Joint-bounded crescentic scars formed by subglacial clast-bed contact forces: Implications for bedrock failure beneath glaciers
Glaciers and ice sheets are important agents of bedrock erosion, yet the precise processes of bedrock failure beneath glacier ice are incompletely known. Subglacially formed erosional crescentic markings (crescentic gouges, lunate fractures) on bedrock surfaces occur locally in glaciated areas and c...
Published in: | Geomorphology |
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Main Authors: | , , , |
Other Authors: | , , , |
Format: | Article in Journal/Newspaper |
Language: | English |
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Elsevier
2017
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Online Access: | http://hdl.handle.net/1893/25503 https://doi.org/10.1016/j.geomorph.2017.03.021 http://dspace.stir.ac.uk/bitstream/1893/25503/1/Crescentic_Scars_FINAL_AAM_withFigs_tables.pdf |
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ftunivstirling:oai:dspace.stir.ac.uk:1893/25503 |
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record_format |
openpolar |
institution |
Open Polar |
collection |
University of Stirling: Stirling Digital Research Repository |
op_collection_id |
ftunivstirling |
language |
English |
topic |
Conchoidal fracture Subglacial erosion Joint Rock strength |
spellingShingle |
Conchoidal fracture Subglacial erosion Joint Rock strength Krabbendam, Maarten Bradwell, Tom Everest, Jeremy D Eyles, Nick Joint-bounded crescentic scars formed by subglacial clast-bed contact forces: Implications for bedrock failure beneath glaciers |
topic_facet |
Conchoidal fracture Subglacial erosion Joint Rock strength |
description |
Glaciers and ice sheets are important agents of bedrock erosion, yet the precise processes of bedrock failure beneath glacier ice are incompletely known. Subglacially formed erosional crescentic markings (crescentic gouges, lunate fractures) on bedrock surfaces occur locally in glaciated areas and comprise a conchoidal fracture dipping down-ice and a steep fracture that faces up-ice. Here we report morphologically distinct crescentic scars that are closely associated with preexisting joints, termed here joint-bounded crescentic scars. These hitherto unreported features are ca. 50–200mm deep and involve considerably more rock removal than previously described crescentic markings. The joint-bounded crescentic scars were found on abraded rhyolite surfaces recently exposed (<20years) beneath a retreating glacier in Iceland, as well as on glacially sculpted Precambrian gneisses in NW Scotland and various Precambrian rocks in Ontario, glaciated during the Late Pleistocene. We suggest a common formation mechanism for these contemporary and relict features, whereby a boulder embedded in basal ice produces a continuously migrating clast-bed contact force as it is dragged over the hard (bedrock) bed. As the ice-embedded boulder approaches a preexisting joint in the bedrock, stress concentrations build up in the bed that exceed the intact rock strength, resulting in conchoidal fracturing and detachment of a crescentic wedge-shaped rock fragment. Subsequent removal of the rock fragment probably involves further fracturing or crushing (comminution) under high contact forces. Formation of joint-bounded crescentic scars is favoured by large boulders at the base of the ice, high basal melting rates, and the presence of preexisting subvertical joints in the bedrock bed. We infer that the relative scarcity of crescentic markings in general on deglaciated surfaces shows that fracturing of intact bedrock below ice is difficult, but that preexisting weaknesses such as joints greatly facilitate rock failure. This implies that ... |
author2 |
British Geological Survey Biological and Environmental Sciences University of Toronto orcid:0000-0003-0947-3309 |
format |
Article in Journal/Newspaper |
author |
Krabbendam, Maarten Bradwell, Tom Everest, Jeremy D Eyles, Nick |
author_facet |
Krabbendam, Maarten Bradwell, Tom Everest, Jeremy D Eyles, Nick |
author_sort |
Krabbendam, Maarten |
title |
Joint-bounded crescentic scars formed by subglacial clast-bed contact forces: Implications for bedrock failure beneath glaciers |
title_short |
Joint-bounded crescentic scars formed by subglacial clast-bed contact forces: Implications for bedrock failure beneath glaciers |
title_full |
Joint-bounded crescentic scars formed by subglacial clast-bed contact forces: Implications for bedrock failure beneath glaciers |
title_fullStr |
Joint-bounded crescentic scars formed by subglacial clast-bed contact forces: Implications for bedrock failure beneath glaciers |
title_full_unstemmed |
Joint-bounded crescentic scars formed by subglacial clast-bed contact forces: Implications for bedrock failure beneath glaciers |
title_sort |
joint-bounded crescentic scars formed by subglacial clast-bed contact forces: implications for bedrock failure beneath glaciers |
publisher |
Elsevier |
publishDate |
2017 |
url |
http://hdl.handle.net/1893/25503 https://doi.org/10.1016/j.geomorph.2017.03.021 http://dspace.stir.ac.uk/bitstream/1893/25503/1/Crescentic_Scars_FINAL_AAM_withFigs_tables.pdf |
genre |
glacier Iceland |
genre_facet |
glacier Iceland |
op_relation |
Krabbendam M, Bradwell T, Everest JD & Eyles N (2017) Joint-bounded crescentic scars formed by subglacial clast-bed contact forces: Implications for bedrock failure beneath glaciers. Geomorphology , 290, pp. 114-127. https://doi.org/10.1016/j.geomorph.2017.03.021 http://hdl.handle.net/1893/25503 doi:10.1016/j.geomorph.2017.03.021 WOS:000405056100009 2-s2.0-85017577546 529849 http://dspace.stir.ac.uk/bitstream/1893/25503/1/Crescentic_Scars_FINAL_AAM_withFigs_tables.pdf |
