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Interim Essay: Origin of certain disrupted-bedrock features on the Isthmus of Avalon
The Avalon Peninsula contains a significant accumulation of noteworthy disrupted-bedrock features, extending over a wide area. The geographic distribution of major frost-heave features is nonuniform, with most of the significant features concentrated on the Isthmus of Avalon and on the north-central portion of the Carbonear sub-peninsula. This essay will focus on observations recorded on the Isthmus of Avalon. The features of interest include large frost-heaved bedrock monoliths, wide areas of abundant bedrock frost heave, "restricted" (see below) plucked hillsides and fissures. Many similar features are found elsewhere in the world, but the disrupted-bedrock features on the Isthmus of Avalon possess some unusual attributes that make them worthy of special attention.
Here are the attributes considered noteworthy:
1) Relict bedrock frost heave is observed where joint blocks have been dislodged from substrate and then shifted or aligned horizontally in a manner that indicates the frost-heave took place beneath glacial ice. Furthermore, evidence suggests that the indicated glacial flow was overriding the local topography, implying that ice thicknesses were in excess of 100 meters.
2) Abundant instances of plucking are seen where little or no rock has been removed from the site. When a hillside shows evidence of plucking, it is typical for broken rock to be entrained in glacial flow and then carried down-ice a considerable distance. There are many instances of this type of plucking on the Avalon. However, there are also many instances, particularly on the Isthmus, where the plucking process has disrupted a glacially-smoothed hillside, but left the resulting fragments shifted by less than 1 meter. This “restricted plucking” then resembles a frost-wedging process or a frost-heave process more than it resembles conventional plucking. Nevertheless this limited plucking seems to have been guided in terms of location, orientation and severity by glacial flow patterns in the area.
3) Plucking on ascending slopes and double-sided plucking have been observed. Some hills and ridges show “restricted plucking” (the kind described in 2 above) on the wrong (glacial ice ascending) side or on two opposite sides of the same landform, but with the plucking on the down-ice (lee) side being more intense than the plucking on the up-ice (stoss) side. Ascending-side (backward) plucking is typically subdued compared to descending-side plucking occurring in nearby areas. This “backward” plucking, although subdued, is clearly different in character (and more severe) than exfoliation (pressure rebound fracturing).
Each of the above attributes is suggestive of frost-heave-like bedrock disruption occurring beneath glacial ice. Furthermore, the observations suggest that local ice movement did not provide the principal energy source acting to disrupt the rock. Rather, the disrupted-bedrock features appear to have more in common with periglacial features where thermally-driven processes such as ice segregation or the expansion of water upon freezing generated the rock-shifting stress. The fact that the features include evidence of deep subglacial origin complicates any explanation of their occurrence. Seasonal temperature fluctuations are completely attenuated by as little as 10-20 meters of overlying glacial ice. There is thus no obvious temperature gradient to drive conventional periglacial frost-heave processes occurring deep under glaciers. Even if the observed subglacial bedrock disruption features are assumed to have originated from purely hydraulic processes, there is difficulty in explaining the removal of the heat of crystallization when groundwater freezes near a deeply ice-buried (hence insulated) rock surface.
Here is a possible model to explain the apparent observations of subglacial frost-heave on the Isthmus of Avalon:
The Isthmus rocks have been subjected to low-grade regional metamorphism giving rise to an extensive and deep-seated system of longitudinal and cross joints. The rocks have been repeatedly stressed by fluctuating ice loadings during the Wisconsin and previous glaciations. Loading and unloading the rocks at intervals has widened joints and kept precipitated minerals from obstructing flow paths in joints. Thus the bedrock did not become impermeable in spite of the long elapsed time since tectonic activity ceased.
The joint system yielded an abundance of failure planes that were exploited by ice loading during intervals when glaciation was mainly temperate. Since the failure planes were frequently misaligned with ice flow directions, basal slip glaciation scoured the landscape in a rough and irregular manner. The resultant topography was very rugged on the 10-100 meter scale.
In the interval following the last glacial maximum, a thick temperate ice sheet overlay the Isthmus of Avalon. The weight of this ice sheet and its long-term presence ensured that the bedrock joint system became saturated with pressurized groundwater. In making this assumption, is is worth noting that ice loading will tend to increase the porosity of heterogeneous bedrock (see Technical Note 04). This porosity increase stems primarily from uneven elastic rock deformation and is largely reversible upon unloading. During unloading, bedrock will thus tend to expel stored groundwater toward the surface.
