Glaciers: Retreat, Moraines, Valleys, Fjörds

Glaciers are overwhelmingly dramatic. In this article, we will present their key features and explain how important they are. But, before starting, let’s have a look at one of the monsters we’ll be working on. The following figure displays an ice tongue of the Vatnajökull‎ in Iceland.


If you enjoyed this picture, keep reading, as I’m sharing in this article other great pictures of glaciers I’ve taken! Like that of the Perito Moreno in Argentina:

Generalities about Glaciers

But what’s a glacier? Is it just a large block of ice?

Basically, yes. Maybe not just a large block, but rather a gigantic accumulation of ice, which can be as large as the size of a country. And its enormity makes it very long to melt. It therefore stays throughout seasons, years and centuries as a single colossal mass of ice.

But where did the ice come from?

From the snow that has been accumulating on the top of the glacier.

Wait. Isn’t there a difference between ice and snow?

Yes there is. As you’re picturing it, snow is lighter and more fragile. Indeed, water drops that eventually turned into snow were solidified high in the sky in a very slow manner. Because of this, it gave water molecules time to structure themselves into the geometries which minimize the energy of the block of solidified water. The structure leaves also a lot of gaps which are filled by air. The geometry is based on 120° angles and can take different forms depending on temperature and humidity, as displayed in the following figures of snowflakes.


So what turns snow into ice?

Snowflakes get crushed!

Well, I crush snow to do snowballs, or even worse, I sky on snow, but it remains snow…

That’s because you are way too weak, at least compared to glaciers! You have to picture snow being covered by thick layers of snow.

How thick?

Tens, hundreds or even thousands of meters of snow! This creates an enormous pressure that smashes the snowflakes into dense solidified water, also known as ice. That’s why, even though Mount Cook, the summit of New Zealand, may be covered in snow, the glaciers flowing down are made of almost ice only.

Mount Cook

It looks like the ice even becomes blue…

Yes! As the ice gets crushed, it gets rid of air and becomes very dense. This is accentuated by surface water melting due to sun rays and filling deeper layers, as it eventually freezes back. So, the ice which comes out of glacier tongues is extremely dense frozen water. Now, the blue property really is a property of water molecules. In particular, the bonds oxygen-hydrogen tend to absorb red frequencies of light. What’s left is blue.

Retreat and Advance

You have probably heard that glaciers have been retreating. This phenomenon is attributed to global warming.

Is this true?

As you’d expect it intuitively, heat does melt glaciers and diminishes their size. Ice can even evaporate directly. These phenomenons which diminish the size of glaciers are called ablation. They occur rather in areas of glaciers which are warmer. This usually corresponds to lower elevations or area more exposed to sun. These areas are called ablation zones.

But I guess that this isn’t the end of the story…

Indeed. Some glaciers are actually advancing despite warming or extension of ablation zones. This has been the case, for instance, for Fox Glacier in New Zealand. Not only does this glacier extend to the rainforests, but it’s also been advancing for the two last decades!

Fox Glacier

Really? How is it possible? How can it go down to the rainforest?

Let’s first answer your last question. Because glaciers are extremely huge and dense, it takes time to melt them, even if the temperature is high. In fact, in the early 1800s, Frederic Tudor took advantage of this, as he shipped huge blocks of ice from New England to the Caribbean islands, and even to India! You can find out more about this in the introduction of my article on the frontier of cold.

OK, so glaciers melt slowly. This doesn’t explain how they can be flowing down rainforests…

Another important feature of glaciers is that, counter-intuitively, they behave like viscous fluids. This is due to the enormous pressure they are under, and can only be observed for large masses of ice. To simplify, you can think of these glaciers as dense ice cream. They flow downhill. They typically advance a few centimeters per day, and a few hundreds of meters a year, as displayed in the following fast-forward video from French TV show c’est pas sorcier.

Unfortunately, for copyright infringements, the video has been removed from YouTube. I hope one day such great shows will be allowed to be embedded in articles like mine, because I’d argue that this contributes for these videos to reach their full potentials.
But I guess this speed depends on the slope, right?

