Pahoehoe is a distinct type of basaltic lava flow defined by its smooth surface texture. The term is a Hawaiian word meaning “smooth, unbroken lava,” used globally by geologists. This lava is characteristic of effusive eruptions, where magma flows out calmly rather than exploding violently. Pahoehoe is commonly associated with shield volcanoes, such as those in Hawaii, where the magma is hot and flows easily.
The Distinctive Appearance of Pahoehoe
The defining characteristic of pahoehoe is its smooth surface, which contrasts sharply with other lava types. As the flow cools, the thin, pliable crust wrinkles and coils, creating the recognizable “ropy” texture that resembles twisted strands of rope. This continuous, undulating surface can also appear billowy or hummocky depending on the flow conditions.
Freshly cooled pahoehoe can exhibit a silvery-gray, metallic blue, or iridescent sheen. This bright color quickly dulls to a dark, charcoal-black as the surface oxidizes and weathers. The surface is typically continuous and fragile, often creating hollow, shelly crusts where gas bubbles have escaped just beneath the surface.
How Pahoehoe Lava Forms and Flows
Pahoehoe forms from highly fluid, hot basaltic lava with low viscosity. This fluidity is maintained by low effusion rates and slow, laminar flow velocity, often moving between 10 and 100 meters per hour. The movement begins when a thin, flexible skin rapidly cools and congeals on the surface of the molten stream.
The hotter lava beneath the crust continues to flow downstream, dragging the plastic skin along. This shearing action causes the skin to fold and wrinkle, permanently creating the coiled, rope-like pattern upon final cooling. This insulating crust facilitates the formation of lava tubes, which allow the molten rock to travel long distances without significant heat loss.
The flow advances slowly by a series of small, bulbous protrusions called “toes” that break out from the cooled crust. As pressure builds, the lava breaches the thin front, creating a new lobe that quickly develops its own smooth crust. This continuous process allows the flow field to spread out as a network of overlapping flows.
Pahoehoe Versus ‘A’ā Lava
The primary distinction between pahoehoe and its counterpart, ‘A’ā lava, lies in their viscosity and flow dynamics, though they often share the same basaltic chemical composition. Pahoehoe is the less viscous, more fluid end of the spectrum. Conversely, ‘A’ā lava, whose name is said to mimic the sound of walking on it, has a higher viscosity and is much thicker.
The difference in viscosity leads to dramatically different flow textures. ‘A’ā is characterized by a rough, jagged, and clinkery surface of broken rock fragments. While pahoehoe flows slowly and smoothly, ‘A’ā flows are more turbulent and advance as a steep, bulldozing front of sharp chunks. This turbulent movement shatters the crust as it forms, preventing a continuous, smooth skin.
A pahoehoe flow can transform into an ‘A’ā flow if it experiences an increase in flow rate, a drop in temperature, or encounters a steep slope. These changes increase the shear strain, causing the thin, smooth crust to break up and fragment, resulting in the characteristic clinkers of ‘A’ā. Once the flow becomes ‘A’ā, the process cannot reverse to form a smooth pahoehoe surface again.