Ice Ages and geological timescales

Key questions

What exactly is an ‘Ice Age’? 
How has the amount of ice on Earth changed over time? And how do we know? 
What is the ‘Geological Timescale’, and how do Ice Ages fit into it?

The Earth’s climate has not remained the same throughout Earth’s history. In fact, large changes have occurred, with swings in average temperature and sea level far beyond what most people might imagine. If you could travel back in time to when there were still dinosaurs (just before 65 million years ago) and visit what is now southern England, not only would the climate be much warmer than now, but you would be underwater – more than 200 metres deep!

On the other hand, if you visited southern England about 20,000 years ago (a mere blink of an eye in geological time), you would find it much colder (like northern Alaska today) and a trip to the beach would require you to travel much further: sea level was about 120 metres lower than now and what is presently the English Channel was dry land. You wouldn’t need to travel very far north before meeting the edge of an ice sheet which covered most of the British Isles.

Times in Earth’s history when the planet has been relatively cold (with glaciers and ice sheets) are referred to as ‘Ice Ages’. During other times in Earth’s history it has been too warm across the planet for glaciers to exist anywhere.

Technically speaking, we’re living during an ‘Ice Age’ today – in the sense that we have a world with glaciers. However, the present time is a relatively warm phase within a period of geological time when glaciers and ice sheets have typically been larger and more extensive than now.

To understand these dramatic changes, we need to take a long view of the Earth and to think outside of the time frames that we’re used to. The Geological Timescale provides a framework for looking at the Earth’s deep past; and when compared with this, the last time an ice sheet extended near to what is now London really was just an instant ago.

Starter

The Earth formed around 4,600 million years ago, an amount of time which is very difficult for us to imagine. More amazing still, scientists think that the Solar System is ‘middle-aged’, so the Earth likely has another 4,000 million years or so ahead before the Sun becomes a red giant star and swallows it up. 

To give some perspective on Earth’s age, if we compare the whole history of the Earth so far to a 24 hour day, then all of recorded human history would fit into the last quarter of a second of that day!

Over this enormous time span, the surface of the Earth, the atmosphere, and the climate have been continuously changing. From about 55 million years ago (the last 17 minutes of our 24 hour day), the long-term trend in Earth’s climate switched back to one of cooling. By 2.6 million years ago (inside the last minute of our 24 hours) the climate became cool enough for large glaciers to expand in the Northern Hemisphere (Antarctica had already grown its ice sheet), and geologists mark this time as the beginning of Earth’s most recent Ice Age – the Quaternary Period.

Watch this YouTube video from BBC Science website for a short introduction to the Ice Age. 

Over this most recent geological period, glaciers have expanded and retreated many times. The most recent time when glaciers reached their peak extent was about 20,000 years ago, a time called the Last Glacial Maximum (LGM).

Download the Ice Age introduction PowerPoint and look through the slides. As you do so, take notes on the following questions:

How has the Earth’s climate and environment been changing during the Quaternary Period? 
How was the world different from today during the Last Glacial Maximum?
Go to Illinois State Museum website for another good source for general information on Ice Ages.

Main Activity

The Geological Timescale divides up the Earth’s history into different sections of time, each section having a name and a clearly defined start and end point. Also, large sections of time are divided into a number of subsections; and in turn, each of these subsections is divided into its own, smaller subsections of time. This allows geologists to ‘zoom in or out’ of geological time when studying and talking about the history of the Earth.

All geologists in the world work with the same Geological Timescale, and this is essential so that geological features (such as rock layers) in different countries can be compared correctly in time across the world. It is a similar idea to the reason people in different countries use the same calendar!

The largest sections of geological time are called ‘Eons’, and in each Eon there are a number of ‘Eras’. Eras contain ‘Periods’ which, in turn, contain ‘Epochs’. To pinpoint ourselves right now in the Geological Timescale, we are living in the Holocene Epoch, of the Quaternary Period, of the Cenozoic Era, of the Phanerozoic Eon!

So that you can become more familiar with the Geological Timescale, download and complete The Geological Timescale task. In this task you will answer questions (and mark in some of the major features of geological time onto a time column diagram). To do this go to the British Geological Survey’s website.

At the end of the task, you are also be asked to look at this BBC website.

To finish the main part of this lesson, download and look at the Ice Age Timescale slides. Notice that the Quaternary Period (or Ice Age) is subdivided into the Pleistocene and Holocene Epochs.

Plenary

After the Last Glacial Maximum, the climate warmed and the great ice sheets that once covered much of North America and Europe melted away to eventually disappear. As the ice sheets shrank back, forests recolonized areas that were previously covered by ice, and there was a movement of many different kinds of animals taking advantage of these changes. With shrinking ice sheets also came rising sea level, as H2O that was locked up in glaciers found its way back to the oceans via rivers that were full of glacial meltwater.

It was this rise in sea level that created the British ‘Isles’ as we know them today – what is now England was cut off from the rest of the continent only when sea level had risen high enough to flood the low lying land in between, creating the English Channel about 9,000 years ago.

However, the changes from the last glacial into our present (relatively warm) time did not happen in a smooth and gradual way. Download and complete the Post-glacial Timescale task to learn more about how the Earth’s climate and environment have changed since the Last Glacial Maximum up to the present day.

To finish this lesson, download and look through the Ice Age evidence and causes PowerPoint slides. After looking through the images, create a fact file on a sheet of paper that lists some different types of fieldwork evidence for past changes. Also note down some of the causes for the climate changes that have occurred.