Question

The oldest rocks on Earth are REALLY old - about 4.6 billion years old - but all of the oldest ones are found on land and not in the ocean. Around the mid-ocean ridge, the rocks are very young (only 9 million years old). Why is there a difference in the ages of rocks on land versus in the ocean?

Answer 1

Why are the oldest rocks on the ocean floor only 200 million years old while the oldest rocks on the continents are almost 4 billion years old?

Explanation: The oldest oceanic crust is about 260 million years old. This sounds old but is actually very young compared to the oldest continental rocks, which are 4 billion years old. ... It is due to the process of subduction; oceanic crust tends to get colder and denser with age as it spreads off the mid-ocean ridges.