1 Tolbachik, Russia (181)
It is hard to argue with naming an unexpected eruption that produced 20+ km lava flows from multiple fissures the recipient of the 2012 Pliny for Volcanic Event of the Year. In late November, reports began to filter in that a new eruption had started on the slopes of Tolbachik, one of a cluster of Kamchatka volcanoes that includes Bezymianny and Kliuchevskoi. However, unlike its more active neighbors, Tolbachik hadn't erupted in over 36 years. Much like that last eruption in 1976, the latest Tolbachik eruption was a fissure eruption that coalesced into singular vents with long -- and I mean long -- a'a lava flows. As of late December, the eruption was ongoing, steaming away on the snowy slopes of the remote volcano.
Image: Lava flows snaking from the erupting vent on Tolbachik, seen on December 15, 2012. KVERT/Yu. Demyanchuk.