The deglaciation that commenced following the last glacial maximum was not monotonic, but rather was interrupted by reversions back to colder-climate conditions. Most notable among the post-LGM cold intervals was the Younger Dryas cold period. The Younger Dryas interval changed the character of the depleting temperate glaciers on the Isthmus of Avalon, converting them to polythermal glaciers or to cold-based glaciers. It is these non-temperate glacial conditions that are relevant to explaining the observed unconventional occurrences of ice-induced bedrock disruption on the Isthmus.
When analyzing subglacial bedrock disruption features, a key question arises. Did glacial activity cause the occurrence of the features, or did glacial activity only mediate the occurrence of the features? Conventional plucking is an example of a subglacial process caused by the movement (and associated stresses) of glacial ice. It is reasonable to deduce that in the case of the subglacial frost-heaved bedrock and the “restricted plucking” observed on the Isthmus of Avalon, glacial ice flow mediated the occurrences, but did not necessarily cause them. This deduction does not exclude the possibility that subglacial hydraulic processes were powered by the weight of thick, upslope temperate glacial ice. Rather, the concept includes this possibility, along with the possibility that pressurized groundwater originated primarily from the ice-unloading process accompanying deglaciation.
When pressurized groundwater from any source moves toward the rock-ice interface beneath a glacier, the greatest flow will tend to occur where ambient pressure at the ice-rock interface is at a minimum. Hence, hydraulically driven rock disruption will predominate in areas where glacial ice pressure is low, as, for instance where glacial ice is thin or on the lee side of an ice-flow obstruction. In this way glacial flow and thickness will influence the occurrence of groundwater-driven rock disruption processes, even when glacial flow is not the principal underlying energy source for those processes.
The process of heat transfer by ice movement (advection) may help to explain the maintenance of a thermal gradient at the deep ice-bedrock interface sufficient to prevent heat buildup at this interface. Given that groundwater travels through bedrock joints as liquid, it is most convenient to assume that freezing in the joints is prevented simply by temperatures exceeding the pressure melting point. Since frost-heave forces are delivered by ice, not water, it is necessary to conclude that freezing occurs in joints, fissures and voids before the subglacial ice-bedrock interface is reached. This in turn demands that the overlying glacier be cold-based and that heat given off as groundwater crystallizes is removed upward into the ice, preventing the basal ice from becoming temperate.
With ice depths exceeding 100 meters, heat transfer from the base of the ice (presumed near zero degrees C) to air at a seasonal average temperature of -10 degrees C is very gradual because of the weak temperature gradient (less than 0.1 degrees C per meter). This weak gradient could account for removal of heat from intruding groundwater water freezing at the glacier base in cases where the groundwater flow is very gradual. This possibility cannot be excluded when considering disrupted bedrock features that might have taken 1000 years to form. Complex advection patterns, associated with creep of a cold-based glacier over a rugged topography could significantly assist in removing basal heat. The benefit of attributing part of the heat removal process to advection lies in attempting to explain why groundwater freezing is localized to the ice-bedrock interface and does not occur too early (hindering groundwater flow) or too late (allowing groundwater leakage). Unlike thermal conduction, the process of heat transfer via advection can act only to the ice-bedrock interface.
A final consideration relates to the climatic conditions that prevailed on the Isthmus of Avalon during the Younger Dryas. Observations suggest that little or no basal-slip glacial activity took place after widespread, often fragile, frost-heave features were emplaced on the Isthmus. Given that some of these features formed in a deep subglacial environment, it is necessary to conclude that glaciation at the time of frost-heave occurrence was cold-based and remained cold-based effectively up to the point of total ice depletion. This would imply a radically different precipitation pattern than is now observed on the Avalon Peninsula. Specifically, snowfall on the glaciers would need to be insufficient to replace ice losses from creep flow, summer ice-surface melting/runoff, and sublimation. In essence, the ice cap on the Isthmus of Avalon seems to have entirely evaporated away or otherwise benignly dissipated during the Younger Dryas.
Summary:
In summary, certain special relict frost-heave features observed on the Isthmus of Avalon were potentially formed as a consequence of pressure release when cold glaciers gradually sublimed away or otherwise dissipated during the Younger Dryas stadial. As bedrock was unloaded of ice burden, groundwater flowed to the surface and froze upon approach to the glacier-bedrock interface. Bedrock then failed in tension in a process resembling plucking. Glacial ice movement mediated but did not cause the bedrock disruption features. This description may account for a significant portion of the bedrock frost heave and restricted plucking occurrences seen on the Avalon Peninsula.
