Pahoehoe vs Aa: Two Textures From the Same Basalt

Two Hawaiian words for two textures

Pāhoehoe and ʻaʻā are Hawaiian terms, adopted worldwide by geologists, for the two dominant surface textures of basaltic lava. Pāhoehoe is smooth, glossy and often folded into ropy, billowing patterns as its thin, still-plastic crust drags and wrinkles over the faster-moving liquid beneath. ʻAʻā is the opposite: a jumbled, spiny mass of loose, sharp-edged fragments called clinker riding on top of a slower, denser core. Both can erupt from the exact same vent, with the exact same chemical composition — the difference is entirely in how the lava was treated on its way to solidifying.

Shear rate is the real variable

As basaltic lava cools it starts crystallising, and the growing crystals raise its effective viscosity. If the flow is moving gently — low shear rate, typically found close to a vent, on shallow slopes, or in tube-fed flows insulated from rapid cooling — the thin surface crust can stretch and fold plastically without breaking, producing the smooth, ropy pāhoehoe texture. If the same lava is subjected to a high shear rate — a steeper slope, faster effusion, or simply having already crystallised further and stiffened — the crust cannot deform fast enough to keep up and instead fractures into countless jagged fragments, which is the birth of ʻaʻā.

crust deformation rate < crust yield strain rate  →  stretches, folds  → pahoehoe
crust deformation rate > crust yield strain rate  →  fractures, tumbles → aa

  key drivers of shear rate: slope, effusion rate, crystallinity, cooling rate

The one-way transition

A single flow commonly starts as pāhoehoe near the vent, where it is hot, fast and low in crystal content, and transitions to ʻaʻā further downstream as it cools, crystallises and slows. This pāhoehoe-to-ʻaʻā transition is effectively one-directional: once shearing has broken the surface into clinker and raised the local crystal fraction, the flow does not spontaneously smooth back out, because the very process that caused the fracturing — rising crystal content and effective viscosity — does not reverse itself within an active flow. Geologists use this asymmetry as a diagnostic tool: finding ʻaʻā downslope of pāhoehoe on an old flow tells you the direction the lava was travelling.

Why the texture matters for hazard and travel distance

Pāhoehoe often advances through insulated lava tubes beneath its own crust, which dramatically reduces heat loss and lets the flow travel far greater distances than its surface temperature alone would suggest — some Hawaiian pāhoehoe flows have reached the ocean many kilometres from source through tube systems. ʻAʻā, by contrast, advances as a blocky, tumbling front with a rubbly clinker layer constantly being carried over the top and buried at the base, a self-armouring process that is visually dramatic but generally less efficient at conserving heat, so ʻaʻā flows tend to be shorter and thicker for a given eruption volume.

Modelling the transition

A simple model tracks crystallinity and local shear rate together: crystallinity rises with time-integrated cooling, viscosity rises with crystallinity following an empirical relationship that steepens sharply above roughly 40-50% crystals by volume, and once the resulting local shear stress on the crust exceeds its fracture strength, that patch is reclassified from pāhoehoe to ʻaʻā and stays that way for the rest of the simulation — a compact rule set that reproduces the real-world one-way transition and lets you watch a smooth flow visibly roughen as it advances and cools.

Frequently asked questions

Try it live

Everything above runs in your browser — open Pahoehoe vs Aa Lava and change the parameters while it is running. Nothing is installed, nothing is uploaded, the whole model lives in one tab.