Fossilization | How to Collect Fossils | Search The Web! | Fossils Main Page | Digging | Tar Pits
****JavaScript based drop down DHTML menu generated by NavStudio. (OpenCube Inc. - http://www.opencube.com)****
|
| You must have heard people talking about the age of fossils and rocks. But rocks can be billions of years old, and still look like they were when they were first formed, so how can we know how old they are? Over the years, different kinds of rocks will form over a single area. If you were to cut the land in half and look at it, you would see different coloured layers, each one representing the formation of a different rock, and a different period in that area's history. These layers are called strata. The strata at the top are younger than the strata at the bottom. Also, any geological feature is younger than any rock it cuts across. So we know which fossils are oldest by their position in the rock strata. |
| Now we know which one's are older and younger than each other, but we still don't have an absoulute age for any of them. Luckily, scientists have worked out a method that is extremely accurate for dating rocks, it's called Geometric Dating. |
| Most igneous rocks contain some unstable isotopes. This means that every second they have a small chance of decaying. It isn't a slow process like with humans. Our body has to wear out through use, then we die. But not with unstable isotopes. They can't wear out, either they decay, or they don't decay. When they decay, they don't just dissapear. They turn into different isotopes. The original isotope is called the parent isotope, and the isotope it decays into is called the daughter isotope. |
| For example, U-235 atoms have a 50:50 chance of decaying in 704,000,000 years. So, after 704 million years after they were formed, half of the U-235 atoms would have decayed into Pb-207 atoms, the daughter isotopes. After another 704 million years, half of the remaining U-235 atoms would have decayed into Pb-207 atoms. This is called a half-life. |
| By studying the ratio of U-235 atoms to Pb-207 atoms, we can determine how old a rock is. Some rocks can't be dated, however, like slate and limestone. We can find out the relative ages of these by comparing them to rocks that have been dated in nearby strata. If a layer of limestone is above a layer that has been dated at 750,000 years old, and below a layer that has been dated at 500,000 years old, we know its age must be between 750,000 and 500,000 years. In order to date rocks like these, you'd need a laboratory with special equipment, and you'd also need to know how to do the procedure. Needless to say, don't try to date your own fossils using this method, but now you know how it's done. |
|
|