Magma is molten rock trapped within the Earth’s crust or mantle, typically at temperatures between 650°C and 1,200°C. While it is molten rock, magma is not a uniform fluid. This extreme heat causes rock to melt, forming a silicate mixture. Magma is a dynamic, high-temperature mixture that incorporates materials in three distinct physical states: liquid, solid, and gas.
The Multi-Phase Nature of Magma
Magma is a multi-phase substance, existing as a combination of liquid, solid, and gas components simultaneously. The liquid portion is known as the melt, primarily a molten mixture of oxygen and silicon, often termed silicate melt. This liquid serves as the matrix holding the other components in suspension.
The solid phase consists of various mineral crystals suspended within the melt. As magma cools during ascent or storage, certain minerals crystallize out of the liquid, creating a crystal-rich slurry. The proportion of these suspended crystals significantly impacts the material’s overall properties.
The gaseous phase is referred to as volatiles. These are substances like water vapor (\(\text{H}_2\text{O}\)), carbon dioxide (\(\text{CO}_2\)), and sulfur dioxide (\(\text{SO}_2\)) that are dissolved within the melt under immense pressure deep underground. High pressure keeps these gases in solution, similar to carbonation in a sealed bottle.
Factors Governing Magma Flow and Behavior
The physical behavior of magma, particularly its ability to flow, is described by its viscosity, which is its resistance to movement. Viscosity determines whether magma will produce a gentle, flowing eruption or a violent, explosive one. This resistance to flow is controlled by three primary factors that interact within the multi-phase mixture.
Temperature has a direct inverse relationship with viscosity. Magma at higher temperatures flows more easily because the heat breaks the bonds between atoms in the melt, allowing them to move past each other more freely. Cooler magma becomes more sluggish and sticky, which increases its viscosity.
The magma’s chemical composition, specifically its silica (\(\text{SiO}_2\)) content, is the second control. Silica molecules link together into complex chains and networks, a process called polymerization, which acts like internal friction. Magmas with high silica content, such as rhyolite, have a high degree of polymerization and are highly viscous, resisting flow. Conversely, magmas with lower silica content, like basalt, exhibit low viscosity, allowing them to flow more readily over long distances.
The third factor is the content of dissolved volatiles. Dissolved gases act as a flux, disrupting the bonds in the silicate melt and lubricating the mixture until the pressure drops and they bubble out of solution.
Magma’s Subsurface Environment
Magma originates in the Earth’s upper mantle or crust where high temperature and lower pressure conditions favor the melting of rock. Common formation zones include tectonic plate boundaries, such as where plates pull apart or one plate dives beneath another, and mantle plumes that create hotspots.
This molten material is less dense than the surrounding solid rock, causing it to buoyantly rise toward the surface. It often collects in large underground reservoirs known as magma chambers within the crust.
The immense pressure from overlying rock layers keeps volatile gases dissolved within the melt, preventing their escape. This pressure is also a factor in keeping the rock molten, as high pressure raises the melting point of rock.
A crucial distinction in terminology is based purely on location: the material is called magma only while it remains beneath the Earth’s surface. Once extruded onto the surface, whether through a fissure or a volcanic vent, it is referred to as lava. Magma that solidifies underground forms intrusive igneous rocks, while lava solidifies on the surface to form extrusive igneous rocks.