5 January 2013
Robert Wills is a PhD student at Caltech. On 29th December 2012 he was walking with his family on Tennessee Beach, north of the Golden Gate Bridge in San Francisco, California. This beach features a spectacular arch 30 m above the ground cut into the cliff face. The day was stormy with large waves. Robert describes on his blog what happened next:
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. This was exciting enough that we kept our cameras out and ready and our attention on the arch. I was hoping to get another such small rockslide because I liked how it looked in a picture, a brown waterfall emanating from a hole in the wall. 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.
Remarkably, he caught the entire event on camera with an incredible set of photos. I thoroughly recommend that you visit his website to take a look, it actually has two different set of images of the same event (the other set caught by Chris Wills of the California Geological Survey). As a taster I will reproduce (with Robert’s permission) three images from the sequence:
There are many more images on Robert’s website. Do take a look – they are astonishing perspectives on a collapse event.