How can we get an "absolute" date for the layer of rocks and fossils shown by the arrow?
; MM_swapImage('rlays','layage','clonus/radiomet/radlayl1.gif')){kind=small}
Notice that the layer we are interested in is between two layers of volcanic ash.
Due to their chemical composition, volcanic ashes are easy to date.
If we analyze samples of the two ash layers, we can find an absolute age for each of them.
Then, we can use the ages of the two ash layers to narrow down the age of the layer that's in between.
; MM_swapImage('rlays','layage','clonus/radiomet/radlayl2.gif'))
Using radiometric dating, we get an age for the upper volcanic
ash layer of 468 million years.
(mya = millions of years ago)
The lower one is 470 million years old.
Which would be the correct age range for the layer we are interested in?
a.) 470-472 million years
b.) 465-472 million years
c.) 465-467 million years
d.) 468-470 million years
Ooops! Take another look at the layers. Hint: Using the Law of Superposition, the age range should fall in-between the ages of the layers on top and bottom. Try again.
Great! Using the Law of Superposition, the layer on the bottom is older and the layer on top younger. So the ages of "our" layer is in-between these two. It is between 468 and 470 million years old.
; MM_swapImage('rlays','layage','clonus/radiomet/radlayl3.gif')){kind=link}