What comes out of a steam vent?

How it works

Near volcanoes, there is very hot rock, and sometimes magma, not far underground. Rainwater soaks down, meets the hot rock and heats up into scalding water and steam. The steam rushes up through cracks in the rock and puffs out of holes in the ground.

Water vapour itself is actually invisible. The white cloud you see forms when the steam meets cooler air and turns into millions of tiny water droplets, like a little cloud. That's why steam vents look most dramatic on cold days.

The underground gases include hydrogen sulfide, a gas containing sulphur that smells like rotten eggs. Yellow crystals of sulphur often form on the rocks around vents. In some spots the gases are strong enough to be harmful, so always stay on the marked paths.

Where to see it in New Zealand

Rotorua is famous for its eggy smell, with steam vents, bubbling mud pools and geysers in and around the city, at places such as Kuirau Park and Whakarewarewa. Near Taupō, geothermal power stations use underground steam to make electricity.

Try it together

On a cold morning, breathe out and watch the white puff appear. After a hot shower, look at the droplets on the mirror. Both work just like a steam vent's 'smoke': invisible water vapour cools and turns into tiny droplets you can see.