Magma

Magma is the parent material of all igneous rocks. While often used interchangeably with “lava,” the distinction is simple but significant: magma exists underground, while lava is magma that has breached the surface. This subterranean molten rock acts as the engine for volcanoes and is a key driver of the rock cycle.

The Three Components of Magma

Magma is rarely just a simple liquid. It is a complex multiphase substance composed of:

1. The Melt: The liquid portion, made of mobile ions of common elements like silicon, oxygen, aluminum, potassium, and calcium.
2. Solids: Mineral crystals that have begun to freeze out of the melt as it cools.
3. Volatiles: Dissolved gases that remain trapped in the liquid at high pressure. The most common are water vapor (H₂O), carbon dioxide (CO₂), and sulfur dioxide (SO₂). When magma rises, these volatiles expand, driving explosive eruptions.

Physicochemical Properties

• Temperature: Magma temperatures range from roughly 700°C for high-silica rhyolites to over 1,300°C for low-silica basalts.
• Viscosity: This measures the magma’s resistance to flow. High-silica magma is polymerized (sticky) and flows poorly, while low-silica magma is fluid. Viscosity is the primary factor determining whether a volcano erupts effusively (flows) or explosively (ash).

How Magma Forms

Contrary to popular belief, the Earth’s mantle is not a liquid ocean of magma; it is solid rock that flows very slowly over geological time. Magma only forms under special conditions:

1. Decompression Melting: Occurs at divergent boundaries (mid-ocean ridges). As mantle rock rises toward the surface, the pressure drops faster than the temperature, allowing the rock to melt.
2. Flux Melting: Occurs at subduction zones. Water released from a sinking tectonic plate rises into the hot mantle wedge above. This water lowers the melting point of the mantle rock, much like salt melts ice on a road.
3. Heat Transfer: Very hot basaltic magma rising from the mantle can pool at the base of the continental crust, melting the surrounding rock to create new, silica-rich magmas.

Magma Evolution

Magma rarely stays the same composition from source to surface. It changes through Magmatic Differentiation:

• Fractional Crystallization: As magma cools, high-temperature minerals (like olivine) crystallize first and sink. This removes magnesium and iron from the melt, leaving the remaining liquid richer in silica.
• Assimilation: Magma can melt and incorporate the surrounding country rock as it rises.
• Mixing: Two different magma bodies may meet and mix in a chamber, creating a hybrid composition.

Chemical Classification

Magma is classified by its silica (SiO₂) content:

• Mafic (Basaltic): ~50% silica. Hot, fluid, and dark-colored. Examples include the lavas of Hawaii and Iceland.
• Intermediate (Andesitic): ~60% silica. Explosive and common in stratovolcanoes like Mt. St. Helens.
• Felsic (Rhyolitic): >70% silica. Cool, sticky, and light-colored. These form domes and supervolcanoes.

Planetary Perspective

Magmatism is not unique to Earth. The dark “seas” (maria) on the Moon are vast plains of ancient basaltic magma. Io, a moon of Jupiter, is the most volcanically active body in the solar system, erupting ultra-hot silicate magmas driven by tidal heating. Conversely, icy moons like Enceladus may produce “cryomagma”—slushy water and ammonia—proving that magma is a state of matter defined by its environment.