Age determinations from radioactive-isotope decay methods are ...

[vicky]

Age determinations from radioactive-isotope decay methods are usually performed on a mineral crystal. Assume that a rounded grain of feldspar is plucked from a sandstone, analyzed, and determined to be 200 million years old.
 
  What does this tell you about the age of the sandstone? What other methods can be used to determine when the sand in the sandstone was deposited?
  What will be an ideal response?

Question 2

Why do geologists sometimes change the absolute ages of the boundaries in the geologic time scale?
 
  What will be an ideal response?

[onowka]

Answer to Question 1

We know that the crystal was formed 200 m.y. ago. This is the maximum age of the rock, and we can only say that the sandstone is less than 200 m.y. Analysis of the cementing agent can give us information about the minimum age of the rock. Fossil content can further refine the age, as well as determining the age of minerals in any igneous rocks that cross-cut the sandstone.

Answer to Question 2

The ages of the boundaries in the geologic time scale change because they get revised as new absolute age information becomes available.

[vicky]

Excellent

[kishoreddi]

Thanks for the timely response, appreciate it