Bárðarbunga, the huge Icelandic volcano subject to weeks of ominous earthquakes, has erupted.

Iceland raised its aviation alert to red as the volcano stirred to life from beneath the massive Vatnajokull glacier, what experts call a "subglacial eruption." Seismic data show lava is actively melting the ice cap over it.

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Melissa Pfeffer, of the Icelandic Meteorological Office, told reporters it is not clear when the Bárðarbunga eruption will reach open air, given the 330 to 1300-foot thickness of the glacier. Immediate concerns were for a "jokulhlaup," a massive burst of melt-water surging down the slopes of the volcano.

However, the more ice and water there are in a volcanic eruption, the more ash that an eruption is likely to have. A 2010 eruption of the similarly capped Eyjafjallajökull volcano produced an ash cloud responsible for a week of international aviation chaos and 100,000 flights cancelled.

World aviation authorities have since created contingencies for diverting air travel to prevent such delays in the future.

However, it is not known how long Bárðarbunga will continue to erupt. The 1783 eruption of another Icelandic volcano, Laki, lasted eight months. The eruption belched so much ash into the atmosphere and acted as a shield that catastrophically blocked sunlight, which led to the 1784 "Year Without A Summer" and set off famines across Europe.

The Eyjafjallajökull eruption lasted from April to August.

Bárðarbunga is in a remote area of Iceland, and not near population centers. Tourist and hikers were evacuated yesterday.

A no-fly zone has been declared around Bárðarbunga, but standing flight patterns have not yet been affected.

Iceland straddles a point on the Earth's crust where two tectonic plates, the North American and Eurasian, are spreading apart. The result is constant volcanism. Iceland counts 180 volcanoes, of which 18 have erupted since the island was settled in 900 AD.

TagsBardabunga, Iceland, Volcano