Glaciers and Glaciation: Formation, Flow, and Mass Balance

Core Idea

A glacier is a thick mass of ice on land that moves under its own weight. Glaciers form where more snow falls each year than melts, so the snow builds up, compacts into firn, and turns into dense glacial ice. Ice moves by slow internal deformation and by sliding over a wet base, always flowing from the zone of accumulation toward the zone of ablation, where ice is lost. Whether a glacier's front advances or retreats depends on its mass balance, the yearly difference between ice gained and ice lost.

How It's Best Learned

Model glacier flow with a slab of silicone putty or thick dough on a tilted board, and mark lines on it to watch how the middle moves faster than the edges. Then draw a long profile of a valley glacier with the accumulation zone, equilibrium line, and ablation zone, and use it to explain advance and retreat. Use paired historical and modern photos of a real glacier to measure retreat, and connect the landforms left behind to the processes that made them.

Common Misconceptions

• A retreating glacier is flowing backward uphill. (Ice always flows downhill; the front retreats when melting at the front removes ice faster than flow delivers it.)
• Ice is too brittle to flow. (Near the surface ice cracks into crevasses, but under the pressure of tens of meters of ice above it, ice deforms slowly like a very stiff plastic.)
• Glaciers exist only in polar regions. (Glaciers also occur on high mountains near the equator, such as Kilimanjaro and the Andes, where cold temperatures at altitude let snow build up.)

Explainer

A glacier begins with snow that does not melt. In cold places, such as polar regions and high mountains, more snow can fall in winter than melts in summer. Year after year the snow piles up. Its own weight compacts the lower layers, and the snow grains partly melt, refreeze, and grow. Air is squeezed out. The snow becomes firn, a grainy material, and finally glacial ice. When the ice is thick enough, it begins to move.

There are two main types. Valley glaciers flow down mountain valleys. Ice sheets spread across whole landmasses. Today the Antarctic and Greenland ice sheets hold about 99 percent of Earth's glacier ice, and ice covers roughly 10 percent of the land.

Glaciers move in two ways. In internal flow, ice crystals deform and slide past each other under pressure, so deep ice behaves like a very stiff plastic. The top few tens of meters are not under enough pressure to flow and crack into crevasses instead. In basal sliding, meltwater at the base lets the glacier slip over its bed. The center of a valley glacier moves faster than its sides, which are slowed by friction.

Every glacier has an upper zone of accumulation, where it gains snow, and a lower zone of ablation, where it loses ice by melting, sublimation, and calving. The equilibrium line separates them. The yearly difference between gain and loss is the mass balance. A positive balance makes the front advance. A negative balance makes it retreat, even though the ice keeps flowing downhill.

Moving ice reshapes the land. It plucks loose blocks and grinds rock with the debris it carries. Glaciers carve U-shaped valleys, bowl-shaped cirques, sharp ridges called arêtes, and flooded valleys called fjords. When ice melts, it drops till, an unsorted mix of sizes, often in ridges called moraines. Meltwater streams leave sorted outwash.

In 1837 the naturalist Louis Agassiz argued that much of northern Europe had once been covered by ice. He was right. During the last glacial maximum, about 20,000 years ago, ice sheets covered much of North America and northern Europe, and sea level was about 120 meters lower than today. The crust beneath those ice sheets is still rebounding now.