Lava is molten rock that has erupted from a volcano and flowed onto the Earth’s surface. Despite originating from a similar source, primarily basaltic magma, this liquid rock solidifies into formations with starkly different appearances and textures. The physical conditions under which lava cools and moves determine which of the two dominant types, pāhoehoe or ‘a’ā, will form. Understanding the differences requires examining their final surface textures, initial properties, and dynamic flow processes.
Distinctive Surface Characteristics
The most immediate difference between the two lava types lies in their solidified surface texture. Pāhoehoe, which translates to “smooth” or “unbroken,” is characterized by a sleek, billowy, or ropy surface that often resembles coiled fabric or draped material. When fresh, pāhoehoe appears dark and shiny, sometimes displaying a glassy sheen where it cooled rapidly.
In sharp contrast, ‘a’ā lava presents a rough, jagged, and fragmented surface. This flow consists of a massive, dense core moving beneath a layer of sharp, broken lava pieces known as clinker. Walking across an ‘a’ā flow is extremely difficult and requires sturdy footwear, while pāhoehoe feels relatively stable and smooth underfoot.
The Governing Factors: Temperature and Viscosity
The formation of these distinct surfaces is fundamentally controlled by the molten lava’s temperature and viscosity. Viscosity, the resistance of a fluid to flow, is the most important factor determining the final lava type. Pāhoehoe forms from less viscous, more fluid lava, while ‘a’ā forms from comparatively more viscous lava.
Temperature directly controls viscosity, as the two properties are inversely related. Pāhoehoe flows originate from hotter lava, often erupting above 1100°C. This high heat keeps the lava highly fluid, allowing it to move easily with less internal friction.
Conversely, ‘a’ā flows are associated with cooler, thicker lava. The transition from pāhoehoe to ‘a’ā occurs when the temperature drops below approximately 1200°C. Both types are commonly basaltic, meaning the difference is due to the thermal state and the forces acting upon the moving mass, not major chemical variation.
Formation Mechanisms and Flow Behavior
The difference in viscosity and temperature dictates the mechanisms by which the flows advance and solidify. Pāhoehoe’s low viscosity allows it to develop a thin, elastic skin upon exposure to the air, while the lava beneath remains liquid. As the molten interior moves forward, this pliable crust stretches, folds, and wrinkles, creating the characteristic ropy or billowy textures.
This mechanism promotes the formation of lava tubes, which are natural conduits where the outer walls and ceiling solidify, creating an insulated tunnel. These tubes are highly effective at preventing heat loss, allowing pāhoehoe to travel great distances from the vent. The flow advances steadily by the propagation of small, localized lobes or “toes,” making it generally slow, steady, and predictable.
In contrast, the higher viscosity of ‘a’ā lava causes the cooling surface to form a thick, brittle crust almost immediately. As the dense lava mass continues its forward movement, the internal stress, known as shear strain, is too great for the crust to stretch or fold. This stress causes the crust to repeatedly fracture and break apart into sharp, angular fragments.
The resulting clinker is carried along the flow’s surface, where it is constantly churned and dropped at the flow’s front. An ‘a’ā flow advances by bulldozing this layer of self-generated debris, which acts as a rubbly rampart. Although the initial velocity of an ‘a’ā flow can be faster than pāhoehoe, the constant fragmentation and exposure to the air cause it to lose heat more rapidly, leading to a shorter overall flow distance compared to tube-fed pāhoehoe. Once a pāhoehoe flow transitions to ‘a’ā due to cooling or increased strain, the process is irreversible.