Above is the natural arch at Tennessee Beach, California, before it collapsed on Dec. 29, 2012, and…

…this above is the same seaside cliff minutes after tons of rock came tumbling down onto the beach.

The entire collapse was witnessed by Robert Wills, a PhD student at Caltech. Here is an excerpt from his account:

“As we were admiring the waves, a crack and the ensuing sound of a waterfall of rocks caught our attention and everyone on the beach spun to see a small stream of rock flowing down the cliff face below the arch… I could never have expected what happened next. Two minutes later, just as I was hoping, a few chunks of rock fell from the underside of the arch and I started my camera. Then the arch started to buckle in on itself and squeeze out rock from beneath it. With this, the collapse of the arch shortly followed and the entire surrounding hillside started to slip off into the ocean in a thundering roar as boulders the size of a piano crashed into the surf and the sand sending up a large splash of debris that got me a little nervous despite my 100-meter distance. It all lasted less than 10 seconds and left the beach quiet in comparison, the roar of the surf nothing compared to the thunder of the rockslide. The small crowd of 15 or 20 people stood in awe, wondering if the show was over. The main event was over, with only one smaller rockfall in the next 20 minutes, but the scene was still fascinating, with waves washing over the fresh pile of rock turning the ocean brown.”

Below is a small version of the entire sequence. The larger version can be found here. Thanks to landslide researcher Dave Petley of The Landslide Blog for bringing this to my attention. Petley’s blog is filled with rare events like this one, which are increasingly being captured on camera.

Image courtesy: Chris Wills, California Geological Survey