Volcanoes are natural forces that erupt with a wide range of intensity, from gentle, flowing lava to sudden, devastating explosions. The difference in their power is determined by complex geological processes deep beneath the surface. The key to this variance lies in the magma’s composition and how that composition controls the release of trapped gases. This understanding allows for a more accurate assessment of volcanic hazards worldwide.
Defining Volcanic Explosivity
The scientific community measures the power of a volcanic eruption using the Volcanic Explosivity Index (VEI), a standardized scale from 0 to 8. The VEI quantifies an eruption’s magnitude based on the volume of ejected material and the height of the eruption column. Because the scale is logarithmic, each increasing number represents an eruption approximately ten times more powerful than the last. The fundamental driver behind any explosive eruption is the rapid buildup of gas pressure within the magma chamber. When magma ascends, dissolved gases separate to form bubbles, and if these bubbles cannot escape easily, the internal pressure becomes overwhelming.
Magma Viscosity and Silica Content
The ability of gases to escape from magma is dictated by viscosity, the material’s resistance to flow. Low-viscosity magma allows gas bubbles to move upward and escape gradually, resulting in a gentle eruption. Conversely, high-viscosity magma acts like thick, sticky tar, trapping the gas bubbles and preventing their release. This difference in viscosity is directly controlled by the magma’s silica content (silicon dioxide).
Magmas with low silica content (45–55%) are referred to as mafic and are highly fluid, leading to low viscosity. The simple silica structures do not link together extensively, allowing the magma to flow easily. In contrast, felsic magmas have a high silica content (over 70%), causing the silica structures to polymerize into complex chains. These interconnected chains dramatically increase the magma’s resistance to flow, resulting in high viscosity. High-viscosity magma seals the volcano’s vent, allowing gas pressure to build to a catastrophic level before a violent release.
The Highly Explosive Stratovolcano
The most explosive common type of volcano is the Stratovolcano, also known as a Composite Volcano. These towering, symmetrically conical mountains are constructed from alternating layers of hardened lava flows, ash, and fragmented debris. Stratovolcanoes are associated with silica-rich magma, such as andesite or dacite, which is the high-viscosity material that traps gas and leads to violent eruptions. This magma is often found at subduction zones, fueling the explosive nature of these volcanoes.
When internal pressure overwhelms the overlying rock, the resulting eruption sends massive columns of ash and gas high into the atmosphere. A deadly hazard of these eruptions is the pyroclastic flow, a fast-moving cloud of superheated gas and volcanic fragments that can travel over 100 miles per hour. Extreme explosive events can empty the underlying magma chamber, causing the summit to collapse inward and form a large basin known as a caldera. Examples like Mount St. Helens and Mount Pinatubo demonstrate the potential for Stratovolcanoes to rank high on the VEI scale.
Contrast: Shield Volcanoes and Effusive Eruptions
In contrast to Stratovolcanoes are Shield Volcanoes, characterized by their broad, gently sloping profiles resembling a warrior’s shield. These structures are built by repeated eruptions of low-viscosity, basaltic magma, which has a low silica content. The fluidity of this magma allows it to flow easily over great distances before solidifying, preventing the formation of a steep-sided cone.
Because this low-viscosity magma does not resist the movement of gas bubbles, volatile compounds escape continuously and gently. This results in effusive eruptions, where lava pours out in calm flows rather than exploding violently. Shield volcanoes, such as those found in Hawaii, rarely score higher than a VEI of 0 or 1.