A Mountain of Glass Foam
The northeastern coast of Lipari is dominated by a startling landscape: the brilliant white slopes of the former pumice quarries. This is what remains of Monte Pilato, a volcanic cone that has been systematically excavated for centuries, leaving a 200-meter-high rock face that looks like a marble cliff from a distance. The rock is pumice, an igneous stone formed during explosive volcanic eruptions.
Pumice is a foam of volcanic glass. It is created when super-heated, gas-rich magma is violently ejected from a volcano. The rapid decrease in pressure allows dissolved gases like water and carbon dioxide to expand and form bubbles, just like opening a carbonated beverage. This frothy magma then cools and solidifies almost instantly, trapping the gas pockets, or vesicles, within the rock. The composition of Lipari's pumice is rhyolitic, meaning it is high in silica (about 71%) and low in iron and magnesium oxides, which gives it its white color. This high porosity, which can be up to 90% by volume, results in a very low density—often less than that of water, allowing it to float. The last major pumice-forming eruption on Lipari occurred around 700 AD, blanketing parts of the island in a thick layer of this buoyant stone.