When magma erupts at the surface it can form different types of volcanoes depending on the viscosity, or stickiness, of the magma, the amount of gas in the magma, and the way in which the magma reached the surface.
Different types of volcanoes include stratovolcanoes, shield, fissure vents, spatter cones and calderas.
These volcanoes are majestic giants with steep sides and a symmetrical cone shape. They form from very thick, viscous, or sticky, lava that won't flow easily. The lava therefore builds up around the vent forming a volcano with steep sides – we call this a stratovolcano and it has a familar triangular shape.
Because the magma is so viscous, gas can't leave the magma, therefore, when the magma rises to the surface the gas pressure builds up inside the volcano, resulting in an explosive eruption.
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.
Mauna Kea and Mauna Loa are shield volcanoes. They are the world's largest active volcanoes, rising nearly 9 km above the sea floor around the island of Hawaii.
As magma rises, it will find the easiest route to reach the surface. If it rises up through a long fracture, fountains of lava can form a ‘curtain of fire’ which we call a fissure eruption. The Eyjafjallajökull eruption in 2010 began with a fissure eruption on the side of the volcano.
When magma contains too much gas to form an effusive lava flow, but too little to form an explosive eruption, it erupts from the volcano as blobs of magma which fall close to the vent forming a low, steep-sided cone that we call a spatter cone.
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. An example of a caldera is Yellowstone in North America.