Evidence suggests that the oldest rocks on Earth are about 4000 million years old. So the Earth must be older than this - about 4.5 thousand million years old. But the structure of the Earth has not stayed the same all that time.
The rock cycle
Rocks are weathered and eroded continually. If new rocks and mountains were not being formed, the continents would eventually be worn down to sea level.
Sedimentary rocks like these have layers. They are evidence of sedimentation, the process by which weathered and eroded rock accumulates on the sea bed. Sedimentary rocks often contain fossilised remains of ancient animals and plants.