AaEdit
Aa refers to a distinctive type of lava flow characterized by a rough, clinkery surface and a blocky, jagged texture. This form of lava flow, typically associated with basaltic compositions, contrasts with the smoother, rope-like surface of pahoehoe lava. Aa flows occur when the surface crust fractures as the lava advances, exposing a still-molten interior that breaks into angular fragments. The result is a crust that appears broken into sharp, irregular blocks, which can accumulate into a rugged surface layer over the cooling flow. Aa is a common feature in many basalt-dominated volcanic settings, including shield volcanoes and rift zones, and is observed in regions such as Kilauea and Mauna Loa in the Hawaiian Islands, as well as in various locations around the globe where basaltic volcanism is active. For readers seeking broader context, the study of aa is closely linked to topics like lava, basalt, and volcano.
Aa flows are an important part of the spectrum of how lava behaves after eruption. They typically form from lava that is broadly similar in composition to pahoehoe but becomes more viscous as it cools and as the surface crust thickens. The surface crust of an aa flow is brittle and prone to fracturing, so as the flow continues to advance, the crust shatters and slides forward in blocks. This process produces a rubble-like surface and a front that is often thick and uneven. The interior lava remains hot and mobile, allowing the flow to continue to move even as the surface becomes highly disrupted. The texture and shape of aa surfaces are influenced by factors such as lava temperature, gas content, eruption rate, and underlying topography, as well as the mechanical properties of the crust formed on cooling.
Geologic characteristics
Texture and appearance: Aa surfaces are rough, with coarse, clinker-like blocks ranging from small to several meters across. The crust is thickened and fragmented, giving the flow a visibly uneven, jagged exterior. Inside the crust, molten lava continues to advance, feeding the growth of new blocks as the flow progresses. See how this contrasts with the ropey, smooth surface of Pahoehoe.
Composition and viscosity: Aa commonly arises in basaltic systems, where lava is relatively silica-poor and rich in magnesium, iron, and calcium. While still fluid enough to flow, the lava associated with aa is more viscous than pahoehoe, which promotes crustal rupture and fragmentation.
Temperature and cooling: The exterior cools rapidly upon contact with the ambient air or cooler ground, forming a brittle crust that is prone to breaking. The interior remains hot, underpinning continued movement and the formation of fresh blocks at the leading edge.
Physical extent: Aa flows can advance tens to hundreds of meters per day in favorable conditions, and their fronts often remain relatively stable as blocks accumulate in front of the advancing lava.
Formation and texture
The shift from a smoother pahoehoe-style lava to an aa texture is tied to the evolving balance between cooling, crust formation, and internal lava flow. When the crust thickens and becomes brittle, it fails under gravitational and rheologic stresses, producing blocks that roll or slide forward. As new blocks form at the flow front, the surface becomes a mosaic of angular fragments, creating the characteristic rough appearance. This texture can persist for considerable distances, giving aa flows their distinctive rugged silhouette along the landscape.
In some cases, aa surfaces may be punctuated by areas of fresher, less-fragmented lava, producing a patchwork of textures across a single flow field. Researchers study these patterns to infer eruption dynamics, crust formation rates, and the thermal regime of the lava. See lava flow dynamics for broader context.
Distribution and examples
Aa flows are a hallmark of basaltic volcanism and are commonly observed at shield volcanoes and fissure-fed eruptions. Notable examples include flows from Kilauea and Mauna Loa in the Hawaiian Islands, where abundant aa lava has built up rough, step-like slopes and fronts. Aa also appears in volcanic settings outside Hawaii, including parts of Iceland and other regions with extensive basaltic volcanism. The presence of aa in a given lava field often reflects a balance of eruption rate, lava temperature, and the mechanical behavior of the crust as it cools and fractures.
In recent volcanic episodes, aa flows have served as visible indicators of ongoing effusive activity, allowing scientists to monitor lava supply, flow direction, and potential impacts on nearby communities and infrastructure. See volcanic hazard and shield volcano for related topics.
Differences with pahoehoe
While both aa and pahoehoe are basaltic lava textures, they differ primarily in surface morphology and mode of fragmentation. Pahoehoe lava forms a smooth, ropy surface as the lava advances with a relatively cohesive, low-friction crust. Aa lava, by contrast, develops a thick, brittle crust that breaks into angular blocks, producing a rough, clinkery surface. The flow fronts of pahoehoe tend to advance more smoothly, whereas aa fronts advance in a more segmented, stepwise manner due to block formation and movement. The transition between these textures is a well-known aspect of basaltic volcanism and is often influenced by cooling rates, eruption intensity, and underlying slope.
Hazards and impacts
Aa flows can pose significant hazards to nearby settlements, roads, and infrastructure due to their rough, abrasive surface and the potential for rapid block movement. While often slower in overall advance than some pahoehoe flows, aa fronts can advance unpredictably as new blocks form and become dislodged. The high surface area and exposed fresh lava also contribute to radiant heat, which can threaten vegetation, structures, and equipment. In addition, as with other lava types, gas emissions from aa flows contribute to respiratory hazards and can affect air quality in the surrounding region. Effective monitoring and risk communication rely on understanding the behavior of aa flows in the context of ongoing eruption dynamics and topography. For broader context, see volcanic gas and volcanic hazard.