Why Do Glaciers Flow Like Rivers When Ice Is So Hard?
― Crushed Ice Slowly Changes Shape
Ice is hard. Hit it, and it cracks. Yet glaciers creep down valleys over thousands of years. It turns out that when ice is squeezed hard enough for long enough, instead of breaking, it slowly changes shape. That is the whole secret behind why glaciers flow.
Take an ice cube fresh from the freezer and squeeze it hard in your hand. Nothing happens — it stays exactly the same shape. Drop it, and it shatters. That is what ice is like.
Yet photos of mountains show white bands of ice winding down valleys like rivers, deep cracks running across their surface.
Why does ice, which is supposed to be rigid, appear to "flow"? And why do those cracks form on the surface of something that is flowing?
There are two main reasons
Glaciers can be over a hundred metres thick. That weight constantly presses on the ice below, which — instead of cracking — slowly changes shape. Countless tiny shifts like this add up, and the whole mass creeps downhill.
Heat rising from the ground, plus friction from the ice rubbing against rock, can melt a little ice at the base. That thin film of water helps the glacier slide, so the whole mass shifts over the rock beneath it.
The first is about the ice itself changing shape; the second is about what happens between the glacier and the ground. Let's look at each in turn.
What does it mean for hard ice to "slowly change shape"?
Inside ice, water molecules are stacked like neat layers of sheets. The bond between one sheet and the next is weaker than the bonds within a sheet. So when a strong force is applied, the sheets can shift slightly against each other.
Picture pushing a deck of cards at an angle from above. Each individual card barely moves, but the whole deck slumps sideways. Something similar happens inside ice.
The catch is that this shifting is extremely slow. Most glaciers are said to move only tens of metres a year. Watching one, you would never notice it moving at all. Some fast-moving glaciers reportedly advance tens of metres in a single day, but even that is far too slow to see with the naked eye.
The key point is that this "shape-changing" motion only happens where the ice is under strong pressure. Near the surface of a glacier, there is less ice piled on top, so the pressure isn't enough. That ice stays the hard, brittle kind we all know. So the upper layers can't keep up with the slow motion of the ice below, and they tear apart. These are the deep cracks called crevasses. Take a look at Figure 1.
Where does the water at the base come from?
Three things happen at once at the base of a glacier. First, heat rises from deep inside the Earth — the ground everywhere is constantly losing a little heat outward. Second, friction between the sliding ice and the rock generates heat, the same way rubbing your hands together warms them up.
The third reason is a little more surprising. Under strong pressure, the melting point of ice drops slightly. In other words, under pressure, ice can melt even below 0°C. This effect is tiny — even at the base of a thick glacier, the drop is said to be less than one degree. Still, if the base temperature is already close to 0°C, that small shift can tip the balance between melting and staying frozen.
The thin layer of water this produces makes it easier for the glacier to slide over the rock. Even glaciers of the same thickness can move at very different speeds, depending on whether their base is frozen solid or cushioned by water.
Glacier ice isn't frozen water — it starts as fallen snow, which gets crushed by the weight of snow piling on top of it, squeezing out air as it slowly turns to ice. That means deeper ice is older, and the tiny bubbles trapped inside it are thought to be samples of ancient air.
Valleys carved by rivers tend to have a sharp, V-shaped bottom. Glaciers, on the other hand, scrape away at the base and sides across their whole width at once, which is thought to produce wide, bowl-shaped, U-shaped valleys. In Japan, too, this shape is believed to survive in places like the Northern Alps.
Summary
Glaciers flow because ice has two sides to it: a "brittleness" that cracks, and a "softness" that slowly changes shape. The surface cracks, the deep ice deforms, and the base slides — put all three together, and a block of ice creeps down a valley like a river.
Squeeze ice hard enough, for long enough,
and it stops breaking — and starts flowing.
For more on ice's strange properties, see Why Does Ice Float on Water?; for how mountains get reshaped over vast timescales, see How Did Fossil Shells End Up on Top of a Mountain?; and for how snow crystals form, see Why Are Snowflakes Six-Sided?.
- Lean an old bar of chocolate upright in a room for a few days. In warm weather, the bottom will gradually bulge and change shape. Hit it, and it snaps cleanly — yet given time, it deforms.
- Drop some honey onto a plate and leave it for 10 minutes. Right after it lands it forms a little mound, but it gradually spreads flat. The same substance looks completely different depending on whether the force is "fast" or "slow."
- Stack about 50 sheets of paper, press down on top with your hand, and shift it sideways. Each individual sheet barely shifts, but the top and bottom of the stack end up far apart. This is close to what happens inside ice.
Testing this with real ice would take years. Try these substitutes instead, to get a feel for how even hard things change shape given enough time.
Want to go deeper? ― Terms, formulas, and textbook linksFrom middle-school science to university-level courses, each section is labelled by level
- MSCovered in middle-school science
- HSCovered in high-school "Basic Physics / Basic Earth Science"
- HS+High-school advanced content, or textbook sidebar material
- UnivNot covered in high school — university-level content (glaciology, geophysics)
- ResearchNot yet settled even at university level — an active research question
MSTerms: this phenomenon has a name
- Plastic deformation: a change in shape that doesn't reverse once the force is removed. The way glacier ice slowly changes shape over time is an example.
