Pahoehoe is a term used globally in geology to classify a specific type of basaltic lava flow, characterized by its smooth, undulating surface texture. The name comes from the Hawaiian language, reflecting the flows’ common occurrence in the Hawaiian Islands. This lava is associated with hot, fluid magma that has a low viscosity, allowing it to flow easily and spread out. Its distinctive appearance forms billowy or rope-like surfaces.
Defining the Ropy Surface
The most immediate characteristic of a pahoehoe flow is its continuous, smooth surface, often described as billowy, glassy, or folded into rope-like patterns. The Hawaiian word pāhoehoe means “smooth, unbroken lava.” It is also connected to the Hawaiian word hoe, meaning “to paddle,” which refers to the swirling appearance of the solidified surface.
The color of the cooled pahoehoe lava ranges from silvery when very fresh to a dark black as it ages and weathers. This smooth texture results from the lava’s high fluidity, which allows the outer skin to stretch and wrinkle without fracturing as the molten material moves beneath it. The surface can also develop hummocks, or small, rounded hills, as the lava inflates beneath the thin, flexible crust.
The Formation Process
Pahoehoe lava forms when highly fluid, low-viscosity basaltic magma is erupted at high temperatures, often around 1,100 to 1,200 degrees Celsius. The low viscosity allows the lava to flow slowly, often at speeds ranging from one to a hundred meters per hour. This calm movement permits a thin, continuous skin to rapidly cool and solidify on the surface, acting as an insulating layer.
As the molten lava moves beneath this thin, plastic crust, the outer layer is dragged, folded, and rumpled. This internal movement creates the characteristic ropey texture as the interior gently deforms the cooling skin. The insulating crust helps the lava maintain its heat and low viscosity over long distances, which is the primary mechanism for lava tube formation. Tubes form when the sides and roof of a flow solidify, allowing the hot, fluid lava to continue flowing efficiently inside a well-insulated channel.
Distinguishing Pahoehoe from ‘A’ā Lava
Pahoehoe is one of two major types of basaltic lava flow, the other being ‘a’ā. The two are distinguished by their vastly different surface textures. The difference arises not from a change in chemical composition, as both are typically basalt, but primarily from the physical conditions of the flow, such as temperature, viscosity, and flow rate. Pahoehoe’s smooth, ropy surface contrasts sharply with ‘a’ā’s rough, jagged, and blocky texture, which is often described as clinkery.
‘A’ā lava forms when the flow has a higher viscosity, moves more quickly, or undergoes greater internal strain, which causes the cooling crust to shatter continuously. Unlike the pliable skin of pahoehoe, the thicker crust of ‘a’ā cannot stretch and breaks into sharp, angular fragments that are then carried along by the flow. While a pahoehoe flow can sometimes transition into an ‘a’ā flow due to cooling or increased speed over a steep slope, the reverse transition is extremely rare.