But I also know that heat is generated inside the earth via radioactive decay of elements including uranium. I have read that when magma cools and crystalizes the uranium 238 isotopes 'lock into place' and begin the decay process.
I understand the process of the U-238 - Pb-206 decay chain, but I'm unsure how the resulting age represents the age of the rock being dated and not simply the age of the earth.
