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Ice Ages

Ok, let's talk about something other than dinosaurs just this once.

Reconstruction of a woolly mammoth
Image credit:

Everyone has heard of the ice age, right? Not only is it a series of cartoon films, but it also refers to certain periods in Earth's history. You read that right, periods as in plural. There has been more than one ice age throughout Earth's history, so when asking the question 'when was the ice age?', the answer would be 'which one?'. Most people, of course, are normally referring to the most recent ice age which, believe it or not, is still ongoing!

Yes, we are indeed still in an ice age, but you may be surprised as there are still some extremely hot places in the world. Also, if this is the case, why are we all complaining about global warming? Well, to answer those questions, we need to look at what an ice age is...

What is an ice age?

So, what is an ice age? Well, the Earth fluctuates between various periods of greenhouse warming and vastly lowered temperatures. During the periods of lower temperatures, continental ice caps will form on both poles of the planet. The presence of these ice caps is what defines an ice age. If there are ice caps, it's an ice age, if there are no ice caps, it's not an ice age. Simple!

So why are there still arid deserts, tropical jungles and Badlands reaching temperatures of 50°C+ every year? Well the reason we don't look out the window and see what is often illustrated of the ice age is because we are currently in what is known as an interglacial period. During any ice age, the average global temperature will still fluctuate every 100,000 years or so, with a spike in temperature for a couple thousand years, leading to a shrinkage in the ice caps. This spike is known as an interglacial period and is a completely normal part of any ice age. The reason that global warming is such a worry is that we should be near the end of the natural interglacial period, with the fluctuations in temperature occurring slowly enough for evolution to react accordingly. Rather than getting colder, however, temperatures are rising due to anthropogenic effects (AKA us humans), meaning the current interglacial period isn't just being dragged out, it's being dragged in the opposite direction at an alarming rate, being too quick for natural life to adapt.

How many have there been?

So, if this isn't the first ice age, how many have there been?

In total, there have been six ice ages (including the current one). For the sake of ease, I'll give a quick summary of them in chronological order. You guys know how much I love a timeline!


Around 2.9 billion years ago, during the Mesoarchean era, we have our first glaciation! Named after the supergroup of rocks it is found in, the Pongola glaciation has shown evidence in varied places such as Switzerland and South Africa and lasted around 120 million years. This glaciation wasn't exactly a large one though (not like ones further on the list), which is odd considering the sun is estimated to have been much less powerful back then. The reason glaciations don't appear as common at this time is due to the high amount of greenhouse gases present, locking in any heat that does come in. It seems quite clear what caused the Pongola ice age then, since it happens to coincide with the first forms of life that fed via photosynthesis, ridding the Earth of those pesky greenhouse gases and cooling it right down.


Another glaciation named after the supergroup it was found in, the Huronian ice age is actually a series of smaller ice ages that occured over a period of around 200 million years. This took place right at the start of the Paleoproterozoic era and is thought to have been caused by the same photosynthesising organisms that caused the Pongola ice age. This has a much stronger confirmation however, since it coincides with an event known as the Great Oxygenation Event (GOE), a time when oxygen saw an exponential increase from the waste of photosynthesis. Unfortunately for those greedy little organisms, this eventually spelled disaster for them, since their process of feeding and excreting meant there wasn't much left other than the toxic oxygen to feed on. In other words, the oldest known mass extinction event was caused by the equivalent of a human drowning in their own excrement...yikes.

Graph showing the oxygen increase during the Great Oxygenation Event.
Estimated evolution of atmospheric PO2. The upper red and lower green lines represent the range of the estimates. 1 GA= 1 billion years BCE. Image credit


Ok, I'm cheating slightly with this one just for the sake of ease, but during the Neoproterozoic, there were actually a total of 4 ice ages. The first of these, the Sturtian glaciation, occured around 715 million years ago (during the Cryogenian period) and lasted for around 35 million years. This is possibly the most famous glaciation second to the most recent one, often nicknamed the 'snowball Earth' period. This namesake was given because geological evidence shows that during this time, Earth resembled a giant (you guessed it) snowball, with ice and snow covering virtually the entire planet!

Artist's rendition rendition of the Snowball Earth
Rendition of the Snowball Earth. Art by Oleg Kuznetsov

A similar environment is seen 30 million years later, known as the Marinoan glaciation, which lasted for around 15 million years. As to what caused them? Scientists can't quite agree, but it could be either the depletion of greenhouse gases reaching a crescendo, or an impact winter from an steroid strike.

Moving onto the Ediacaran period, some 580 million years ago, we have the Gaskiers ice age. Unlike many others on this list, this was actually a relatively short glaciation, lasting under 340 thousand years. 40 million years after this, we the Bayknourian ice age, lasting around 5.5 million years. These weren't quite as big as what we see on the 'Snowball Earth', but they do align quite closely with the small and weird Ediacaran fossils that started cropping up, meaning things were finally starting to warm up and organisms finally started evolving to use all that oxygen that was all around.


The next ice age was at the end of the Ordovician period. Known as the Andean-Saharan ice age, this one was between 450-420 million years ago. Evidence is seen for this from mostly anomalies in oxygen isotopes as well as characteristic glacial deposits (which I'll go into later on). If you have read the Dino-gen post on the big 5 extinction events, you'll know that this glaciation played a major role in the first big extinction event on Earth, wiping out 80% of life and being caused by volcanic winters.


Our next ice age is the Karoo glaciation. Also known as the Late Paleozoic Icehouse, it started right at the end of the Devonian, around 360 million years ago, and didn't end until the Late Permian, around 255 million years ago. At this time, the super continent known as Gondwana was transforming into the supercontinent Pangea, so many warm water oceans were being closed up. As a result, there were many places in the world with large land-ice sheets, meaning it counts on this list.


After that, no other ice ages occurred all the way through the time of the dinosaurs, right up until the Late Cenozoic glaciation, which is the one we are currently in! Having started around 34 million years ago, this is the ice age that most normally think about when they hear the term. Just to reiterate what I said earlier, all ice ages occur in pulses, with glacial and interglacial periods. The last glacial period that most will often associate with this ice age was from 115,000 to 11,000 years ago and the interglacial after that is ongoing.

Charles R. Knight painting of Neanderthals during the last ice age

Giveaway features of past ice ages

Of course, geologists aren't just making all of this up. Glacial deposits exist all across the rock record, showing us when these ice ages were. The main thing to remember when looking at glacial deposits is that, no matter how large the ice sheet is, glaciers move (albeit very slowly) and carry debris with them. This debris is almost always in the form of rocks of various size and, once the ice begins melting, the deposit it leaves behind is called 'till'.

A photograph of a geological till
A typical till. Image credit:

Many of these melting glaciers are often moving downhill and, once they reach the bottom, create what is known as a morraine.

An aerial photograph of a morraine
That'sa moraainne

There are various other types of deposits that are somewhat derivative of these (i.e. the mostly involve the deposit of till), but those are types that I will have to cover in a future post!

For now, until next time!

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