1. Upper Mantle: This is the outermost layer of the mantle, extending from the base of the crust (Moho discontinuity) down to about 410 km depth. It's composed mainly of peridotite, a dense rock rich in olivine and pyroxene. The upper mantle contains the lithosphere (rigid, brittle, and includes the crust) and the asthenosphere (semi-solid, ductile, and can flow).
2. Transition Zone: This layer lies between 410 km and 660 km depth. It's characterized by changes in mineral composition and density due to increasing pressure. This zone is where the mineral olivine transforms into a denser form.
3. Lower Mantle: This is the largest and densest layer of the mantle, extending from 660 km to the core-mantle boundary (2,890 km). It's mainly composed of silicate minerals, but the pressure and temperature are so extreme that the minerals are compressed into different crystal structures.
It's important to note that these layers are not distinct and have a gradual transition between them. The boundaries are defined by changes in mineral composition, density, and physical properties.
