The Mid-Atlantic Ridge is a mostly-underwater mountain range formed by plate tectonics. Magma emerges through the crust from rifts along the ridge, cooling to create new crust. Here the ridge crosses Iceland.

Photo: Trey Ratcliff, http://www.flickr.com/photos/stuckincustoms/2245301149/. Licensed under Creative Commons Attribution-Noncommercial-Share Alike 2.0 Generic