Glaciers carve out landforms from rocks they travel over, creating distinctive landforms like striations. As they pass over these grooves, their effects erode into the rock beneath, often stripping away surface vegetation or even jagged points from rocks that once stood upon their own.
Erosion occurs through two main processes: plucking and abrasion. Unfortunately, these processes are difficult to observe unless you know exactly where they’re happening.
1. The ice is too thick
Glaciers erode rocks and sediment by means of abrasion. Their movements scrape the bedrock surface, leaving tool marks from microscopic scratches to centimeter-deep gouges over hundreds of meters long. Furthermore, glaciers also carry debris between locations; this process produces the characteristically streamlined landforms associated with glaciated landscapes.
As is generally understood, glacial abrasion works best when glacier ice moves rapidly over loose bedrock allowing basal slip to occur. Unfortunately, this may not always be possible for cold-based glaciers.
As best we can do is search for evidence of erosion left by glaciers during their time of occupation, we must look for signs of it in their deposits. To do this, long profiles of glaciated valleys should be examined; typically these reveal that erosion rates vary widely from site to site due to glacial hydrologic phenomena which in turn depend on physical properties of both ice and surrounding terrain that influence glacial hydrologic phenomena and thus influence erosion mechanisms.
2. The ice is too thin
Glaciers’ enormous weight (up to 10-4 to 10-2 meters per annum at their base) acts to pick up rocks and stone and grind them down into smaller and smaller pieces, a process known as abrasion.
Rock particles contained within glacial ice are known as tools. As these tools abrade against surfaces below them, erosion occurs; evidence of this process can be seen by striations patterns on glaciated landscapes.
Abrasion depends on a number of factors, including density and concentration of basal debris as well as subglacial deposition. For instance, having too much debris may increase friction between ice and bed and slow abrasion down; glaciers tend to be more effective at this when their basal debris load is lower; this kind of erosion is what gives glaciated landscapes their distinctive landforms such as fjords, deep valleys and cirques that stretch back over great distances.
3. The ice is moving too fast
Glaciated landscapes tend to experience erosion rates that are controlled by glacier’s sliding velocity over bedrock and deposition of debris (Hallet 1979; Nolan Motyka Echelmeyer Trabant 1995). Erosion typically proceeds more rapidly under glaciers than interglacial basins.
Although most glaciers move at a rate too slow for human eyes to detect, some move much faster – prompting scientists to be concerned if climate change continues apace as these could melt so rapidly that sea levels would rapidly increase.
So this summer, a team led by Fiammetta Straneo of Woods Hole Oceanographic Institution in Massachusetts has been visiting Greenland’s Jakobshavn Glacier to better understand what makes it tick. One of their methods includes dropping 90 rubber duckies reminiscent of those found in bathtubs into drains on its surface to see what happens with them; but to truly comprehend glaciers they require an understanding of both how fast and why they move.
4. The ice is moving too slow
Glaciers can erode their surroundings, leaving behind unique landforms. But for erosion to occur, the glacier must first rub against bedrock or other rocks, an action known as abrasion.
Theoretically, more rock fragments embedded in a glacier’s base should speed its ability to erode bedrock more rapidly1. In reality, however, friction increases and can slow the glacier down when moving over its base surface.
However, glaciers still leave behind an impressive mark on the landscape. Their movement leaves long scratches known as glacial striations on rock surfaces as well as carving them into features such as ribbon lakes, fjords, and cliffs; all telling us of a glacier’s former presence here through abrasion, plucking, freeze-thaw weathering or freeze-thaw erosion processes that occurred here.