Sure! The higher the slope, the faster the glacier. This typically explains why Fox Glacier is in the rainforest: It’s because the slope from the mountains to the rainforest is so steep that the glacier is advancing very quickly at this point. To actually find out how far a glacier can go, you need to compare the speed at which it flows with the speed at which it melts.

OK. This explains why it goes into the rainforest. But not why it’s advancing… After all, it is getting warmer, isn’t it?

I can’t certify that it’s warmer now at Fox Glacier than two decades ago, but I can provide an explanation for the advancement of the glacier even if it’s actually getting warmer! As I said, what we need to compare is the speeds at which the glacier flows and at which it melts. Now, if it’s getting warmer, it does melt faster. But it could still be advancing…

if it’s flowing faster!


But why would it flow faster? The slope can’t be changing, can it?

No it’s not. At least, not in a couple of decades. There’s actually another important factor which can increase the speed of the glacier. It’s its weight. The heavier the glacier, the faster it will flow down. Just like ice cream.

And the mass of the glacier comes from snow… So the more it snows, the faster the glacier, right?

Exactly! Now, snow mainly falls in high elevations. These areas which accumulate snow of glaciers are called accumulation zones. And as you said, the more they accumulate snow that falls, the heavier the glacier gets, the faster it flows down and the more it advances. Overall, glaciers retreat because they melt, and they advance because it snows. Another way of understanding it is the volume of glacier varies depending on the difference between the mass of fallen snow and the mass of melted ice. This is displayed in the following picture:

So I guess that it’s been snowing more than usual in the last decades over Fox glacier…

Indeed it has! And I’ve been given an interesting explanation by a New Zealand guide. She said that the development of Australian cities like Melbourne and Sidney may have created more humid air masses which flow to New Zealand’s west coast. This could have resulted in increasing precipitations, and, thus, the advancement of Fox Glacier.

Unfortunately, I haven’t been able to check this information, and this would be awesome if you could refer me to some articles to corroborate what she said.

Dangers of Glaciers

Glaciers are breathtaking. It’s highly recommended not to venture on glaciers without guides! They’re also very dangerous.

Why? After all, they only move a few centimeters every day…

The main danger is crevasses. These holes can be tens to hundreds of meters deep.

But I’d be able to see them, right?

Not necessarily. Fresh snow may have covered these holes. Even worse, it may have constructed natural bridges over crevasses. These bridges may totally cover the hole, while being not solid enough to support your weight!

But wait. You said that glaciers were like ice cream. There’s no such holes in ice cream, is there?

Ice under huge pressure does behave like ice cream. And, overall, glaciers do undergo huge pressures because of their own weights. But it’s not the case of the ice at their surface! This ice behaves much more like a solid object. And since this ice goes through large variations of slopes which implies strong forces all over the object, it can break.

Amusingly, the crevasses created by the breaking of ice eventually collapses, as the glacier arrives at weaker or more stable slopes. This means that a ski trapped in the a crevasse can eventually be found surrounded by ice in the ablation zone of the glacier years later. Less amusingly, if you fall into a crevasse and can’t get out, you’ll eventually die trapped and frozen under meters of ice…

The glaciers of the video seem to have white and black lines…

Indeed. As explained in the video, crevasses that form in winter are filled with snow, while crevasses that form in summer are filled with dusts. This means that later on on the glacier, we have successions of white and black lines, which correspond to the season in which a crevasse at the level of the lines was formed. These lines are known as Forbes bands.

OK… Crevasses sound dangerous indeed. But if I only stay near them, I’d be fine, right?

Yes. Although there are several glaciers it is dangerous to live near.


Formations of subglacial water lakes can lead to floods. The worst example of this concerns glaciers on volcanoes, which can provoke catastrophic jökulhaups. To know more about these, read my article on geological wonders of Iceland. But glaciers can also create other lakes which don’t even need to be subglacial to be dangerous.

What are you talking about?

As glaciers advance, they drag huge rocks. When they stop advancing and start retreating, these rocks are left. They form what’s known as moraines. These moraines form natural dams, creating lakes like the lac blanc in the Alps in France.

Le Lac Blanc

In other cases, huge lakes can be formed by moraines. But as the lakes get larger, the pressure on these moraines increase. They then may yield suddenly, resulting in the enormous lakes being poured instantaneously into valleys. These floods can easily destroy villages of valleys.

