A lava flow is a river of molten rock that moves across the ground after erupting from a volcano or volcanic vent. These flows can travel anywhere from a few meters to tens of kilometers, depending on their composition, temperature, and the terrain they cross. Lava erupts at temperatures up to about 1,140°C (2,080°F) and doesn’t fully solidify until it cools below roughly 1,000°C (1,800°F), a process that can take months for thicker flows.
What Determines How Lava Flows
The single biggest factor controlling a lava flow’s behavior is its silica content. Silica is a mineral compound that acts like a thickener: the more silica in the lava, the more viscous (sticky and slow-moving) it becomes. Basalt lava, which has the lowest silica content, flows the most freely. Rhyolite, at the other end of the spectrum, is so thick it barely moves at all. Temperature matters too. As lava cools even slightly, its viscosity increases dramatically, which is why the leading edge of a flow often creeps along while hotter lava farther back moves faster.
Four main factors control how fast and far lava travels: the lava’s viscosity, the steepness of the slope, whether the flow is spreading out freely or funneled through a narrow channel, and how quickly new lava is being supplied from the vent. Fluid basalt flows can reach 10 km/h (6 mph) on steep slopes, though on gentle terrain they typically move at less than 1 km/h, roughly walking pace. When basalt is confined inside a channel or underground tube on a steep slope, it can exceed 30 km/h (19 mph). Thicker andesite flows manage only a few kilometers per hour and rarely extend more than 8 km (5 miles) from the vent. The thickest lavas, dacite and rhyolite, often pile up over the vent as steep-sided mounds called lava domes, advancing less than a few meters per hour.
Pahoehoe vs. Aa: The Two Main Surface Types
Geologists classify basaltic lava flows into two forms, both named with Hawaiian words. Pahoehoe has a smooth, billowy, or ropy surface. It forms when lava is hot and fluid enough to stretch and fold as it moves, creating textures that look like coiled rope or wrinkled skin. Aa (pronounced “ah-ah”) has a rough, jagged, clinkery surface made of broken, spiny chunks of lava called clinkers. The same eruption can produce both types: pahoehoe often transitions into aa as it travels farther from the vent and loses heat, becoming stiffer and breaking apart rather than flowing smoothly.
When lava erupts underwater or flows into the ocean, it forms a distinctive shape called pillow lava. The outer surface chills almost instantly on contact with water, creating rounded, ellipsoidal masses roughly the size and shape of pillows. These stack tightly together as fresh lava continuously pushes out through cracks in the cooling shell. Pillow lavas are found on ocean floors worldwide and in ancient rock formations that were once submerged.
Lava Tubes and Long-Distance Transport
One of the more remarkable features of lava flows is their ability to build their own underground plumbing. Lava tubes form when a fast-flowing river of lava develops a cooled, hardened crust on top while the interior stays molten and superheated. The process works similarly to ice forming on a winter river: the slower-moving edges cool first, and a solid roof gradually extends inward from both sides until the surface is completely sealed.
Once insulated, the lava inside actually gets hotter because it loses so little heat to the air. This superheated flow can then carve deeper into the ground beneath it through a process called thermal erosion, creating a wider, deeper conduit. Lava tubes are the reason flows can travel great distances from a vent without cooling and stopping. When the eruption eventually ends, the molten lava drains out and leaves behind a hollow tunnel. Some of these tubes are large enough to walk through and have become popular tourist destinations, like those in Hawai’i Volcanoes National Park.
How Lava Flows Cool
Cooling is a slow process, especially for thick flows. During the 2018 eruption of Kīlauea in Hawai’i, lava reached a maximum temperature of about 1,140°C (2,080°F). Once the entire flow cooled below 1,000°C (1,800°F) it had solidified on the surface, but the interior remained extremely hot. For a flow roughly 4.5 meters (15 feet) thick, studies estimated it would take more than 130 days to cool to just 200°C (290°F). Thicker flows take proportionally longer. This means that even months after an eruption ends, the ground can remain too hot to walk on safely.
Damage and Diversion Efforts
Lava flows destroy virtually everything in their path. Unlike other volcanic hazards such as ash fall or gas emissions, a lava flow is a wall of rock that buries structures, roads, and land permanently. The silver lining is that most flows move slowly enough for people to evacuate, though property loss is typically total.
Humans have tried to fight back with mixed results. In 1973, when Eldfell volcano erupted on Iceland’s Heimaey island, crews pumped billions of gallons of seawater onto an advancing aa lava flow for five months to protect the island’s only harbor. The rough, broken surface of the aa flow actually helped, allowing seawater to seep deep into the interior and cool the core, not just the surface. The flow front thickened and slowed as a result. The eruption ended before the harbor was lost, though no one can say for certain whether the effort would have held indefinitely.
On Mount Etna in 1991-1992, Italian authorities tried a different approach when lava threatened the town of Zafferana Etnea, 9 km downslope. Workers built a massive barrier, 234 meters long and 21 meters high, about 2 km above the town. It delayed the flow for a month before being overtopped. Three additional barriers were built and also overtopped. Finally, explosives were used to breach the feeder lava tube supplying the flow, redirecting lava into an artificial channel. After four failed attempts, the fifth succeeded. Cut off from its supply, the flow heading toward Zafferana stalled. By June 1992, the eruption had weakened enough that diversion was no longer necessary.
The lesson from both cases is that lava diversion requires continuous, costly effort sustained for as long as the eruption lasts. A single barrier or a few days of water spraying is not enough. And as the USGS has noted, these efforts may only delay the inevitable if an eruption persists for years rather than months.
What Happens to Land After a Flow
Fresh lava is sterile rock. The common assumption is that it slowly weathers into fertile volcanic soil, but the reality is more nuanced. Research on lava flows in Oregon and Hawai’i has shown that the soil forming on young lava flows comes primarily from external sources, not from the rock breaking down in place. In Hawai’i, windblown dust from as far away as Asia accounts for up to 25% of the soil on volcanic landscapes and provides critical nutrients like phosphorus. Ocean spray contributes calcium, another essential soil nutrient. The lava itself weathers far too slowly to explain the soil and vegetation that eventually colonize it.
Plants can grow on bare lava, but they establish far more successfully where wind, water, or other forces have deposited a layer of sediment on top. In tropical climates with warm temperatures and heavy rainfall, soil develops faster. In cooler, drier environments, a lava flow can remain largely barren for thousands of years. This process, called primary succession, is one of the slowest forms of ecological recovery on Earth.