Geology.com has a spectacular sequence of images of a rockfall in progress in Yosemite National Park that are well worth a look. The images (which are explicitly copyrighted, so I have not reproduced them here) were collected by a well-known photographer, Herb Dunn, in the canyon of the Merced River.
The pictures show a fall from what appears to be a previously active rockfall scar. Below the scar there is a large area of rockfall debris. The falls appear to occur in granitic rocks, with the detachment being controlled by a series of large joints.
The failure in this case was apparently quite large - the bock was 30 m x 15 m - and appears to have come from a wedge failure at the top of the scar. As a result of the large fall height and substantial volume, the boulders have fragmented as they have impacted the slope, creating a spectacular plume of dust that rolls downwards. The rocks also disturb the accumulated and weathered scree that they hit, which means that the dust cloud starts to change colour in the third image as the discoloured material is kicked up. The fifth image captures some airborne blocks (see the bottom of the image in the centre), which are quite impressive.
Note that the failure has happened on a beautiful sunny day in August, so the trigger mechanism is not really obvious (thermal expansion maybe, or perhaps just progressive failure).
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