Pahoehoe sounds like a Hawaiian greeting. Aa sounds like the noise you make when you step on it barefoot.
Those aren’t coincidences—both terms come from Hawaiian, where volcanoes have been doing their thing long enough for locals to develop a sophisticated vocabulary for molten rock. Pahoehoe (pronounced pah-hoy-hoy) translates roughly to “smooth, unbroken lava,” while aa (pronounced ah-ah) means “stony rough lava.” The names stuck because Western geologists, arriving in the 19th century, realized Hawaiians had already figured out what they were scrambling to classify.
When Lava Decides Between Silk Sheets and Broken Glass
The difference starts with temperature and viscosity, which sounds boring until you realize it’s the difference between walking across a hardened lava field or shredding your boots. Pahoehoe flows hot—usually above 1,100°C—and behaves like thick honey, creating those gorgeous ropy, billowing surfaces that look almost organic. Its low viscosity means it moves fast, sometimes exceeding 10 kilometers per hour down steep slopes. In 1969, Mauna Ulu on Hawaii’s Big Island produced pahoehoe flows that traveled 12 kilometers to the ocean, taking only hours to make the journey.
Aa, meanwhile, is the volcanic equivalent of a bad attitude.
It flows cooler—often below 1,000°C—and moves slower, fragmenting into sharp, jagged clinkers as it advances. The surface crust breaks apart constantly, tumbling over itself like a slow-motion avalanche of razor-edged rock. During Mount Etna’s 1983 eruption, aa flows crawled through Sicilian villages at barely 100 meters per hour, giving residents time to evacuate but destroying everything in their path with methodical brutality.
The Chemistry That Nobody Talks About But Probably Should
Here’s the thing: both lava types can come from the same eruption. Same magma chamber, same chemical composition, different personalities. The transformation depends on how much the lava gets jostled, cooled, and degassed as it flows. Pahoehoe loses heat and volatiles gradually, maintaining its smooth charactr as gases escape gently. But rough it up—send it down a steep slope, let it cool rapidly, force those gases out violently—and you get aa.
Scientists documented this transformation directly during Kilauea’s Pu’u ‘O’o eruption, which started in 1983 and continued for 35 years. Lava emerged as pahoehoe near the vent, then converted to aa within a few hundred meters as it accelerated downslope. The transition zone looked like someone had run a geological blender across the flow’s surface.
Why Iceland Doesn’t Use Hawaiian Words But Probably Should Consider It
Iceland has its own volcanic terminology—aptly named “hraun” for lava fields—but Icelandic geologists still use pahoehoe and aa because, turns out, Hawaiian volcanologists got there first with the nomenclature. The 2021 Fagradalsfjall eruption produced spectacular pahoehoe flows that mesmerized livestream viewers worldwide, with lava fountains feeding rivers of glowing rock that folded into intricate rope patterns.
Tourists love pahoehoe. It photographs beautifully, with those swirling, almost liquid-looking textures frozen mid-flow. You can walk on cooled pahoehoe in sneakers.
Aa will destroy your sneakers, your ankles, and your enthusiasm for volcano tourism. The Hawaiian Volcano Observatory recorded dozens of injuries from people attempting to cross aa fields without proper gear. Each chunk of aa has multiple sharp edges, and the voids between rocks can swallow a leg up to the knee.
The Industrial Revolution’s Favorite Lava Type That Nobody Knows About
Wait—maybe we should talk about why this matters beyond volcano nerds and unfortunate hikers. Lava type affects everything from hazard prediction to infrastructure planning. Pahoehoe’s fluid nature means it can travel farther and faster, potentially threatening areas kilometers from the vent. But its smooth surface, once cooled, makes decent foundations for roads and buildings. Hawaiian highways regularly cross pahoehoe fields from eruptions centuries old.
Aa’s jagged surface remains nearly unusable without extensive bulldozing and filling. After Kilauea’s 2018 Lower East Rift Zone eruption, recovery crews spent months just clearing aa from roads in Leilani Estates, using heavy machinery to break apart the clinkers. Some homeowners simply abandoned properties buried under aa rather than face the excavation costs.
The 1973 Eldfell eruption in Iceland’s Westman Islands produced mostly aa that buried 400 buildings and threatened the island’s harbor—the economic lifeline for this fishing community. Icelanders responded by pumping seawater onto the advancing aa, cooling it rapidly to halt its progress. It worked, but only because aa’s slower movement gave them time to act. Pahoehoe would have overwhelmed the town before they mobilized the pumps.
