A glacier is a huge mass of ice that flows slowly downhill under its own weight. They cover about 10% of the Earth's land surface and hold nearly 70% of all the fresh water on the planet. Glaciers carved most of the dramatic scenery in the northern half of the world — the fjords of Norway, the Great Lakes, the valleys of the Alps and the shape of Central Park's rocks in New York.
How Snow Becomes a Glacier
A glacier starts where more snow falls in winter than melts in summer. Year after year the snow piles up.
The weight of the new snow squashes the layers beneath. Air is pressed out, the flakes fuse together, and after a few years the snow turns into a grainy substance called firn. Keep piling snow on top and the firn is compressed into solid glacial ice.
This takes decades to centuries. Once the ice is around 50 metres thick, the pressure at the bottom is enough that the ice starts to deform and flow, and at that point it is officially a glacier rather than a large heap of ice.
Why Glacier Ice Is Blue
Fresh snow is white because it is full of air bubbles that scatter all colours of light equally. Glacier ice has had almost all the air squeezed out of it.
Dense, bubble-free ice absorbs the red end of the light spectrum and lets blue through. The thicker the ice, the more red gets absorbed, and the deeper the blue looks.
That is why crevasses and the walls of ice caves glow an intense turquoise, while the surface of the same glacier looks white or dirty grey. The blue is the oldest, most compressed ice.
Two Kinds
Valley glaciers, sometimes called alpine glaciers, sit in mountain valleys and flow downhill between rock walls. Most of the glaciers you can visit are these.
Ice sheets are enormous domes of ice that bury the landscape underneath them and spread outwards in all directions. There are only two left: Antarctica and Greenland. Together they hold about 99% of the world's glacial ice.
Antarctica's is around 14 million square kilometres and up to 4.8 kilometres thick. Greenland's is about 1.7 million square kilometres, and if it melted entirely, global sea level would rise around seven metres.
Glaciers Move
Glaciers flow in two ways at once. The ice deep inside deforms and creeps under pressure, and the whole mass can also slide over meltwater at its base.
Most move between a few centimetres and a couple of metres a day. Some are much faster: Jakobshavn Isbrae in Greenland has been measured at over 40 metres a day, one of the fastest glaciers in the world.
Occasionally a glacier surges, speeding up by ten or a hundred times for months at a time before slowing again. Nobody fully understands why, though water building up underneath seems to be involved.
Make a Glacier
Freeze water in a plastic cup with a little sand and small pebbles mixed in. Once it is solid, take it out and press it firmly along a bar of soap or a lump of modelling clay, then drag it slowly across.
Look at what you have done. The trapped sand and stones have scratched grooves into the surface, and the soap has been scraped away. That is exactly how a glacier works: the ice itself is not the tool, the rock frozen into it is.
Those scratches are called striations, and geologists find them on bedrock all over northern Europe and North America. They point in the direction the ice was travelling, which is how we mapped where the ice sheets went during the last ice age.
The Landscapes They Leave
Rivers cut V-shaped valleys. Glaciers cut U-shaped ones, with steep sides and flat floors, because the whole width of the valley is being scraped rather than just the bottom.
A cirque is the armchair-shaped hollow at the head of a glacier where it started. Where several cirques cut back into the same mountain from different sides, they leave a sharp pyramid peak called a horn. The Matterhorn is the most famous one.
A fjord is a glacial valley that was carved below sea level and then flooded when the ice melted. Norway has hundreds; Sognefjord is over 200 kilometres long and 1,300 metres deep. Chile, New Zealand, Greenland and Alaska all have them too.
What They Drop
Everything a glacier picks up gets dumped when the ice melts, unsorted and all mixed together. This rubble is called till, and a ridge of it is a moraine.
A terminal moraine marks the furthest point the ice reached. Long Island in New York is largely a terminal moraine from the last ice age, and so is much of Cape Cod.
Glaciers also leave erratics — boulders carried far from where they came from and left sitting in a landscape made of completely different rock. Some are the size of houses, and geologists can trace exactly which mountain they were quarried from hundreds of kilometres away.
The Last Ice Age
Around 20,000 years ago, ice sheets covered most of Canada, the northern United States, Scandinavia, northern Britain and large parts of Russia. So much water was locked up in ice that global sea level was about 120 metres lower than today.
That exposed land where there is now sea. Britain was joined to Europe by a plain now called Doggerland under the North Sea, and people lived and hunted there. A land bridge across the Bering Strait connected Siberia to Alaska, and it is one of the routes by which people first reached the Americas.
The ice began retreating around 20,000 years ago and was largely gone from Europe and North America by about 11,700 years ago. The land it had been squashing is still rising — parts of Scandinavia and Canada are lifting by nearly a centimetre a year, a process called isostatic rebound. Some Swedish harbours built a few hundred years ago are now too shallow to use.
Ice Cores Are Time Machines
Because glacier ice forms layer by layer, each year's snow is preserved in order, with tiny bubbles of the air that was around when it fell.
Scientists drill down and pull out cylinders of ice called cores. By analysing the trapped air they can measure exactly how much carbon dioxide was in the atmosphere thousands of years ago, and by looking at the chemistry of the ice they can work out the temperature.
The deepest cores from Antarctica go back around 800,000 years, and a project drilling for older ice is aiming at 1.5 million. Ash layers in the ice date individual volcanic eruptions, and there is even lead pollution recorded from Roman silver mining.
Icebergs
When a glacier reaches the sea it starts to float, and chunks break off. That is called calving, and the chunks are icebergs.
Around 90% of an iceberg is below the water, which is where the phrase about the tip of the iceberg comes from. Ice is slightly less dense than sea water, so just a tenth sticks out.
The iceberg that sank the Titanic in 1912 almost certainly calved from a Greenland glacier. The largest ever recorded, B-15, broke off Antarctica's Ross Ice Shelf in 2000 and was about 11,000 square kilometres — bigger than Jamaica.
Crevasses and Seracs
Ice can bend slowly but it snaps if it is bent quickly. That is why the top 50 metres or so of a glacier is brittle and full of cracks while the ice below flows.
Those cracks are crevasses, and they open where the glacier goes over a bump, around a bend or speeds up. Some are over 40 metres deep. Fresh snow can bridge them over, which is why mountaineers on glaciers rope themselves together — if one person breaks through, the others hold them.
Where a glacier flows over a steep drop it breaks into an icefall, a chaos of leaning towers of ice called seracs, which collapse without warning. The Khumbu Icefall on Mount Everest is the most notorious, and climbers cross it as fast as they can and preferably before dawn, when the ice is coldest and most stable.
They Are Shrinking
Almost every glacier on Earth is losing ice. Global measurements show glaciers outside the ice sheets losing hundreds of billions of tonnes a year, and the rate has been accelerating.
Glacier National Park in the United States had around 150 glaciers when it was named in 1910 and has fewer than 30 today. Iceland held a funeral in 2019 for Okjökull, the first Icelandic glacier declared dead, complete with a plaque addressed to the future.
It matters beyond scenery. Around two billion people depend on rivers fed by mountain glaciers and snow — the Ganges, Indus, Mekong, Yangtze and Rhine all begin in ice. Glaciers act like a savings account, storing water in winter and releasing it slowly in summer, and once they are gone the summer flow goes with them. To keep exploring, read about how rivers work and what Antarctica is like.
