Earth & climate
Geologic time
The Earth is unimaginably old. How rock layers and fossils tell its long story and how researchers divide time into ages.
How old is the Earth? Not a few thousand and not a few million years, but about 4600 million years. That is 4.6 billion years. In this enormous span of time the Earth has changed again and again: continents wandered, seas came and went, and life developed from the tiniest single cell all the way to us.
An unimaginably long story
Because such huge numbers are hard to grasp, a trick helps: imagine the whole history of the Earth as a single year. Then the Earth would form on 1 January, and the dinosaurs would only live in December. The large dinosaurs died out about 66 million years ago, which in this picture would be 26 December. Humans would appear only in the last minutes before midnight on 31 December. Our entire life is just a tiny moment in this story.
up to about 500 °C. Thin solid outer shell, we live on it.
Layers tell the time
In lakes and seas fine mud constantly sinks to the bottom and over time turns into rock. In this way layer grows upon layer. A simple rule applies: the lowest layers were laid down first, so they are the oldest. The higher up a layer lies, the younger it is. A steep slope or a canyon is therefore like an open history book.
Fossils as witnesses of time
Some layers contain , the petrified remains of living things. They reveal which plants and animals lived back then. Because certain species only occurred in certain ages, they can be used to work out the age of a layer. If the same fossil species is found at two far apart places, both layers were about the same age.
Exercises
0 of 6 solvedTime to try it yourself. You can't break anything, every attempt counts.
Which rock layer is normally the oldest?
What do researchers use fossils in the rock layers for?
How many million years ago did the large dinosaurs die out? (answer in millions)
Order these events of the Earth's history from oldest to youngest.
- 1The Earth forms.
- 2The dinosaurs.
- 3The first tiny single cells.
- 4Humans.
The Earth is about 4600 million years old. How many billion years is that? (1 billion is 1000 million)
If the same fossil species is found in two far apart rock layers, then both layers are about ….