Volcano Hazard Zone Calculator

The Volcanic Explosivity Index (VEI), a logarithmic 0–8 scale devised by Newhall & Self in their 1982 paper in the Journal of Geophysical Research and still the scale used by the USGS Volcano Hazards Program and the Smithsonian's Global Volcanism Program to classify eruption size, is the backbone of this calculator: each step up represents roughly a tenfold increase in ejecta volume, from under 10,000 m³ at VEI 0 (gentle effusive lava) to over a trillion cubic meters at VEI 8 (mega-colossal supervolcano eruptions). From your chosen VEI and eruption type — effusive, explosive, phreatic, or pyroclastic, each carrying a different hazard multiplier — the calculator derives an eruption column height and then scales out four separate hazard radii. Pyroclastic flow radius, the most lethal zone since these superheated gas-and-ash flows move at highway speeds and are essentially unsurvivable, grows with both VEI and a type multiplier that's highest for pyroclastic-flow-dominant eruptions. Lahar (volcanic mudflow) distance extends further, especially for snow- and ice-capped summits, since the calculator scales it up with a snow factor tied to your summit elevation input — this is why lower-VEI eruptions at glaciated volcanoes like Nevado del Ruiz can still be catastrophic downstream.

Ashfall radius scales with column height and can extend hundreds of kilometers for large eruptions, while ballistic range (thrown rock and bombs) stays comparatively short, within a few kilometers. The "risk at your distance" score interpolates across these zones to give a single 0–100 number for wherever you specify. Keep in mind this is a simplified empirical scaling model, not a hazard map from an actual volcanic observatory — real hazard zones depend heavily on local topography, prevailing wind, and eruption history that this generalized VEI-based approach cannot capture.