Magma is molten rock found beneath Earth’s surface, distinct from lava, which is magma that has erupted onto the surface. Understanding magma’s composition is important for comprehending its behavior, including its movement through the Earth’s crust and whether a volcanic eruption will be explosive or effusive.
The Core Ingredients of Magma
Magma consists of three primary components: melt, dissolved gases, and suspended solid crystals. The melt is the most abundant part of magma, predominantly composed of silicate minerals rich in silicon (Si) and oxygen (O). These elements form silicate tetrahedra. The melt also contains other elements, including aluminum (Al), iron (Fe), magnesium (Mg), calcium (Ca), sodium (Na), and potassium (K). The specific proportions of these elements define the magma type.
Magma also contains dissolved gases, or volatiles, similar to carbonation in soda. These gases are under immense pressure deep within the Earth. The most common dissolved gases are water vapor (H2O), carbon dioxide (CO2), and sulfur dioxide (SO2). Smaller quantities of hydrogen sulfide (H2S), chlorine (Cl), and fluorine (F) can also be present. These dissolved gases play a significant role in volcanic eruptions, providing the driving force for magma to rise and erupt.
Magma is rarely a pure liquid and often contains suspended solid mineral crystals. These crystals form as the melt cools. Common examples include olivine, pyroxene, feldspar, and quartz. Their presence indicates the magma has begun cooling.
Why Magma Composition Varies
Magma’s composition is not uniform and can vary significantly due to geological processes occurring as it forms and ascends. Its initial composition is largely determined by the type of source rock that melts (mantle or crustal). For instance, melting of mantle rock produces magmas with higher levels of iron, magnesium, and calcium, while melting of crustal material results in magmas dominated by oxygen, silicon, aluminum, sodium, and potassium.
Rocks do not melt uniformly; partial melting occurs because different minerals have different melting temperatures. Minerals with lower melting points melt first, creating a melt chemically different from the original solid rock. This process produces magma that is more silica-rich than the source rock. For example, partial melting of ultramafic mantle rock can yield a more silica-rich mafic magma.
As magma cools, different minerals crystallize out of the melt at specific temperatures, a process called fractional crystallization. When these crystals are removed, they change the chemical makeup of the remaining magma, leading to a wide range of compositions as elements are selectively removed.
Magma’s composition can also be altered through assimilation, where it melts and incorporates surrounding country rock as it rises through the crust. Additionally, different batches of magma can mix, resulting in a hybrid composition combining the characteristics of the original magmas.
How Magma’s Makeup Shapes its Behavior
Magma’s composition directly influences its physical properties and eruption style. Viscosity, a fluid’s resistance to flow, is primarily controlled by silica content. High-silica magmas, such as rhyolitic magma, are highly viscous (thick and sticky). Conversely, low-silica magmas, like basaltic magma, have low viscosity and flow more easily.
The combination of magma’s viscosity and dissolved gas content determines whether an eruption will be explosive or effusive. High-viscosity magma traps gases, leading to immense pressure buildup. When this pressure becomes too great, it results in explosive eruptions, where gases rapidly expand and fragment the rock. In contrast, low-viscosity magma allows gases to escape more easily, which leads to effusive eruptions where lava flows out relatively gently.
Magma’s composition also affects its density, which plays a role in its ascent through the Earth’s crust. Less dense magma rises buoyantly through denser surrounding rock. The level at which magma’s density equals that of the surrounding rock, known as the level of neutral buoyancy, influences where magma accumulates and forms chambers before eruption.