🌋 Lahar: Volcanic Mudflow

About this simulation

A lahar is a fast-moving slurry of volcanic rock, ash, and water that surges down a river valley from a volcano's flanks. Lahars form when heavy rain, a crater-lake breach, or rapid snow/ice melt (often triggered by an eruption) mixes with loose volcanic debris. They can travel tens of kilometres, far outrunning the eruption itself, and are one of the deadliest volcanic hazards — the 1985 Nevado del Ruiz lahar killed over 23,000 people in the town of Armero, Colombia.

How it works

• Slope angle — steeper valley flanks accelerate the flow through gravity.
• Water content — more water lowers viscosity and raises velocity, but dilutes the flow into a faster, less dense hyperconcentrated flood; less water makes a slow, boulder-choked debris flow with immense destructive force.
• Debris supply — how much loose sediment feeds the flow, controlling how thick and continuous the surge appears.
• The brown scar painted onto the valley floor shows the cumulative destructive path — width and darkness scale with how wet and sediment-laden the flow is.

Controls

• Slope angle — steepness of the volcano's flank (5–35°).
• Water content — proportion of water versus solid debris in the flow (10–80%).
• Debris supply — rate at which new material feeds the flow.
• ▶ Trigger lahar — releases a flow from the crater down the valley.
• ↺ Reset slope — clears the deposit trail and rebuilds the terrain.

Did you know?

Lahars can occur years after an eruption, whenever heavy rainfall remobilises loose pyroclastic deposits — which is why volcano observatories keep monitoring slopes long after the ash stops falling.