Hundreds of thousands of Americans have been put on alert for a major volcano which could erupt in the coming weeks.
Mount Spurr has seen increased activity in recent months, with regular earthquakes thought to be the early signs it is about to blow.
Alaska’s government has warned locals to watch for any updates, with the risk currently graded as yellow, meaning it is ‘exhibiting signs of elevated unrest above known background level’.
The volcano is only 80 miles from the state capital, Anchorage, where some 300,000 people live.
They are far enough away that they won’t see lava seeping into their homes, but they would be affected by the ash cloud – which could also have much more far-reaching effects.
Last time Mount Spurr erupted in 1992, the ash cover was so great that it was enough to ‘block out the sun for several hours across Cook Inlet, turning day into “night”,’ the Alaska Volcano Observatory (AVO) said.
Before that, the ashfall in the 1953 eruption was about twice as much again, and also caused darkness over at least 12,000 square miles.
Depending on the size of the eruption, the plume could reach up to 50,000ft, or 15km into the sky.
This would have major effects on global aviation, as well as for the local area.
In 2010, the eruption of Eyjafjallajökull volcano in Iceland caused the largest air traffic shutdown since World War Two. Winds spread the ash across much of northern Europe, grounding planes that could not fly through the dusty atmosphere.
The volcano observatory said: ‘In general, the thicker the ashfall, the more problems it will cause on the ground, especially in populated areas.
‘The 1992 ashfall closed the Anchorage airport for almost a day until it could be cleared, and presented air quality problems for months as the ash was resuspended by winds and vehicle traffic.
‘A variety of other impacts are possible including abrasive damage to mechanical equipment, and disruptions to electrical service.’
Earthquakes below the volcano indicate that magma is active beneath the surface.
An AVO update issued yesterday said: ‘Unrest continues at Mount Spurr volcano. Frequent small volcanic earthquakes were detected beneath the volcano over the past day.
‘Most of the earthquakes that are occurring under the volcano are too small to be located. Nothing unusual was observed in cloudy to partly cloudy webcam and satellite views.
‘Results from the most recent overflight on April 24 show that gas emissions are broadly unchanged from March and remain elevated above background levels.’
The volcano, which is covered in snow, has been spotted ‘steaming’ recently, with all eyes on it to see if further warning signs will indicate an even more imminent eruption, which would mean it moves to an orange or red alert.
AVO said: ‘Based on previous eruptions, changes from current activity in the earthquakes, ground deformation, summit lake conditions, and fumarolic activity [volcanic gases escaping] would be expected if magma began to move closer to the surface.
‘Therefore, if an eruption occurred, it would be preceded by additional signals allowing warning.’
The volcano is monitored using web cams, seismic and infrasound sensors, lightning networks and satellite data.
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