Types of Lava

Aa is the lava that earns its name. Walk across a cooled aa flow and you'll understand — the surface is a chaotic pile of jagged, angular rubble called clinker that will shred boots and skin alike. The Hawaiian name supposedly mimics the cry of pain from crossing it barefoot.

Aa forms when pahoehoe lava loses heat, gains viscosity, and gets churned by the flow's internal shear forces. The surface breaks into rough, angular fragments while the interior stays molten and continues advancing like a slow-motion bulldozer, with the clinker falling off the advancing front and being overridden. Aa flows are thicker and slower than pahoehoe but carry more mass and are far more destructive to anything in their path.

Where to see it: Mount Etna in Italy is an aa factory — most of its recent flows have been aa, including the 2021 eruptions that threatened communities on the volcano's eastern flank. In Hawaii, the 2018 Kilauea lower East Rift Zone eruption produced massive aa flows that buried the Leilani Estates subdivision.

Blocky lava looks like someone dumped a truckload of angular boulders down a hillside — because that's essentially what happens. The blocks have smooth faces (unlike aa's jagged clinker) and form from more evolved, silica-rich lava: andesite, dacite, or rhyolite. This is the lava of stratovolcanoes.

The key difference from aa is composition. Aa is basaltic; blocky lava is felsic. Higher silica content means higher viscosity, lower temperature, and a flow that barely moves. Blocky flows often pile up near the vent as thick, stubby lobes. Some barely flow at all — they're essentially lava domes that ooze over their own margins. The blocks form because the stiff surface fractures into regular, smooth-faced fragments as the interior slowly moves.

Where to see it: The lava dome growing inside Mount St. Helens' crater (2004–2008) is a textbook example of blocky lava — dacitic composition, incredibly viscous, extruding at a rate of about 5 cubic meters per second. Japan's Unzen and Montserrat's Soufrière Hills have produced similar blocky dome complexes.

Pillow lava is the most common lava type on Earth — and the one you're least likely to see being made. It forms when lava erupts underwater (or under ice), and the cold water instantly quenches the outer surface into a glassy rind while molten lava continues to push out from within, forming rounded, pillow-like blobs that stack on top of each other.

Here's a number that puts it in perspective: roughly 70% of Earth's surface is covered by oceanic crust made primarily of pillow basalt. Every mid-ocean ridge — the entire 65,000 km global network — produces pillow lava constantly. It's the most voluminous volcanic product on the planet, but almost all of it happens 2–3 km below sea level where nobody sees it.

Where to see it: The best places to see ancient pillow lava on land are ophiolites — slabs of oceanic crust that tectonic forces have thrust onto continents. The Troodos Mountains in Cyprus have world-famous exposed pillow lavas. Iceland's Reykjanes Peninsula has submarine-erupted pillow lavas exposed by post-glacial land uplift. More dramatically, the 2016–17 Bogoslof eruption in Alaska's Aleutians produced pillow lava during a submarine phase.