A lahar is a slurry of volcanic ash, rock and water that behaves like wet concrete — a non-Newtonian debris flow with real yield strength, not a simple river. This model represents the flow as thousands of particles seeded along a 3D valley mesh, revealed progressively as a front advances downhill; each particle's position, thickness and colour respond live to water content, flow volume and slope.
Trigger a lahar at the head of the valley and watch it surge downhill, thickening the channel, spreading across the flood plain and burying model settlements in its path — the same behaviour recorded at real lahar-prone volcanoes worldwide.
Set Water content for how fluid versus concrete-like the flow is, Flow volume for how much debris is mobilised, and Slope steepness for how aggressive the upper valley is. Press Trigger Lahar to release the flow, drag to orbit the 3D scene, and watch the live readouts.
The 1985 Nevado del Ruiz eruption melted only a fraction of its ice cap, yet the resulting lahars reached Armero — 74 km away — in under three hours, travelling far faster than residents could evacuate on foot.
A lahar is a fast-moving mixture of volcanic debris (ash, pumice, rock) and water that flows down river valleys draining a volcano. It is denser and more viscous than a normal flood — often compared to wet concrete — and can travel tens of kilometres from its source.
Three main triggers: an eruption melting snow or glacial ice on the summit, the sudden breakout of a crater lake, or intense rainfall remobilising loose ash and pyroclastic deposits on the volcano's flanks — sometimes long after the eruption itself.
Higher water content lowers the mixture's yield strength, so the modelled flow moves faster, spreads thinner and wider, and reads more grey-blue. Lower water content keeps more debris in suspension, producing a slower, thicker, browner flow that deposits deeper mud — mirroring real hyperconcentrated-flow versus debris-flow behaviour.
It approximates the total volume of mobilised material in millions of cubic metres (Mm³). Larger volumes activate more particles, spread further past the channel banks, and produce a wider estimated inundation width, similar to how bigger lahars overtop their channels and bury broader areas.
The model reduces flow speed as the valley flattens, matching real lahars: they are fastest and most turbulent on steep upper slopes near the vent, then decelerate and deposit sediment as gradient drops on distal plains and fans.
They are illustrative but grounded in real ranges — historic lahars have reached 20 to 40 m/s (70 to 140 km/h) in steep upper channels and travelled tens of kilometres, matching events like the 1985 Nevado del Ruiz lahars that reached Armero, Colombia in under three hours.
When Nevado del Ruiz erupted on 13 November 1985, hot pyroclastic material melted part of its ice cap. The resulting lahars swept down the Lagunillas valley and buried the town of Armero under metres of mud and debris, killing more than 20,000 people — one of the deadliest volcanic disasters of the 20th century.
It is an educational, qualitative model rather than a calibrated hydraulic solver: it does not solve the full non-Newtonian rheology equations volcanologists use, but it reproduces the right relationships between water content, volume, slope and flow behaviour that make lahars such a severe hazard.