The Geologic Timescale: A Guide

I'm not sure if I've mentioned it on here before but during my time at uni I've worked as a tutor to earn some money during my studies. I've been tutoring students in A-Level and general Pre-Uni level geology helping to prepare them for Earth and Natural Science Oxbridge applications. Earlier this week I was planning out one of those sessions about the geologic timescale and it reminded me that actually before uni I was slightly baffled by what the geologic timescale is, what to do with it, and what I needed to know. So in that spirit I thought I would do a quick post about it on here for anyone starting geology and wanting hep getting their head around it.

International Chronostratigraphic Chart (ICS, 2024)

So, what even is the geologic timescale? Basically it is a way geologists are able to split up the 4.6 billion year history of our planet into smaller chunks so scientists can communicate easily about when things happened. It is defined by the ICS (International Commission on Stratigraphy) and acts as a universal geological language. That being said there are some small discrepancies between countries when we get down to smaller units of time (for example the Carboniferous in the US is divided into the Mississippian and the Pennsylvanian but is not in the UK).

Geologic Time

Before we discuss how the timescale works it is important to understand geologic time, aka the history of the Earth. The planet is 4.6 billion years old and this s a very long time. To put this into perspective the metaphor of a 24 hour clock is often used. Let's say the 4.6 billion year history of the Earth is represented by the 24 hours we have in a day.:

At 12am the Earth forms

At 4am we get the oldest rocks that we see today forming

At 5am the earliest life begins

At 1pm Photosynthesis starts

At 8pm multicellular life evolves

At 10:30pm life moves onto land

At 11pm the dinosaurs evolve

At 11:58pm humans evolve

And recorded human history happens at 11:59:59pm

This is meant to illustrate how much of Earth history took place before life even evolved and how recent human history actually is.

The 24 hour clock analogy for the history of the Earth. Source

The other thing I'm going to mention here is how we talk about geologic time. We tend to refer to things in terms of 'years ago' rather than "years since the Earth formed". So for example the dinosaurs went extinct 66 million years ago. This has been shortened to Ma - meaning million years ago - and Ga - meaning billion years ago (Ga standing for giga-annum). So the biggest mass extinction in Earth history took place at 252Ma and the rise of oxygen on the Earth took place between 2.1Ga and 2.7Ga.

How it works

The image at the start of this post is the official International Chronostratigraphic chart form the ICS but below I have put a more simplified version that can better showcase how the chart works.

So all of the Earth's history is split into sub divisions of time. The biggest of these are the Eons, of which there are only two - the Precambrian (not actually on the graph below but is everything older than the Phanerozoic) and the Phanerozoic (our current eon). From there the eons are divided into eras - Pre-Archean/Hadean, Archean, and Proterozoic for the Pre-Cambrian, and Paleozoic, Mesozoic, and Cenozoic for the Phanerozoic. There are a lot of complex names going on here but don't stress - you don't have to have it memorised you will learn it as you go through your degree. The eras are then split into periods - our current one being the Quaternary, and the periods are then split into epochs and those into stages (not on the graph below).

You definitely don't need to memorise the timescale but if you want to I like to use mnemonics to help, especially for the Paleozoic because it has so many periods. The one I use is:

Can - Cambrian

Orange - Ordovician

Sea turtles - Silurian

Devour - Devonian

Coral - Carboniferous

Perhaps - Permian

This is kinda random because I made it up in my bathroom when I was about 18 but I've used it throughout my whole degree. The Mesozoic and Cenozoic I find easier to remember as there is less periods in each era but making up mnemonics for them would work as well!

These different names are used to refer to key events in the history of time, for example the big 5 mass extinction events are named after the periods they took place in/between. We have the End Permian, the Cretaceous-Paleogene, and the Jurassic-Triassic extinctions for example. These are not the only events named after when they took place - we often refer to different marine ecosystems by when they lived such as the 'Mesozoic fauna' or 'Cambrian Trilobites". Other events also take their names form when they took place, the Neoproterozoic Oxygenation Event or the Paleocene-Eocene Thermal Maximum being key examples.

Creating the Timescale

So I've mentioned that the geologic time scale is set by the ICS but how exactly do they define the boundary between two periods? They use 'a golden spike".

Officially golden spikes are GSSPs (Global Boundary Stratotype Section and Point) and they are literal golden spikes that are hammered into a section of rock that marks the boundary between two units of geologic time. For the exact boundary to be defined they have to meet a certain criteria, they must be:

• Distinct and easily identifiable

• The layer of rock must be continuous and global

• Present in many different types of rock formations

• Ideally in a location that is easy to reach and protected

A great example of a golden spike is the one at the boundary between the Cretaceous and the Palaeogene. This is when we had the KPg mass extinction and the dinosaurs went extinct. At this point in time there is a global layer of iridium rich rock due to the meteorite impact that hit Earth (the meteorite was rich in iridium). The GSSP for this is located in El Kef in Tunisia.

Some examples of golden spikes from different geologic boundaries.

Golden spikes don't have to just be in sedimentary rocks. The boundary between the Cryogenian and Ediacaran is in metamorphic rocks, and the golden spike for entering the Anthropocene (our current epoch) is in sediments in Lake Crawford in Canada. Setting the Anthropocene golden spike was quite controversial as not everyone believes the Anthropocene should be defined as an epoch. This is a very interesting debate across geology, broader science, and even politics. More about the Anthropocene being defined as an epoch from the IUGS can be read here.