Pahoehoe

Pahoehoe is a type of lava flow defined by a smooth, unbroken surface that often develops a ropey or ropy texture. The term is Hawaiian in origin, used to describe the distinctive surface features observed on cooled lava.

Pahoehoe typically forms from low-viscosity, basaltic lava. As the molten rock flows, a thin crust cools and stiffens while the interior continues to move. The surface fabric is then stretched and folded, producing smooth, billowy fields and characteristic ropes or folds. The texture can vary from glossy, glassy to more matte, depending on cooling rate and mineral content.

Lava moving beneath the crust can inflate the surface, creating lobes or toes that advance as a coherent unit. The crust may blister and peel, and in some cases a pahoehoe flow develops lava tubes when the interior continues to move after the surface crust thickens and solidifies.

Pahoehoe is common in basaltic eruptions at oceanic hotspots and shield volcanoes, such as those in Hawaii, where it forms the familiar smooth surfaces of many lava flows. It can occur with other low-viscosity lavas, though higher-viscosity compositions tend to produce a'a, a rough, clinker-covered surface.

A'a and pahoehoe represent different end-members of lava-flow behavior. Pahoehoe can transition into a'a as cooling and fragmentation increase, producing a rougher surface. The texture and progression of pahoehoe provide insights into eruption dynamics, lava composition, and flow conditions.