op_rights |
This item has been embargoed for a period. During the embargo please use the Request a Copy feature at the foot of the Repository record to request a copy directly from the author. You can only request a copy if you wish to use this work for your own research or private study. Accepted refereed manuscript of: Krabbendam M, Bradwell T, Everest JD & Eyles N (2017) Joint-bounded crescentic scars formed by subglacial clast-bed contact forces: Implications for bedrock failure beneath glaciers, Geomorphology, 290, pp. 114-127. DOI:10.1016/j.geomorph.2017.03.021 © 2017, Elsevier. Licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International http://creativecommons.org/licenses/by-nc-nd/4.0/ http://creativecommons.org/licenses/by-nc-nd/4.0/ 2018-04-05 [Crescentic_Scars_FINAL_AAM_withFigs_tables.pdf] Publisher requires embargo of 12 months after online publication. |
op_rightsnorm |
CC-BY-NC-ND |
op_doi |
https://doi.org/10.1016/j.geomorph.2017.03.021 |
container_title |
Geomorphology |
container_volume |
290 |
container_start_page |
114 |
op_container_end_page |
127 |
_version_ |
1766009770662166528 |
spelling |
ftunivstirling:oai:dspace.stir.ac.uk:1893/25503 2023-05-15T16:21:47+02:00 Joint-bounded crescentic scars formed by subglacial clast-bed contact forces: Implications for bedrock failure beneath glaciers Krabbendam, Maarten Bradwell, Tom Everest, Jeremy D Eyles, Nick British Geological Survey Biological and Environmental Sciences University of Toronto orcid:0000-0003-0947-3309 2017-08-01 application/pdf http://hdl.handle.net/1893/25503 https://doi.org/10.1016/j.geomorph.2017.03.021 http://dspace.stir.ac.uk/bitstream/1893/25503/1/Crescentic_Scars_FINAL_AAM_withFigs_tables.pdf en eng Elsevier Krabbendam M, Bradwell T, Everest JD & Eyles N (2017) Joint-bounded crescentic scars formed by subglacial clast-bed contact forces: Implications for bedrock failure beneath glaciers. Geomorphology , 290, pp. 114-127. https://doi.org/10.1016/j.geomorph.2017.03.021 http://hdl.handle.net/1893/25503 doi:10.1016/j.geomorph.2017.03.021 WOS:000405056100009 2-s2.0-85017577546 529849 http://dspace.stir.ac.uk/bitstream/1893/25503/1/Crescentic_Scars_FINAL_AAM_withFigs_tables.pdf This item has been embargoed for a period. During the embargo please use the Request a Copy feature at the foot of the Repository record to request a copy directly from the author. You can only request a copy if you wish to use this work for your own research or private study. Accepted refereed manuscript of: Krabbendam M, Bradwell T, Everest JD & Eyles N (2017) Joint-bounded crescentic scars formed by subglacial clast-bed contact forces: Implications for bedrock failure beneath glaciers, Geomorphology, 290, pp. 114-127. DOI:10.1016/j.geomorph.2017.03.021 © 2017, Elsevier. Licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International http://creativecommons.org/licenses/by-nc-nd/4.0/ http://creativecommons.org/licenses/by-nc-nd/4.0/ 2018-04-05 [Crescentic_Scars_FINAL_AAM_withFigs_tables.pdf] Publisher requires embargo of 12 months after online publication. CC-BY-NC-ND Conchoidal fracture Subglacial erosion Joint Rock strength Journal Article AM - Accepted Manuscript 2017 ftunivstirling https://doi.org/10.1016/j.geomorph.2017.03.021 2023-02-02T23:17:15Z Glaciers and ice sheets are important agents of bedrock erosion, yet the precise processes of bedrock failure beneath glacier ice are incompletely known. Subglacially formed erosional crescentic markings (crescentic gouges, lunate fractures) on bedrock surfaces occur locally in glaciated areas and comprise a conchoidal fracture dipping down-ice and a steep fracture that faces up-ice. Here we report morphologically distinct crescentic scars that are closely associated with preexisting joints, termed here joint-bounded crescentic scars. These hitherto unreported features are ca. 50–200mm deep and involve considerably more rock removal than previously described crescentic markings. The joint-bounded crescentic scars were found on abraded rhyolite surfaces recently exposed (<20years) beneath a retreating glacier in Iceland, as well as on glacially sculpted Precambrian gneisses in NW Scotland and various Precambrian rocks in Ontario, glaciated during the Late Pleistocene. We suggest a common formation mechanism for these contemporary and relict features, whereby a boulder embedded in basal ice produces a continuously migrating clast-bed contact force as it is dragged over the hard (bedrock) bed. As the ice-embedded boulder approaches a preexisting joint in the bedrock, stress concentrations build up in the bed that exceed the intact rock strength, resulting in conchoidal fracturing and detachment of a crescentic wedge-shaped rock fragment. Subsequent removal of the rock fragment probably involves further fracturing or crushing (comminution) under high contact forces. Formation of joint-bounded crescentic scars is favoured by large boulders at the base of the ice, high basal melting rates, and the presence of preexisting subvertical joints in the bedrock bed. We infer that the relative scarcity of crescentic markings in general on deglaciated surfaces shows that fracturing of intact bedrock below ice is difficult, but that preexisting weaknesses such as joints greatly facilitate rock failure. This implies that ... Article in Journal/Newspaper glacier Iceland University of Stirling: Stirling Digital Research Repository Geomorphology 290 114 127 |