Rocks of the same age across oceans
Topic: Continental Drift, Sea-floor Spreading and Plate Tectonics · NCERT: Class 11, Ch 4 "Distribution of Oceans and Continents"
Meaning
Rocks of the same age across oceans is one of the main pieces of evidence for continental drift. Radiometric dating uses the steady decay of elements such as uranium and potassium to find a rock's age. It shows that some very old rock belts match on coasts that now face each other across an ocean. These rocks are far older than the ocean between them. The oldest marine deposits (sea-laid sediments) on both coasts date only from the Jurassic period. So the ocean did not exist before then, and the two coasts must once have been joined.
Example
A belt of 2,000-million-year-old rocks on the coast of Brazil matches rocks in western Africa. The oldest sea deposits on both coasts are only Jurassic, which shows that the South Atlantic opened much later.
Don't confuse with
- Jigsaw fit of continents: The jigsaw fit matches the shapes of coastlines. This evidence matches the ages of rocks.
Related concepts
- Jigsaw fit of continents
- Radiometric dating
- Tillite
- Fossil evidence for continental drift
- Mesosaurus