The Avalon Peninsula contains a significant accumulation of noteworthy disrupted-bedrock features, extending over a wide area. The geographic distribution of major frost-heave features is nonuniform, with most of the significant features concentrated on the Isthmus of Avalon and on the north-central portion of the Carbonear sub-peninsula. This essay will focus on observations recorded on the Isthmus of Avalon. The features of interest include large frost-heaved bedrock monoliths, wide areas of abundant bedrock frost heave, "restricted" (see below) plucked hillsides and fissures. Many similar features are found elsewhere in the world, but the disrupted-bedrock features on the Isthmus of Avalon possess some unusual attributes that make them worthy of special attention.
Here are the attributes considered noteworthy:
1) Relict bedrock frost heave is observed where joint blocks have been dislodged from substrate and then shifted or aligned horizontally in a manner that indicates the frost-heave took place beneath glacial ice. Furthermore, evidence suggests that the indicated glacial flow was overriding the local topography, implying that ice thicknesses were in excess of 100 meters.
2) Abundant instances of plucking are seen where little or no rock has been removed from the site. When a hillside shows evidence of plucking, it is typical for broken rock to be entrained in glacial flow and then carried down-ice a considerable distance. There are many instances of this type of plucking on the Avalon. However, there are also many instances, particularly on the Isthmus, where the plucking process has disrupted a glacially-smoothed hillside, but left the resulting fragments shifted by less than 1 meter. This “restricted plucking” then resembles a frost-wedging process or a frost-heave process more than it resembles conventional plucking. Nevertheless this limited plucking seems to have been guided in terms of location, orientation and severity by glacial flow patterns in the area.
3) Plucking on ascending slopes and double-sided plucking have been observed. Some hills and ridges show “restricted plucking” (the kind described in 2 above) on the wrong (glacial ice ascending) side or on two opposite sides of the same landform, but with the plucking on the down-ice (lee) side being more intense than the plucking on the up-ice (stoss) side. Ascending-side (backward) plucking is typically subdued compared to descending-side plucking occurring in nearby areas. This “backward” plucking, although subdued, is clearly different in character (and more severe) than exfoliation (pressure rebound fracturing).
Each of the above attributes is suggestive of frost-heave-like bedrock disruption occurring beneath glacial ice. Furthermore, the observations suggest that local ice movement did not provide the principal energy source acting to disrupt the rock. Rather, the disrupted-bedrock features appear to have more in common with periglacial features where thermally-driven processes such as ice segregation or the expansion of water upon freezing generated the rock-shifting stress. The fact that the features include evidence of deep subglacial origin complicates any explanation of their occurrence. Seasonal temperature fluctuations are completely attenuated by as little as 10-20 meters of overlying glacial ice. There is thus no obvious temperature gradient to drive conventional periglacial frost-heave processes occurring deep under glaciers. Even if the observed subglacial bedrock disruption features are assumed to have originated from purely hydraulic processes, there is difficulty in explaining the removal of the heat of crystallization when groundwater freezes near a deeply ice-buried (hence insulated) rock surface.
Here is a possible model to explain the apparent observations of subglacial frost-heave on the Isthmus of Avalon:
The Isthmus rocks have been subjected to low-grade regional metamorphism giving rise to an extensive and deep-seated system of longitudinal and cross joints. The rocks have been repeatedly stressed by fluctuating ice loadings during the Wisconsin and previous glaciations. Loading and unloading the rocks at intervals has widened joints and kept precipitated minerals from obstructing flow paths in joints. Thus the bedrock did not become impermeable in spite of the long elapsed time since tectonic activity ceased.
The joint system yielded an abundance of failure planes that were exploited by ice loading during intervals when glaciation was mainly temperate. Since the failure planes were frequently misaligned with ice flow directions, basal slip glaciation scoured the landscape in a rough and irregular manner. The resultant topography was very rugged on the 10-100 meter scale.
In the interval following the last glacial maximum, a thick temperate ice sheet overlay the Isthmus of Avalon. The weight of this ice sheet and its long-term presence ensured that the bedrock joint system became saturated with pressurized groundwater. In making this assumption, is is worth noting that ice loading will tend to increase the porosity of heterogeneous bedrock (see Technical Note 04). This porosity increase stems primarily from uneven elastic rock deformation and is largely reversible upon unloading. During unloading, bedrock will thus tend to expel stored groundwater toward the surface.
