Extrusive igneous rocks

Magma that reaches the Earth's surface and flows as hot streams or is explosively blown out of a volcano is called lava. Lava has a range of compositions, a variety of high temperatures, and flows at various speeds.

The chemical composition of magma closely relates to how explosive and hazardous a volcanic eruption will be. The variation in the amount of silica (SiO2) in igneous rocks is used to describe the variation in composition of igneous rocks—and the magmas that formed them. Rocks with low amounts of silica (basalt, gabbro) are known as mafic rocks, whereas rocks with high concentrations of silica (rhy-olite, granite) are known as silicic or felsic rocks.

All magmas have a small amount of gas dissolved in them, usually comprising between 0.2-3 percent of the magma volume, and this is typically water vapor and carbon dioxide. The gases typically control such features as how explosive a volcanic eruption can be with greater abundances of gases leading to more explosive eruptions.

Magmas exhibit a wide range in temperatures. Measuring the temperature of an erupting volcano is difficult, since temperatures typically exceed 930°F (500°C) and melt most thermometers. Also, the volcano may explode, killing the people who try to measure its temperature. Therefore, temperature is measured from a distance using optical devices, yielding temperatures in the range of 1,900-2,200°F (1,040-1,200°C) for basaltic magma, and as low as 1,155°F (625°C) for some rhyolitic magmas.

Magmas can move downhill at very variable rates. For example, in Hawaii magma often flows downhill in magma streams at about 10 miles per hour (16 km/hr), destroying whole neighborhoods, whereas in other places it may move downhill so slowly as to be hardly detectable. At the other end of the spectrum some explosive volcanic ash clouds move downhill at speeds of several hundred miles (km) per hour, destroying all in their path. The measure of the resistance to flow of magma is called viscosity. The more viscous a magma, the less fluid it is. Honey is more viscous than water. The viscosity of magma depends on its temperature and composition. Higher-temperature magma such as basalt tends to have a higher fluidity (lower viscosity) than lower temperature magma such as rhyolite, explaining why basaltic flows tend to move over large distances, whereas rhyolitic magmas form large steep sided domes around the volcanic vent they erupted from. Magmas with more silica in them (like rhyo-lite) are more resistant to flow because the silica molecule forms bonds with other atoms (mostly oxygen), forming large chains and rings of molecules that offer more resistance to flow than magmas without these large interlocking molecules.

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