Excellent question Matt! To find out how old the Earth is, scientists look for the oldest rock on the entire planet and then figure out how old that rock is. But finding the oldest rocks can be difficult. Since the Earth is an active planet with plate tectonics, rock is constantly recycled – broken down into magma far below the surface, deep inside the Earth, and then brought back up to the surface once more. But difficult does not mean impossible!
Scientists have found incredibly old rocks and have been able to find out how old they are through a process called radiometric dating. Some of the oldest rocks found on Earth are in the Canadian Shield, dating from 2.5-4.3 billion years old. In fact, the oldest rock in Nova Scotia is a part of that Shield. While not rocks, tiny zirconium silicate crystals which are minerals can also tell us that the Earth is at least 4.3 billion years old – but these samples are found at its surface and the Earth must be older than anything just at the surface. To help scientists further improve their estimate of the age of the Earth, they have looked at rock samples from other objects in the Solar System. Analysis of samples from meteorites, the Moon and Mars have helped to put the Earth’s age at 4.5 billion years old, give or take 50 million years.