- Basal sliding: the whole glacier sliding over the bedrock beneath it. It depends heavily on whether there is water at the base.
- Crevasse: a deep crack in the surface of a glacier. It forms because the brittle upper ice can't keep pace with the slow motion of the ice below, and tears.
MSHSDo the math: how much pressure is at the base of a glacier?
Consider a glacier 100 metres thick. Every square metre at its base carries the full weight of the ice stacked above it. Let's convert that into something familiar: atmospheric pressure.
| In symbols | P = ρ × g × h |
| In words | Pressure at the base = density of ice × gravitational acceleration × thickness of ice |
| Where it comes from | The weight of the column of ice sitting on one square metre at the base (density × volume × gravitational acceleration), divided by that 1 m² area. It's the same "hydrostatic pressure" formula used for the bottom of a body of water |
| P | Pressure at the base of the glacier (in pascals = newtons per square metre) |
| ρ | Density of ice (kg/m³) |
| g | Gravitational acceleration (m/s²). The force of gravity on 1 kilogram |
| h | Thickness of the glacier (m) |
| Density of ice (weight per cubic metre) | about 900 kilograms |
| Thickness of glacier considered | 100 metres |
| Gravitational force on 1 kilogram (rough figure) | about 9.8 newtons |
| Weight of ice sitting on 1 m² at the base (kilograms) | 900 × 100 = 90000 |
| Force this exerts on the base (newtons) | 90000 × 9.8 = 882000 |
| How many times atmospheric pressure (about 100000) | 882000 ÷ 100000 ≒ 8.8 |
In other words, the base of a 100-metre-thick glacier is under about 9 atmospheres of pressure — comparable to being 90 metres deep in water. Here, density is measured in kilograms per cubic metre, force in newtons, and pressure in newtons per square metre. Sustained over a long time, this pressure is what sets the ice slowly deforming.
HSHS+Why is the surface fastest, not the base?
HSThe deeper the ice, the greater the pressure, so the deeper ice deforms more actively. But the distance we actually see a point move is the sum of all the shifting in every layer below it. Since the surface sits on top of the largest total stack, it ends up moving fastest of all. That's why the arrows in Figure 1 get longer toward the top.
HS+Ice is not like a simple liquid such as syrup: doubling the applied force doesn't just double the rate of deformation. In fact, the rate of deformation is said to scale roughly with the cube of the applied force. This strong sensitivity is why even a small increase in thickness can make a glacier suddenly move much faster.
UnivTreating ice as an "extremely slow flow"
In university-level glaciology, glaciers are treated as a flow in which the applied stress and the rate of deformation are not proportional. How the ice crystals are aligned inside, how much water it contains, and how much fine grit is mixed in all affect how easily it deforms. The relationship between stress and deformation rate is known as Glen's flow law, under which the rate of deformation scales roughly with the cube of the stress. At the base, it's also thought that ice melts on the upstream side of bumps in the rock and refreezes on the downstream side as it flows over them — another mechanism believed to help glaciers move.
📖 For the derivation and further reading: Hydrostatic pressure (Japanese Wikipedia) ／ Glaciers (Japanese Wikipedia)
ResearchWhat's still not fully understood
- The plumbing of basal water Whether the water at a glacier's base forms a broad, thin film or gathers into narrow channels like small rivers is thought to depend on location and season, but it's hard to observe directly, and much remains unknown.
- Why speeds sometimes surge Some glaciers are known to move dozens of times faster than usual, but only for a few months at a time. What triggers this is still debated.
- Predicting what happens once ice reaches the sea The rate at which glaciers and ice sheets flow into the ocean feeds directly into predictions of future sea-level rise. This remains one of the areas where forecasts vary the most.
In other words, everything in this article reflects "the best explanation we have so far." What's happening at the base, in particular, may well be revised as research continues.
Textbook connections (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| MS | Science: force and pressure, changes of state | How the weight of ice presses on the base; how ice melts |
| HS | Basic Earth Science: how landforms develop / Basic Physics: force and pressure | Bowl-shaped valleys, calculating basal pressure |
| HS+ | Physics: deformation of matter / Earth Science: glacials and interglacials | Why stress and deformation rate aren't proportional |
| Univ | Glaciology / geophysics | Treating ice as a flow, refreezing at the base |
| Research | Glacier and ice-sheet observation and modelling | Plumbing of basal water, sudden speed changes |
| ― | Everyday connections | Shape of mountain valleys, ancient air trapped in ice |
- Japanese Society of Snow and Ice (ed.), Encyclopedia of Snow and Ice, Asakura Publishing (日本雪氷学会 編『雪と氷の事典』朝倉書店)
- National Institute of Polar Research (国立極地研究所), "The Science of the Antarctic and Arctic" explainer pages
- W. S. B. Paterson, The Physics of Glaciers, Elsevier
- Japan Meteorological Agency (気象庁), "Climate Change Monitoring Report," sections on sea level and snow/ice
※This article is a general-audience science explainer. The figures given are approximations meant to help illustrate the underlying mechanisms. If you are visiting snow- or ice-covered areas, please follow the guidance and instructions of local authorities and site managers.