Work out volcano eruption volume instantly with clear inputs, formula shown and shareable results.
Eruption volume is deposit area times average thickness, but tephra is porous, so the figure is converted to dense rock equivalent by scaling with the density ratio against magma at about 2500 kg/m3. 120 square kilometres at 0.35 m gives 0.042 cubic kilometres bulk, about 0.017 DRE - a VEI 4 event.
Bulk volume
volume = deposit area x average thickness
Dense rock equivalent
DRE = bulk volume x deposit density / magma density
Because tephra deposits are up to 60% void space. Comparing eruptions needs the equivalent volume of solid magma.
By erupted bulk volume on a logarithmic scale: VEI 4 is 0.1-1 km3, VEI 5 is 1-10 km3, and each step is a tenfold increase.