how can scientists tell how old some rocks are?

Question

and what does this tell us about when the earth was formed?

Answer 1

Both carbon and uranium dating are used, and others are used as well. Carbon dating (accurate for about 50,000 years) is used in sedimentary rocks containing organic material in sufficient amounts to test (usually pieces of wood or peat are used). Uranium dating is good for the entire age of the earth (but is less accurate in younger beds). What happens in both cases is the radioactive material is gathered and the amount of various isotopes or daughter products is determined. Using relatively simple mathematics they can tell how old the radioactive material is. If the radioactive material has not been contaminated then that age will be the age of the rock.

In the case of metamorphic rocks the radioactive date can be reset when the metamorphism occurs, so the date is the date of the metamorphism.

For sedimentary rocks there are two types of age determination: absolute age and relative age. Relative age means the geologist can determine that one rock is older or younger than others, by the position of the rock beds. There is no direct information as to the actual number of years, however. But-- if the rocks in question interbed with nearby igneous rocks or rocks that contain sufficient radioactive material, a really good estimate of the absolute age can be found. Finally, using fossils in an unknown bed as compared to identical fossils in a bed with an absolute age (found by one of the methods I mentioned above) elsewhere in a region or (less precisely) somewhere else in the world, the absolute age can be found.

Answer 2

It is probable that earth was formed by accretion. Accretion means the gradual building up from a nucleus - like a pearl or an onion.

We don't know how that might affect the rocky elements - so scientists, when they date a rock, say, "Hmmm, we haven't used 300 million for a while!"

They will say they used radiometric dating processes. But if you don't know where or how the rocky element was formed - how is it possible to establish a baseline?

So the true answer is they are guessing.

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Answer 3

Life has a habit of concentrating particular forms of carbon. It likes the more radioactive varieties. So living stuff has a characteristic high concentration of carbon-14 relative to carbon 12.

When the living stuff dies, it stops collecting carbon-14, and that then starts to decay, turning slowly back into carbon-12. It decays at a very predictable rate. So by measuring the concentration of carbon-14 relative to carbon-12 you can tell how long ago it was that the stuff you've got was alive. That tells you the age of the rock.

That's radio-carbon dating. I think they have other ways to add to that nowadays.

Answer 4

When rocks form, they include a sample of specific elements. Some of these elements occur in nature as radioactive isotopes that decay according to a known half-life. Thus, studying the percentages of certain isotopes can allow for a calculation of age.
http://en.wikipedia.org/wiki/Rock_Dating


Also, rocks occur in strata, or layers. Understanding the geology of layers with respect to other layers can infer an age.

Answer 5

The rocks near a breach in the ocean (mid ocean ridge; closer to the ridge) are the newer rocks and as it produces new surface, the rocks get pushed back so older rocks are found on the edge of the ridge. Hope that helped, although I could have explained better w/ a diagram! lol

Answer 6

By carbon dating

Answer 7

they can