Ask An Expert: How Are Ice Caves Formed?

Ice caves are a popular winter tourist attraction here in Iceland, with visitors travelling from far a wide to witness the other-worldy beauty of these incredible natural structures. Grapevine journalists even had a chance to visit one themselves for this issue. But how are ice caves actually formed? We reached out to Joaquín Belart, a postdoctoral glaciology researcher and Coordinator at the National Land Survey of Iceland, to explain all.

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“Glaciers contain networks of subglacial rivers that merge and unify into a main river when exiting the glacier. Ice caves are most commonly formed at the edge of the glacier, where the subglacial rivers melt the ice as they carve their way out. Other ice caves can be formed as vertical tunnels (also called ‘moulins’) near the glacier margin. Moulins are created when a river runs over the surface of the glacier, and eventually sculpts its way into the glacier, creating a series of galleries. The glacier rivers reduce their flow drastically during the winter, leaving these caves nearly empty of water and making them easier to access. The colder temperatures also make the ice stiffer, which makes the ice caves more stable and safer to visit during the winter.

“Ice caves in Iceland often have a unique feature: they contain tephra, or volcanic ash layers visible within the ice. These are the result of the country’s various volcanic eruptions, and mark the date that the ash originally fell on top of the glacier. So if you see a black line running along the wall of an ice cave, you are looking directly at a piece of volcanic history.”