Glaciers and the Ice Ages

Rivers of ice

A glacier looks frozen and still, but it is really a slow-moving river made of ice. Glaciers form on high mountains and across whole polar continents, and over thousands of years they have shaped some of the most dramatic scenery on Earth — from the fjords of Norway to the peaks of the Alps. They also hold most of the planet's fresh water, which makes them important not just to geologists but to everyone.

How a glacier is born

A glacier forms wherever more snow falls each year than melts away. Each winter's snow buries the last, and the weight squeezes the lower layers. Air is pressed out and the snow turns to dense, blue-tinged glacial ice. Once the ice is thick enough — tens of metres — gravity makes the whole mass begin to flow slowly downhill, even over fairly flat ground.

This is the same idea behind the three states of water: water locked as solid ice can still creep and reshape the land, given enough time and weight.

Glaciers as sculptors

Moving ice is one of Earth's most powerful tools of weathering and erosion. As a glacier grinds forward, it:

• Plucks rock by freezing onto it and tearing chunks away.
• Abrades the ground, using the embedded rocks like sandpaper to scratch and polish the bedrock.

The results are easy to spot once you know them:

• U-shaped valleys: rivers cut narrow V-shaped valleys, but glaciers gouge out broad, flat-bottomed, steep-sided U-shapes.
• Fjords: U-shaped valleys later flooded by the sea, like those in Norway and New Zealand.
• Jagged peaks and ridges where glaciers eat into a mountain from several sides.
• Moraine: the heaps of rocky rubble a glacier carries and dumps when it melts. Lines of moraine mark where the ice once reached.

What is an ice age?

An ice age is a long stretch of Earth's history when global temperatures drop enough for huge ice sheets to spread across the continents. During the most recent one, ice up to several kilometres thick covered most of Canada and northern Europe. The last glacial maximum was about 20,000 years ago; so much water was locked in ice that sea levels were roughly 120 metres lower than today, and you could have walked between Britain and France.

Ice ages are not a single event but a series of cold glacial periods separated by warmer interglacial gaps. We are living in an interglacial right now.

What triggers the cold

Why does the ice come and go? The main pacemaker is a set of slow changes in Earth's motion called Milankovitch cycles:

• The shape of Earth's orbit around the Sun changes over about 100,000 years.
• The tilt of its axis wobbles between about 22° and 24.5°.
• The axis itself slowly wobbles like a spinning top.

These shifts change how much sunlight reaches the poles, nudging the planet in and out of ice ages. Levels of greenhouse gases then amplify the change — a link you can explore in the greenhouse effect.

Why glaciers matter today

Glaciers store about 69% of the world's fresh water, and meltwater feeds rivers that billions of people drink from. Today, most glaciers are shrinking as the climate warms, which raises sea levels and threatens water supplies. Scientists drill deep ice cores from glaciers because the trapped air bubbles preserve a record of the atmosphere stretching back hundreds of thousands of years — a frozen diary of Earth's climate.

Try it yourself: model glacial erosion

1. Freeze water in a small plastic tub to make a block of ice. Before it fully freezes, press some sand and small grit into the bottom.
2. Pop out the ice block and find a bar of soft soap or a slab of modelling clay.
3. Press the gritty side of the ice onto the soap and drag it slowly across, pushing down firmly.
4. Look at the surface: the grit will have scratched grooves, just like a real glacier abrading bedrock.

Those scratches, called striations, are exactly what geologists hunt for in the field to prove that a glacier once passed over a landscape — long after the ice itself has melted away.