Molten rock that flows across a planet’s surface is known as lava, and the resulting landforms are called lava flows. These outpourings of liquid rock are a fundamental process of volcanism. The most common type, basaltic lava, produces two primary morphological forms. These two distinct types of lava flows are universally known by their Hawaiian names: Pāhoehoe and ‘A‘ā. These terms describe the appearance of the solidified surface, which reflects the flow’s behavior and conditions during emplacement.
Pāhoehoe: The Smooth, Ropy Flow
Pāhoehoe lava is characterized by a smooth, continuous surface that often appears billowy, undulating, or like coiled rope. This distinctive, glass-like texture forms because the lava has low resistance to flow, allowing a thin, elastic skin to congeal on the surface. As the liquid lava moves beneath this thin crust, the skin is dragged and wrinkled, creating the signature ropy appearance. Pāhoehoe flows typically advance slowly, often as a series of small, bulbous lobes, or “toes.”
This fluid nature allows the flow to cool and solidify on the exterior while the interior remains incandescent, which leads to the formation of extensive lava tubes. These tubes act as insulated conduits, allowing the molten rock to travel long distances without significant heat loss. The flow field tends to be relatively thin, ranging from less than a meter to a few meters thick.
‘A’ā: The Rough, Jagged Flow
In contrast to the smooth texture of Pāhoehoe, ‘A‘ā lava flows are defined by an extremely rough, fragmented, and jagged surface. The surface is composed of angular, sharp blocks of broken lava called clinker. The movement of ‘A‘ā is typically faster and shallower than Pāhoehoe, advancing as a steep-sloping wall of these fragmented chunks. As the pasty lava core moves downslope, the cooled surface fragments are carried along, tumbling down the leading edge and then being buried by the advancing flow. This process creates layers of rubbly fragments on both the top and bottom of the final flow unit.
Walking across an ‘A‘ā flow is difficult and slow due to the loose, unstable nature of the clinker and the sharp edges of the fragments. Unlike the tube-fed Pāhoehoe, ‘A‘ā flows usually travel in open channels, which promotes faster cooling and the development of this fragmented surface. The flows themselves tend to be thicker, often measuring several meters high.
The Underlying Causes of Different Lava Textures
The formation of either Pāhoehoe or ‘A‘ā is determined primarily by the physical conditions of the lava, namely its temperature and viscosity. Viscosity is the lava’s resistance to flow, and it is the most important factor influencing the final texture. Pāhoehoe forms from hotter, less viscous (more fluid) lava.
The lower viscosity allows the surface crust to stretch and deform without breaking as the molten interior continues to move. This minimal fracturing preserves the smooth, continuous surface texture. Pāhoehoe lavas generally maintain a high temperature, often in the range of 1,100 to 1,200 degrees Celsius, which keeps the lava highly fluid.
Conversely, ‘A‘ā forms from cooler, more viscous lava, or from lava that has experienced a higher degree of shear strain. As the flow moves, the higher viscosity causes the cooling surface crust to break constantly into sharp, angular fragments rather than stretching smoothly. This constant fracturing is what creates the rough, clinkery surface.
A Pāhoehoe flow can transition into an ‘A‘ā flow if it experiences significant heat loss or an increase in flow speed over a steep slope. Both conditions increase the lava’s viscosity and the mechanical stress, forcing the smooth crust to shatter into the characteristic jagged blocks. Once the transition to ‘A‘ā occurs, the process is irreversible.