| MadSci Network: Earth Sciences |
This is known from radiometric dating of rocks. In radiometric dating, scientists measure the amounts of different isotopes of certain elements in a rock sample. Since radioactive isotopes are known to decay at a certain constant rate, the amounts remaining in the rock will tell how old it is. For igneous rocks this is the time when it solidified from molten magma.
The oldest rocks ever found on Earth are actually slightly younger than this, about 3.8 billion years old. Most rock found on the Earth's surface is much younger because Earth's crust is "recycled" by plate tectonics. The radioactive "clock" in the rock is reset when it is melted or heated to high temperature.
So if the oldest rocks ever found on Earth are 3.8 billion years old, how do we know that the Earth is older than that?
Because we have some rocks that are not from the Earth!
Moon rocks and meteorites are older than any Earth rocks we know of. The Moon rocks collected on the Apollo missions and most meteorites that have hit the Earth are about 4.5 billion years old. The Moon doesn't have plate tectonics, so its crust is not recycled. The rock now on the Moon's surface was formed when the Moon was formed. Meteorites also would have been made when the solar system was formed, and of course the Earth would have been made at the same time as the rest of the solar system.
By the way, the search for the world's oldest rock still continues. Every decade or two someone finds a rock that is a little older and sets a new record. I think the current record is a rock from Canada found by Sam Bowring. Who knows? Maybe someday you could find a little remnant chunk of the Earth's original crust hidden away in some remote wilderness! Good luck hunting!
You can find more information about the age and origin of the Earth at the
UCMP Web Geological Time Machine
I hope this helps.
---- Sean Sherlock, Geologist