just a few km wide and very short. One recognizable feature is a crater that is formed at their top.

Volcanoes are distinguishable by their composition and structure and they may take different forms due to the varied material that erupts from them; some may grow huge and tall, while others may not. Most volcanoes fall into the following types of volcanoes; stratovolcanoes or composite volcanoes, shield volcanoes, cinder volcanoes and lava dome volcanoes.

Magma is a combination of of molten rock and chemical compounds that develops below the earth's Surface at temperatures that range from 1,300°- 2,400° F (700°-1,300° C). Magma forms in areas known as magma chambers and may remain there until it cools and solidifies forming igneous rock.

It may occur that the magma does not cool inside the magma chamber, but finds its way to the Surface of the earth through a crack in the crust of the earth.

Magma can remain underneath the earth´s crust for a long time, but if the pressure and surrounding rocky environment allow, magma may rise to the surface, finding the easiest way out, which is through a volcano crater. Once magma erupts through a volcano crater, it may do it explosively or gently, depending on factors, including silica (SiO2) content, temperature and gases. Highly viscous (felsic) lava may produce very explosive eruptions, sending volcanic material very high into the sky. Less viscous lava (andesitic or mafic or even ultramafic) produce less explosive eruptions.

Composite or stratovolcanoes are the tallest types of volcanoes; they are formed by layers of felsic lava and ash and magmatic material. They often get a canonical shape after several layers build on top of the other over several eruptions. These types of volcanoes eruptive lava is highly viscous; therefore when it flows out of the vent it cools and hardens before spreading large distances.

Stratovolcanoes are also called composite volcanoes because of the way in which each layer of hardened lava builds on top of another to give them their characteristic shape. The ash, cinder and other material spreads around the vent, then the lava that is erupted flows on top of it and cools and hardens, forming a solid layer that will serve as the foundation on which the next layer will build up. They may develope slopes of anywhere from 6-10° at the flanks, increasing to 30-40° until reaching the top. They are most common at subduction zones-where oceanic plates submerge under continental plates.

These are the largest volcanoes on the planet by extension at the base and they get their name because of the resemblance their structure has to that of a warrior´s shield. The lava that forms them is highly fluid or with low viscosity and can travel a greater distance forming broad sheets of lava that spread in all directions to give shape to a dome that looks like a shield; consequently they don´t develop steep slopes, like those of stratovolcanoes.

Their lava is principally mafic (highly fluid lava) that may erupt continuously over long periods of time, building a gentle slope 2.3 degrees each time that adds to the shape of the volcano. When totally formed, they may have developed slopes, ranging from 7-10°. There may be lavas erupting through fractures on the flanks of these volcanoes. Eruptions on these volcanoes are less explosive than those on stratovolcanoes and that relates to their silica (SiO2) temperature and low gas content.

The eruptions of shield volcanoes could become explosive if their lava comes in contact with water. Kilauea volcano, in the pic, is a shield volcano located in Hawaii. One of the largest volcanoes of this type is Mauna Loa in Hawaii with 120 km (75 miles in diameter and rising 4.16 km (13,678 ft.) above sea level.

The following is a list of other types of volcanoes that are very rare or are not continental and have been classified independently; however, they may also fall into the classification of some of the volcanoes mentioned here.

Cinder cones are steep sided mounds with a bowl shaped crater at the summit and that are nearly circular at the base; they develop slopes that are between 30-45 degrees. They are made of loose pyroclastic fragments that pile up around a vent, caused by explosions of lava fountains. When lava is erupted from the vent. It is sent into the air, where it explodes and breaks into pieces; the larger pyroclasts fragments gather close to the vent, while the smaller fragments settle farther away.

Lava rarely erupts through the top of the cinder crater, instead, lava makes its way through the lighter cinders at the base, creating lava flows that harden over time. Cinder volcanoes usually form in the whereabouts of shield volcanoes, stratovolcanoes and calderas. They are the smallest volcanoes, ranging from tens to hundreds of meters tall. Scientists have located 100 cinder cones on the flanks of Mauna Kea in Hawaii.

These volcanoes originate from highly viscous masses of dacite or rhyolite lava that move at a speed of only a few meters per hour. Their high viscosity prevents it from flowing far from the vent. They usually grow by internal intrusion of lava, causing swelling and steep-sided monoliths. They may begin to take their characteristic shape over the span time of just a few months or even years. Over time, the outer surface of the dome dries and breaks into pieces that slide on its sides.

These types of volcanoes regularly form within the craters of stratovolcanoes; however, they may also occur on the flanks of them. The may grow to several hundred meters high and thousands across; one of the biggest domes; (2000 ft. high and 2 miles in diameter), is the one at the summit of Lassen Peak in California. Some domes may take the shape of spines or monoliths of viscous lava that form around, while others form coulees (solid canyons). Recently formed domes may still contain lava that is not completely degasses and a sudden explosion may lead to pyroclastic flows (water vapor, hot gas and volcanic material).