The volcanic rock that floats across oceans for years
After Krakatoa erupted in 1883, rafts of floating stone covered kilometres of sea, in places piled 1.5 metres above the waterline. They drifted around the Indian Ocean for up to twenty years with tree trunks riding among them. The stone was pumice, a volcanic glass so full of bubbles that it floats.
Pumice forms when superheated, pressurised magma is blasted out of a volcano and both cools and depressurises in an instant. Dropping the pressure makes dissolved gases such as water and carbon dioxide come out of solution, much like the fizz released when a fizzy drink is opened. Because the rock chills at the same moment, the bubbles are frozen in place inside glass, leaving a solid that is 64 to 85 percent empty space.
Its dark cousin, scoria, comes from runnier magma and has bigger bubbles with thicker walls, so it is denser and usually sinks. Pumice floats until water finally soaks in, which can take years. Some types have long tubular bubbles that give a silky texture and can make the rock look remarkably like wood. The Romans called it spuma maris, froth of the sea, believing it was hardened sea foam, and its modern name traces back to a Latin word linked to foam.
Underwater eruptions can launch vast floating fields without anyone noticing the blast. The Havre Seamount in the Pacific erupted in July 2012, the largest known deep-ocean eruption, and was only discovered when huge sheets of pumice up to five metres thick were spotted at sea. Rafts from eruptions near Tonga in 1979, 1984 and 2006 carried marine life with them as they spread.
On land the rock is simply scooped up. It lies in loose deposits, so open pits need only bulldozers and power shovels, not blasting. Italy and Turkey led world output in 2011 with 4 and 3 million tonnes, out of about 17 million tonnes mined globally.
Source: Pumice
