Magma rises to the surface because it is less dense than the surrounding rocks and a structural zone in the Earth's crust enables movement. When it reaches the surface, magma is referred to as lava. Magma may collect under the surface in an area called a magma chamber until it cools and forms igneous rock, or it may escape through the surface in a volcanic eruption.
Magma is comprised of a mixture of molten rock, dissolved gases and crystals. Temperatures can range between 1300 and 2400 degrees Fahrenheit depending upon the type of magma. There are three general magma types: basaltic, andesitic and rhyolitic. The three magma types reflect their chemical composition.
The gases dissolved in magma are primarily water vapor with some carbon dioxide and minor amounts of chlorine, sulfur and fluorine gases. Rhyolitic magma tends to contain a greater amount of gas than basaltic or andesitic magma. The gases present in magma expand in relation to the decrease in pressure within magma rising closer to the surface.
The explosive eruptions of magma through the Earth's surface occur when the rising magma contains a high gas content and has a low viscosity. Exploding bubbles within the magma reaching the surface can propel clots of lava through the air that cool and solidify as they fall. The larger solidified chunks of magma are called pyroclasts, and the smaller and less destructive particles are referred to as volcanic ash. The most destructive form of magma eruption is the type that results in a rush of lava flow down the flanks of a volcano.