Pāhoehoe (pronounced “pah-HOY-hoy”) is a type of lava flow with a smooth, billowy, or rope-like surface. It’s one of two main forms that basaltic lava takes, the other being ʻaʻā, which is rough and jagged. Both terms come from Hawaiian, where these lava types are easiest to observe in action. Despite looking completely different once they cool, pāhoehoe and ʻaʻā can have identical chemical compositions. What separates them is how the lava moves and cools.
How Pāhoehoe Forms
When lava first erupts from a volcanic vent, it’s extremely gas-rich, sometimes more than 85 percent bubbles. At this stage the lava is closer to foam than liquid rock. As it flows away from the vent, it cools slowly and moves at a relatively gentle pace, allowing a flexible skin to form on the surface. That skin gets dragged and folded by the still-moving lava underneath, creating the signature ropy wrinkles that make pāhoehoe one of the most photographed geological features on Earth.
The flows are typically thin, ranging from a few inches to a few feet thick, and tend to be narrow and elongated. They advance not as a single wall of material but through small individual lobes or “toes” that break through cracks in the cooled crust and slowly spread outward. These toes creep around obstacles in their path rather than bulldozing through them. Average flow-front speeds range between 1 and 10 meters per hour, though some Hawaiian flows have been clocked at up to 20 meters per hour. For comparison, ʻaʻā flows can move at speeds up to 2,000 meters per hour.
The Role of Lava Tubes
One of pāhoehoe’s most important features is its ability to build lava tubes. As the flow moves away from the vent, it naturally forms channels. A cooled crust eventually roofs over these channels, creating insulated underground tunnels. These tubes keep the lava hot and fluid over surprisingly long distances, allowing flows to travel many kilometers from the eruption site without losing much heat.
Over time, the lava inside these tubes can actually remelt older rock surfaces, widening the tunnels into large conduits. This insulated delivery system is a big reason pāhoehoe flows can reach the ocean. During Kīlauea’s ongoing eruptions in Hawaii, pāhoehoe flows fed by tube systems have repeatedly traveled 11 kilometers or more from the vent to the coast. Once the eruption stops and the tubes drain, they’re left behind as hollow caves, some of which are large enough to walk through.
Surface Types and Subtypes
Not all pāhoehoe looks the same. Geologists recognize several distinct surface forms:
- Ropy pāhoehoe is the classic version, with wrinkled, rope-like folds created as flowing lava drags its cooling skin downstream. It’s the most widely recognized and photographed subtype.
- Shelly pāhoehoe forms in gas-rich flows where bubbles rise and create thin crusts at the surface. Walking on it feels like stepping on large eggshells, and it crunches underfoot.
- Smooth pāhoehoe has a glassy, nearly flat surface with minimal folding.
- Slabby, spiny, and toothpaste pāhoehoe represent transitional forms where the lava was starting to cool or slow in ways that disrupted the smooth surface but didn’t produce full ʻaʻā roughness.
On flat terrain, pāhoehoe can also create features called tumuli: hill- or plateau-shaped mounds that crack and bulge upward as fresh lava continues to inject beneath a thickening crust. These can keep growing as long as lava feeds into them from below.
What Makes It Different From ʻAʻā
The transition from pāhoehoe to ʻaʻā comes down to two factors: how thick (viscous) the lava becomes and how much it’s being deformed as it moves. Pāhoehoe forms when lava is hot, fluid, and moving gently enough that its surface can stretch in a flexible, ductile way. If the lava cools and crystallizes past roughly 50 percent solid crystals, or if it suddenly speeds up (say, by flowing over a steep cliff), it can no longer stretch smoothly. The surface fractures into rough, jagged chunks, and the flow becomes ʻaʻā. This transition is a one-way street: pāhoehoe can become ʻaʻā, but ʻaʻā doesn’t revert back.
The practical difference is dramatic. An ʻaʻā flow advances as a steep wall of broken lava rubble that pushes through everything in its path like a bulldozer. A pāhoehoe flow creeps forward in small lobes, flowing around trees and structures rather than knocking them down. Both are destructive, but they destroy in very different ways.
Temperature and Physical Properties
Active pāhoehoe on Kīlauea has been measured at about 1,144°C (2,091°F), with a viscosity around 380 pascal-seconds. To put that viscosity in perspective, it’s roughly the consistency of thick honey or warm taffy, fluid enough to flow and fold but not so runny that it splashes. This relatively low viscosity is what allows the smooth surface to form in the first place.
Field measurements from Kīlauea’s Puʻu ʻŌʻō eruption in 2016 confirmed these values, and they align with decades of earlier observations on Hawaiian lavas. The effusion rate during active episodes can run about 2 cubic meters per second, with individual flow events resurfacing more than 300,000 square meters of ground per day.
Where to Find Pāhoehoe
Hawaii is the most famous location for pāhoehoe, particularly Kīlauea and Mauna Loa on the Big Island. Kīlauea’s decades-long eruption cycle has produced vast fields of fresh pāhoehoe that are easily accessible in Hawaiʻi Volcanoes National Park. But pāhoehoe isn’t unique to Hawaii. It forms wherever low-viscosity basaltic lava erupts at moderate rates, including Iceland, the Galápagos Islands, and parts of the East African Rift. Older pāhoehoe surfaces can also be found in places like the Columbia River Basalt Group in the Pacific Northwest, though millions of years of weathering have dulled their original textures.
Underwater, the equivalent of pāhoehoe is pillow lava, which forms rounded, bulbous shapes as lava extrudes into cold seawater. The process is similar: a flexible skin forms rapidly on the outside while molten material continues to push through from within.