Transcript: With the radioactive decay process we can make an estimate for the age of the Earth. However it's a difficult technique to apply reliably. The technique only works if the parent and daughter atoms in the decay from one element to another or to an isotope are trapped in the same location. So really radioactive decay only works as an age estimate after the last time that a rock solidified. So we need to find rocks that are very old and have not melted or re-solidified. Such rocks are very rare on the surface of the earth. If you picked up a typical rock anywhere you might travel, it would only be one or two hundred million years old. Geologists, however, have worked hard to find places, typically in the Antarctic plateau, in Greenland, and a few other places near Australia, where igneous rocks are preserved from the earliest phases of the Earth's existence. The oldest rocks measured in this way give an age of about 4.0 billion years. To this we add the age of the Earth when it was essentially molten to deduce an age of 4.5 or 4.6 billion years. These age estimates on Earth were confirmed when we brought back moon rocks through the Apollo missions. The moon does not have geological activity. Moon rocks were measured to be 4.0 billion years in general with some chips as old as 4.4 or 4.5 billion years.