Geological Time

When we talk about something being “recent”, what do we mean?

In day to day terms recent can mean this week, this month, or this year. It implies that an event is contemporary, that it didn’t occur too long ago. Context is all important. When we talk about history we consider “recent events” to be those that have had a direct impact on the present. In human terms that could means the last century or so. 

Geologists have a very different concept of time. When we study the way landscapes have evolved “recent” can mean a few million years. These time scales are very hard for us to wrap our heads around. It is easy to imagine a year, because we all experience them. A century is about the length of the longest human lifespans, and that probably isn’t a coincidence. It is a long period of time, but one which is easy for most of us to visualise and understand. We know how long we have lived, and can easily imagine the lives of our parents and grandparents. It’s when we try to imagine the passage of time before then that we can run into trouble.

Once we get into longer timescales everything becomes a bit harder to picture. How many generations have passed since the dawn of civilisation?  How long ago did the dinosaurs die out? We can work out the answers to these questions, but it takes a lot of practice to put historical events in perspective. One oft cited example from the field of archaeology is that Cleopatra VII lived closer to the present day than to the time when the pyramids were built. This takes a lot of people by surprise; after all they were both part of a period which we, in the popular consciousness at least, define quite loosely as “Ancient Egypt”. The fact that this “period” lasted for thousands of years, including numerous different civilisations and cultures is easy for those of us who aren’t historians to forget. 

We run into similar problems when talking about the far larger spans of time associated with palaeontology and geology. Anatomically modern humans have only actually existed for around 200,000 years. Our various ancestor species existed before that, and it is believed that human like species have been using tools for around 2.5 million years. This seems like an amazing length of time, but in the grand scheme of things it is the blink of an eye when compared to the periods over which other successful groups of species have existed. 

The first dinosaurs are believed to have emerged during the middle Triassic period; a staggering 232 million years ago. This means that the last dinosaurs, which lived around 65 million years ago lived closer to the modern day than they did to the time when their earliest ancestors evolved. Humanity, and its descendant species, will have to do very well if we are to last for even a fraction of this length of time. 

Millions of years are all very well but the age of the earth is far, far longer. The earth formed 4.5 billion years ago. Just to confuse things there are actually two definitions of “billion”. What we’re talking about here is 1x10⁹ a one with nine zeros after it.  to write this out the long way the world is 4,500,000,000 years old. 

It is likely that life formed quite “rapidly”, probably within the first 0.5 billion years. Many of the processes that shape the planet move quite slowly. One good example of this is the movement of the tectonic plates. Many dinosaurs lived on a massive supercontinent called Pangea, at a time when all of the landmass of the earth formed one massive block. From around 200 million years ago Pangea gradually split apart forming the continents as we know them today. 

The movement that results in this dramatic change is so slow that it is hard to observe. In order to figure out the past arrangement of the continents we have to examine numerous lines of evidence to reconstruct a jigsaw puzzle millions of years in the making. The further back in time we got the harder this process becomes, as evidence such as the fossil record becomes more fragmentary. 

But some of you will likely have noticed that the breakup of Pangea is pretty recent in geological terms. There is evidence to suggest that Pangea was the latest of eight supercontinents which have existed over the course of Earth’s history. The earliest, usually referred to as “Ur” likely formed around 3 billion (3x10⁹) years ago. Earth had been around for 1.5 billion years before this first supercontinent formed, but the processes which build up continental crust are very slow. Ur was the first time enough of this landmass came together to form a full continent. It took numerous cycles to produce the amount of landmass which we know today. Continents gradually merged and rifted, with more landmass being produced all the time. 

Looking back through time is never easy, and there is frequently a lot of uncertainty involved. Fortunately the science of geology has developed numerous tools for peering back in time. We can date objects using the ratios of radioactive elements, and extract cores of soil or ice to determine what was happening thousands of years ago. We can use the fossil record, and the stratigraphic layers built up over time to go back even further. These techniques and many others allow us to work out what processes are occurring on scales which no one human could observe.