Over summer the beaches of Sydney have seen the arrival of a ‘pumice raft’. The high tide line has been marked by a distinctive row of small light weight rocks which floated in on the tide. The phenomenon caused much comment amongst beach goers and gave children an exciting new material for their sandcastles. As usual a search in the Powerhouse Museum collection turned up something interesting; samples of pumice collected in 1908 by the party who made the first ascent of Mount Erebus in Antarctica. The party included Sir Douglas Mawson and Dr T. W. Edgeworth-David and the climb was undertaken during Sir Ernest Shackleton’s British Antarctic Expedition.

Mount Erebus is an active volcano and Antarctica’s second highest peak at 3,794 metres above sea level. The climbing party consisted of six men including geologists T W Edgeworth David and Douglas Mawson. They took six days to climb the volcano and one to descend it, enduring extreme temperatures, ‘lava bombs’ and billowing clouds of steam in the process. Considering the perils the explorers faced the party escaped relatively unharmed and all managed to make it to the summit except for Phillip Brocklehurst who suffered frost-bitten toes on the fifth day of the expedition (his 21st birthday) and ended up having one of his big toes amputated on return to the base. The pumice samples in our collection were presented to the museum by Sir Ernest Shackleton himself through the University of Sydney in 1909.

Pumice is formed in violent volcanic explosions by the rapid cooling and depressurisation of super heated material (usually silica). As this material meets the atmosphere trapped gasses ‘boil’ away leaving a very porous, lightweight rock that will float on water. The pumice washing up in Sydney is from an underwater volcano, the Havre Seamount which lies about 800km North/East of New Zealand. The Havre Seamount erupted in July 2012 forming an enormous pumice raft, variously estimated to be between 19,000 and 26,000 square Kilometres in surface area.

Post by Lynne McNairn, Web and Social Technologies