Basaltic lava can freeze into two strikingly different surface textures even though the underlying rock chemistry is nearly identical. Pahoehoe (Hawaiian for "smooth, unbroken") forms a thin, glassy, continuously deforming skin that folds into ropy, billowy lobes as the still-molten interior keeps advancing beneath it. A'a (Hawaiian for "stony, rough lava") forms when that skin cannot keep up with the shear of the flow — it fractures into a jagged, clinkery rubble that is carried along on top of, and tumbles ahead of, the moving flow front.
A single eruption can produce both textures — and even transition from pahoehoe to a'a mid-flow — simply because the lava speeds up, cools faster, or travels farther from the vent. Geologists call this the "pahoehoe-to-a'a transition," and it is essentially irreversible: once a'a forms, it does not revert to pahoehoe downstream.
An interactive 3D lava field whose surface texture morphs continuously between smooth, ropy pahoehoe and jagged, rubbly a'a as you adjust effusion rate, cooling rate, and slope.
Both textures form from chemically similar basaltic lava. The difference comes down to shear stress on the cooling crust: fast supply, rapid cooling, and steep slopes fracture the skin into a'a clinker, while slow, steady, well-insulated flow lets the skin fold smoothly into pahoehoe ropes.
Drag the flow rate, cooling rate, and slope sliders and watch the computed a'a index and surface geometry respond live. Toggle the thermal glow to see the hot vent zone, and hit regenerate for a fresh flow pattern.
A flow can start as pahoehoe near the vent and turn to a'a farther downstream purely because it speeds up or cools faster — a one-way transition that geologists use to read the history of a lava field.