A lava dome is a mound of thick, slow-moving lava that piles up directly over a volcanic vent instead of flowing away like the rivers of lava you might picture in Hawaii. Because the lava is too viscous to travel far, it bulges upward and outward into a steep-sided, roughly circular mass that can grow from a few meters to several kilometers across. Lava domes are one of the more quietly dramatic features in volcanology, building over weeks, months, or even years, and they can be surprisingly dangerous.
How a Lava Dome Forms
Most lava flows are fed by relatively fluid basaltic magma, the kind that runs downhill in glowing streams. Lava domes form from something much stickier. The magma is typically rich in silica (usually dacite or rhyolite), which makes it so viscous that instead of spreading across the landscape, it squeezes out of the vent like toothpaste and heaps up around the opening.
The dome grows in one of two ways. In endogenous growth, fresh magma pushes into the dome’s interior, inflating it from the inside like a balloon. This creates an onion-like layered structure, with older material shoved outward by newer material at the core. In exogenous growth, new lava breaks through the surface and piles on as visible lobes. The distinction matters for safety: exogenous growth tends to build steeper, less stable shapes that can collapse into fast-moving avalanches of hot rock. Endogenous growth, on the other hand, can trap volcanic gases inside the dome, raising the risk of explosive blowouts.
Four Main Types of Lava Domes
Not all lava domes look the same. Geologists recognize four broad types based on shape and behavior.
- Tortas (low lava domes): Named after the Spanish word for cake, these form on relatively flat ground. The lava spreads outward but not far, producing a flat-topped, roughly circular mound. They range from a few meters thick to nearly a kilometer, with diameters stretching several kilometers. They grow from the inside out, filling in near the vent and pushing older layers to the edges. The Andes mountains have many classic examples.
- Peléean domes: Named after the famous 1902 dome on Mount Pelée in Martinique, these are the steepest of all lava domes. They look circular like tortas from above, but instead of a flat top, they sprout tall vertical spines of solidified lava that give them a jagged, craggy profile. Those spines frequently crumble, leaving rings of rocky debris (talus) around the base. Peléean domes usually form on large composite volcanoes. The lava dome that grew at Mount St. Helens between 2004 and 2008 is a modern example.
- Coulées: These are a hybrid between a dome and a lava flow. When thick lava erupts on a steep slope, gravity can coax it slowly downhill. Most coulées travel only a few kilometers, though some reach well beyond 10 km. A striking example is Chao in the Andes, which oozed nearly 14 km downslope with a flow front over 700 meters high. Coulées often display huge parallel ridges on their surface called ogives, formed by the intense pressure of the creeping flow.
- Upheaved plugs: The rarest type. The lava here is so stiff that it gets pushed straight up out of the vent like a piston, sometimes carrying chunks of surrounding rock along with it. These pillar-like extrusions can rise dramatically above the surface before eventually toppling.
How Big Can They Get?
Lava domes vary enormously in size. Some are modest bumps a few tens of meters across. Others are massive. The dome that grew inside Mount St. Helens’ crater starting in October 1980 reached about 305 meters (1,000 feet) above the crater floor over the course of several eruptive episodes, each adding millions of cubic meters of new lava. The spine that grew on Mount Pelée beginning in November 1902 rose to roughly 305 meters (1,000 feet) before it collapsed in mid-1903.
Growth rates vary too. When Chile’s Chaitén volcano erupted in 2008, its dome grew at an estimated 45 to 66 cubic meters per second during the first few months, one of the highest rates ever recorded for a dome-building eruption. By contrast, some domes creep along so slowly that changes are only detectable with instruments.
Why Lava Domes Are Dangerous
Lava domes don’t produce the dramatic fountains or fast-moving lava flows that dominate disaster movies, but they are responsible for some of the deadliest volcanic events in recorded history. The core problem is instability. As a dome grows, its steep flanks can collapse without much warning, sending avalanches of superheated rock and gas (called pyroclastic flows) racing downhill at highway speeds. The 1902 collapse of Mount Pelée’s dome destroyed the city of St. Pierre on Martinique, killing roughly 30,000 people.
Even when a dome doesn’t collapse outright, trapped gases can build pressure inside it until the dome explodes. This combination of collapse risk and explosion risk makes actively growing lava domes one of the most closely watched volcanic hazards.
How Scientists Monitor Dome Growth
Because the shift between quiet growth and catastrophic failure can happen quickly, volcanologists use an array of tools to track dome behavior in real time. GPS stations record the precise three-dimensional position of points on or near the dome’s surface. By comparing positions over time, scientists can detect bulging, tilting, or sliding that might signal an impending collapse.
Tiltmeters, which work like ultra-sensitive carpenter’s levels, pick up subtle changes in the angle of the ground. During the 2004 to 2008 dome-building eruption at Mount St. Helens, tiltmeters recorded thousands of tiny tilting events alongside the unusually regular “drumbeat” earthquakes that accompanied the eruption. Satellite imagery, including infrared sensors that can map heat patterns across a dome’s surface, helps scientists estimate whether growth is happening internally or on the surface, which in turn shapes hazard forecasts.
Getting instruments close to an active dome is risky, so during the Mount St. Helens eruption, the Cascades Volcano Observatory developed portable instrument packages nicknamed “spiders.” These self-contained GPS and sensor bundles could be slung beneath a helicopter and dropped onto otherwise inaccessible terrain near the vent, giving scientists real-time data without putting anyone on the ground.
How Domes Differ From Other Volcanic Features
It’s easy to confuse a lava dome with a cinder cone or a small shield volcano, but they form through very different processes. A cinder cone is built from fragments of volcanic ash, cinders, and other debris (collectively called tephra) that pile up around a vent during explosive eruptions. The material is loose and granular, not solid lava. Shield volcanoes sit at the opposite extreme: they’re made almost entirely of very fluid lava flows that travel great distances and build broad, gently sloping mountains, like those in Hawaii.
A lava dome falls in between. It’s made of solid lava, not loose debris, but that lava is far too thick to flow more than a short distance. The result is a compact, steep-sided mass that stays close to its vent. Some lava domes sit alone, while others grow inside the craters of larger composite volcanoes, plugging the vent like a cork and setting the stage for future explosive eruptions.