The age of the Earth
All our evidence for changes in the Earth comes from looking at rocks. Folds and fossils in sedimentary rocks, radioactive dating and the weathering of ancient craters show that the oldest rocks are about 4000 million years old. That means the Earth must be at least as old as this.
The only thing that we have been able to observe directly is the Earths crust, which is the very thin outer rocky layer.
Evidence from earthquakes shows that the Earth has a very dense core surrounded by a solid mantle.
The Earth is almost a sphere. These are its main layers, starting with the outermost:
Note that the radius of the core is just over half that of the Earth.