Planet Earth

A planets surface provides geologists with clues as to what is happening inside. But many of these clues are ambiguous because so many other processes (impacts and erosion, for example) contribute to surface characteristics. Most of the surface of the Earth is less than 100 million years old, and even its oldest rocks are less than 4 billion years old, so the record of the origin of our planet has been erased many times. Part of this erasure is due to erosion by wind and water, and part is due to the continual recycling of material back into the interior and the repaving of the ocean basins by seafloor spreading.