This is scary indeed!

Fortunately, in developed countries, artificial deviations of water can avoid such disasters. However, there are other areas, especially in the Himalayas, where kilometers of lakes are withhold by fragile moraines. This is probably the major risk linked with the retreat of glaciers in these regions.

What about global warming? People often say that retreat of glaciers will result in rise of sea level?

The example of Fox Glacier shows how much more complicated the relationship between global warming and retreat of glaciers can be. Indeed, for instance, the glaciers of Antartica might still be advancing. However though, in other areas like in the Arctic, the retreat of glaciers is obvious. Worse, in many cases, it has quickly accelerated in the last decades. This is the case, for instance, of the vallée blanche in the Alps:

La Vallée Blanche

Overall, glaciers do seem to be retreating. And as the Earth keeps warming, retreat of glaciers will accelerate. Thus, an increase of sea level is to be expected. This could have catastrophic consequences for coastal towns and cities. In particular, the Netherlands, which are largely built under sea level, may be constantly flooded in decades.

Glaciers over Geological Times

What if I now told you that the Earth had gone through ice ages? If you don’t believe me, check this video of the Ice Age!

Humm… Doesn’t look that scientifically convincing!

Indeed. Still, my point is that, at some point in the past, the weather was much colder, which means that glaciers were much larger than they are today. In fact, Canada and Northern Europe were covered by two gigantic glaciers!Does this mean that sea level was much lower?

Yes! In fact, you could easily walk from Russia to Alaska, from Spain to Marocco and from Italy to Tunisia. The following figure is an image of what Europe probably looked like:

Europe in Ice Age

I understand now the title! Glaciers do shape the Earth!

Yes. But the level of the sea is not the only aspect of the Earth shaped by glaciers. As glaciers advanced for millions of years, they dug their way through mountains. The removed rocks were then carried to the sea or formed moraines. In particular huge lateral moraines were created as rocks were left on the sides of the glaciers. This created enormous valleys. These valleys are profound and have a flat bottom, as the weight of the glaciers was evenly spread laterally. Such valleys are called U-shaped valleys, and are found in mountainous areas. One of the most impressive one is the Yosemite valley in California:

Yosemite Valley

Is it possible that glaciers have dug so deep that the bottom is below sea level?

Yes! This has happened in colder areas, such as Alaska, Norway or New Zealand. These U-shaped valleys filled with sea water are called fjords. The most renowned fjord is Milford Sound in New Zealand. Surrounded by thousands of meters high mountainous cliffs, its width is measured kilometers in width and its length in tens of kilometers. And it gets deeper than 400 meters! As a result, the fjord is filled with whales, dolphins, seals and even some sea monsters like giant squids! Enough of talking… Let’s enjoy the view:

Milford Sound

I guess that given enough time, glaciers do shape the wonders of Earth…

Yes! They do! But that’s not all. Gigantic glaciers can also help us go back in time!

What do you mean?

Think about it. Gigantic glaciers, like the one in South Pole, are accumulations of snow. But the glaciers in South Pole are actually 4,700 meters thick! They have actually accumulated the snow of a few millions of year! By retrieving a column of snow, scientists have managed to analyze 800,000 year-old snow!

OK… But what good is there in this? I mean, snow is just snow, no matter how old it is, right?

There’s a bit of air in snow! More interestingly, carbon dioxide is captured by snow. By analyzing snow and dating snow, scientists have analyzed the evolution of climate along the last hundreds of thousands of year. In particular, they have found a stunning correlation between warm periods and high concentration of carbon dioxide. This indicates how much pollution affects global warming.

Let’s Conclude

If there’s one thing I hope you’ll remember from this article is that glaciers are awesome! Not only are they impressive to look at, they are also marvels of science with complex interesting structures. However, they are also dangerous objects because they involve enormous masses with powerful forces. Finally, they are a major force that shaped the Earth into what it now looks. Let me end by a suggestion. If you’ve never seen a glacier, go and check one out for your next vacation! You are more than welcome to blame me if you didn’t enjoy it, but if you do blame me, I’ll suspect you to be lying!