Glaciation is the process by which large masses of ice form, spread across land, and reshape the terrain beneath them. It happens when snow accumulates faster than it melts over many years, compressing into dense ice that eventually begins to flow under its own weight. Glaciation has repeatedly transformed Earth’s surface, carving valleys, flattening mountains, and lowering sea levels by locking up enormous volumes of water as ice.
How Snow Becomes a Glacier
A glacier starts as nothing more than snow that refuses to melt. In areas where winter snowfall exceeds summer melting year after year, permanent snow patches build up in the mountains. Fresh snow is about 90 percent air, but as new layers pile on top, the weight compresses the snowflakes into coarser, denser grains. Meltwater seeps down and refreezes, filling gaps between grains.
The intermediate stage between snow and glacial ice is called firn, which is compacted snow that has survived at least one full summer. Firn takes roughly a year to form in temperate climates, but in extremely cold regions like interior Antarctica, the process can stretch to 100 years. Firn becomes true glacier ice once the interconnecting air passages between grains are completely sealed off, trapping air only as isolated bubbles.
Once the ice mass grows thick and heavy enough, gravity takes over. The ice begins to flow downhill, deforming internally as individual ice grains slide past one another under pressure. Many glaciers also slide along their base, with meltwater acting as a lubricant between ice and bedrock. The center of a glacier moves faster than its edges, where friction with valley walls slows it down. The moment a permanent snow patch shows signs of flow, it officially qualifies as a glacier.
Two Main Types of Glaciation
Glaciation takes two broad forms depending on scale. Alpine glaciation occurs in mountainous terrain, where ice fills preexisting stream valleys and flows downhill like a slow river. These glaciers are confined by the topography around them, carving deeper into the valleys they follow. You can see alpine glaciation today in the Himalayas, the Andes, the Alps, and the mountains of Alaska.
Continental glaciation is far more massive. Continental glaciers are ice sheets that spread across entire landmasses, flowing outward in all directions from a central accumulation zone rather than following valleys. These ice sheets can grow more than 3,000 meters thick. Today, continental glaciation exists only in Greenland and Antarctica, but during past ice ages, similar sheets buried much of North America and northern Europe.
What Triggers Ice Ages
Earth’s orbit around the Sun isn’t perfectly steady. It shifts in three predictable ways over thousands of years, and these shifts control how much solar energy different parts of the planet receive. Together, they’re known as orbital cycles, and they’re the primary driver of glaciation over geologic time.
The first is the shape of Earth’s orbit, which stretches from more circular to slightly more oval and back again. This cycle has a relatively small effect on total solar energy but appears linked to the timing of major glacial periods. The second is the tilt of Earth’s axis, which wobbles between 22.1 and 24.5 degrees over a roughly 41,000-year cycle. When the tilt is smaller, seasons become milder: winters are warmer (less snow melts) and summers are cooler (less ice melts), allowing ice sheets to gradually build up at high latitudes. The third is precession, a slow wobble in the direction Earth’s axis points, cycling roughly every 23,000 years. Precession changes when in Earth’s orbit each hemisphere experiences summer and winter, making seasonal contrasts more extreme in one hemisphere and less extreme in the other.
For most of the last million years, ice ages followed a roughly 100,000-year cycle. Before that, they came about every 41,000 years, matching the tilt cycle. The interaction between all three orbital patterns determines whether ice advances or retreats, though scientists still debate the relative importance of each.
Atmospheric carbon dioxide plays a reinforcing role. During glacial peaks, CO2 concentrations dropped to around 185 parts per million. During warmer interglacial periods, they rose to about 285 ppm. Lower CO2 means less heat is trapped in the atmosphere, which helps ice sheets persist and grow.
How Glaciers Reshape the Land
Moving ice is one of the most powerful erosive forces on Earth. Glaciers reshape terrain through two main processes. Plucking occurs when meltwater seeps into cracks in bedrock beneath the glacier, freezes, and pries loose chunks of rock that the glacier carries away. Abrasion happens when rocks embedded in the base of the glacier grind against the bedrock below, scouring it smooth and leaving parallel scratches called striations.
These processes create distinctive landforms. Alpine glaciation carves bowl-shaped depressions called cirques into mountainsides, and when multiple cirques erode toward each other, they leave behind narrow, knife-edge ridges called arêtes. Glaciers widen and deepen the valleys they flow through, transforming V-shaped river valleys into broad U-shaped valleys. When a U-shaped valley meets the sea, it fills with water and becomes a fjord.
Continental glaciation leaves its own signatures. As ice sheets advance and retreat, they deposit massive piles of sediment called moraines along their edges and at their farthest extent. Drumlins are smooth, elongated hills of glacial sediment shaped by the flow of ice over them. Eskers are long, winding ridges of sand and gravel deposited by meltwater streams that once flowed through tunnels inside or beneath the glacier. Much of the rolling terrain across the northern United States, Canada, and Scandinavia owes its shape to these processes.
The Last Glacial Maximum
The most recent peak of glaciation, called the Last Glacial Maximum, occurred roughly 26,000 to 20,000 years ago. About 8 percent of Earth’s total surface was buried under ice. Massive ice sheets covered Canada down to the Great Lakes region, blanketed Scandinavia, and extended across northern Russia. Sea levels dropped approximately 125 meters (410 feet) below present levels as so much ocean water was locked up in ice, exposing land bridges like the one connecting Siberia to Alaska.
Life adapted by retreating to refugia, pockets of habitable land that remained unglaciated, typically along mountain peripheries and southern lowlands. These refugia preserved populations of plants and animals that would later recolonize interior regions as the ice melted. Research on alpine species shows that postglacial recolonization often followed specific corridors, particularly transversal valleys where open, forest-free habitat allowed species to spread. Small, stable refugia on the edges of mountain ranges were especially important for preserving genetic diversity through glacial periods.
Glaciation Today and Looking Ahead
Earth is currently in an interglacial period, a warm stretch between glacial advances. As of the year 2000, glaciers outside the Greenland and Antarctic ice sheets covered about 705,000 square kilometers and held an estimated 122 trillion tonnes of ice. That ice is shrinking. Glaciers have lost an average of 273 billion tonnes of ice per year since 2000, and the rate is accelerating: losses increased from 231 billion tonnes per year in the early 2000s to 314 billion tonnes per year more recently.
Under natural orbital conditions alone, the next glacial period would likely begin around 50,000 years from now. Some researchers have estimated it could come as soon as 10,000 to 20,000 years, while others place it at 40,000 to 60,000 years, depending on the modeling approach. Historical carbon emissions of roughly 500 billion tonnes of carbon are unlikely to significantly delay that timeline. However, continued large-scale emissions could push the onset of the next glaciation further into the future by keeping atmospheric CO2 levels well above the thresholds that have historically allowed ice sheets to grow.