Water, ash, and the great unknown of explosive volcanic eruptions

This section of brown Icelandic soil (top) contains 800 years of ash deposits erupted from five different volcanoes. The black layers, 5-10 cm (2-4 in) thick, are from Katla Volcano. A white arrow points to a closeup of the 1755 Katla ash deposit (lower left). The ash looks like specks of dust in the sample bag (lower right), but microprobe imaging reveals how complex the grain shapes are. Photos by J. Schmith.