2. El Hierro, Canary Islands (194)
We might not have had much warning when it came to Nabro, but the eruption to the south of El Hierro in the Canary Islands had months of precursory seismicity that had many watching the island closely to see if and when an eruption might occur. As early as the end of July and the beginning of August, thousands of small earthquakes had occurred under the island that had not seen an eruption in potentially 2,500 years. By late August, deformation was observed at the volcano, a sure sign of magma intrusion and by October, a submarine eruption had started to the south of El Hierro. As the eruption continued, the submarine eruption produced "jacuzzis" (see above) and strange "coconuts" were found on the surface, likely the product of mixing magmas, but by December, it appeared that the eruption was ending with most of the activity occurring beneath hundreds of meter of seawater.
Image: A "jacuzzi" formed by submarine eruptions to the south of El Hierro as seen on November 23, 2011. Image courtesy of INVOLCAN.