13.1 Glacier Formation and Types

Glaciers, massive ice bodies formed over centuries, shape our planet's landscapes. They require specific conditions to develop, including cold temperatures and abundant snowfall. Understanding glacier formation and types is crucial for grasping their impact on Earth's surface.

Alpine and continental glaciers are the two main types, each with unique characteristics. Alpine glaciers carve mountain valleys, while continental ice sheets cover vast landmasses. These icy giants play a vital role in Earth's climate system and water cycle.

Glacier Formation Conditions

Climatic Requirements

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• Glaciers form in areas where annual snowfall exceeds annual snowmelt, leading to a persistent accumulation of snow and ice over many years
• Glacier formation requires consistently cold temperatures, typically below freezing for most of the year, to prevent significant melting of accumulated snow and ice
• Sufficient precipitation in the form of snow is necessary to build up the mass of the glacier over time, with the amount varying depending on factors such as latitude, elevation, and local climate conditions

Geographic Factors

• Glaciers are more likely to form at high latitudes or high elevations where temperatures remain cold throughout the year, such as in polar regions (Arctic, Antarctic) or mountainous areas (Alps, Himalayas, Andes)
• Topography plays a role in glacier formation, with high-altitude basins, valleys, and plateaus providing favorable conditions for snow accumulation and ice buildup
• The orientation of mountain slopes can influence glacier formation, with north-facing slopes in the Northern Hemisphere and south-facing slopes in the Southern Hemisphere receiving less direct sunlight, promoting cooler temperatures and reduced melting

Alpine vs Continental Glaciers

Alpine Glaciers

• Alpine glaciers, also known as valley glaciers, form in mountainous regions and are confined by the topography of the surrounding valleys and slopes
  • Alpine glaciers are typically smaller in size compared to continental glaciers, with lengths ranging from a few hundred meters to several kilometers (Mer de Glace, France; Pasterze Glacier, Austria)
  • The movement of alpine glaciers is influenced by the steepness of the terrain, with gravity causing them to flow downslope through valleys and basins
• Alpine glaciers often have a distinct accumulation zone at higher elevations and an ablation zone at lower elevations, separated by the equilibrium line
• Examples of alpine glaciers include the Aletsch Glacier in Switzerland, the Baltoro Glacier in Pakistan, and the Malaspina Glacier in Alaska

Continental Glaciers

• Continental glaciers, also called ice sheets, are large masses of ice that cover vast areas of land, often spanning hundreds of thousands of square kilometers
  • The two main continental glaciers in the world today are the Antarctic Ice Sheet and the Greenland Ice Sheet, which cover most of their respective landmasses
  • Continental glaciers are not constrained by surrounding topography and can flow outward in all directions from a central dome or divide
• The immense weight and thickness of continental glaciers cause them to depress the underlying land surface, forming basins and altering the landscape on a large scale
• Continental glaciers can be several kilometers thick at their center and gradually thin out towards the edges, where they may terminate in ice shelves or calve into the ocean (Ross Ice Shelf, Filchner-Ronne Ice Shelf)
• During past glacial periods, continental glaciers covered much larger areas, including parts of North America (Laurentide Ice Sheet) and Europe (Fennoscandian Ice Sheet)

Snow Accumulation and Metamorphism

Accumulation and Mass Balance

• The process of glacier formation begins with the accumulation of snow in areas where snowfall exceeds snowmelt, leading to a positive mass balance
• Accumulation occurs primarily through direct snowfall, but can also include other processes such as wind-blown snow, avalanches, and hoarfrost formation
• The rate of snow accumulation varies depending on factors such as latitude, elevation, and local climate conditions, with higher accumulation rates generally occurring in maritime climates and at higher elevations

Snow Metamorphism

• Over time, the accumulated snow undergoes a process called metamorphism, which transforms the snow into denser, more compact ice
  • Metamorphism occurs due to the pressure exerted by the weight of overlying snow, causing individual snow crystals to deform, break down, and recrystallize into larger, more tightly packed ice grains
  • As the snow compacts and recrystallizes, air spaces between the grains are reduced, increasing the density of the snow and eventually turning it into solid glacial ice
• The transformation of snow into glacial ice through metamorphism typically takes several years to several decades, depending on factors such as the amount of annual snowfall, temperature, and the presence of meltwater
• Firn is an intermediate stage in the metamorphism process, characterized by partially compacted snow that has survived at least one summer melt season but has not yet fully transformed into glacial ice
  • Firn has a density between that of fresh snow and glacial ice, typically ranging from 400 to 830 kg/m³
  • The presence of a firn layer is important for the survival of glaciers, as it helps to insulate the underlying ice and reduce surface melting

Glacier Zones

Accumulation Zone

• The accumulation zone is the upper part of a glacier where annual snowfall exceeds annual snowmelt, resulting in a net gain of snow and ice mass
  • The accumulation zone is typically located at higher elevations where temperatures are colder, allowing for the preservation and buildup of snow
  • The boundary between the accumulation zone and the ablation zone is called the equilibrium line, which represents the point where annual accumulation equals annual ablation
• Snow accumulation in this zone occurs through direct snowfall, wind-blown snow, and avalanches, which gradually compacts and transforms into glacial ice through metamorphism
• The accumulation zone is essential for maintaining the mass balance of a glacier, as it provides the necessary input of snow and ice to offset losses in the ablation zone

Ablation Zone

• The ablation zone is the lower part of a glacier where annual snowmelt and ice loss (ablation) exceed annual snowfall, resulting in a net loss of glacial mass
  • Ablation processes in this zone include melting, sublimation, and calving (the breaking off of ice chunks at the glacier's terminus)
  • The ablation zone is typically located at lower elevations where temperatures are warmer, promoting increased melting and ice loss
• Surface meltwater in the ablation zone can form supraglacial streams and rivers, which may eventually flow into moulins (vertical shafts) or crevasses, transporting water to the base of the glacier
• The size and extent of the ablation zone can vary depending on factors such as climate, elevation, and the glacier's overall mass balance, with a larger ablation zone indicating a negative mass balance and potential glacier retreat

Glacier Terminus

• The terminus, also known as the snout or toe, is the lowest end of a glacier where the ice stops and melts or calves into water (for tidewater glaciers)
  • The position of the terminus can advance or retreat over time, depending on the balance between accumulation and ablation processes in the glacier system
  • A retreating terminus indicates that ablation is exceeding accumulation, resulting in a net loss of glacial mass, while an advancing terminus suggests the opposite
• The terminus of a land-terminating glacier may be marked by a distinct ice cliff, where the glacier ends abruptly and melts, forming a proglacial lake or stream
• Tidewater glaciers, which terminate in the ocean, often have a calving front at their terminus, where large chunks of ice break off and form icebergs (Jakobshavn Glacier, Greenland; Hubbard Glacier, Alaska)
• The behavior and dynamics of the glacier terminus can provide important insights into the overall health and stability of the glacier, as well as its response to changing climatic conditions