Why have scientists been unable to determine the exact age of the Earth?
The early Earth was molten and remained so for at least several hundred million years. The first rock solidified from melt roughly 4.25 billion years ago based on radioactive decay dating methods. Because only rock and certain mineral assemblages can be dated scientifically, the age of the Earth must be older than the oldest terrestrial rock of Earth origin. The oldest rocks that have arrived on Earth from asteroid or meteor impacts have been dated to the presumed age of the Earth, roughly 4.6 billion years.
