How Glaciers Move

Glaciers travel downhill under the quiet insistence of gravity and the slow reshaping of their own ice. On the molecular level, ice is arranged in stacked layers whose bonds are strong within each layer but comparatively weak between them. When stress and strain remain proportional, ice behaves like an elastic solid — stiff, resisting change. But once a glacier grows thick enough, usually beyond 50 m (160 ft), the pressure overwhelms these interlayer bonds. The ice begins to yield, flowing plastically rather than bending elastically. From that point on, the glacier moves as a layered river of crystallized water.

This behavior is captured by the Glen–Nye flow law:

Σ = k τⁿ

where:

Σ = shear strain (flow) rate
τ = stress
n = a constant between 2–4 (commonly 3 in most glaciers)
k = a temperature-dependent constant.

Flow is slowest near the base and along valley walls, where friction anchors the ice. Velocities increase toward the center and near the surface, where resistance weakens and deformation lessens. At the glacier’s skin — its uppermost ice — the accumulated movement of all underlying layers reaches its greatest speed.

Where the ice lies thickest, erosion intensifies. Deep ice masses scour and deepen valleys, exaggerate pre-existing terrain, and carve fjords whose plunging profiles reflect the immense pressures funneled into them. High peaks rising above ice sheets — nunataks — remain largely untouched, eroding only a few meters over more than a million years.

Early glacial science offered competing theories. Some believed meltwater freezing within the glacier forced it to expand; others argued that pressure-driven melting and refreezing (regelation) explained ice motion. James Forbes, in the 1840s, provided the correct framework: glaciers behave like exceptionally slow, viscous fluids, deforming under their own weight — a truth that took decades to be fully accepted.

Fracture zone and cracks

The top 50 m (160 ft) of a glacier are rigid because they are under low pressure. This upper section is known as the fracture zone and moves mostly as a single unit over the plastic-flowing lower section. When a glacier moves through irregular terrain, cracks called crevasses develop in the fracture zone. Crevasses form because of differences in glacier velocity. If two rigid sections of a glacier move at different speeds or directions, shear forces cause them to break apart, opening a crevasse. Crevasses are seldom more than 46 m (150 ft) deep but, in some cases, can be at least 300 m (1,000 ft) deep. Beneath this point, the plasticity of the ice prevents the formation of cracks. Intersecting crevasses can create isolated peaks in the ice, called seracs.

Crevasses can form in several different ways. Transverse crevasses are transverse to flow and form where steeper slopes cause a glacier to accelerate. Longitudinal crevasses form semi-parallel to flow where a glacier expands laterally. Marginal crevasses form near the edge of the glacier, caused by the reduction in speed caused by friction of the valley walls. Marginal crevasses are largely transverse to flow. Moving glacier ice can sometimes separate from the stagnant ice above, forming a bergschrund. Bergschrunds resemble crevasses but are singular features at a glacier's margins. Crevasses make travel over glaciers hazardous, especially when they are hidden by fragile snow bridges.

Below the equilibrium line, glacial meltwater is concentrated in stream channels. Meltwater can pool in proglacial lakes on top of a glacier or descend into the depths of a glacier via moulins. Streams within or beneath a glacier flow in englacial or sub-glacial tunnels. These tunnels sometimes reemerge at the glacier's surface.

Subglacial processes

Beneath the seemingly solid glacier, where ice meets rock, most of the drama of glacial motion unfolds — often within just a few meters of contact. The temperature, texture, and softness of the bed dictate whether the glacier will slide smoothly, deform the underlying sediments, or remain anchored in place.

A soft, porous bed can accommodate movement as the sediment itself shifts, sometimes carrying the glacier atop it like a slow, living conveyor. Where the bed is frozen or hard, sliding occurs only if a thin film of meltwater lubricates the interface. The characteristics of this layer — its porosity, pressure, and distribution — determine how readily the ice can glide.

