Plate tectonics - Earth’s layers | Britannica.com

Plate tectonics - Earth’s layers | Britannica.com: Plate tectonics - Plate tectonics - Earth’s layers: Knowledge of Earth’s interior is derived primarily from analysis of the seismic waves that propagate through Earth as a result of earthquakes. Depending on the material they travel through, the waves may either speed up, slow down, bend, or even stop if they cannot penetrate the material they encounter. Collectively, these studies show that Earth can be internally divided into layers on the basis of either gradual or abrupt variations in chemical and physical properties. Chemically, Earth can be divided into three layers. A relatively thin crust, which typically varies from a few kilometres to 40 km (about 25 miles)
Walt Armour
A lot of information is contained in this link. Plate information, volcanoes, and interior of the earth.