15 November 2019
The Blackhawk landslide
Posted by Dave Petley
The Blackhawk landslide
The Blackhawk landslide is without doubt one of the most impressive rock avalanches on Earth. It is located at 34.393, -116.773 in the Lucerne Valley on the escarpment that divides the San Bernardino Mountains to the south from the Mojave Desert to the north, in California. This is a Google Earth image of the landslide source and deposit:-
A Google Earth image of the Blackhawk landslide in the Lucerne Valley of California.
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As the image shows, this is a landslide on a very grand scale. With an estimated volume of about 300 million cubic metres, the landslide extends over a distance of about 9 km from the crown to the toe, with a deposit that is up to 2 km wide and up to 30 m thick. The fall height is estimated to be about 1,200 m in total, making this a highly mobile landslide.
The event is not recent. Dating of the deposit suggests that it may have occurred about 18,000 years ago, although there is huge uncertainty in that date. However, in the dry desert environment of inland California the landslide is exceptionally well-preserved, and of course the landslide mass contrasts with the valley floor, rendering the mass highly visible.
This is a landslide that is surprisingly poorly investigated. There is a very nice PhD thesis from 1959 by Ronald Shreeve that is online, which describes the geology and mechanics of the landslide. This was in the days in which a PhD thesis could be just 84 pages long (there are lessons to learn from that!). Shreeve describes the geology, and tries to explain the exceptional mobility of the landslide, hypothesizing that it may have moved on a lubricating basal air layer. There is also a nice blog article about it on the excellent Looking for Detachment blog. Finally, there is a book chapter from 1978 about the landslide by Brann Johnson, but even the University of Sheffield subscription to Science Direct does not provide access to that one.
This is landslide that would really benefit from a revisit using up to date techniques. There is exquisite hummocky morphology in the landslide deposit, captured well in the this 2006 Google Earth image:-
Google Earth image of the Blackhawk landslide in California
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And to me, the now dissected landslide source area suggests that this might have been quite a complex event too. This is a Google Earth image of the landslide scar area; I have annotated the approximate boundaries of the upper part of the deposit:-
Google Earth image of the source area of the Blackhawk landslide.
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The implication appears to be a highly unusual morphology of the landslide scar. It is not clear to me how the mechanics of this part of the landslide motion would have operated – is there a large volume of stalled material still within the scar area?
Acknowledgement and footnote
Many thanks to my friend funkenbeachin for pointing out this landslide, and for the discussions about it. He has hypothesised that there may be other events in this area as well – take a look at the image below. It is not hard to believe that there is more than one landslide deposit on the valley floor:-
Wide angle Google Earth view of the valley floor around the Blackhawk landslide
Dave Petley is the Pro-Vice-Chancellor (Research and Innovation) at the University of Sheffield in the United Kingdom. His blog provides a commentary on landslide events occurring worldwide, including the landslides themselves, latest research, and conferences and meetings.
I wish there were a larger frame with this area embedded — just being spoiled. I mean I did try to find it on google earth, which was enlightening in itself and fun, but I didn’t find it
Thanks for the post, I live very close to this landslide and its always captured my imagination. there is a surprising lack of in depth information about it.
I cheked out the landslide on Google Earth as my mind could not process the description given for the landslide relative to the screen grabs. Looking at the landslide from the side and more or less in profile made it much easier to comprehend what I was reading and seeing.
Thanks for sharing! This is a most excellent landslide. Really surprised more have not studied it.
Nothing compared to mt. St. Helens
What is your estimate of the duration of the slide event? How do you come to that conclusion?
https://www.google.com/maps/place/CA-247,+California/@34.4052903,-116.7857973,8867m/data=!3m1!1e3!4m5!3m4!1s0x80c4c16c25b6727b:0x2d5a5aa59082cc47!8m2!3d34.4177909!4d-116.741477
I live 20 miles from the landslide area and have driven past it many times in the last 34 years. I had always thought it was an unusual geological formation? It was probably only 5 years ago someone had told me about the ancient landslide out past Lucerne Valley, I knew exactly where and what they were talking about when they told me!
I visited the site in 2014 and blogged about it with annotated graphics and on-the-ground images. The actual post is here: https://earthly-musings.blogspot.com/2014/12/southern-californias-blackhawk.html.
It its unfortunate that the site is not protected as a geo-heritage site and much vandalism and trash dumping is occurring here. Nonetheless, a very impressive site!!
I finally found it :-):
https://www.google.com/maps/@34.523292,-116.7546151,4456a,35y,180h,75.87t/data=!3m1!1e3?hl=en-GB
Thanks for the post. Looks like the quarry on the north-west side of the landslide toe might give a nice glimpse into its make-up. ‘click’, do you live close enough to take a look?
Shreve’s thesis eventually became a 1968 Geological Society of America Special Paper (#108). I first learned about this landlide from John Shelton’s 1966 book, “Geology Illustrated”, and have always thought of it as one of the examples behind Jay Melosh’s acoustic fluidization process (recent, free access, with references: https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2016JF004093