3.2 The Nature of Magma (Molten Lava), a Mixture of Liquids, Solids and Gases

Magma is completely or partly molten material, which on cooling solidifies to form an igneous rock. Most magmas consist of three distinct parts, a liquid component, a solid component, and a gaseous phase.

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The liquid portion, called melt, is composed of mobile ions of those elements commonly found in Earth’s crust. Melt is made up mostly of ions of silicon and oxygen along with lesser amounts of aluminum, potassium, calcium, sodium, iron, and magnesium.

The solid components, if any, in magma are silicate minerals that have crystallized from the melt. As a magma body cools, the size and number of crystals increases. During the last stage of cooling, a magma body is mostly a crystalline solid with only minor amounts of melt.

The gaseous components of magma, called volatiles, are materials that will vaporize, or form a gas, at surface pressures. The most common volatiles found in magma are water vapor, H2O, carbon dioxide, CO2, and sulfur dioxide, SO2, which are confined by the immense pressure exerted by the overlying rocks. These gases tend to separate from magma as it moves toward the surface, a low-pressure environment, where they may generate a steam eruption. Further, when deeply buried magma bodies crystallize, the remaining volatiles form hot, water-rich fluid that migrates through the surrounding rocks. These hot fluids play an important role in metamorphism.

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