Glaciology Explained: How Glaciers Shape the Earth

How Glaciers Form and Move

Glaciers begin as persistent snowfields in regions where winter snowfall exceeds summer melting year after year. As snow accumulates, the weight of overlying layers compresses the lower snow into dense, granular ice called firn. With continued burial and compaction over decades, firn recrystallizes into solid glacial ice with a density of about 850 to 900 kilograms per cubic meter. The boundary between the upper zone where snow accumulates (the accumulation zone) and the lower zone where ice melts or calves into water (the ablation zone) is called the equilibrium line. A glacier grows when accumulation exceeds ablation and retreats when ablation exceeds accumulation, though the ice itself continues to flow downhill regardless of whether the glacier is advancing or retreating.

Glacial ice moves through two primary mechanisms. Internal deformation (also called plastic flow or creep) occurs when the weight of the ice causes individual ice crystals to deform and slide past one another along crystal planes, much like a deck of cards being slowly pushed from behind. This process dominates in cold, polar glaciers frozen to their beds. Basal sliding occurs when the base of the glacier reaches the pressure melting point and a thin film of meltwater lubricates the contact between ice and bedrock, allowing the entire glacier to slide forward on its bed. Basal sliding is the dominant flow mechanism in temperate glaciers, where meltwater is abundant, and can account for up to 90 percent of the glacier total movement. Glacial flow rates range from a few centimeters per day for slow polar glaciers to several meters per day for fast-moving outlet glaciers, with occasional surges reaching tens of meters per day.

Types of Glaciers

Glaciers are broadly classified into two categories based on size and setting. Alpine (or valley) glaciers form in mountainous terrain, flowing downhill through valleys carved by rivers and earlier glaciations. They range from small cirque glaciers occupying bowl-shaped hollows near mountain summits to large valley glaciers extending tens of kilometers. When multiple valley glaciers flow out of a mountain range and merge on the lowlands at the base, they form a piedmont glacier, a broad lobe of ice spreading across flat terrain. The Malaspina Glacier in Alaska, roughly 3,900 square kilometers in area, is the classic example.

Continental ice sheets are vastly larger, covering entire landmasses and flowing outward in all directions from central domes of maximum ice thickness. Only two continental ice sheets exist today: the Antarctic Ice Sheet (roughly 14 million square kilometers, containing enough ice to raise global sea level by about 58 meters if fully melted) and the Greenland Ice Sheet (roughly 1.7 million square kilometers, containing about 7 meters of potential sea level rise). During the last glacial maximum, about 20,000 years ago, additional ice sheets covered much of North America (the Laurentide and Cordilleran ice sheets), northern Europe (the Scandinavian Ice Sheet), and parts of South America and Asia. Ice shelves are floating extensions of ice sheets that project over the ocean. The Ross Ice Shelf in Antarctica, roughly the size of France, is the largest. Ice shelves buttress the glaciers that feed them, and their collapse can dramatically accelerate ice flow from the interior.

Glacial Erosion

Glaciers are extraordinarily effective agents of erosion, capable of removing enormous volumes of rock and reshaping landscapes on a grand scale. Glacial erosion operates through several mechanisms. Plucking (or quarrying) occurs when meltwater seeps into fractures in the bedrock beneath the glacier, freezes, and bonds to the ice. As the glacier moves forward, it pulls blocks of rock from the bed and incorporates them into the ice. Abrasion occurs when rocks and sediment embedded in the base of the glacier scrape across the bedrock like sandpaper, grinding the surface smooth and carving parallel scratches called striations that record the direction of ice flow. Meltwater erosion beneath and in front of the glacier also contributes, as pressurized water flowing through subglacial channels carves potholes, channels, and tunnels in the bedrock.

