A glacier is a large, persistent body of dense ice that forms on land and moves slowly under the influence of its own weight and gravity. This massive, flowing structure is a powerful geological agent that reshapes the terrain it covers. The movement of the ice mass and the dramatic landforms it leaves behind often lead to confusion about whether the glacier is a feature of the landscape or a force that creates features. Understanding this distinction requires looking closely at how the ice forms, moves, and interacts with the Earth.
Understanding Glaciers as Geological Features
A glacier is considered a landform, defined as any natural feature of the Earth’s surface. The sheer mass, distinct shape, and persistent presence of the ice body satisfy this definition, classifying it as a dynamic, ice-related landform. Glaciers range from confined valley glaciers, which flow down pre-existing stream valleys in mountainous regions, to vast continental ice sheets. The Antarctic and Greenland ice sheets are prime examples of the largest type, covering immense areas and obscuring the underlying topography.
The moving body of ice is distinct from the landforms it generates, such as the scars and deposits left on the bedrock and soil. A glacier’s mass occupies a specific space on the Earth’s surface for an extended period, shaping the landscape beneath it. The existence of the ice itself, whether an alpine glacier nestled between mountain peaks or a massive continental sheet, is a feature of the terrain. Therefore, the ice mass is both a landform and the engine for creating many others.
The Physical Process of Glacier Formation
The process of glaciation begins with the accumulation of snowfall that must exceed the amount of ice and snow lost through melting or evaporation. This persistent, surplus snow transforms under the pressure of the layers above it. The initial light, fluffy snow compacts, forcing out air and recrystallizing into denser, rounded ice grains known as firn.
As more snow accumulates, the increasing weight compresses the firn further, eventually squeezing out nearly all the trapped air. This densification converts the granular firn into hard, interlocking crystals of glacial ice. Glaciers form only where this cycle of accumulation and compaction occurs, typically above the snow line where winter snowfall is not entirely lost in the summer. The uppermost area where this net gain of mass occurs is called the zone of accumulation.
How Glaciers Change the Landscape
As the dense ice mass flows downslope, it modifies the terrain through mechanical processes and the action of meltwater. The primary erosional mechanisms are abrasion and plucking. Abrasion occurs as rock fragments frozen into the base of the ice scrape and grind against the underlying bedrock, acting like giant sandpaper. This action leaves behind long, parallel scratches called glacial striations, and can produce a smooth, polished surface on the rock.
Plucking, also known as quarrying, is a powerful process where the glacier tears away large blocks of rock from the bedrock. Meltwater penetrates cracks and joints in the rock, and as it freezes and expands, it weakens and fractures the rock. As the glacier moves, it pulls these loosened blocks free, incorporating them into the flowing ice. The moving ice transports this unsorted mixture of rock, sediment, and debris, collectively known as till, either on its surface, within the ice, or dragged along its base.
The presence of meltwater at the glacier’s base is a significant factor, acting as a lubricant that reduces friction and allows the ice to slide more rapidly across the bedrock. This sliding motion enhances the efficiency of both plucking and abrasion, increasing the glacier’s erosive power. Meltwater systems, flowing in channels beneath or within the ice, also transport and deposit sediment, contributing to landscape alteration.
Features Left Behind by Glacial Activity
The erosional and depositional actions of glaciers leave behind a distinct suite of landforms visible long after the ice has retreated. Erosional features, which dominate in mountainous areas, include the amphitheater-shaped hollows called cirques, carved into mountain sides at the head of a glacial valley. When two cirques form back-to-back or side-by-side, they sharpen the separating ridge into a narrow, knife-edge crest known as an arête. Glaciers also transform the V-shaped valleys carved by rivers into broad, deep troughs with a characteristic U-shaped cross-section.
Depositional features are created when the glacier melts and releases its load of till, the unsorted mixture of sediment it carried. Moraines are the most common depositional features, forming mounds or ridges of till.
- Terminal moraines are deposited at the glacier’s furthest point of advance.
- Lateral moraines are deposited along the glacier’s sides.
- Drumlins are streamlined, teardrop-shaped hills composed of till, indicating the direction of ice flow.
- Glacial erratics are large boulders transported long distances and deposited on a different type of bedrock, serving as evidence of the glacier’s journey.