When magma erupts at the surface, as lava, it can form different types of volcanoes depending on the viscosity, or stickiness, of the magma, the amount of gas in the magma, the composition of the magma and the way in which the magma reached the surface. Strictly speaking there are two broad types of volcano – a stratovolcano and a shield volcano, although there are lots of volcanic features that can form from erupted magma (such as cinder cones, or as lava domes) as well processes that shape volcanoes. In this section you can find out the difference between stratovolcanoes and shield volcanoes as well as lava domes and calderas.
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Viscosity is important in volcanology. An eruption of highly viscous magma tends to produce steep-sided volcanoes with slopes that are about 30-35 degrees. That’s because the viscous volcanic material doesn’t flow that far from where it is erupted, so it builds up in layers forming a cone-shaped volcano known as a stratovolcano. Shield volcanoes, on the other hand, have gentle slopes that are less than 10 degrees and erupt more fluid lavas called basalt. When a shield volcano erupts the basalt can flow great distances away from the vent to produce broad, gentle slopes.
Where a volcano produces low viscosity, runny, lava it spreads far from the source forming a volcano with gentle slopes. This type is called a shield volcano. Most shield volcanoes are formed of fluid basaltic lava flows. Mauna Kea and Mauna Loa are shield volcanoes. They are the world’s largest active volcanoes, rising over 9 km above the sea floor around the island of Hawaii.
Stratovolcanoes have relatively steep sides and are more cone-shaped than shield volcanoes. They are formed from viscous, or sticky, lava that does not flow easily. The lava therefore builds up around the vent forming a volcano with steep sides. Stratovolcanoes are more likely to produce explosive eruptions due to gas building up in the viscous magma.
Andesite (named after the Andes Mountains), is perhaps the most common rock type of stratovolcanoes, but stratovolcanoes also erupt a wide range of different rocks in different tectonic settings.
The Soufrière Hills volcano, on the Caribbean island of Montserrat, is well known for its lava dome complex at the summit of the volcano, which has gone through phases of growth and collapse. As viscous lava is not very fluid, it cannot flow away from the vent easily when it is extruded. Instead it piles up on top of the vent forming a large, dome-shaped mass of material.
Magma is stored beneath a volcano in a magma chamber. When a very large explosive eruption occurs which empties the magma chamber, the roof of the magma chamber can collapse forming a depression, or bowl on the surface which has very steep walls. These are calderas and can be tens of miles across. Calderas could be volcanoes formed during an eruption that removes the summit of a single stratovolcano. Caldera-forming eruptions can remove massive portions of a single stratovolcano. The top can literally be blown off!