• The glacier’s weight and motion can dilate sediments beneath it, rearranging them from a tightly packed order into a looser jumble, increasing pore space and lowering fluid pressure.
• Pressure can also compact underlying sediments, squeezing air and water from pore spaces, a process often irreversible in frozen soils.
• Particles ground by abrasion and fracture may reduce space between grains, though the constant motion of the ice can counteract this, generating heat and subtle rearrangements.

Bed properties vary across space and time. Changes in underlying geology or the roughness of bedrock, including protruding boulders, can dramatically slow the glacier. Ice flows around obstacles by melting under intense pressure on the upstream side and refreezing downstream, a delicate dance of motion and pause.

Fluid pressure beneath the glacier also affects movement. High pressures provide a buoyant lift, reducing friction and enabling faster flow. In the fastest ice streams, effective pressure can nearly vanish, letting the ice float on a thin cushion of water, almost independent of the weight above.

Basal melting and sliding

Glaciers may also advance by sliding along their beds, lubricated by meltwater that lowers basal friction. This water can form from pressure-induced melting, friction, or geothermal heat. Variations in meltwater at the glacier’s base can accelerate flow, particularly in temperate or warm-based glaciers.

The forces governing this motion can be described as:

τD = ρ g h sin α

where τD is the driving stress, ρ the ice density, g gravity, h the ice thickness, and α the surface slope in radians.

Basal shear stress, τB, depends on bed temperature and softness. The effective shear stress, τF, is the smaller of τB and τD, controlling the glacier’s plastic flow rate.

Thicker ice depresses the melting point of water beneath it, enabling basal melt in a feedback loop: pressure fosters melting, friction generates heat, and faster flow increases basal temperature, accelerating motion. Some Antarctic glaciers, under certain conditions, can surge at rates up to about a kilometer per year, though such events are rare and episodic. Eventually, the glacier may thin as accumulation fails to match transport, conduction cools the base, and the flow slows — often until the cycle begins anew.

Beneath the surface, subglacial water channels exert a profound influence on movement. Water may flow rapidly through pipe-like tunnels or spread in thin sheets, occasionally triggering surges. The draining and redistribution of subglacial lakes can subtly shift the ice above, leaving traces in the surface topography.

Speed

A glacier’s pace is shaped largely by friction. Ice at the bottom moves more slowly than ice above, and in alpine glaciers, valley walls slow the edges relative to the center.

Typical glacial movement averages around 1 meter (3 ft) per day, though stagnant zones may see almost no motion — in some Alaskan valleys, trees even take root atop abandoned glacial sediments. At the other extreme, fast-moving glaciers like Greenland’s Jacobshavn Isbræ can surge 20–30 meters (70–100 ft) per day. Flow is influenced by slope, ice thickness, snowfall, confinement, basal temperature, meltwater availability, and bed hardness.

Occasionally, glaciers enter periods of dramatic acceleration called surges. During these events, normally slow ice suddenly races forward, driven by factors such as bed failure, accumulated meltwater, or excessive mass exceeding a critical tipping point. In rare cases, flow rates have reached 90 meters (300 ft) per day.

Where glaciers move faster than about 1 km per year, glacial earthquakes can occur — large, sudden shifts within the ice. In Greenland, these seismic events peak during summer months, and their frequency has increased notably since the early 2000s, reflecting the dynamic interplay between ice, meltwater, and the underlying terrain.

Ogives

Ogives, or Forbes bands, trace the annual rhythm of a glacier in undulating light and dark bands across its surface. These alternating ridges and troughs are born from seasonal motion: ice descending from icefalls is fractured and ablated in summer, then layered with new snow in winter, creating a subtle, wave-like pattern.

The width of one dark and one light band often corresponds roughly to the glacier’s annual progress. Sometimes these features appear as simple color bands or gentle undulations rather than pronounced waves, yet each ogive silently records the passage of time and the glacier’s slow, patient journey down the mountainside.

• Types of Glaciers
• Formation of Glacier
• Structure of Glacier
• Geography of Glaciers
• Glaciers and Climate Change
• Glaciers as Water Reservoirs
• Glaciers in Culture and Myth
• Glaciers and Geology
• Glaciers and Biodiversity
• Glacier Tourism and Recreation
• Future of Glaciers