Hawaii Kilauea volcano spews out blue lava

For several years Paris-based photographer Olivier Grunewald has been documenting the Kawah Ijen volcano in Indonesia, where dazzling, electric-blue fire can often be seen streaming down the mountain at night.

"This blue glow—unusual for a volcano—isn't, of course, lava, as unfortunately can be read on many websites," Grunewald told National Geographic in an email about Kawah Ijen, a volcano on the island of Java.

The glow is actually the light from the combustion of sulfuric gases, Grunewald explained.

Those gases emerge from cracks in the volcano at high pressure and temperature—up to 1,112°F (600°C). When they come in contact with the air, they ignite, sending flames up to 16 feet (5 meters) high.

Some of the gases condense into liquid sulfur, "which continues to burn as it flows down the slopes," said Grunewald, "giving the feeling of lava flowing."