Measurement and theory of ploughing boulder movement
Analysis of the characteristics, distribution and rate of movement of ploughing boulders reveals several attributes that throw light on the mechanism of boulder movement. Ploughing boulders occur only in areas of active solifluction, on frost-susceptible soils with low plastic and liquid limits. Dur...
| Published in: | Permafrost and Periglacial Processes |
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| Main Author: | |
| Format: | Article in Journal/Newspaper |
| Language: | English |
| Published: |
2001
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| Online Access: | https://research-portal.st-andrews.ac.uk/en/researchoutput/measurement-and-theory-of-ploughing-boulder-movement(7f92cc82-7097-44bf-b246-04069bedeca0).html https://doi.org/10.1002/ppp.389 http://www.scopus.com/inward/record.url?scp=0034829693&partnerID=8YFLogxK |
| Summary: | Analysis of the characteristics, distribution and rate of movement of ploughing boulders reveals several attributes that throw light on the mechanism of boulder movement. Ploughing boulders occur only in areas of active solifluction, on frost-susceptible soils with low plastic and liquid limits. During movement they rotate to adopt an alignment of least resistance. Excavations revealed thick ice lenses under the base of a boulder in winter, and emergent shear planes in frontal ridges. Rates of boulder movement average from zero to a few centimetres per year and are exponentially related to gradient. The differential thermal conductivity (diffusivity) of the boulder and surrounding soil implies that the seasonal freezing plane descends more rapidly through the boulder, causing ice-lens growth at the boulder base; conversely, the thaw plane reaches the base of the boulder whilst the sun-rounding soil is still frozen. A limit-equilibrium model of boulder movement by gelifluction suggests that movement results from trapping of excess water under boulders during thaw, elevation of sub-boulder porewater pressures to above-hydrostatic levels and consequent reduction in shearing resistance at the base of the boulder. Movement is initiated by thaw of soil immediately downslope, but allows trapped water to escape, reducing porewater pressures and allowing stability to be regained. This model appears consistent with the observed characteristics, distribution and behaviour of ploughing boulders. Copyright (C) 2001 John Wiley & Sons, Ltd. |
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