How to Date Rocks

How do Geographers date rocks ? Give them flowers!

Jokes aside, the way Geologists date rocks is not too complicated. It all stemmed from Albert Einstein's theory that radioactivity is the energy for which chemical energy is emitted from any substance, rock or soil over time (energy change).

1. Collect a sample - if you have a large rock segment, cut a piece out or gather some smaller rocks from the environment. This will mean one can easily transport the rock to the rock laboratory. 
2. Using specialised equipment in the rock laboratory, the rock sample can be crushed and made into a power form, which has the consistency of sugar. 
3. The main thing that determines how old the rock is, is it's mineral contents. How much minerals is embedded in the rock tells us how old it is because of Einstein's theory of radioactivity. 
4. Scientists have to determine the amount of Lead and Uranium in the sample. Uranium radioactively converts into Lead over time as it decays and reaches "half life". To do that, a mass spectrometer must be used to separate out the large Uranium particles and the small, tiny Lead particles that the former disintegrates into. 
5. We know the rate of decay for Uranium (the total time). Therefore, the amount of uranium to lead can be proportioned (in a ratio) and this can tell us how old the rock is. 

Dating rocks is all about proportion of Uranium to Lead particles in the sample. Rocks form as sand and soils deposit layer by layer, over many millions of years. If any pebbles are deposited over it, they get buried on top of more layers of sand, silt and soil until gradually all we see is the top layer of young grass growing on a hill.

If asked to determine how old a rock is just by looking - this is what you could do:

1. Spot any fossils embedded in the rock. These tell you which time era the rock was formed in, and if you're good at paleontology, then the rest is quite simple. 
2. Categorize the rock type - is it sedimentary, igneous rock or metamorphic rock ? This can tell you generally the age of the rock and how it is formed.