You may first think of fiery orange eruptions when you think of magma, but the different types of magma behave differently. Three types of magma exist on Earth today, and an extinct fourth type is unable to form on our cooled planet. Each magma type has a different chemical make-up, though the types are all mostly made up of silica and other minerals. Underground pressure builds up to push magma to the surface, sometimes in a huge explosion and sometimes in a slow-moving trickle.
Basaltic magma is made up of 45 to 55 percent silica. It is high in iron, magnesium and calcium and is low in potassium and sodium. The temperature of basaltic magma is around 1,000 to 1,200 degrees Celsius, based on laboratory measurement and limited field operations. Scientists find it difficult to determine an exact temperature because it is dangerous to get close enough to magma to measure it. Basaltic magma is the most fluid of the three types of magma, though it is still 10,000 to 100,000 times less fluid than water. It also has the lowest amount of gas of the three types. Basalt magma is found at Earth's "hot spots," such as Hawaii. When it cools, basaltic magma becomes basalt rock.
Andesitic magma is the "middle" magma in terms of heat; fluidity, or speed of flow; and gas content. It is probably 800 to 1,000 degrees Celsius. It is made up of 55 to 65 percent silica with average amounts of iron, magnesium, calcium, potassium and sodium. When cooled, andesitic magma becomes andesite rock. Andesitic magma is somewhat explosive, coming to the crust of the Earth at reverse fault areas. In these spots, two plates in the Earth's crust move against each other in opposite directions, and the top piece moves up toward the surface rather than toward the planet's center. Small- to medium-sized earthquakes also release andesitic magma. An example of andesitic magma is the eruption of Mount St. Helens in Oregon.
Felsic-type magmas include ryolitic and dacite magmas, which are very thick and slow-moving. Felsic magma seems to form when the Earth's crust melts with seawater. It is highly explosive when erupting from volcanoes. Felsic magma is made up of 65 to 75 percent silica. It is low in iron, magnesium and calcium and is high in potassium and sodium. Compared to other types of magma, felsic magma is cool, reaching 650 to 800 degrees Celsius, and is high in volatile, or unstable, gases and melted rocks. Felsic magma forms huge calderas, like the Yellowstone caldera. When cooled, it forms ryolite or granite rock.
Today, our planet is too cool for ultramafic, or komatiite, magma to form. This is probably a good thing, since ultramafic magma would be the hottest and fastest-flowing of the magmas, running almost as fast as water down a volcano. In the distant past, ultramafic lava could reach as high as 1,600 degrees Celsius. Now lava rarely reaches above 1,200 degrees.
- Tulane University Department of Earth & Environmental Sciences: Volcanos, Magma and Volcanic Eruption
- Geomania: Magma Formation
- "The Changing Earth: Exploring Geology and Evolution"; James S. Monroe and Reed Wicander; 2008
Nadia Nygaard has been writing and editing since 2005. She is published in "Farm and Ranch Living" and has edited projects as diverse as grant proposals, medical dissertations and tenant law handbooks. She is a graduate of the University of Washington with a Bachelor of Arts in English and women's studies.