Deep Time: Earth’s History
- Earth & Space
- Ages 11–14 · Grades 6–8
What you’ll find out
- Superposition, cross-cutting relationships and index fossils put rock layers in order (relative dating).
- Radiometric dating measures the absolute age of rocks from steady radioactive decay.
- The geologic time scale organises Earth’s 4.6-billion-year history; humans occupy a tiny fraction of it.
No sign-up and no real names. In the lesson you check your understanding as you read, and practise until it sticks.
Reading time in the rocks
Earth is about 4.6 billion years old, a span so long it is often called deep time. Geologists reconstruct Earth’s history by studying rocks. In undisturbed layers, the lowest layers formed first and are therefore oldest; this is called the law of superposition. A body of igneous rock that cuts across layers must be younger than the layers it cuts. Certain fossils are found only in rocks of a particular age, so they help geologists match up layers in different places and put events in order.
The geologic time scale
Geologists can also measure the actual age of some rocks using radiometric dating. Certain atoms in minerals decay into other atoms at a steady, known rate, so measuring how much has decayed reveals how long ago the rock formed. Combining these ages with relative dating, scientists built the geologic time scale, which divides Earth’s history into eons, eras and periods. If all of Earth’s history were squeezed into one day, dinosaurs would appear at about 11 p.m. and humans only in the final few seconds.
Curriculum links: NGSS MS-ESS1-4 · England KS3: Earth and atmosphere · Cambridge Lower Secondary Stages 7–9: Earth and space · NCERT Social Science, Classes 6–8