The landscapes produced by glacial erosion are among the most dramatic on Earth. Cirques are bowl-shaped depressions carved into mountainsides where glaciers originate. When cirques on opposite sides of a ridge erode headward and meet, they produce a narrow, knife-edged ridge called an arete. When three or more cirques erode into a single peak, they create a pyramidal horn, the Matterhorn in the Alps being the most famous example. U-shaped valleys (also called glacial troughs) are river valleys that have been widened, deepened, and straightened by glacial erosion, transforming the original V-shaped river valley into a broad U-shape. Hanging valleys form when tributary glaciers, being smaller and shallower than the main glacier, erode less deeply, leaving their valleys perched high above the main trough floor after the ice retreats. Waterfalls often cascade from hanging valleys into the main valley below, as at Yosemite Falls in California. Fjords are glacial troughs that have been flooded by the sea after the ice retreated, producing the deep, narrow coastal inlets found in Norway, New Zealand, Chile, and British Columbia.

Glacial Deposition

Glaciers transport enormous quantities of sediment ranging from fine clay to house-sized boulders, all carried within, on top of, or beneath the ice. When glaciers melt, this material is deposited in characteristic landforms. Till is the unsorted mixture of clay, sand, gravel, and boulders deposited directly by the ice without reworking by water. Moraines are ridges or mounds of till marking the positions of a glacier edge. Terminal moraines mark the farthest advance of the glacier, lateral moraines form along the valley walls, and medial moraines form where two glaciers merge and their lateral moraines combine. Ground moraine is the relatively flat, hummocky layer of till deposited beneath the glacier as it retreats, forming the gently rolling landscape that covers much of the northern United States and northern Europe.

Glaciofluvial deposits are sediments carried and sorted by meltwater streams flowing from the glacier. Outwash plains (also called sandur) are broad, flat areas of sorted sand and gravel deposited by braided meltwater streams in front of the glacier. Eskers are sinuous ridges of sand and gravel deposited by streams flowing through tunnels beneath the glacier. Kames are mounds of sand and gravel deposited in depressions on or against the ice surface and left behind when the ice melts. Kettles are depressions formed when blocks of ice buried in glacial sediment eventually melt, causing the surface to collapse. Kettle lakes, including Walden Pond in Massachusetts, fill these depressions. Drumlins are streamlined, elongated hills of till shaped by the overriding glacier into smooth, teardrop forms aligned with the direction of ice flow. Fields of drumlins, sometimes containing thousands of individual hills, are found across New York State, Ireland, and Scandinavia.

Glaciers and Climate

Glaciers are both indicators and drivers of climate change. Because their size responds directly to the balance between snowfall and melting, glaciers provide a visible, physical record of climate trends. The retreat of mountain glaciers worldwide since the mid-19th century, and the accelerating loss of ice from Greenland and Antarctica in recent decades, are among the most compelling indicators of global warming. Glacier ice cores, drilled from the deep interior of ice sheets, preserve a record of past atmospheric conditions stretching back hundreds of thousands of years. Trapped air bubbles in the ice record past concentrations of carbon dioxide, methane, and other gases, while the chemical composition of the ice itself records temperature and precipitation patterns. The Vostok and EPICA ice cores from Antarctica extend the climate record back 800,000 years, revealing the regular cycle of glacial and interglacial periods driven by variations in Earth orbit (Milankovitch cycles).

Glaciers also influence climate through powerful feedback mechanisms. The ice-albedo feedback is one of the most important: ice and snow reflect 80 to 90 percent of incoming solar radiation back into space, keeping the surface cold and preserving the ice. When warming causes ice to melt, the exposed land or ocean absorbs more solar radiation, warming the surface further and causing more melting. This positive feedback loop amplifies both warming and cooling trends, helping to explain why climate transitions between glacial and interglacial states, once initiated by orbital changes, proceed rapidly. The fresh water released by melting glaciers can also disrupt ocean circulation patterns. The most dramatic example occurred roughly 12,800 years ago, when a massive pulse of glacial meltwater into the North Atlantic disrupted the Atlantic meridional overturning circulation, triggering a return to near-glacial conditions (the Younger Dryas cold period) that lasted over a thousand years.