Based on the observation that deposits of large rock avalanches consist predominantly of intensely fragmented rock debris, it is proposed that the processes of rock fragmentation are significant causes of the peculiar distribution of mass in these deposits, and of the correspondingly long runout. Rock fragmentation produces high-velocity fragments moving in all directions, resulting in an isotropic dispersive stress within the translating rock mass. A longitudinal dispersive force consequently acts in the direction of reducing mass depth and tends to cause the rear part of the avalanche to decelerate and halt and the front part to accelerate. The result is greater longitudinal spreading of the travelling mass compared with nonfragmenting granular avalanches. The longer runout results from this additional fragmentation-induced spreading.
Physical modelling of part of prehistoric Waikaremoana landslide shows that the blockslide must have hit the valley wall at c . 40 m/s, after sliding 2 km on a 5.5–8° slope, in order to form the 150-m high mound of debris known as Raekahu. Both the blockslide and a distal rock avalanche were in simultaneous motion when the impact occurred. Finely ground rock on the slide plane suggests that a mechanism of dynamic rock fragmentation may explain the low friction necessary for acceleration to 40 m/s. When a rock particle fractures in a confined space, an isotropic dispersive pressure equal to the rock's Hugoniot elastic limit (in the GPa range) at the ambient pressure and strain rate may be exerted on its surroundings. Beneath the 275-m thick block, about one particle in 15–30 or so fragmenting at any instant (with lower density for higher rock strength at higher strain rate), could completely support the weight of the block by fragmentation pressure; but then there would be no frictional resistance (and hence no further fragmentation). Self-regulation of the process may explain the apparent coefficient of friction of c . 0.1 in the blockslide. Low friction through dynamic fragmentation may apply widely to blockslides with a basal layer of comminuted rock.
Abstract Determining the shear‐velocity dependence of dry granular friction can provide insight into the controlling variables in a dry granular friction law. Some laboratories believe that the quality of this study is at the forefront of the discipline for the following reasons. Results suggest that granular friction is greatly affected by shear‐velocity ( v ), but shear experiments over the large range of naturally occurring shear‐velocities are lacking. Herein we examined the shear velocity dependence of dry friction for three granular materials, quartz sand, glass beads and fluorspar, across nine orders of magnitude of shear velocity (10 −8 –2 m/s). Within this range, granular friction exhibited four regimes, following a broad approximate “m” shape including two velocity‐strengthening and two velocity‐weakening regimes. We discuss the possible physical mechanisms of each regime. This shear velocity dependence appeared to be universal for all particle types, shapes, sizes, and for all normal stresses over the tested range. We also found that ultra‐high frequency vibration as grain surfaces were scoured by micro‐chips were formed by spalling at high shear velocities, creating ∼20 μm diameter impact pits on particle surfaces. This study provides laboratory laws of a friction‐velocity ( μ ‐ v ) model for granular materials.
Research Article| March 01, 2007 Orbital forcing of mid-latitude Southern Hemisphere glaciation since 100 ka inferred from cosmogenic nuclide ages of moraine boulders from the Cascade Plateau, southwest New Zealand Rupert Sutherland; Rupert Sutherland 1GNS Science, P.O. Box 30-368, Lower Hutt, New Zealand Search for other works by this author on: GSW Google Scholar Kyeong Kim; Kyeong Kim 2Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona 85721-0092, USA Search for other works by this author on: GSW Google Scholar Albert Zondervan; Albert Zondervan 3GNS Science, P.O. Box 30-368, Lower Hutt, New Zealand Search for other works by this author on: GSW Google Scholar Mauri McSaveney Mauri McSaveney 3GNS Science, P.O. Box 30-368, Lower Hutt, New Zealand Search for other works by this author on: GSW Google Scholar Author and Article Information Rupert Sutherland 1GNS Science, P.O. Box 30-368, Lower Hutt, New Zealand Kyeong Kim 2Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona 85721-0092, USA Albert Zondervan 3GNS Science, P.O. Box 30-368, Lower Hutt, New Zealand Mauri McSaveney 3GNS Science, P.O. Box 30-368, Lower Hutt, New Zealand Publisher: Geological Society of America Received: 09 Jun 2005 Revision Received: 03 Sep 2006 Accepted: 14 Sep 2006 First Online: 08 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 GEOLOGICAL SOCIETY OF AMERICA GSA Bulletin (2007) 119 (3-4): 443–451. https://doi.org/10.1130/B25852.1 Article history Received: 09 Jun 2005 Revision Received: 03 Sep 2006 Accepted: 14 Sep 2006 First Online: 08 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Rupert Sutherland, Kyeong Kim, Albert Zondervan, Mauri McSaveney; Orbital forcing of mid-latitude Southern Hemisphere glaciation since 100 ka inferred from cosmogenic nuclide ages of moraine boulders from the Cascade Plateau, southwest New Zealand. GSA Bulletin 2007;; 119 (3-4): 443–451. doi: https://doi.org/10.1130/B25852.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Cosmogenic nuclide (Be-10) exposure dating of moraine boulders in the Cascade Valley, southwest New Zealand, reveals three phases of glaciation with similar maximum magnitude since 100 ka. In this area, 8–10 lateral moraines were deposited during the Last Glacial Maximum (LGM) at 22–19 ka, and >15 lateral moraines and three end moraines were deposited during recession after the LGM. Also, three exposure ages of 29–33 ka from pre-LGM deposits may indicate increased weathering and erosion at the onset of the LGM in New Zealand, as has been suggested by other studies. An exposure age of 57.8 ± 2.7 ka from one of the highest moraines, combined with previous studies of cave speleothems, glacial features offset by the Alpine fault, the Vostok dust record, and sediment cores, supports the inference that a significant glacial phase culminated at 66–58 ka. A cluster of five exposure ages from older moraines reveals a glacial phase with at least three advance-retreat cycles at 79.0 ± 3.9 ka. Correlation between the ages of glacial periods and the timing of Southern Hemisphere summer insolation minima suggests that orbital forcing has played a first-order role in regulating glacial extent in New Zealand. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
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