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. So far, oldest dated Earth rocks are 3.96 billion years. Older rocks include meteorites and moon rocks, where Moon rocks from the Lunar highland are about 4.5 billion years old, mare basalt rocks are 3.2 - 3.8 billion years old. Meteorites are all older than 4.5 billion years. Most of the evidence for an ancient Earth is contained in the rocks that form the Earth's crust.
Earth is geologically active.
Earth 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.