Magma

Magma is the molten or semi-molten material found beneath the Earth's surface.

It serves as the essential source for all igneous rocks. While people often use the term lava, that name actually refers to magma only after it has been extruded onto the surface. Evidence of magmatism has also been discovered on other terrestrial planets and some natural satellites. Beyond just liquid rock, magma often contains suspended solid crystals and gas bubbles. This mixture of solid, liquid, and gas phases is common as magma approaches the surface and pressure drops.

Magma is produced by the melting of the mantle or the crust in various tectonic settings. On Earth, these settings include subduction zones, continental rift zones, mid-ocean ridges, and hotspots. Once formed, these melts migrate upward through the crust. They are often stored in magma chambers or trans-crustal crystal-rich mush zones. During this storage period, the magma's composition can change through several processes. These include fractional crystallization, where crystals form and separate, and contamination with existing crustal melts. Magma can also change through magma mixing or degassing, which is the loss of dissolved gases.

Most silicate magmas are dominated by oxygen and silicon. These are the two most abundant chemical elements in the Earth's crust. Other elements like aluminium, calcium, magnesium, iron, sodium, and potassium are also present. Petrologists, who study rocks, often describe magma composition by the mass fraction of various oxides. The amount of silica in the melt is especially important. It directly influences the temperature and the viscosity, which is the thickness or resistance to flow of the liquid.

Scientists classify silicate magmas into four distinct chemical types based on their silica content. Felsic or silicic magmas contain more than 63% silica. These include rhyolite and dacite magmas. Because of the high silica, they are extremely viscous. At 700 °C, rhyolite magma can have a viscosity of 10^8 cP, while cooler rhyolite can reach 10^11 cP. For comparison, water has a viscosity of only about 1 cP. This high viscosity often leads to explosive eruptions. However, they can sometimes erupt effusively to form lava domes or thick, short flows called coulees.

Intermediate magmas, such as andesite, contain between 52% and 63% silica. These magmas are typically hotter than felsic ones, often ranging from 800 °C to 1,200 °C. They are also lower in aluminium but usually richer in magnesium and iron. Their viscosity is lower than felsic magma, typically around 3.5 million cP at 800 °C. This is slightly thicker than smooth peanut butter. Intermediate magmas often show a tendency to form phenocrysts, which are visible crystals within the rock. These crystals often include amphibole or pyroxene.

Mafic or basaltic magmas have a silica content between 45% and 52%. They are characterized by high ferromagnesian content and typically erupt at temperatures between 1,000 °C and 1,200 °C. Their viscosity is relatively low, between 10^4 and 10^5 cP. This is similar to the thickness of ketchup. Because they are fluid, basaltic lavas can flow long distances. This often creates low-profile shield volcanoes or wide flood basalts. In underwater environments, they can form structures known as pillow lavas.

Ultramafic magmas contain less than 45% silica and represent an extreme in composition. An example is komatiite, which contains over 18% magnesium oxide. These magmas are thought to have erupted at incredibly high temperatures, sometimes exceeding 1,600 °C. At these temperatures, the liquid is highly mobile with a viscosity as low as 100 to 1,000 cP, similar to light motor oil. While these were common in the past, no modern komatiite lavas are known today. This is because the Earth's mantle has cooled too much to produce such highly magnesian magmas.

Studying magma is difficult because it is hidden deep underground. Most scientific observations occur after magma transitions into lava flows. However, scientists have encountered magma in situ, or in its original place, only three times during geothermal drilling. Two of these instances occurred in Iceland, and one occurred in Hawaii. These rare moments provide direct data on the physical and chemical properties of molten rock before it reaches the surface.