A volcano is an opening in the surface of a planet (or moon) that allows hot material to escape from the magma chamber below the surface. When the hot material, such as lava, ash, and gas make their way to the surface, the volcano erupts. The style of eruption depends on the type of magma. Eruptions can be explosive, sending hot mixtures of ash and gas high into the sky; or they can be calm, only sending small amounts of lava down the slope.
Several factors determine the explosiveness of a volcanic eruption. These include: dissolved gases, water vapor, temperature, and composition. The composition determines the viscosity, or the resistance to flow, of the magma. Magmas with a high viscosity are thick and slow moving, and have a high silica content. Low viscosity magmas are thin and runny, and have a low silica content.
The three types of magma are:
There are three ways the magma can make it to the surface:
Subduction zones, mid-ocean ridges, and hot spots. Image modified from Nasa SpacePlace.
1) Subduction When tectonic plates bump into each other (converge) one of the tectonic plates can be pushed under another one deep into the Earth under the crust (called subduction). The tectonic plate that was forced into the Earth then melts from the high temperature and high pressure and can eventually rise to the surface as magma and form a volcano from the melted crust. The big volcanoes in Washington are formed this way.
2) Mid-Ocean Ridges When tectonic plates move away from each other (diverge) the hot, buoyant magma beneath the crust rises to fill the space. This typically happens in oceanic crust underwater and forms “black smokers”.
3) Hot Spots The third way that volcanoes can form is at a hot spot inside the earth. Scientists are still figuring out exactly why hot spots happen where they do, but the basic idea is that magma rises and pushes its way to the surface through the tectonic plate. Yellowstone and the Hawaiian islands are two famous examples of hot spot volcanoes.