Here's the most violent event ever observed on Mars, tons of rock, dust and ice plummeting down a 700-meter cliff at 15 meters per second. It's one of four avalanches the High Resolution Imaging Science Experiment (HiRISE) camera on the Mars orbiter observed on the north polar scarps of Mars. The white material at the top of the cliff is carbon-dioxide ice, and it's possible spring sunshine caused the ice to expand and break, sending loose dust and ice hurtling down the slope. Another possible explanation: A Marsquake. Click through for the full set of images.
You can see in the bottom picture that the one cloud of debris is originating from the middle of the cliff face. It's harder to tell with the top one, but neither appear to be coming from the layer of frozen CO2 at the top of the cliff. It might be a quake since both avalanches are occurring simultaneously several hundred meters apart, but it looks more energetic. With the cliff illuminated by the Sun, it could be some a couple of pockets of volatiles heating up enough to blow out once the surface had been illuminated for a while. Damned coincidental. Too bad they couldn't keep the camera pointed there for a couple of hours to see how often that kind of thing happens.