ACOUSTIC FLUIDIZATION: WHAT IT IS, and IS NOT. H. J. Melosh1 1Earth Atmosphere and Planetary Sciences, Purdue University, West Lafayette in 47907
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Bridging the Gap III (2015) 1004.pdf ACOUSTIC FLUIDIZATION: WHAT IT IS, AND IS NOT. H. J. Melosh1 1Earth Atmosphere and Planetary Sciences, Purdue University, West Lafayette IN 47907. [email protected] Acoustic Fluidization: I introduced the concept of acoustic fluidization in 1979 to explain the astonish- ingly low strength and viscosity exhibited by the col- lapse of impact craters and the formation of central peaks in lunar craters [1]. The name I gave to this pro- cess was not well chosen for the geologic communi- ty—“vibrational fluidization” might have been more apt--but that name was already in use in another con- text and would have proven confusing [2]. The principal motivation for introducing this pro- cess was to explain the transition from simple to com- plex craters on the Moon. Mechanical analysis of the collapse of simple craters indicates that the strength of the underlying material is only about 3 MPa [3]. Fur- thermore, the rise of central peaks in craters larger than Figure 1. Strain rate vs. stress in sand fluidized by about 15 km diameter on the Moon suggests that the strong vibrations at a frequency of about 300 Hz. Σ is post-impact debris can flow as a fluid with a viscosity the ratio between the rms vibration amplitude and the in the vicinity of about 109 Pa-sec [4], implying a overburden pressure, and the yield stress is measured Bingham rheology for the material surrounding the in the absence of vibrations. The solid theoretical crater. How could dry, broken rock debris on the sur- curves are fit to the data with no free parameters. face of a planet lacking water, air or even clay minerals flow like a liquid? My solution was to invoke the ac- tion of strong vibrations in the rock debris broken by Tests of Acoustic Fluidization? A recent study [12] the shock from the impact, which I proposed could argues against the efficacy of acoustic fluidization in briefly fluidize the debris. The reality of such strong the formation of a well-exposed central peak of a crater vibrations near an impact is supported by ground mo- on Mars on the basis of the mapped exposures’ failure tion measurements near large explosions on Earth [5], to conform to the assumptions of the Block Model. In and the nonlinear dependence of strain rate on stress particular, it was stated that the apparent continuity of predicted by the theoretical model is consistent with the mapped units and the lack of matrix separating the phenomenological Bingham rheology [6]. Moreo- observable discrete blocks is evidence against the pro- ver, the predicted relation between stress and strain cess. However, I believe that this is a case of taking a rate as a function of the amplitude of vibrations was simplified heuristic model too literally. The Block verified by measurements with a rotational viscometer Model, with its discrete blocks separated by thick lay- [7], as shown in Figure 1. ers of matrix is only a conceptual model to motivate The acoustic fluidization model has been widely equations that are similar to those of the full acoustic applied to hydrocode modeling of impact crater col- fluidization model, which never postulated such a lapse through the approximate scheme of the “Block structure. Acoustic fluidization only supposes that a Model” [8], with great success, although at the cost of mass of dry rock debris, which is potentially quite ho- introducing three empirical parameters; one that relates mogeneous, is fluidized by the fluctuating pressures the amplitude of the vibrations to the strength of the induced by strong vibrations (acoustic waves) travers- initial shock wave, another that specifies an effective ing the mass. Individual clasts or larger rock units, viscosity (it essentially chooses an effective frequency normally pushed tightly against each other by the pres- or wavelength of the vibrations) and one that specifies sure of overlying rock debris, suddenly become free to the decay rate of the vibrational energy. slip during a transient fluctuation of the pressure field The full acoustic fluidization model also envisions to low pressure. The mechanism is very similar to one a feedback between flow and the vibrational energy proposed (and observationally verified) for the flow of field [9,10], which is not captured by the Block Model. rock debris