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, theGuardian 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.
Seen from the Vatnajökull ice cap, lightning streaks through an ash cloud billowing from the erupting Grímsvötn volcano on May 22.
So-called volcanic lightning is born of the same ingredients as lightning in a regular thunderstorm, Martin Uman, a lightning expert at the University of Florida in Gainesville, told National Geographic News in 2010. Those ingredients include water droplets, ice, and possibly hail all interacting with each other and with airborne particles—in this case ash from the eruptions—to cause electrical charging, Uman said.