Lava, the molten rock that escapes from a volcano, presents a dramatic and powerful display of the Earth's internal heat. While often visualized as a singular, glowing red river, the reality is far more complex, with compositions and behaviors varying significantly based on chemical makeup and temperature. Understanding these distinctions is crucial for volcanologists attempting to predict eruption styles and for appreciating the diverse landscapes formed by volcanic activity. The viscosity, gas content, and mineral structure of each flow dictate its movement, surface texture, and potential hazard level.
Classification by Silica Content
The primary method for categorizing these flows is based on silica content, which directly influences viscosity. High-silica compositions are thick and resistant to flow, while low-silica versions are fluid and travel great distances. This chemical difference dictates the type of volcanic structure that forms and the nature of the eruption itself.
Felsic Lava
Felsic flows contain 60% or more silica, making them highly viscous. This stickiness traps immense amounts of gas, leading to explosive eruptions rather than gentle flows. When these materials do reach the surface, they form thick, blocky masses that pile up around the vent. The high gas pressure within this type of melt often results in violent Plinian eruptions, characterized by ash columns that can reach the stratosphere.
Intermediate Lava
Intermediate compositions, such as andesitic flows, contain between 57% and 60% silica. These materials exhibit moderate viscosity, allowing for a balance between explosive activity and effusive flow. Andesitic compositions are common in stratovolcanoes, where alternating layers of ash and hardened rock create the classic conical shape. The behavior of these flows is highly variable, sometimes advancing slowly and other times collapsing violently as lava domes.
Mafic Lava
Mafic flows, including basalt, have the lowest silica content, usually under 57%. This low viscosity allows gases to escape easily, resulting in relatively calm and effusive eruptions. These fluids can travel kilometers from the source, creating vast sheet floods known as lava plateaus. The low gas content and fluidity mean that mafic eruptions are generally less hazardous to human life, though they can destroy infrastructure in their path.
Behavioral and Textural Types
Beyond chemical composition, scientists classify these melts by how they move and solidify. This behavioral approach focuses on the surface texture and flow dynamics observed during an eruption. Two primary types dominate the volcanic landscape: ʻAʻā and Pāhoehoe.
ʻAʻā Lava
Named for the Hawaiian word for "burning," ʻAʻā is a rough, clinkery surface. This type forms when a relatively thin layer of solidified rock breaks apart as the molten interior continues to push forward. The result is a fragmented, jagged landscape that is difficult to traverse. ʻAʻā flows are typically associated with mafic compositions but can occur with intermediate types, featuring steep fronts and chaotic internal structures.
Pāhoehoe Lava
In contrast, Pāhoehoe flows have a smooth, ropy, or billowy surface texture. This appearance is caused by a thin, flexible crust forming over a still-moving, molten interior. The material beneath continues to flow, twisting the cooled surface into intricate shapes that resemble coiled rope or ocean waves. This type of flow is generally associated with lower volumes and more fluid mafic compositions, creating some of the most visually stunning volcanic scenery.