The deglaciation that commenced following the last glacial maximum was not monotonic, but rather was interrupted by reversions back to colder-climate conditions. Most notable among the post-LGM cold intervals was the Younger Dryas cold period. The Younger Dryas interval changed the character of the depleting temperate glaciers on the Isthmus of Avalon, converting them to polythermal glaciers or to cold-based glaciers. It is these non-temperate glacial conditions that are relevant to explaining the observed unconventional occurrences of ice-induced bedrock disruption on the Isthmus.
When analyzing subglacial bedrock disruption features, a key question arises. Did glacial activity cause the occurrence of the features, or did glacial activity only mediate the occurrence of the features? Conventional plucking is an example of a subglacial process caused by the movement (and associated stresses) of glacial ice. It is reasonable to deduce that in the case of the subglacial frost-heaved bedrock and the “restricted plucking” observed on the Isthmus of Avalon, glacial ice flow mediated the occurrences, but did not necessarily cause them. This deduction does not exclude the possibility that subglacial hydraulic processes were powered by the weight of thick, upslope temperate glacial ice. Rather, the concept includes this possibility, along with the possibility that pressurized groundwater originated primarily from the ice-unloading process accompanying deglaciation.
When pressurized groundwater from any source moves toward the rock-ice interface beneath a glacier, the greatest flow will tend to occur where ambient pressure at the ice-rock interface is at a minimum. Hence, hydraulically driven rock disruption will predominate in areas where glacial ice pressure is low, as, for instance where glacial ice is thin or on the lee side of an ice-flow obstruction. In this way glacial flow and thickness will influence the occurrence of groundwater-driven rock disruption processes, even when glacial flow is not the principal underlying energy source for those processes.
The process of heat transfer by ice movement (advection) may help to explain the maintenance of a thermal gradient at the deep ice-bedrock interface sufficient to prevent heat buildup at this interface. Given that groundwater travels through bedrock joints as liquid, it is most convenient to assume that freezing in the joints is prevented simply by temperatures exceeding the pressure melting point. Since frost-heave forces are delivered by ice, not water, it is necessary to conclude that freezing occurs in joints, fissures and voids before the subglacial ice-bedrock interface is reached. This in turn demands that the overlying glacier be cold-based and that heat given off as groundwater crystallizes is removed upward into the ice, preventing the basal ice from becoming temperate.
With ice depths exceeding 100 meters, heat transfer from the base of the ice (presumed near zero degrees C) to air at a seasonal average temperature of -10 degrees C is very gradual because of the weak temperature gradient (less than 0.1 degrees C per meter). This weak gradient could account for removal of heat from intruding groundwater water freezing at the glacier base in cases where the groundwater flow is very gradual. This possibility cannot be excluded when considering disrupted bedrock features that might have taken 1000 years to form. Complex advection patterns, associated with creep of a cold-based glacier over a rugged topography could significantly assist in removing basal heat. The benefit of attributing part of the heat removal process to advection lies in attempting to explain why groundwater freezing is localized to the ice-bedrock interface and does not occur too early (hindering groundwater flow) or too late (allowing groundwater leakage). Unlike thermal conduction, the process of heat transfer via advection can act only to the ice-bedrock interface.
A final consideration relates to the climatic conditions that prevailed on the Isthmus of Avalon during the Younger Dryas. Observations suggest that little or no basal-slip glacial activity took place after widespread, often fragile, frost-heave features were emplaced on the Isthmus. Given that some of these features formed in a deep subglacial environment, it is necessary to conclude that glaciation at the time of frost-heave occurrence was cold-based and remained cold-based effectively up to the point of total ice depletion. This would imply a radically different precipitation pattern than is now observed on the Avalon Peninsula. Specifically, snowfall on the glaciers would need to be insufficient to replace ice losses from creep flow, summer ice-surface melting/runoff, and sublimation. In essence, the ice cap on the Isthmus of Avalon seems to have entirely evaporated away or otherwise benignly dissipated during the Younger Dryas.
Summary:
In summary, certain special relict frost-heave features observed on the Isthmus of Avalon were potentially formed as a consequence of pressure release when cold glaciers gradually sublimed away or otherwise dissipated during the Younger Dryas stadial. As bedrock was unloaded of ice burden, groundwater flowed to the surface and froze upon approach to the glacier-bedrock interface. Bedrock then failed in tension in a process resembling plucking. Glacial ice movement mediated but did not cause the bedrock disruption features. This description may account for a significant portion of the bedrock frost heave and restricted plucking occurrences seen on the Avalon Peninsula.
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