A swarm of small earthquakes that struck western Saudi Arabia last year was the rumbling of a volcano, geologists said.

More than 30,000 minor quakes occurred between April and June last year within an ancient solidified lava field called Harrat Lunayyir, damaging some buildings in the nearby town of Al Ays and prompting the authorities to evacuate 40,000 people from the region.

Most of the quakes were tiny, measuring less than two on the scale of magnitude, but several were hefty, delivering a jolt of up to 5.4.

US and Saudi geologists probing the incident conclude that ground deformation, detected by satellite radar, and the shockwaves’ seismic signature and depth all point to a cause that is volcanic.

The ground ruptured dramatically along eight kilometres, they found. It ripped open to a width of 45 centimetres as a tentacle of magma probed forward just beneath the surface.

Because magma has now come so close to the surface, the chance of an eruption has increased, the experts say.

But the hazard is low, given the remoteness of the site and the expected type of eruption, a slow-moving lava flow.

Saudi Arabia’s geology is best known for the oil-drenched sedimentary rocks of the east that are the source of its bounty in hydrocarbons.

Less familiar to the general public is the peninsula’s western side, which is home to around 180,000 square kilometres of lava fields, known in Arabic as harrat, that were formed over the past 30 million years.

On a human scale, volcanic eruptions in Saudi Arabia are rare, occurring with frequencies of hundreds of years.

The best known event was a 52-day eruption in 1256 that sent flows of lava “like a red-blue boiling river” into the holy city of Medina, according to contemporary accounts.