Older rocks include meteorites and moon rocks with dates on the order of 4.6 billion years . Led to the discovery of over 200 isotopes. Many radioactive elements can be used as geologic clocks. Each element decays at its own nearly constant rate. Once this decay rate is known, geologists can estimate the length of time over which decay has been occurring by measuring the amount of radioactive parent and the amount of stable daughter elements.
Earth is geologically active.
Has a hot, molten interior. Rocks are remelted and their internal clocks are reset. Also, rocks on Earth's surface are acted on by erosion and weathering. Sediment will turn into sedimentary rock over time. Older rocks are buried deeply under younger rocks.