Volcanic eruptions vary widely in their intensity and style. Some volcanoes gently ooze molten rock, while others explode with great force. The difference between a quiet, flowing eruption and a catastrophic, explosive event depends on the fundamental properties of the molten rock, or magma, beneath the surface. Understanding the chemical makeup of magma is essential for predicting how a volcano will behave, as this composition determines factors like temperature and thickness, which directly control the violence of an eruption.
Defining Magma Composition: Mafic vs. Felsic
Magma is categorized primarily by its chemical composition, with the amount of silica acting as the main differentiator. Felsic magma is rich in silica, typically containing more than 65% by weight, along with lighter elements like aluminum, sodium, and potassium. Felsic magmas are generally cooler, with temperatures ranging from approximately 650 to 800 degrees Celsius. The high silica content causes these magmas to form extensive molecular chains, making them exceptionally thick and resistant to flow, a property known as high viscosity.
In contrast, mafic magma is low in silica, often containing between 45% and 55%. Mafic magmas are rich in heavier elements like iron and magnesium, which gives them their name. These magmas are significantly hotter, with eruption temperatures commonly between 1000 and 1200 degrees Celsius. The lower silica content and higher temperature result in a runnier consistency, giving mafic magma a much lower viscosity compared to its felsic counterpart. Mafic magma flows easily, similar to warm oil.
How Viscosity and Gas Content Drive Explosivity
All magma contains dissolved gases, known as volatiles, held in solution by intense pressure deep within the Earth. The primary volatiles are water vapor and carbon dioxide, along with smaller amounts of sulfur and chlorine gases. As magma rises toward the surface, the confining pressure decreases, causing these dissolved gases to form bubbles, much like opening a carbonated beverage. The expansion of these gas bubbles is the driving force behind all volcanic eruptions.
The violence of an eruption is determined by how easily these gas bubbles can escape the molten rock. In low-viscosity mafic magma, the bubbles migrate quickly and continuously through the fluid, escaping gradually and resulting in a gentle eruption. This process is known as effusive volcanism, where the gas bursts at the surface, and the lava flows easily away.
However, the high viscosity of felsic magma acts as a powerful barrier, trapping the expanding gas bubbles within the thick melt. As the magma continues to rise, the gas bubbles swell, and pressure within the magma chamber builds rapidly. When the internal gas pressure exceeds the strength of the surrounding rock, the magma body shatters violently. This fragmentation turns the liquid rock into tiny solid fragments, creating a massive explosion that ejects ash, gases, and rock fragments high into the atmosphere.
Contrasting Eruption Styles and Hazards
Felsic magma is substantially more explosive than mafic magma because its high viscosity effectively corks the volcanic vent, facilitating immense pressure buildup. Felsic eruptions are characterized by violent explosions that produce towering eruption columns of ash and gas. These eruptions form steep-sided stratovolcanoes, commonly found at continental plate boundaries, such as Mount St. Helens.
The most dangerous hazards associated with felsic eruptions are fast-moving pyroclastic flows—superheated clouds of gas and ash that race down the volcano’s flanks. Widespread dispersal of volcanic ash can disrupt air travel, destroy crops, and collapse buildings. In contrast, mafic eruptions are typically effusive, forming broad, gently sloping shield volcanoes, exemplified by those in Hawaii, such as Kilauea. Hazards from mafic volcanoes are primarily restricted to the lava flows themselves, which are slow-moving and generally pose a low immediate threat to human life.