Explosion at Yellowstone National Park forces tourists to run for safety

A rare explosion of steam, water and rock forced tourists to run for safety at Yellowstone National Park in the US.

Footage showed a column of black and grey vapour rising in the air as people warned each other to "back up".

The hydrothermal explosion happened on Tuesday at Biscuit Basin, a collection of hot springs near the famous Old Faithful Geyser in Wyoming.

A wooden walkway was left covered in silt, but officials said no injuries had been reported as they closed the area.

Yellowstone sits on a dormant volcano, but the explosion - which sent debris an estimated 100 feet into the sky - is not believed to be related to a change in the volcanic system.

What is a hydrothermal explosion?

This is when there is a rapid ejection of boiling water, steam, and rock fragments.

Water with a temperature of around boiling point sitting beneath the ground rapidly turns into steam because of a drop in pressure.

This can be caused due to a clogged passage in the natural network beneath the park, creating a build-up of pressure and eruption of steam.

Read more on Sky News:Video shows Olympian whipping horse in trainingDramatic footage shows whale capsizing boat

How dangerous are Yellowstone's thermal features?

Despite no one being hurt in the explosion, at least 22 people are known to have died from injuries relating to the thermal features since 1890.

Visitors are warned to stick to the designated boardwalks, as some of the features have a thin crust which may be covered with a form of acidic water.