How Glaciers Form and Move

Ice covers about ten percent of Earth's land surface, most of it in the ice sheets of Antarctica and Greenland, with the rest spread across smaller mountain and valley glaciers on every continent except Australia. None of that ice arrived as a single frozen block. It built up one snowfall at a time, over years, decades, or in the largest ice sheets, hundreds of thousands of years, and it is not sitting still: glacial ice flows, slowly but continuously, under the pressure of its own weight.

From Snowflake to Glacial Ice

A glacier begins wherever more snow falls in winter than melts the following summer, so that a layer survives year over year instead of disappearing. As new snow buries old snow, the buried layers compress under the growing weight above them. Fresh snow is mostly trapped air, but as it compacts, air pockets shrink and connect, and individual snow crystals bond together into a denser, granular material called firn, which looks more like coarse sugar than fresh snow. Continued burial and pressure eventually squeeze out most of the remaining air and fuse the firn into solid glacial ice, a transition that can take anywhere from a few years in wet, temperate climates to several centuries in the driest, coldest polar regions.

Why Ice Under Pressure Flows Like a Very Slow Liquid

Ice seems rigid on a human timescale, but under the sustained pressure of tons of ice stacked above it, it deforms rather than staying fixed, a behavior materials scientists call plastic flow. Individual ice crystals inside a glacier slip past one another along internal crystal planes, similar in principle to how a deck of cards can shift when pushed from the side even though no single card bends. This internal deformation lets a glacier's ice slowly migrate downhill from where snow accumulates toward its lower edge, typically moving anywhere from a few centimeters to a few meters per day depending on ice thickness, slope, and temperature.

Basal Sliding: When the Whole Glacier Slips

Where the base of a glacier reaches the melting point, usually because pressure from the ice above lowers the melting point slightly or because geothermal heat from the ground below warms the ice-rock boundary, a thin layer of meltwater can form underneath the glacier. That water acts as a lubricant, letting the entire mass of ice slide over the underlying rock rather than moving only through internal deformation. Glaciers that reach this state, called warm-based or temperate glaciers, generally move considerably faster than cold-based glaciers frozen solid to the ground beneath them, and basal sliding is the dominant reason certain glaciers, particularly fast-moving outlet glaciers that drain large ice sheets, can travel many meters or even kilometers per year.

Accumulation, Ablation, and Whether a Glacier Grows or Shrinks

Every glacier has an accumulation zone, typically its higher, colder upper reaches where more snow falls than melts, and an ablation zone, its lower reaches where more ice is lost to melting, evaporation, and calving than is replaced by new snowfall. The boundary between these two zones is called the equilibrium line, and its position shifts from year to year with weather and from decade to decade with climate. A glacier's overall mass balance, the net difference between accumulation and ablation over a full year, determines whether it is advancing, retreating, or roughly stable, and long-term monitoring of that balance is one of the primary ways scientists track how mountain glaciers are responding to a changing climate. Researchers at the National Snow and Ice Data Center maintain reference data on glacier mass balance measurements collected from monitoring stations around the world.

Glaciers as a Water Source

Beyond their role in the landscape, glaciers function as natural water reservoirs, storing precipitation as ice during colder or wetter periods and releasing it gradually as meltwater during warmer months. Downstream communities across much of High Mountain Asia, the Andes, and parts of the western United States depend on this seasonal meltwater to supplement river flow through the dry season, and the systems that ultimately deliver that water to households rely on the same basic processes described in how water treatment works to make glacier-fed river water safe to drink. Because glacial meltwater eventually reaches the ocean, sustained glacier loss also contributes, alongside thermal expansion of seawater, to the slow rise in global sea level that compounds with the daily rise and fall driven by tides in low-lying coastal areas.