Molten rock extruded onto the Earth’s surface during a volcanic eruption is known as lava. Its behavior is dictated by its temperature and chemical makeup. As this material flows and cools, it solidifies into various distinct forms, which geologists categorize based on their surface texture. One of the most common types of lava flow is ‘a’a, a term adopted globally from the Hawaiian language, recognized immediately by its rough, jagged appearance.
Identifying the Rough Texture
The defining characteristic of ‘a’a lava is its fragmented and spiny surface, composed of broken lava blocks called clinkers. These clinkers are sharp, angular pieces of cooled lava that pile up on the flow’s surface, creating a chaotic and difficult terrain. The Hawaiian name ‘a’a is fitting, as it is thought to relate to a sound of pain or difficulty, accurately describing the experience of trying to traverse this rock.
The structure of an ‘a’a flow is layered. Fragmented clinkers form a carapace over a massive, dense, and still-molten interior core. As the flow advances, cooled blocks from the top tumble down the steep front, are overridden, and ultimately form a rubbly layer at the bottom. This clinker layer can be up to a couple of meters thick, acting as an insulating covering for the hot, flowing center.
The Mechanics of ‘A’a Formation
The formation of the characteristic rough surface is a direct result of the lava’s physical properties and the stresses it undergoes while moving. ‘A’a flows are formed from lava that is more viscous, meaning it has a higher resistance to flow compared to other types. This higher viscosity is often caused by the lava losing heat and gases, or by a greater crystal content, as it travels away from the vent.
As this thicker, pasty lava moves, its surface cools quickly, forming a brittle crust. Because the lava underneath continues to push forward, the solidifying surface is constantly subjected to significant internal friction, known as high shear stress. This intense shearing action rips the brittle crust apart, breaking it into the jagged, spiny clinkers carried along on the flow’s surface. ‘A’a flows typically move as thick sheets or in open channels.
How ‘A’a Differs from Pahoehoe
‘A’a lava is most frequently contrasted with pāhoehoe lava, and the distinction lies in their flow dynamics and resulting surface textures. While both types often share the same chemical composition, pāhoehoe has a smooth, billowy, or ropy surface, unlike the rough, clinkery appearance of ‘a’a. This textural difference is primarily driven by the initial viscosity and flow conditions.
Pāhoehoe lava is less viscous and flows more fluidly, allowing a thin, continuous skin to form that wrinkles into rope-like folds without fracturing. Although ‘a’a is the more viscous of the two, pāhoehoe lava is often slightly hotter when first extruded from the vent. A single flow can transition from pāhoehoe to ‘a’a if it cools, loses gas, or encounters a steeper slope, which increases the shear stress and forces the surface to break up. Once the crust is broken and the ‘a’a morphology is established, the flow cannot revert back to smoother pāhoehoe form.