In a remote corner of Tanzania stands an astonishing mountain called Ol Doinyo Lengai, where lava fountains harden in midair then shatter like glass.

Unlike other volcanoes, which spew silica-rich basalts, Ol Doinyo Lengai is the only active volcano on Earth that erupts with a carbonatite, a type of igneous rock rich in carbonates such as calcite and dolomite. Usually carbonatites are intruded into other formations and rarely reach Earth's surface as eruptions.