Ice is an extraordinary material with the power to transform landscapes. Through erosion processes such as plucking and abrasion, it sculpts an amazing variety of landforms into existence.
Some examples of landforms include horns, aretes, truncated spurs, ribbon lakes and mountain ranges. Let’s examine each of these landforms individually to learn their formation!
As glaciers move down mountains, they can occasionally tear off large chunks of rock known as plucking, leaving smooth surfaces (known as the stoss side) on one side and rough surfaces on the other (lee side). This process leaves behind cool marks known as striations marks.
Plucking is more prevalent with valley glaciers due to frictional forces between them and bedrock causing melting to penetrate cracks called joints in rock layers, leading them to widen, loosening large pieces of rock known as joint blocks and eventually plucking them out by hand.
Glacial plucking has created some truly incredible landscapes, from corries, aretes, pyramidal peaks, U-shaped valleys and U-shaped lakes to ribbon lakes, moutonnee roches moutonnee and crag and tail formations.
Abrasion is the process by which glaciers erode rock. Glaciers contain rocks of various sizes and shapes, which move downslope with them as the glacier moves, grinding against and scraping over bedrock, leaving behind scratches known as striations scars in its path.
These striations reflect the direction of ice movement and help shape landforms such as stoss-and-lee topography and roche moutonnees. This type of erosion is particularly significant on medium-sized glacial landforms such as cirques, rock basins and drumlins but has less of an impactful role for smaller landforms such as ribbon lakes or roche moutonnees due to basal sliding or meltwater’s influence over abrasion.
Freeze-thaw weathering is a physical/mechanical process that weakens rock to make it easier for glaciers to pick up and carry away. This creates piles of angular rocks called scree slopes or blockfields at the bases of mountains or cliffs.
Rainwater or melting snow seeps into cracks, joints and other porous spaces in rocks where water accumulates during rainstorms or melting snowmelt, becoming trapped when temperatures drop at night and freezing temperatures expand cracks by 10% and pry parts apart of the rock surface.
Repeated cycles of freezing and thawing cause havoc for rocks, gradually breaking them up into smaller and smaller pieces that make them vulnerable to chemical weathering processes.
As rivers move over hilly landscapes, they must navigate around features known as interlocking spurs that protrude from the valley floor. Over time, their flow erodes these features, gradually shaping them into intricate designs that become permanent features in their environment.
Harder rocks tend to be eroded more by rivers than soft ones, leading to interlocking spurs on river landscapes. When glaciation occurs, its sheer force can cut right through these spurs creating truncated spurs.
These troughs can then be filled with water to form ribbon lakes – something seen frequently on mountains like Kilimanjaro and Mount Kenya.
As glaciers move across a landscape, their movement causes erosion that produces stunning landforms such as aretes, corries, rock lips terminal moraines and ribbon lakes.
Glaciers often erode harder rock more quickly than soft rocks, leading to ridges of hard rock forming at the head of valleys or ribbon lakes being left behind as a result of moving glaciers across both.
Changes in glacier flow rate caused by expansion or compression can result in uneven erosion across different areas, for instance eroding more of one cirque back wall than another and creating ribbon lakes in valley floors.
Crag and Tail
Crag and tail formations on resistant rock surface created by glaciers are characteristics of resistant terrain with steep stoss faces carved out of hard, compacted rock, contrasting to gently sloping lee (or tail) sides that are less resistant. Crag and tail formation is produced when glaciers plucked and abraded resistant rock to create these features.
Horned crag and tails tend to form in areas with higher topographic elevation, while conventional ones occur at lower points, suggesting they form in different subglacial environments. We propose that their formation requires basal temperature distribution to undergo a transition from fully frozen-bed conditions to partially thawed beds with frozen patches at higher elevations – inhibiting till transport across crags while permitting accumulation of two tails at its flanks.