We have all seen the striations, grooves and rock pavements carved by glacial flow; as well as large ridges of sorted sand and gravel deposited by melting glacial water known as eskers and drumlin fields.
What remains less clear are the processes that create landscape features. Finding out how glacial erosion takes place requires finding out which term best describes it.
Plucking
Glacial erosion has left its mark on many land features, from U-shaped valleys and U-shaped lakes to U-horned valleys, horns, moraine, drumlins and kettle lakes. Glaciers move enormous masses of ice called glaciers down mountain valleys causing this erosion process.
Glaciers use an erosion process known as plucking to erode rocks and earth below them, breaking off large pieces and leaving behind grooves known as glacial striae.
Glaciers carry bits of rock of all sizes that differ in type and strength from surrounding bedrock; these fragments of material are known as erratics; melting glaciers deposit them as unsorted deposits called till.
Plucking rates depend on the location of a glacier. Warm-based (temperate) glaciers may erode more effectively due to being closer to their pressure melting point throughout their thickness, permitting basal sliding with increased rates of erosion1.
Abrasion
Glaciation’s profound impact on long-term landscape evolution is well documented, yet our understanding of its driving and controlling forces for erosion at continental shields, shelves and passive margins remains limited. A hierarchy of internal and external controls such as climate, lithology, topographic relief and thermomechanical ice sheet configuration all play a part in its efficacy worldwide – something simple environmental proxy measures such as latitude or climate regime can’t capture.
Our new 4D landscape reconstruction reveals that glacial erosion across the Northern Fennoscandian Shelf and Arctic domain was time-transgressive, with increased efficacy under dynamic marine-based ice sheets from Glaciation II-III onwards, and episodic episodes of preferential erosion at the interface between Shield and TMF provinces. This insight allows us to contextualise contemporary process studies which support erosion rates orders-of-magnitude higher than considered norm7,8; furthermore, time-depth profiles show bulk patterns were controlled primarily through focused and selective incision patterns of erosion within platform provinces or shaley basement provinces respectively.
Basal Slip
As ice flows over bedrock it deforms rapidly to accommodate surface irregularities, leading to rapid deformation known as basal slip. This sliding action wears away at bedrock by wearing away particles of rock and mineral held within moving ice, wearing away at it as tools are released as it erodes further downstream – leaving behind debris fields at its path’s end – responsible for creating the characteristic stoss-and-lee topography that has come to characterise glacial erosion processes.
Erosion differs from abrasion because ice movement creates tensional rather than compressive pressure in bedrock, increasing plucking rates in areas with effective tensional stress regimes.
Although it is impossible to know for certain whether any given bedrock landform was created by basal slip, evidence of disruption indicates it likely is. Conversely, cold-based glaciers move only via ice creep and do not create such dramatic landforms.
Deposition
Glacial erosion is an inexorable geological process that shapes continental shields, mountain ranges and passive margins across the planet. Unfortunately, its rate can be difficult to measure directly; as a result, estimates of erosion rates are often made using theoretical models9 or proxy measures10.,11.
Glacial erosion is driven primarily by basal sliding; however, the surface of a glacier can also be subjected to gouging and scraping from debris embedded within it1. This erosion process involves either scoring bedrock surfaces by larger rock fragments that are carried across by the ice, or polishing them by silt-sized particles dragged across by it2. Climate should logically have an enormous influence on glacial erosion since basal sliding depends on temperature conditions and melt availability; however, empirical evidence of any relationship remains limited 11.