Lava Flows Explained: Aa, Pahoehoe, and the Forms of Molten Rock
When a volcano erupts effusively, it sends rivers of molten rock across the landscape. But not all lava flows are alike. The same volcano can produce flows that look utterly different, from glassy, smooth surfaces that ripple like rope to harsh, broken expanses of jagged rubble. The forms that lava takes, named with Hawaiian words that have entered the global scientific vocabulary, reveal how the lava behaved as it cooled and moved.
What controls a lava flow
The behaviour of a lava flow depends mainly on the lava's temperature, its composition, and how much gas it contains. Hotter, more fluid basaltic lava flows easily and can travel far, while cooler or more silica-rich lava moves sluggishly. The rate of cooling, the slope of the ground, and the supply of fresh lava all shape how a flow advances and what surface it forms.
Pahoehoe: the smooth, ropey flow
Pahoehoe, a Hawaiian word, describes lava with a smooth, billowy, or ropey surface. It forms from fluid, fast-moving basaltic lava that retains its heat and a flexible skin as it flows. As the surface cools slightly while the lava beneath keeps moving, it wrinkles into the characteristic ropey folds. Pahoehoe can advance in delicate, glowing toes and lobes, building smooth, undulating sheets of rock.
Aa: the jagged, clinkery flow
Aa, also a Hawaiian word, describes lava with a rough, broken, clinkery surface of sharp, angular fragments. It forms from lava that is cooler, more viscous, or moving faster than pahoehoe, so that its surface tears apart rather than folding. Walking on an aa flow is treacherous, its loose, razor-sharp blocks giving the flow its harsh, forbidding character. The same eruption can produce both aa and pahoehoe as conditions change.
From pahoehoe to aa
A single flow can transform from pahoehoe to aa as it travels. As lava moves downslope and cools, loses gas, or speeds up, smooth pahoehoe can break up into rough aa. This transition is a vivid illustration of how the physical state of the lava, rather than its origin, determines the form it takes, and it helps scientists read the conditions under which a flow formed.
Blocky lava and viscous flows
More silica-rich lavas, such as andesite and dacite, are far more viscous and produce blocky lava flows, with surfaces of large, angular blocks. These thick, slow-moving flows advance only short distances and can build steep-fronted lobes. At the most viscous extreme, such lava barely flows at all and instead piles up into lava domes.
Lava tubes and channels
As a pahoehoe flow advances, its surface can crust over while the molten interior keeps moving, forming a lava tube. These natural conduits insulate the lava, allowing it to travel great distances with little heat loss. When the eruption ends and the lava drains away, it can leave behind hollow tubes and caves, a distinctive legacy of effusive eruptions found at volcanoes like those of Hawaii and Iceland.
Reading the rock
For geologists, the surface texture of a solidified lava flow is a valuable record. The presence of pahoehoe, aa, or blocky lava reveals the composition and behaviour of the lava that formed it, while features like lava tubes, channels, and flow fronts help reconstruct how the eruption unfolded. Reading these forms allows scientists to interpret eruptions that happened long ago.
Explore on the map
Lava flows in all their forms are found at effusive volcanoes worldwide, from the shields of Hawaii and Iceland to the basaltic fields of many volcanic regions. Explore these volcanoes on the interactive map — filter by type to see where the rivers of molten rock have shaped the Earth's surface.