Dinosaur fossils can found rocks dating
Relative age dating also means paying attention to crosscutting relationships.Say for example that a volcanic dike, or a fault, cuts across several sedimentary layers, or maybe through another volcanic rock type.The narrower a range of time that an animal lived, the better it is as an index of a specific time.No bones about it, fossils are important age markers.On the other hand, the half-life of the isotope potassium 40 as it decays to argon is 1.26 billion years.So carbon 14 is used to date materials that aren’t that old geologically, say in the tens of thousands of years, while potassium-argon dating can be used to determine the ages of much older materials, in the millions and billions year range.
This method works because some unstable (radioactive) isotopes of some elements decay at a known rate into daughter products. Half-life simply means the amount of time it takes for half of a remaining particular isotope to decay to a daughter product. Good discussion from the US Geological Survey: geochronolgists just measure the ratio of the remaining parent atom to the amount of daughter and voila, they know how long the molecule has been hanging out decaying. So to date those, geologists look for layers like volcanic ash that might be sandwiched between the sedimentary layers, and that tend to have radioactive elements.
For example, which is older, the bricks in a building or the building itself?
Are there repairs or cracks in the sidewalk that came after the sidewalk was built?
We can compare the amount of radioactive elements in a rock to the amount of specific non-radioactive elements in a rock, do some math and determine how old the rock is. So, if we find a rock that has uranium in it, we can compare it to the amount of lead in the rock to find out how old the rock is.
If you have the right kind of rocks, this method is very accurate.
I also like this simple exercise, a spin-off from an activity described on the USGS site above.