Cosmic rays reveal erupting volcano's guts

COSMIC radiation has been used to peer inside an erupting volcano, a technique that could allow eruptions to be predicted.

Short-lived particles called muons are produced in Earth's atmosphere when charged particles from space slam into gas molecules. These muons can travel through solid rock, though some get absorbed, and the percentage lost depends on the mass of material along their path.

A team led by Hiroyuki Tanaka of the University of Tokyo, Japan, had already shown that muons can reveal the mass of material inside a volcano. Now the team reports observations of Japan's mount Asama during an eruption on 2 February that spewed ash up to 20 kilometres away.

Measurements from before and after the eruption show that between 11,000 and 70,000 tonnes of material left the volcano, agreeing with estimates of the total ash fall at 50,000 tonnes.

The technique might eventually be able to show shifting magma or other changes portending an eruption, says physicist Roy Schwitters of the University of Texas in Austin. Muons are easier to interpret than seismic waves. "The images you get are not confused by whether the rock is fractured or other details," Schwitters says.