'Breathing' magma cap discovered less than 4km under Yellowstone National Park

A "breathing" cap of magma is stopping one of the largest active volcanic systems in the world from exploding in the US, scientists have discovered.

Yellowstone National Park is known to sit on top of a huge pool of active magma, which causes bubbling mud pots and steaming geysers. Now, geologists have discovered a cap of magma rich with volatiles, or dissolved gases that can cause eruptions, just 3.8km below Yellowstone's surface.

It is thought this cap may be helping to stop the volcanic system from erupting by constantly releasing gases - and pressure - from underneath its surface. "It looks like the system is efficiently venting gas through cracks and channels between mineral crystals," said researcher Brandon Schmandt from Rice University in Texas.

He likened the system to "steady breathing" with bubbles rising and releasing through the rock. Researchers from Rice University, the University of New Mexico, the University of Utah and the University of Texas at Dallas published their findings in the Nature science journal.

In order to make their discovery, the team used a 53,000lb vibroseis truck, usually used for oil and gas exploration, to create tiny earthquakes and send seismic waves underground. Read more from Sky News:Ofcom sets out new rules to protect children onlineCould the local elections reshape British politics?Thousands evacuated in New Jersey Because Yellowstone is so busy and big, they had to do the experiment at night and use more than 600 seismometers to collect their data.

The seismic waves reflected off layers underneath the Earth's surface and were then recorded at the surface.They revealed the cap to be at around 3.8km depth. Previously, scientists thought the top of Yellowstone's magma system could be anywhere between three to eight kilometres deep.

This uncertainty meant it was hard to tell how the magma system today compares with conditions before eruptions in the past..