26: Glaciers and Ice Sheets

Over the last few years the news has been full of stories of glaciers melting, retreating and disappearing, but what actually are they and why does their melting matter? Never fear, in this week we shall be exploring just that! We will look back into the mists of time, question why glaciers may grow and shrink, and learn about why walking on glaciers can be quite a risky business, even for well train climbers and mountaineers. Happy reading!

Mendenhall Glacier calving (Picture: Gillfoto/Wikimedia Commons/ CC BY-SA 4.0)


Glaciers are basically giant masses of thick ice made by the building up of snow over many years, crushing snow crystals at the bottom until it is very dense. The surface snow often melts and falls again each seasons, whilst the harder ice below remains (well, in theory…). They can be ‘small’ from a few hundred meters in size, to really quite large covering up to hundreds of kilometres. However, glaciers aren’t just any old lump of dense snow.

What makes them special is the fact that they flow. Though they are solid, the sheer weight of more snow accumulating at the top of the glacier causes other parts of the glacier to flow. This is because the weight and pressure of the ice causes the bottom layer between it and the ground to melt slightly, allowing it to slide. Though slow, this movement involves incredible amounts of power and glaciers have the ability to carry huge rocks and carve out valleys into the landscape. Many valleys and dips in mountainsides seen today were created by glaciers that were around thousands of years ago.

File:Earth from Space Elephant Foot Glacier, Romer Lake, Greenland (31489912472).jpg
A picture from space of the Elephant Foot glacier in Greenland, a picture perfect example of a glacier flowing. Most have a bit more of a jagged appearance at their edge (Picture: Pierre Markuse/ Wikimedia Commons/ CC BY 2.0)

Where are they?

Glaciers are also more common than you’d think. 10% of earths land surface is covered by them, this being both in the Arctic and Antarctic but also high up in the mountains ranges of every continent apart from Australia (unless you count New Zealand as part of the continent as it has a few).

You get two main types of glaciers- alpine glaciers and ice sheets. Alpine glaciers are the ones you find up mountains that flow down through valleys, pushing and carving out the landscape as they go. Ice sheets on the other hand aren’t restricted to mountains. They are thick blankets of ice that can be found in flat areas, covering entire mountains and in the case of Greenland and Antarctica can cover almost entire land masses.

During the last ice age ice sheets completely covered about a third of earth’s land surface, going as far south as Germany in Europe and New York in North America. As so much water was trapped as ice on top of land, it meant that sea levels were lower, so Britain was connected to France by a large land bridge and the boot of Italy looked a lot more like an ankle boot.

An artist’s impression of what the world looked like during the last Ice Age (the Pleistocene). Glaciers came a lot further south than they are today, and lower sea levels meant that there was land in places like Southeast Asia which is now submerged. (Picture: jesusgamarra/DeviantArt)

What happens when they move?

Through a combination of weight, gravity and a slippy underside, glaciers flow. Different parts move at different speeds, so for example the bottom moves slower because it is scraping along the ground, compared to the middle. Because of these differences in speed, tension can start to build up creating massive cracks called crevasses which can open up quickly and be very deep. These can be deadly to climbers and mountaineers, especially when a light dusting of new fresh snow covers the crevasses from sight.

Meltwater can also start to form pipelines when flowing down into these cracks. These pipelines are called moulins and can go all the way to the base of the glacier. It is inadvisable to fall into one of these either.

As they move, glaciers can flow over things over time, covering them up. This can be regular land and mountains, but this can also include active volcanoes. When the volcano erupts it can be quite dramatic and dangerous, as not only molten lava but also water, ice and rocks get spewed out into the surrounding landscape.

So we know that glaciers flow and move, but what kind of speeds are we looking at? Many move at a glacial rate (hence the phrase), i.e. just a few centimetres a year. Others known as galloping glaciers can move at up to 50 m in a day, so it varies quite a lot on the local conditions. With speeds like this you’d think that glaciers would have taken over the earth by now. Luckily temperature and water have taken care of that.

When a glacier meets the coast, it starts to float on top of the water. Here it forms huge ice cliffs, the edges of which start to break off in a process known as calving. This is a noisy and violent process, and the result is that you get big chunks of ice floating in the ocean which are, you guessed it, ice bergs! Over time being in the water and floating off to warmer climates causes these ice bergs to melt and so the glaciers are stopped from taking over.

Inside a Moulin in Juneau, Alaska. (Picture: glacierguide/reddit)

What effect does this have on us and our planet?

If our planet was colder, then the glaciers might flow faster than they melt and we would have another ice age where the ice sheets would take over. For the record another ice age is actually expected in a few thousand years as part of the planets cycles of warming and cooling. If the planet warms up, either from natural temperature cycles or say, I dunno, faster-than-normal warming from human activities also known as climate change *cough*, then the glaciers will melt faster than they form. This means more water will be entering the oceans causing the sea level to rise, which is more than just inconvenient for folks with a nice house by the seaside. Not only will more water enter the seas, but more cold water will enter the seas. There are some who are concerned that this change in ocean temperature might disrupt the currents in the ocean, notably the Gulf Stream.

The Gulf Stream is a warm current of water that starts at the Gulf of Mexico and West Indies, and flows up to Northern Europe. This warm water in turn creates a warmer climate on land, which is why despite being at the same latitude, Alaska and Scotland have quite different climates.

Glaciers are important in relation to water in many ways. Another is that glaciers high up in the mountains can provide freshwater that grows from streams into rivers which many people, animals and plant rely on. The river Ganges, which is very important to people in India for all kinds of aspects of life, is one of these rivers. It starts at the Gangotri glacier in the Himalayas and so the millions of people downstream relying on it are strongly affected by what happens to this glacier. Losing glaciers in many places would mean less freshwater would be available for creatures to sustain themselves, as once it reaches the oceans this freshwater will become salty and unpalatable.

These downsides are all dependent on if glaciers are melting at a faster rate. Are they? Unfortunately many are yes, from those on top of Mount Kilimanjaro being replenished too slowly from lack of precipitation, to spectacularly giant hunks of Antarctica (one which was 160 square miles in size) breaking off and melting into the ocean. However, for full disclosure there are some areas where glaciers are growing, such as where increasing temperatures are actually causing more snowfall, allowing the glaciers to replenish faster and get bigger. This is currently happening in the Upper Indus River Basin in Pakistan.

File:Glaciers and Icebergs at Cape York.jpg
A group of icebergs, loose chunks of glacier, drifting off into the oceans of the world to melt away (Picture: Brocken Inaglory/ Wikimedia Commons/ CC BY-SA 3.0)

Earth’s landscape

Glaciers play a big role on earth. They carve out the lakes, valleys and mountains around us, the soil that they move around can create rich farmlands, they make rivers, adjust the sea level and add an extra scary challenge for enthusiastic climbers who are hiking across them. They grow and shrink over millennia creating ice ages and land bridges, and are also caught up in the impacts of the accelerated climate change going on right now. That’s quite a repertoire for what is essentially a big old pile of ice!

For more info:

National Geographic article on glaciers (pretty pictures included): https://www.nationalgeographic.org/encyclopedia/glacier/

A nice video to show you why you don’t want to jump on a glacier’s edge :

Music and intro: kongano.com and Netflix’s Our Planet (I own rights to neither)

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