How do we know how old the Earth is?

Question

I understand that the geologists are convinced that the earth is quite old, but I can not always understand their texts and films. Can anyone give a simple explanation?

Answer 1

Certain minerals have a known and constant rate of radioactive decay. These minerals also always start with a known level of radioactivity at the time they cooled from molten lava and became solid.

So, all you have to do is measure the current level of radioactivity, and then figure out the amount of time it took to get from the initial level of radioactivity (when the rocks the minerals are in became a solid) to the current level of radioactivity, and you have the age of the rock.

Answer 2

The most important way is looking at isotope ratios. The earth is filled with many minerls that are very slightly radioactive. They can be found in most rocks. Over time, radioactive elements, through the process of giving off particles, turn into slightly different elements. This happens at a predictable rate. So you can tell how old a rock is by looking at how much of a particular element has changed into a different one.