Sulfur and metals can hitch a ride on bubbles rising in molten magma. This could explain why some volcanoes spew out more sulfur than expected, and how metal ores can form in the crust nearby.

Sulfur-rich magma normally sinks to the bottom of magma chambers. A team led by Jim Mungall at the University of Toronto in Canada used lab studies and mathematical modelling to show that magma droplets, which contain metals, can form on the surface of vapour bubbles. Droplets that do not reach the surface cool and form rocks that are rich in sulfur, copper and gold.

In another study, Jon Blundy and his team at the University of Bristol, UK, used lab experiments to conclude that sulfur-rich gases interact with salty, copper-rich fluids inside a magma chamber to form thick deposits of copper-based minerals — similar to those that provide three-quarters of the world's copper.

Nature Geosci. http://dx.doi.org/10.1038/ngeo2373; http://dx.doi.org/10.1038/ngeo2351 (2015)