I know that we can find how old the Earth is by knowing how much uranium ultimately decayed into lead. Uranium-238 has a very long to decay, about 4.5 billion years. So that is how we are able to understand how old the Earth is.
The reason is that there are radioactive elements that are in the center of the earth.
Explanation:
If the earth were only a rock cooling from the time of its birth. It should have already cooled completely thousands of years ago. However, volcanoes continue to shed hot magma and this is proof that there is a source of heat in the center of the earth. This source of heat is due to the radioactivity of elements such as Uranium and Thorium that were enclosed during the formation of the earth. These elements slowly disintegrate emitting particles that collide with others emanating heat. For example, the half-life of uranium is 4.5 billion years, which means that it decays very slowly.
