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?

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.