A cloud of smoke and ash billows from Iceland's Grímsvötn volcano on Saturday. The country's most active volcano, Grímsvötn sits beneath the Vatnajökull ice cap in the southeastern part of the island. The current eruption is the first for this peak since 2004, according to the New York Times.
In April 2010 another Iceland volcano, Eyjafjallajökull, stirred to life, releasing ash plumes that ultimately grounded a hundred thousand flights. (Related pictures: "Iceland Volcano Spews Giant Ash Clouds [April 2010].")
Grímsvötn's 12-mile-high (19-kilometer-high) ash cloud prompted the country's four international airports to cancel flights on Sunday. Winds are pushing the ash westward toward the United Kingdom, where airline officials are preparing for possible impacts to London's Heathrow Airport by the end of the week, the Guardian newspaper reported. (Read more about why ash is so dangerous to airplanes.)
Even so, Grímsvötn is not expected to hinder air traffic across Europe with the same severity as Eyjafjallajökull's 2010 eruption, the Times reported. For example, the weight of Grímsvötn's ash particles will make them drop to the ground faster, according to the newspaper.