|The Blackhawk slide, California|
Photo by Kerry Sieh
Such landslides are not common, but occur frequently enough that there are some eyewitness accounts. One such slide was the Elm rockfall in Switzerland in 1881. It buried a village and killed 115 people. This event was described in German by Albert Heim* and in English by Ken Hsu**. (If anyone knows of an English translation of Heim's work, I would much appreciate a copy!!)
One eyewitness of the Elm slide described the airborne trajectory:
"Then I saw the rockmass jump away from the ledge. The lower part of the block was squeezed by the pressure of the rapidly falling upper part, became disintegrated and burst forth into the air. .. The debris shot with unbelievable speed northward toward the hamlet of Untertal and over and above the creek, for I could see the alder forest by the creek under the stream of shooting debris."
Hsu suggested that the word "sturzstrom", which literally means "fallstream" be adopted to describe these slides and that it be defined as "a stream of very rapidly moving debris derived from the disintegration of a fallen rock mass of very large size; the speed of a sturzstrom often exceeds 100 km/hr, and its volume is commonly greater than a million cubic meters."
In spite of the geologic evidence and eyewitness accounts, there is still no general agreement about the mechanism by which the sturzstroms achieve their long runout distances. Shreve (1968) proposed that the avalanches floated over a layer of compressed air; Goguel (1978) proposed that a layer of steam generated by friction allowed the great mobility; Erismann (1979) proposed that friction allowed melting; others (Fineburg, Bagnold, McSaveney, Campbell) proposed various mechanisms related to the behavior of granular flow. Melosh proposed an acoustic fluidization model, in which high-frequency (acoustic) fluctuations in the local pressure around grains relieved the overburden pressure, allowing the material to move even in the absence of a large driving pressure. There is a good tutorial about acoustic fluidization here, the source of the numbered references below. It is possible that different mechanisms operate in different parts of the landslide trajectory.
See also this post for more discussion of big landslides.
Shreve, R.L. 1968,The Blackhawk landslide: Geol. Soc. America Special Paper 108, 47 pp.
Sreve, R.L., 1966, Sherman landslide, Alaska: Science, 154, p. 1639-1643.
*Heim, Albert, 1882, Der Bergsturz von Elm: Deutsch.Geol. Gesell. Zeitschr., v. 34, p. 74-115, and/or Buss, E., and Heim, Albert, 1881, Der Bergstruz von Elm: Zurich, Wurster and Cie, 163 p., and/or Heim, Albert, 1882, Bergsturz and Menschenleben: Zurich, Fretz and Wasmuth Verlag, 218 p.
**Hsu, Kenneth, Catastrophic Debris Streams (Sturzstroms) Generated by Rockfalls, Geological Society of America Bulletin, v. 86, p. 129-140, 1975.
- Goguel, J. (1978) Rockslides and Avalanches, Edited by B. Voight, 1, 693-705, Elsevier, Amsterdam.
- Erismann, T. H. (1979) Rock Mechanics, 12, 15-46.
- Fineburg, J. (1997) Nature, 386, 323-324.
- Bagnold, R. A. (1956) Philosophical Transactions of the Royal Society of London, A, 225, 49-63.
- McSaveney, M. J. (1978) Rockslides and Avalanches, Edited by B. Voight, 1, 197-258, Elsevier, Amsterdam.
- Campbell, C. S. (1990) Ann. Rev. Fluid Mech., 22, 57-92.
- Iverson, R. M. (1997) Rev. Geophysics, 35, 245-296.
- Melosh, H. J. (1979) JGR, 84, 7513-7520.
- Melosh, H. J. (1987) Debris Flows/Avalanches: Process, Recognition and Mitigation, edited by J. E. Costa and G. F. Wieczorek, 41-49, GSA.