Seracs are dense, large columns or chunks of ice that form when two crevasses meet on a glacier, or when two glaciers meet. They can also be part of an ice-fall.
You can often see seracs at the terminus of a glacier where multiple crevasses meet and chop into each other, forming discrete blocks of ice. Another common cause for seracs is when a glacier stretches out to turn a corner, widen into a plain or move over a rising convex surface – the stretching breaks the surface tension of the glacier’s rigid upper crust. These seracs can appear anywhere in the glacier’s “body,” not just at its terminus.
Like many gigantic ice phenomena, seracs are gorgeous to look at but should be viewed from a safe distance. The house-sized (or larger) chunks fall without any warning; results are usually fatal. (That said, there are plenty of mountaineering adventure sites that show photos of people skiing or hiking right beneath the overhanging lip of a serac – !)
These photos from Whistler backcountry appear thanks to my friend Lars Goeller, who passionately seeks extreme ski locations and returns with amazing photos and stories.
And here are a couple of close-up images of seracs, found through CreativeCommons.org, so you can see the power of the slow but steady glacial movement – strong enough to crack through ice dozens of feet thick.