How old is the Earth
The Earth is about 4.5 billion years old, which is so old it is almost impossible for us to imagine. Geologists use the term 'Deep Time' to describe the geological history of the Earth.
Scientists first began to wonder about the age of the Earth in the 17th century. They noticed that sedimentary rocks created distinct layers, and within the layers they discovered the fossilised remains of unknown creatures. While the geologists realised it must take a long time for sedimentary rock layers to form, they didn't know how long, but even so they dated the Earth to 96 million years old.
Other scientists calculated how long it would take the Earth's surface to cool down to its present temperate, and estimated the age as 100 million years old.
It was not until the discovery of radioactivity in 1896 that scientists realised that not only did radioactive decay provide extra heat in the Earth's core, but the rate at which radioactive elements decay could be used to date the age of rocks (see video).
Rocks not only provide a permanent record of the environment in which they formed, but also a record of the important events in the history of life.
Note: 1 billion = 1000 million.
Geologists divide up Deep Time into Eons, Eras, Periods and Epochs, their boundaries are marked by a change in the environment and the geological record. There have been five mass extinctions during this time, when as much as 80-95% of all life died.
Important events in the history of life on Earth
first life (probably bacteria)
first cells with a nucleus (also known as eukaryotes)
first multi-cell organisms (sponges, seaweed)
first trees and insects
first flowering plants
An interactive infographic exploring Deep Time. If you're interested in mass extinctions, there's an excellent TV series presented by Tony Robinson, Catastrophe, which you’ll find on YouTube.
TASK 21: Compare the timelines below (notice that they don't agree on everything), and create your own time line of the major events in the history of Earth.
If you'd like to make a toilet paper timeline there is a excel table (attached as file at the bottom of the page) showing the length from the start of the roll to important events in geological history. Remember to take photographs and paste them into your notebook.
Next we'll look at how the first rocks were formed on Earth.