During an eruption, volcanoes often release a towering column of material skyward. This column, known as a volcanic aplume, is a defining visual characteristic of explosive volcanic activity.
Understanding an Aplume
A volcanic aplume is a column of super-heated ash, gases, and rock fragments that forcefully rises from a volcanic vent during an explosive eruption. It represents the immediate and visible consequence of magma interacting with the atmosphere. Unlike slower-moving lava flows or ground-hugging pyroclastic flows, an aplume is characterized by its rapid vertical ascent, pushing volcanic material high into the atmosphere.
Formation and Dynamics
The formation of a volcanic aplume begins deep within the volcano, where immense pressure builds as dissolved gases within magma expand. As this gas-rich magma rapidly ascends, the sudden reduction in pressure violently disrupts the magma into fine volcanic ash and coarser rock fragments, known as tephra. This explosive fragmentation propels the mixture upwards at speeds of several hundred meters per second.
The initial upward thrust, called the gas thrust region, is driven by gas expansion and extends for the first one to two kilometers above the vent. Beyond this, the aplume continues its ascent through powerful convection currents. The hot mixture entrains surrounding air, heating it and causing it to expand, which reduces its density and drives further upward movement. This convective thrust accounts for most of the aplume’s height. The ultimate height and shape are influenced by the eruption’s intensity, vent size, and the magma’s gas content and viscosity. Highly explosive eruptions can generate aplumes reaching over 40 kilometers, penetrating the stratosphere. Atmospheric conditions, such as temperature, humidity, and wind, also play a role, with strong winds potentially reducing vertical reach.
As the aplume rises, it reaches an altitude where its density equals the surrounding atmosphere, known as the level of neutral buoyancy. Here, the material spreads out horizontally, forming a characteristic mushroom or umbrella shape. This upper part, the umbrella region, can extend significantly downwind, dispersing volcanic material over vast distances. If the aplume is too dense to be supported by convection, it can collapse, generating hazardous pyroclastic flows down the volcano’s slopes.
Composition and Observable Features
A volcanic aplume is primarily composed of volcanic ash, which consists of tiny fragments of pulverized rock, mineral crystals, and volcanic glass, typically less than 2 millimeters in diameter. This fine particulate matter gives the aplume its dark color. Larger rock fragments, such as lapilli (2-64 mm) and volcanic bombs (larger than 64 mm), are interspersed with the ash but typically fall closer to the vent due to their weight.
A significant portion of an aplume also consists of various gases. Water vapor is the most abundant, often making up 70 to 95 percent of total gas emissions. Other common gases include carbon dioxide, sulfur dioxide, hydrogen sulfide, hydrogen chloride, and hydrogen fluoride. The specific composition of these gases varies depending on the magma type and the volcano’s geological setting.
Atmospheric and Environmental Impact
Once released into the atmosphere, the volcanic aplume disperses its contents, leading to various atmospheric and environmental effects. Fine volcanic ash can be carried thousands of kilometers by wind, forming extensive ash clouds that reduce visibility and degrade air quality. This airborne ash poses a significant hazard to aviation, damaging aircraft engines, scratching cockpit windows, clogging sensors, and causing engine flame-outs due to its abrasive nature and low melting point. Over 90 aircraft have sustained damage from flying through volcanic ash clouds.
Volcanic gases within the aplume also influence atmospheric chemistry. Sulfur dioxide, a common volcanic gas, reacts to form sulfate aerosols, which can reflect incoming solar radiation and lead to a temporary localized cooling effect. Halogen emissions, such as hydrogen chloride and hydrogen fluoride, can affect stratospheric ozone chemistry. The overall impact on weather patterns is temporary and regional, depending on the eruption’s scale and aplume dispersal.