In situ exhumation from bedrock of large rounded boulders at the Giant’s Causeway, Northern Ireland: an alternative genesis for large shore boulders (mega-clasts).

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Abstract

Very large boulders (mega-clasts) are found on some coasts. The size and position ofthe boulders has been used to suggest that contemporary marine processes, actingwithin their normal spatial and energy range, are unlikely to have moved them.Explanations for the presence of such boulders include transport by infrequent veryhigh energy marine processes (storms or tsunamis), mass movement from backingcliffs, transport by ice, or exhumation from glacial deposits. This paper advances analternative explanation which does not involve transport by any of the marine orglacial processes, or gravity. It is proposed that, in a very specific geological andtopographic setting, large boulders are exhumed in situ by storm-waves acting onheavily weathered jointed basalts. Eventually wave action liberates residual blocksfrom the deeply weathered matrix. These liberated boulders will be mobile only ifthey lie within wave competence, and the larger ones will remain as stationaryresiduals. The same in situ weathering processes, followed by removal of the friablematrix material debris by wave action, also progressively round the boulders.Consequently, despite their appearance of being transport-rounded, the larger bouldershave not transported at all. In specific locations, the assumption that the presence, androunding, of such large clasts in the shore zone can be attributed to marine transportcan lead to erroneous interpretations of very high-energy storm-wave (or tsunami)activity.
LanguageEnglish
Pages25-35
JournalMarine Geology
Volume283
DOIs
Publication statusPublished - 2010

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exhumation
clast
bedrock
wave action
tsunami
energy
mass movement
glacial deposit
weathering
basalt
gravity
ice
matrix
coast
in situ
material
removal

Cite this

@article{c9110a7a94594702ae5839d9d2f19ec7,
title = "In situ exhumation from bedrock of large rounded boulders at the Giant’s Causeway, Northern Ireland: an alternative genesis for large shore boulders (mega-clasts).",
abstract = "Very large boulders (mega-clasts) are found on some coasts. The size and position ofthe boulders has been used to suggest that contemporary marine processes, actingwithin their normal spatial and energy range, are unlikely to have moved them.Explanations for the presence of such boulders include transport by infrequent veryhigh energy marine processes (storms or tsunamis), mass movement from backingcliffs, transport by ice, or exhumation from glacial deposits. This paper advances analternative explanation which does not involve transport by any of the marine orglacial processes, or gravity. It is proposed that, in a very specific geological andtopographic setting, large boulders are exhumed in situ by storm-waves acting onheavily weathered jointed basalts. Eventually wave action liberates residual blocksfrom the deeply weathered matrix. These liberated boulders will be mobile only ifthey lie within wave competence, and the larger ones will remain as stationaryresiduals. The same in situ weathering processes, followed by removal of the friablematrix material debris by wave action, also progressively round the boulders.Consequently, despite their appearance of being transport-rounded, the larger bouldershave not transported at all. In specific locations, the assumption that the presence, androunding, of such large clasts in the shore zone can be attributed to marine transportcan lead to erroneous interpretations of very high-energy storm-wave (or tsunami)activity.",
author = "John McKenna and Derek Jackson and Andrew Cooper",
year = "2010",
doi = "10.1016/j.margeo.2010.09.005",
language = "English",
volume = "283",
pages = "25--35",

}

TY - JOUR

T1 - In situ exhumation from bedrock of large rounded boulders at the Giant’s Causeway, Northern Ireland: an alternative genesis for large shore boulders (mega-clasts).

AU - McKenna, John

AU - Jackson, Derek

AU - Cooper, Andrew

PY - 2010

Y1 - 2010

N2 - Very large boulders (mega-clasts) are found on some coasts. The size and position ofthe boulders has been used to suggest that contemporary marine processes, actingwithin their normal spatial and energy range, are unlikely to have moved them.Explanations for the presence of such boulders include transport by infrequent veryhigh energy marine processes (storms or tsunamis), mass movement from backingcliffs, transport by ice, or exhumation from glacial deposits. This paper advances analternative explanation which does not involve transport by any of the marine orglacial processes, or gravity. It is proposed that, in a very specific geological andtopographic setting, large boulders are exhumed in situ by storm-waves acting onheavily weathered jointed basalts. Eventually wave action liberates residual blocksfrom the deeply weathered matrix. These liberated boulders will be mobile only ifthey lie within wave competence, and the larger ones will remain as stationaryresiduals. The same in situ weathering processes, followed by removal of the friablematrix material debris by wave action, also progressively round the boulders.Consequently, despite their appearance of being transport-rounded, the larger bouldershave not transported at all. In specific locations, the assumption that the presence, androunding, of such large clasts in the shore zone can be attributed to marine transportcan lead to erroneous interpretations of very high-energy storm-wave (or tsunami)activity.

AB - Very large boulders (mega-clasts) are found on some coasts. The size and position ofthe boulders has been used to suggest that contemporary marine processes, actingwithin their normal spatial and energy range, are unlikely to have moved them.Explanations for the presence of such boulders include transport by infrequent veryhigh energy marine processes (storms or tsunamis), mass movement from backingcliffs, transport by ice, or exhumation from glacial deposits. This paper advances analternative explanation which does not involve transport by any of the marine orglacial processes, or gravity. It is proposed that, in a very specific geological andtopographic setting, large boulders are exhumed in situ by storm-waves acting onheavily weathered jointed basalts. Eventually wave action liberates residual blocksfrom the deeply weathered matrix. These liberated boulders will be mobile only ifthey lie within wave competence, and the larger ones will remain as stationaryresiduals. The same in situ weathering processes, followed by removal of the friablematrix material debris by wave action, also progressively round the boulders.Consequently, despite their appearance of being transport-rounded, the larger bouldershave not transported at all. In specific locations, the assumption that the presence, androunding, of such large clasts in the shore zone can be attributed to marine transportcan lead to erroneous interpretations of very high-energy storm-wave (or tsunami)activity.

U2 - 10.1016/j.margeo.2010.09.005

DO - 10.1016/j.margeo.2010.09.005

M3 - Article

VL - 283

SP - 25

EP - 35

ER -