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Types of Magma
Three basic types of magma exist, defined by their different elemental makeups. Basaltic magma has the highest concentrations of iron, calcium and magnesium and a low concentration of potassium and sodium. Andesitic magma contains an intermediate amount of elements, while rhyolitic magma contains the lowest amounts, except for a higher presence of potassium and sodium.
Magma and Gas
All magmas contain dissolved gases, and the gases must be present to cause an explosive eruption. Gas confined in an area, such as a magma chamber, expands as it heats up and mixes with other elements. Gas expands in a low-pressure environment. Gas compositions include the combination of primarily water and vapor and some carbon dioxide. Other elements include sulfur, fluorine and chlorine. Slight changes in gas composition exist in some magma, such as rhyolitic magma, which has a higher gas volume than basaltic magma.
Magma Temperature
A threshold temperature of the different types of magma must exist to cause an explosive eruption. The temperatures measured by scientists are approximate because measuring the temperature sources can be risky. Basaltic magma must reach 1,800 to 2,200 degrees. Andesitic magma erupts at 1,500 to 1,800 degrees, while rhyolitic magma must reach 1,200 to 1,500 degrees.
Viscosity
Viscosity defines the thickness of magma, or its resistance to flow. Magma thickness depends on its composition and temperature. Magma that contains higher silica content has higher viscosity, as well as lower temperature magma types. The lower the temperature, the higher the viscosity and more resistance to flow. Higher temperatures move magma faster, such as with basaltic magma. Rhyolitic and andesitic magmas, even though they might reach their temperature thresholds, have higher viscosity and flow more slowly. Viscosity has a direct impact on the explosive nature of magma.
Eruption Phase
The eruption phase begins when the magma reaches a depth or pressure where the dissolved gas separates from the magma and forms its own phase. The gas creates bubbles that expand with increased pressure and temperature. The gas grows in volume with the reduced pressure, until it finds a release point (fracture or channel) where it can escape. The sudden change in pressure excites and expands the bubbles further, causing an explosive release when they breach the surface. An eruption depends on the temperature, type of rock material, amount of gas present and viscosity. Magma that exits to form on the Earth's surface is called lava.