Ice sheet

Lore & Background

Ice sheets are immense continental-scale masses of glacial ice that cover surrounding terrain and exceed 50,000 km² in area. The two existing ice sheets are the Antarctic ice sheet, covering about 98% of Antarctica and averaging over 2 km in thickness, and the Greenland ice sheet. They are the largest glacial bodies on Earth, distinguished from smaller ice caps or alpine glaciers. An ice sheet may contain multiple ice domes—topographic highs from which ice flows outward—and is typically drained by ice streams and outlet glaciers along its periphery. Although the surface is cold, the base is generally warmer due to geothermal heat; in places, melting occurs and the meltwater lubricates the base, allowing faster flow and creating fast-flowing channels called ice streams. Even stable ice sheets are in constant motion, with ice gradually flowing from the central plateau toward the margins. The slope is low near the plateau but steepens at the margins due to an imbalance between high accumulation in the center and lower accumulation with higher ablation at the edges. This imbalance increases shear stress until the glacier begins to flow. Flow velocity and deformation increase as the equilibrium line is approached. Ice sheet growth marks glacial periods within ice ages; historically, the Laurentide and Cordilleran ice sheets merged with the Greenland Ice Sheet to cover much of North America during the Last Glacial Maximum, while the Weichselian Ice Sheet covered Northern Europe and the Patagonian Ice Sheet covered southern South America. Their retreat marked the start of the current Holocene interglacial.

Reader's Guide

Ice sheets are significant because they are the largest glacial bodies on Earth and their retreat or disappearance marks the beginning of interglacial periods, such as the current Holocene. The ongoing retreat of the Greenland and Antarctic ice sheets plays a major role in climate change and sea level rise. Marine ice sheet instability (MISI) can occur when ice sheets grounded below sea level thin, as seawater is denser than ice. The West Antarctic Ice Sheet, grounded below sea level, is vulnerable to rapid ice loss, particularly the Thwaites and Pine Island glaciers, which have been rapidly thinning and accelerating. Ice shelves stabilize ice sheets; their thinning or collapse can accelerate glacial flow. Understanding these dynamics is crucial for predicting future sea level rise.

Did You Know?

• The two existing ice sheets are the Antarctic ice sheet (14 million km²) and the Greenland ice sheet (1.7 million km²).
• Lakes larger than ~300 m in diameter can drain to a glacier's base in 2–18 hours, causing surges.

Vastness and Geographic Reach

The Laurentide Ice Sheet was the dominant geological feature of North America's Pleistocene epoch, a colossal mass of frozen water that repeatedly blanketed millions of square miles across the continent from 2.58 million years ago through the present. At its most extreme, the ice stretched from the Rocky Mountains eastward, swallowing nearly all of Canada east of the Rockies and plunging deep into the northern United States. Its southern boundary at peak advance reached roughly the 38th parallel mid-continent, touching the present-day locations of Chicago, St. Louis, Boston, and New York City. Three principal ice centers organized this vast system: the Keewatin dome over the western interior plains, the Labrador dome across eastern Canada and the northeastern US, and the Cordilleran sheet along the Pacific to the Rocky Mountains' eastern front. The Keewatin dome itself spawned four or five major lobes radiating from a high point over west-central Kivalliq, flowing toward Manitoba, Hudson Bay, the Gulf of Boothia, and the Beaufort Sea. The Labrador dome, meanwhile, surged across Maine, the Maritime Provinces, and into the Gulf of St. Lawrence, while the Appalachian Ice Complex pushed over New Brunswick, Nova Scotia, and the Magdalen Shelf.

Sculpting a Continent's Landscape

The Laurentide Ice Sheet did not merely sit upon the land; it actively carved and reshaped it. Its passage gouged out the five Great Lakes and scattered numerous smaller lakes across the Canadian Shield, stretching from the eastern Northwest Territories through northern Canada and the upper Midwest—Minnesota, Wisconsin, Michigan—eastward to the Finger Lakes, Lake Champlain, and Lake George in New York, and across the northern Appalachians into New England and Nova Scotia. The ice left behind a rich inheritance of glacially scoured valleys, moraines, eskers, and glacial till defining much of southern Canada's and the northern US's surface geology. It also repeatedly altered the shape, size, and drainage of the Great Lakes; near the end of the last glaciation, for instance, Lake Iroquois sprawled far beyond modern Lake Ontario's borders and discharged its waters down the Hudson River to the Atlantic. Even after the ice vanished, its removal triggered measurable isostatic rebound: the ground beneath New York has risen more than 150 feet since the roughly 2,000-foot-thick ice that once buried Manhattan melted away around 10,000 BC. Today, the Barnes and Penny Ice Caps in Nunavut stand as the oldest surviving ice remnants of the entire Laurentide system.

A Lever on Global Climate

Far from being a passive frozen blanket, the Laurentide Ice Sheet actively reorganized atmospheric and oceanic circulation across the globe. By its sheer mass, it deflected the polar jet stream southward—away from its usual path over Montana and Minnesota—channeling moisture into the Southwestern United States and granting that otherwise arid region abundant rainfall during glacial periods. This stood in stark contrast to much of the rest of the world, which grew exceedingly dry, though analogous effects from European ice sheets boosted winter precipitation in Afghanistan, parts of Iran, possibly western Pakistan, and North Africa. The sheet's cycles of growth and melt also disrupted ocean circulation. When the ice melted, enormous volumes of low-salinity freshwater poured into the Arctic Ocean via the Mackenzie River, believed to have disrupted the formation of North Atlantic Deep Water—that dense, cold, saline current flowing from the Greenland Sea. This interruption of thermohaline circulation triggered the brief Younger Dryas cold snap and a temporary re-advance of the ice. The ultimate collapse of the Laurentide sheet, by raising global sea levels, is even suspected to have indirectly influenced the development of European agriculture.

The Final Retreat and Its Aftermath

The Laurentide Ice Sheet's final chapter was neither swift nor simple. The last major advance had covered most of northern North America between roughly 95,000 and 20,000 years before present, but the retreat that followed was punctuated by dramatic reversals. The Younger Dryas episode, driven by disrupted thermohaline circulation, caused a temporary re-advance of the ice. In the far north, the ice clung to Nunavik, Quebec, until as recently as 6,500 years ago. In the New York region, the ice that once stood about 2,000 feet above Manhattan began melting around 16,000 BC and was fully gone by approximately 10,000 BC. After the Younger Dryas ended, the sheet retreated rapidly, shrinking to the Canadian Shield before that last remnant too deglaciated. At peak thickness the ice reached 3 kilometers in Nunavik, though it thinned dramatically toward its margins where nunataks—peaks poking through the ice—were common in hilly terrain. The sheet's southern margin at times included the sites of coastal Northeastern towns and cities, and it extended up the Missouri River valley to the northern slopes of the Cypress Hills, where it merged with the Cordilleran Ice Sheet.

Gallery