A series of quakes in Saudi Arabia is a reminder that ancient volcanic fields aren't never fully asleep.
A swarm of 30,000 earthquakes in northwest Saudi Arabia stunned geologists.
The quakes were caused by magma pushing up through the crust, causing a five-mile-long crack to appear.
Other places might be just as vulnerable to surprise volcanoes and earthquakes.
In May 2009 an ancient, dead lava field in northwest Saudi Arabia woke up and triggered 30,000 small earthquakes, creating a jagged five-mile-long crack in the ground.
Despite being far from any currently active volcanic areas, the Harrat Lunayyir lava field shows all the signs of being charged up from below with a fresh supply of magma in preparation for a new eruption.
The Harrat Lunayyir reawakening is a reminder to people all over the world that just about any volcanic field could come back to life, said U.S. Geological Survey geologist John Pallister.
In the United States, for instance, many cities in western states, like Albuquerque, New Mexico, have been built right up to the edge of an ancient lava fields not at all unlike Harrat Lunayyir, he said.
And there is no reason to believe what happened at Harrat Lunayyir can't happen elsewhere, he said.
"It's the kind of thing you see in a Hollywood movie and rarely in real life," said Pallister who helped the Saudi Geological Survey study the event.
The Saudi government evacuated 40,000 people, just in case there was an eruption.
The swarm of earthquakes -- none exceeding moderate strength -- is exactly the sort of behavior seen around volcanoes like Mount St. Helens or Pinatubo when magma is squeezing up and getting nearer the surface. In fact, the quakes are made by the magma moving.
This is in contrast to the more familiar tectonic earthquakes, which are caused by blocks of crust grinding against each other along a fault, Pallister explained.
The Saudi event is evidence that old remnants of the Red Sea rift zone are still active, despite having moved far from where the major rifting is happening.
"What it tells is us is that we're still getting dribbles," said Pallister of the Harrat Lunayyir event. And although the people evacuated have returned to their homes, "There's potential for eruption in the next several decades."
That's something that needs to be taken into account as development is planned in some other areas, said geologist Cynthia Ebinger of the University of Rochester. Up to now such large ground changes from lots of small quakes has affected only a few stick huts. But that could change.
"There are a lot of things being planned," said Ebinger. "There are other factors like this close to habitations."
As soon as the quakes started, the Saudi Geological Survey got to work and installed extra seismic sensors, Pallister told Discovery News. The data from the seismic network, plus satellite data on the changes in the surface of the ground, were put together with field observations into a report on the unusual incident, which appears in the Sept. 26 issue of Nature Geoscience.
Seismological data pinpointed the moving magma at two kilometers (1.24 miles) below the surface, said Pallister. When that magma was pushed into position, it caused the ground above it to deform -- hence the big crack -- far larger than would be expected if these were just a tectonic earthquakes.
"The deformation is disproportionate to the size of the earthquake," Ebinger observed. Ebinger has studied similar but more damaging intrusions of magma in Ethiopia along the southernmost part of the Red Sea. These and the Saudi event were created by the same volcanic "hotspot" which has caused the crust of the Earth there to weaken and spread apart, with volcanic eruptions filling the void.
In the Afar region of Kenya similar events have produced quakes of less than magnitude 5, yet left fissures in the ground up to 5 meters wide, Ebinger said.