The first evidence of a volcanic eruption beneath Antarctica's ice has been discovered by scientists.
Some 2,000 years ago, a volcanic eruption blew a large hole through hundreds of kilometers of the West Antarctic ice sheet, spewing debris into the atmosphere, say the researchers. And the evidence of the explosion they uncovered suggests this was the biggest eruption Antarctica has seen in the last 10,000 years.
The discovery could also explain why a fast-moving nearby glacier has experienced sudden jolts in its journey towards the sea.
West Antarctica rests on a tectonic rift. Several volcanoes protrude through the ice, including the world's southernmost active volcano, Mount Erebus.
But a subglacial volcano -- one that is locked beneath several hundred metres of ice -- is much harder to detect because the ice conceals the volcanic cone.
Hugh Corr and David Vaughan of the British Antarctic Survey made their discovery using a powerful radar system.
Flying over the Hudson Mountains, which separate the East and West Antarctic ice sheets, they detected a layer of debris the size of New Hampshire in the US (23,000 square kilometres), between 100 and 700 meters beneath the surface of the ice.
The radar signal had been noticed before. But it is so strong that previous researchers had thought that it must be bedrock and mapped it as the bottom of the ice sheet on that assumption.
Corr and Vaughan's radar allowed them to see through the signal to the real bedrock beneath. They deduced that instead of marking the bottom of the ice sheet, the signal comes from a layer of debris, 0.3 millimetres at its thinnest, suspended in the ice.
The elliptical shape of the layer, the fact that it thins towards the edges, and the fact that all of the debris was deposited at the same time led the researchers to conclude that this must be a fine sheet of volcanic debris that was propelled into the air by an eruption and settled back down on the ice around the volcano.
From the amount of debris, Corr and Vaughan estimate that the eruption would have been similar in size to the subglacial eruption at Grímsvötn in Iceland in 2004.
That event produced a hole in the ice several hundred metres across and a plume of ash that rose 12 kilometres into the atmosphere.
Immediately beneath the Antarctic dust, some 1,000 metres high, but just 100 metres under the surface of the ice, Corr and Vaughan identified a mountainous peak. They believe this is the top of the volcano.
"In theory, if a volcano had a violent explosion 2000 years ago, it can go off again any time," says Michael Studinger, of the Lamont-Doherty Earth Observatory at Columbia University.
In practice, monitoring the volcano to predict when it will erupt, will be very difficult. "It's in an absolute stinker of a location," says Vaughan.
Although the new volcano is probably the most recent one to have exploded, researchers have known about another subglacial volcano in Antarctica for some time.
Mount Casertz stands some 600 metres high, but the only sign of its existence from the surface is a large depression in the ice above caused by heat from the active cone melting the ice.
The same process could be happening in the ice around the Hudson Mountains volcano. This, say the researchers, could explain why the nearby Pine Island Glacier has experienced sudden accelerations toward the sea twice in the past few decades.
The topography of the bedrock around the volcano indicates that meltwater would flow off the mountain's flanks and beneath the glacier, lubricating its base and speeding up the movement of the ice.
Corr and Vaughan point out that heat from the volcano could not cause the widespread thinning of the ice that is taking place across Western Antarctica. Climate change, they say, remains the most likely explanation for that phenomenon.