Glacial and Periglacial Processes and Landforms
Glaciers reshape landscapes on a massive scale through erosion, transport, and deposition. Understanding how they work helps explain some of the most dramatic terrain on Earth, from the fjords of Norway to the Great Lakes of North America. This section covers how glaciers erode and deposit material, the landforms they create, and how cold-but-ice-free (periglacial) environments produce their own distinctive features.
Glacial Processes and Landforms
Processes of glacial action
Glaciers don't just sit on the landscape. They actively grind it down, carry material, and dump it somewhere else. There are two main erosion mechanisms:
- Abrasion occurs when a glacier drags rock fragments embedded in its base across the bedrock beneath it. This grinds and polishes the surface, sometimes leaving behind scratches called striations.
- Plucking happens when meltwater seeps into cracks in the bedrock and refreezes, bonding the rock to the glacier. As the glacier moves forward, it rips out entire chunks of rock.
Once material is eroded, glaciers transport it in several ways: on top of the ice (supraglacial), within the ice (englacial), or along the base (subglacial).
When glaciers melt or retreat, they deposit this material. The two main types of glacial sediment are:
- Glacial till: unsorted sediment deposited directly by the ice. It contains everything from fine clay to massive boulders, all jumbled together with no layering.
- Glacial outwash: sediment that has been carried and sorted by meltwater streams flowing away from the glacier. Because water sorts by size, outwash deposits tend to be layered and graded.
Formation of glacial landforms
Glacial erosion and deposition produce a recognizable set of landforms:
- Cirques are bowl-shaped depressions carved into mountainsides at the head of a glacier. Frost wedging and plucking gradually steepen the headwall, deepening the basin over time.
- Arêtes are sharp, knife-like ridges that form when two cirques on opposite sides of a ridge erode headward toward each other, narrowing the rock between them.
- Horns are steep, pyramid-shaped peaks that form when three or more cirques erode into a mountain from different sides. The Matterhorn in the Alps is the classic example.
Moraines are accumulations of glacial till deposited in specific positions relative to the glacier:
- Terminal moraines mark the farthest advance of a glacier, deposited at its snout.
- Lateral moraines form along the sides of a glacier from material that falls onto the ice from valley walls.
- Medial moraines appear as a dark stripe down the middle of a glacier where two glaciers merge and their lateral moraines combine.
Alpine vs. continental glaciation
These two types of glaciation operate at very different scales and produce different landforms.
Alpine glaciation occurs in mountain valleys, where glaciers flow downhill like slow rivers of ice:
- U-shaped valleys have a broad, flat floor and steep sides, carved by the glacier widening and deepening a former V-shaped river valley.
- Hanging valleys are smaller tributary valleys left stranded high above the main valley floor. The main glacier eroded its valley much deeper than the smaller tributary glaciers could, so when the ice melts, the tributary valley "hangs" above, often with a waterfall pouring over the edge.
- Horns (described above) are also products of alpine glaciation.
Continental glaciation involves massive ice sheets spreading across broad, relatively flat terrain. During the last ice age, ice sheets up to 3 km thick covered much of North America and northern Europe.
- Fjords are deeply carved U-shaped valleys along coastlines that flooded with seawater after the ice retreated. Norway's Sognefjord, over 1,300 m deep, is a well-known example.
- Drumlins are streamlined, oval-shaped hills made of glacial till. They're elongated in the direction of ice flow, with a steeper end facing the direction the ice came from. They often occur in clusters called drumlin fields.
- Eskers are long, winding ridges of sorted sand and gravel deposited by meltwater streams that flowed through tunnels beneath the ice sheet.
Periglacial processes and features
Periglacial environments are cold regions near glaciers or at high latitudes where the ground freezes but isn't covered by glacial ice. Repeated freezing and thawing drives most of the processes here.
Permafrost is ground that stays at or below 0°C for at least two consecutive years. It can be hundreds of meters thick in places like Siberia or northern Canada.
- Continuous permafrost covers an entire region with no gaps.
- Discontinuous permafrost is patchy, with unfrozen areas mixed in, typically found at slightly lower latitudes or in areas with local warming (like under lakes).
Only the top layer of permafrost, called the active layer, thaws each summer. This seasonal thawing drives several distinctive features:
- Patterned ground refers to geometric shapes visible on the surface. Ice wedge polygons form when the ground cracks in winter cold, water fills the cracks in spring, and refreezes, gradually widening the wedges over many cycles. Stone circles form when repeated frost heave pushes larger stones outward, sorting them into ring-like arrangements.
- Solifluction is the slow, downslope flow of waterlogged sediment over the still-frozen permafrost beneath. Because the permafrost acts as an impermeable barrier, meltwater saturates the active layer, and gravity pulls the soggy material downhill. This creates lobed, terraced slopes.
Glacial impact on landscapes
Glaciers leave behind clear evidence of their presence, even millions of years after they've melted:
- Glacial striations are parallel scratches or grooves carved into bedrock by rocks embedded in the base of a moving glacier. They're useful because they indicate the direction the glacier was flowing.
- Glacial erratics are large boulders that glaciers transported and deposited far from their source rock. Finding a granite boulder sitting on top of limestone bedrock, for instance, tells you a glacier carried it there.
Glacial lakes form through several mechanisms:
- Kettle lakes form when blocks of ice break off a retreating glacier and get buried in outwash sediment. When the ice melts, it leaves a depression that fills with water. Minnesota's thousands of lakes formed this way.
- Moraine-dammed lakes form when a moraine acts as a natural dam, blocking water drainage in a valley.
Isostatic rebound is the gradual uplift of Earth's crust after the enormous weight of glacial ice is removed. Ice sheets are heavy enough to push the crust down into the mantle. Once the ice melts, the land slowly rises back up. This process is still happening today in Scandinavia and northern Canada, producing raised shorelines and influencing local sea levels.