Lava flow describes the movement of molten rock expelled from a volcanic vent, a process that shapes landscapes and influences geological evolution. Understanding the distinct types of lava flow is essential for assessing volcanic hazards and interpreting the history of planetary surfaces. The behavior of erupted magma depends on viscosity, which is controlled primarily by silica content, gas content, and temperature.
Viscosity and Its Control on Flow Type
The primary factor distinguishing one lava flow from another is viscosity, the resistance of a fluid to flow. Low-viscosity basaltic lavas flow easily, traveling kilometers from their source, while high-viscosity rhyolitic lavas resist movement and tend to pile up near the vent. This difference dictates whether an eruption will produce gentle, spreading flows or violent, blocky formations.
Pahoehoe Flows
Pahoehoe flows are characterized by a smooth, billowy, or ropy surface texture, resulting from low-viscosity basaltic magma. These flows move relatively quickly, forming a flexible crust that wrinkles and folds as the liquid interior advances. They are common in hotspot volcanoes and rift zones, where the magma chemistry allows for efficient heat retention and long-distance travel.
Formation and Surface Features
The ropy appearance of pahoehoe forms when the outer layer cools and solidifies while the molten material beneath continues to flow, stretching the surface into twisted ropes. Occasionally, pahoehoe develops a smooth, glassy surface known as aa when it moves over steep slopes or encounters obstacles. These flows can propagate efficiently, maintaining mobility over long distances due to their minimal friction with the ground.
Aa Flows
In contrast to pahoehoe, aa flows are rough, jagged, and clinkery, composed of broken fragments of lava called scoria. This type of flow occurs when basaltic magma has a slightly higher viscosity or gas content, causing the crust to fracture as it moves. The resulting surface is sharp and unstable, making travel difficult for both humans and wildlife.
Structural Characteristics
Aa flows typically advance more slowly than pahoehoe because the fragmented surface creates significant drag. As the flow travels, the top cools into a rigid carapace that breaks into pieces, while the interior remains fluid. These flows often display steep fronts and can override older lava surfaces, creating complex, multi-layered deposits that record the dynamics of past eruptions.
Block Lava Flows
Intermediate compositions, such as andesite and dacite, often produce block lava flows, which are viscous enough to fracture into large, angular blocks but not so gas-rich as to form pyroclastic material. These flows move slowly, building steep-sided mounds near the vent. The surface consists of massive, meter-sized blocks with little to no ropy or smooth texture.
Behavior and Hazards
Because of their high viscosity, block flows are prone to forming lava domes when they erupt beneath a crater. The fragmentation of the brittle crust generates hazardous rockfalls and can trigger secondary explosions. Historical events, such as those at Montserrat and Unzen, demonstrate how block lava activity poses significant risks to nearby communities through both direct impact and secondary lahars.
Channelized and Tubed Flows
On steep slopes or during high-volume eruptions, lava may organize into narrow channels that concentrate the flow. These channelized flows develop solidified roofs over time, creating lava tubes that insulate the molten interior from atmospheric cooling. The presence of tubes allows lava to travel much farther than surface flows, preserving heat and momentum over kilometers of terrain.
