A lava flow is a stream of molten rock that moves across the ground after erupting from a volcano or volcanic vent. Lava temperatures range from about 650°C to 1,200°C (1,200°F to 2,200°F) depending on the type, and flows can travel anywhere from a slow crawl to 6 miles per hour or faster. Once the molten rock stops moving, it cools and hardens into solid volcanic rock, a process that can take months to decades depending on thickness.
What Lava Is Made Of
Lava is primarily molten silica (silicon dioxide) mixed with varying amounts of iron, magnesium, calcium, potassium, and sodium. The percentage of silica in the melt is the single most important factor in determining how a lava flow behaves. There are three main types:
- Basaltic lava contains 45 to 55% silica, runs at 1,000 to 1,200°C, and flows easily like hot syrup. This is the most common type seen in Hawaii and Iceland.
- Andesitic lava contains 55 to 65% silica, runs at 800 to 1,000°C, and is thicker and slower. It’s common at stratovolcanoes like those in the Cascades and Andes.
- Rhyolitic lava contains 65 to 75% silica, runs at just 650 to 800°C, and is so thick it barely flows at all. It tends to pile up near the vent or erupt explosively.
The silica content matters because silica molecules link together into long chains inside the melt, creating internal friction. More silica means more of these chains, which makes the lava stiffer and more resistant to flow. Think of the difference between pouring water and pouring cold honey. That stiffness, called viscosity, controls nearly everything about how a lava flow moves, how far it reaches, and how dangerous it is.
Why Some Flows Move Fast and Others Crawl
Three factors control how quickly lava travels: its silica content, its temperature, and how much gas is dissolved in it. Higher temperatures make lava more fluid, just as heating honey makes it pour more easily. Low-silica basaltic lava combines high temperature with low viscosity, so it can cover ground quickly.
The fastest recorded lava flow in Hawaii came from the 1950 Mauna Loa eruption, where the flow front advanced from vent to highway at an average of 6 miles per hour. Inside lava channels, where the flow is insulated and concentrated, speeds have been clocked at nearly 35 miles per hour during the 1984 Mauna Loa eruption. By contrast, typical flows from Kīlauea’s Pu’u ‘Ō’ō vent advanced at less than a third of a mile per hour, and pahoehoe flows are even slower than that.
Dissolved gas plays a more complex role. In runny basaltic lava, gas bubbles rise through the melt and escape easily at the surface. In thick, silica-rich lava, the gas gets trapped. Pressure builds until the gas escapes violently, which is why high-silica volcanoes tend to produce explosive eruptions rather than flowing lava.
Pahoehoe vs. ‘A’ā: Two Textures of Flow
Basaltic lava produces two distinct flow types, both named with Hawaiian words. Pahoehoe (pah-HOY-hoy) has a smooth, billowy, ropy surface that’s been compared to the top of a pan of brownies. ‘A’ā (AH-ah) has a rough, jagged, clinkery surface made of broken rock chunks.
The difference comes down to how fast lava is being discharged. When the eruption rate is low, pahoehoe develops. The flow advances by squeezing out individual toes or lobes of lava through small cracks in its cooled crust. When the eruption rate is high, ‘a’ā forms instead. The entire flow front fractures and advances as a single steep wall of lava chunks, pushing through whatever is in its path like a bulldozer.
Under a microscope, the two types look different too. Pahoehoe traps more gas bubbles because they don’t have time to escape, leaving behind small, roughly spherical voids in the rock. ‘A’ā has fewer bubbles, and the ones present are stretched into irregular shapes from the constant churning of the flow. ‘A’ā also contains more crystals because it loses heat faster. A flow can start as pahoehoe and transition to ‘a’ā as it moves downhill, loses heat, or hits steeper terrain, but the reverse almost never happens.
Pillow Lava: Flows Underwater
When lava erupts directly into water, or flows from land into the ocean, it forms rounded, stacked masses roughly the size and shape of pillows. This happens because the outer surface of the lava chills almost instantly on contact with water, forming a glassy skin. Molten lava continues to push through, breaking out of the skin and forming another pillow on top. The result is a pile of closely packed ellipsoidal shapes, sometimes stacked meters deep on the ocean floor. Pillow lavas are some of the most common volcanic rocks on Earth because most volcanic eruptions actually happen along mid-ocean ridges, far from view.
How Long Lava Takes to Cool
A lava flow develops a solid crust quickly, sometimes within hours, but the interior stays molten far longer than most people expect. The crust acts as an extremely effective insulator, trapping heat inside.
USGS data from Hawaii’s 2018 eruption illustrates the timeline. A flow about 4.5 meters (15 feet) thick takes over 130 days just to cool to 200°C, which is still far too hot to walk on. Flows 10 to 15 meters (33 to 50 feet) thick take roughly 8 months to a year and a half to fully solidify. Flows 20 to 30 meters thick could take 2.5 to 6 years. The thickest flows from that eruption, approximately 55 meters (180 feet), may take around 20 years to reach a completely solid state.
For perspective, after the 1959 Kīlauea Iki eruption, the lava lake was about 135 meters (440 feet) deep. It took roughly 35 years to fully solidify, and the interior could still be hot enough today that the rock glows. Reaching actual ambient temperature takes even longer.
What Lava Flows Destroy
Lava flows are among the most destructive volcanic hazards, not because they move fast (most don’t), but because they are unstoppable. Everything in the path of an advancing flow gets knocked over, surrounded, buried, or ignited. Homes, roads, and agricultural land disappear under meters of hardened black rock. Property lines and landmarks become unrecognizable beneath a new, rough landscape.
There’s also an underappreciated hazard: as lava buries vegetation, it generates methane gas. That methane can migrate through underground voids and pockets, then explode when it encounters enough heat. These methane explosions were a real concern during the 2018 Kīlauea eruption, creating dangers well beyond the visible edge of the flow.
Iceland’s ongoing eruptions on the Sundhnúkur crater row offer a recent example of the scale involved. By July 2025, nine eruptions had produced an estimated 26.8 million cubic meters of new lava covering about 3.3 square kilometers. During active periods, lava was flowing at roughly 12 cubic meters per second, and flows from the January 2024 and April 2025 eruptions continue to pose hazards to the nearby town of Grindavík.
How Scientists Track Lava Flows
Volcanologists use a combination of satellite and airborne tools to monitor where lava is going and how fast it’s moving. LiDAR (light detection and ranging) is one of the primary technologies. It fires laser pulses from aircraft or satellites to build highly detailed 3D maps of the terrain, allowing scientists to measure flow thickness, volume, and boundaries with precision. At Mount Etna, researchers have used LiDAR not just to map active flows but also to identify and date older flows based on how their surfaces reflect laser light.
Radar satellites provide another critical tool. Interferometric synthetic aperture radar (InSAR) can detect ground deformation of just a few millimeters, revealing where magma is accumulating underground before an eruption even starts. Thermal imaging from aircraft and satellites tracks the heat signature of active flows in real time, and hyperspectral sensors can map the mineral composition of cooled lava surfaces. Together, these technologies give monitoring agencies the ability to forecast flow paths and issue warnings hours to days before lava reaches populated areas.