in muddy debris streams [13,14], where A recent study has finally begun to lift this restriction pore pressure fluctuations relieve the overburden pres- [11] and provides a more accurate model of how cra- sure within the normally clast-supported matrix. ters collapse. Bridging the Gap III (2015) 1004.pdf I have also suggested that acoustic fluidization is I thus believe that the Martian crater observations responsible for the low coefficients of friction ob- of Johnson and Sharpton [12] as well as the more de- served in large-volume rock avalanches, termed long- tailed observations possible at terrestrial craters, are runout landslides or sturzstrom [1,9,15]. These large, entirely consistent with the action of acoustic fluidiza- rapid mass movements have much in common with tion. On the other hand, the apparent absence of melt crater collapse, including Bingham rheology, low coef- sheets bounding the deformed blocks forming the cen- ficients of friction and laminar viscous deformation tral peaks of terrestrial craters mitigates strongly [16]. Figure 2 illustrates the debris lobe of one such against alternative weakening models that appeal to avalanche that exhibits many of the characteristics of discrete planes of melting in collapsing rock units [22]. the units mapped by Johnson and Sharpton [12]. References: [1] H. J. Melosh, Journal of Geophys- ical Research 84, 7513 (1979). [2] T. R. H. Davies, Rock Mechanics 15, 9 (1982). [3] H. J. Melosh, in Impact and Explosion Cratering, edited by D. J. Roddy et al. (Pergamon Press, New York, 1977), pp. 1245– 1260. [4] H. J. Melosh, Journal of Geophysical Re- search 87, 371 (1982). [5] E. S. Gaffney et al., J. Ge- ophys. Res. Solid Earth 87, 1871 (1982). [6] H. J. Melosh et al., Journal of Geophysical Research 88, 830 (1983). [7] H. J. Melosh et al., Transactions of the American Geophysical Union, EOS 76, suppl., F270 Figure 2. Debris lobe of the McGinnis Peak, AK (1995). [8] H. J. Melosh et al., Annual Review of Earth sturzstrom that was initiated by the 2002 Denali earth- and Planetary Sciences 27, 385 (1999). [9] G. S. Col- quake. Note the coherent, but highly deformed, con- lins et al., J. Geophy. Res. 108, doi: 10.1029 (2003). tacts of distinct debris units in the landslide lobe [17]. [10] H. J. Melosh, Nature 379, 601 (1996). [11] H. Hay et al., 45th Lunar and Planetary Science Confer- It has long been noted that such avalanches, in spite ence 1938 (2014). [12] M. K. Johnson et al., 46th Lu- of speeds from 50 to 70 m/sec, flow in an apparently nar and Planetary Science Conference 46, 1280 (2015). laminar fashion that does not mix distinct rock units, in [13] R. M. Iverson et al., Science 246, 796 (1989). [14] spite of very large strains incurred as the broken rock R. M. Iverson, Reviews of Geophysics 35, 245 (1997). mass flows between its source area and final debris [15] H. J. Melosh, Acta Mechanica 64, 89 (1986). [16] lobe [18]. W. B. Dade et al., Geology 26, 803 (1998). [17] R. W. From a distance, one might infer that the rock in Jibson et al., Earthquake Spectra 20, 669 (2004). [18] the slide lobe remained intact, as is suggested by geo- R. L. Shreve, Science 154, 1639 (1966). [19] B. A. logic maps of many impact crater central peaks. How- Ivanov et al., Lunar Planet. Sci. Conf. XXVII, 589 ever, close inspection reveals that the rock is thorough- (1996). [20] R. T. Laney et al., In: Lunar and Planetary ly shattered and lacks any appreciable tensile strength. Science Conference 9, 2609 (1978). [21] T. Coherent block sizes can range from centimeters to Kenkmann, Geology 30, 231 (2002). [22] L. E. Senft many meters and, in still larger collapses occurring in et al., Earth and Planetary Science Letters 287, 471 impact craters, might even approach tens or hundreds (2009). of meters, as suggested by the boreholes into the Puchezh-Katunki crater’s central peak [19]. Although the flow incorporated large (but evidently deformed) “blocks”, there is seldom evidence of a finer matrix between these units: They are typically separated only by narrow faults and fractures, consistent with the re- quirements of the acoustic fluidization model. Similar observations have also been reported from the well- exposed central peak of the 12 km diameter Kentland, IN (USA) impact crater [20] as well as other craters [21], in which the deformation is largely due to nu- merous small-scale fractures whose motion would normally be opposed by friction and thus require a temporary relief of overburden pressure to slide. .