How do scientists study and measure the four layers of earth?

Scientists use waves to study the different layers of the earth. Usually, they use seismic waves, which are waves generated by earthquakes or nuclear-test explosions. The seismic waves are bent, sped up, or slowed down, or even reflected when they pass through the earth's layers.

Scientist use all sources like geologists and astronomy scientist. They use them one how exact old is Earth and geologists use the tree trunk How old the tree is there could be the eldest tree and the smallest.