Glacial ripping: geomorphological evidence from Sweden for a new process of glacial erosion

Abstract

In low relief Precambrian gneiss terrain in eastern Sweden, abraded bedrock surfaces were ripped apart by the Fennoscandian Ice Sheet. The resultant boulder spreads are covers of large, angular boulders, many with glacial transport distances of 1-100 m. Boulder spreads occur alongside partly disintegrated roches moutonnées and associated fracture caves, and are associated with disrupted bedrock, which shows extensive fracture dilation in the near surface. These features are distributed in ice-flow parallel belts up to 10 km wide and extend over distances of >500 km. Our hypothesis is that the assemblage results from (1) hydraulic jacking and bedrock disruption, (2) subglacial ripping and (3) displacement, transport and final deposition of boulders. Soft sediment fills indicate jacking and dilation of pre-existing bedrock fractures by groundwater overpressure below the ice sheet. Overpressure reduces frictional resistance along fractures. Where ice traction overcomes this resistance, the rock mass strength is exceeded, resulting in disintegration of rock surfaces and ripping apart into separate blocks. Further movement and deposition create boulder spreads and moraines. Short boulder transport distances and high angularity indicate that glacial ripping operated late in the last deglaciation. The depths of rock mobilized in boulder spreads are estimated as 1-4 m. This compares with 0.6-1.6 m depths of erosion during the last glaciation derived from cosmogenic nuclide inventories of samples from bedrock surfaces without evidence of disruption. Glacially disrupted and ripped bedrock is also made ready for removal by future ice sheets. Hence glacial ripping is a highly effective process of glacial erosion.

Glacial Ripping in Sedimentary Rocks: Loch Eriboll, NW Scotland

“…Previous work in Precambrian gneiss terrain of eastern Sweden has identified markers for hydraulic jacking, rock disruption and boulder transport, the components of glacial ripping [1,2]. Here we present a checklist of potential marker features recognised in Sweden (Figure 3).…”

“…Transport involves movement under traction of rock blocks as boulders or rafts at the glacier bed. Extensive covers of locally derived, angular boulders have been termed boulder spreads [1]. Additionally, other features are observed on abraded quartz-arenite pavements which are potentially attributable to glacial ripping.…”

“…Recent work in lowland Sweden has recognized a new process of erosion, termed glacial ripping, that operated beneath the last Fennoscandian Ice Sheet (FIS) [1]. Ripping involved a sequence of process steps that comprise (i) hydraulic jacking and opening of subhorizontal fractures, (ii) consequent disruption of the near-surface rock mass under subglacial traction and (iii) transport and deposition beneath the ice sheet.…”

Thermo-Mechanical Regime of the Greenland Ice Sheet and Erosion Potential of the Crystalline Bedrock

“…For the above processes to take place beneath an ice sheet, at least two conditions must be met: (i) The base must be warm and wet, to allow meltwater to move rapidly and to allow basal slip; (ii) the base must be free of sediment to allow ice/rock contact. Glacial ripping is a newly recognized erosion process that involves the jacking, disruption, and transport of large, angular boulders [15,16]. The process of ripping requires groundwater overpressure, and is discovered beneath the retreating margin of the Fennoscandian Ice Sheet that operates to depths of several meters [16].…”

Data acquisition by digitizing 2-D fracture networks and topographic lineaments in geographic information systems: further development and applications

“…The shallow fracture network remained of interest in establishing the potential of groundwater overpressure and hydraulic jacking of basement fractures (e.g. Pusch et al, 1990;Talbot, 1990Talbot, , 2014Lönnqvist and Hökmark, 2013), as well as playing a potentially important role in a newly recognized erosion mechanism, termed glacial ripping (Hall et al, 2020). For this latter (ongoing) research, one relevant issue is the fracture density of sub-vertical and sub-horizontal fractures as a function of depth in sections with and without fracture dilation; such data were not gathered during the original studies in the 1970s.…”