3.1 Magma composition and properties

Magma, the molten rock beneath Earth's surface, is a complex mixture of liquid rock, crystals, and gases. Its composition and properties directly control how volcanoes behave, from gentle lava flows to violent explosive eruptions.

Magma compositions range from mafic to felsic, and those differences affect everything from how easily the magma flows to how dangerous an eruption can be. Understanding these differences helps geologists predict volcanic hazards and interpret Earth's geological history.

Magma Composition and Properties

Components of Magma

Magma isn't just a uniform pool of melted rock. It's actually three things mixed together: liquid rock (the melt), solid crystals, and dissolved gases.

• Silicate melt forms the liquid portion. It's built from silicon and oxygen bonded together, along with other elements like aluminum, iron, magnesium, calcium, sodium, and potassium. The specific mix of these elements determines what type of magma you're dealing with.
• Solid mineral grains are crystals that have already formed but remain suspended in the melt. Common ones include olivine, pyroxene, amphibole, feldspars, and quartz. Which minerals are present depends on the magma's composition and temperature.
• Dissolved gases (volatiles) include water vapor ($H_2O$), carbon dioxide ($CO_2$), and sulfur dioxide ($SO_2$). These gases stay dissolved under high pressure deep underground, but they become extremely important as magma rises toward the surface.

Factors in Magma Properties

Viscosity is the single most important physical property of magma. It measures resistance to flow: high-viscosity magma is thick and sluggish, while low-viscosity magma flows easily. Three main factors control it:

• Silica content: Higher $SiO_2$ content creates more interconnected molecular chains in the melt, which makes it harder to flow. Alkali metals (Na and K) also increase viscosity.
• Temperature: Hotter magma flows more easily. As magma cools, it becomes more viscous and sluggish.
• Dissolved gas content: This one is a bit tricky. While gases are dissolved, they can actually lower viscosity slightly. But as magma rises and pressure drops, those gases come out of solution (exsolve) and form bubbles, which increases the effective viscosity of the mixture.

Volatiles in Magma Behavior

Dissolved gases, primarily $H_2O$ and $CO_2$, are what drive eruptions. Think of it like a shaken soda bottle: the gas is dissolved under pressure, but release that pressure and the gas rapidly expands.

As magma rises toward the surface, decreasing pressure causes volatiles to exsolve and form bubbles. Whether this leads to a calm eruption or a catastrophic one depends on the combination of volatile content and viscosity:

1. High volatiles + low viscosity → Gases escape easily through the fluid magma. This produces effusive eruptions with gentle lava flows and lava fountains (typical of mafic magmas like those in Hawaii).
2. High volatiles + high viscosity → Gases get trapped because the thick magma won't let them escape. Pressure builds until the magma fragments violently, producing explosive eruptions with ash, pumice, and pyroclastic flows.
3. Low volatiles + high viscosity → Not much gas to drive an explosion, but the magma is still too thick to flow freely. This results in slow-moving, dome-building lava flows.

Mafic vs. Felsic Magma

Magma compositions exist on a spectrum, but the two end-members you need to know are mafic and felsic. An intermediate type also exists, but understanding these two extremes makes the middle easy to fill in.

Mafic magma (the name comes from magnesium + ferric/iron):

• Low silica content (45–52% $SiO_2$)
• Rich in iron (Fe) and magnesium (Mg), giving resulting rocks a dark color
• High temperature (900–1200°C) and low viscosity, so it flows easily
• Typically forms from partial melting of mantle peridotite
• Produces rocks like basalt (extrusive) and gabbro (intrusive)

Felsic magma (from feldspar + silica):

• High silica content (>63% $SiO_2$)
• Enriched in lighter elements like potassium, sodium, and aluminum
• Lower temperature (700–850°C) but higher viscosity, so it flows poorly
• Forms from partial melting of crustal rocks or through fractional crystallization of mafic magma
• Produces rocks like rhyolite (extrusive) and granite (intrusive)