Define the following: Asthenosphere, Mantle, Outer Core, Inner Core.

Here are the definitions for the given terms:

1. Asthenosphere: The asthenosphere is a highly viscous, mechanically weak, and ductile region of the Earth's upper mantle, lying below the lithosphere. It is characterized by its ability to flow slowly, allowing for the movement of tectonic plates.

2. Mantle: The mantle is the thickest layer of the Earth, located between the crust and the outer core. It is primarily composed of solid silicate rocks, but it behaves like a very viscous fluid over geological timescales due to high temperatures and pressures, driving plate tectonics through convection currents.

3. Outer Core: The outer core is a liquid layer of the Earth's core, composed mainly of iron and nickel. It lies above the inner core and below the mantle, and its convection currents are responsible for generating the Earth's magnetic field.

4. Inner Core: The inner core is the Earth's innermost layer, a solid ball composed primarily of iron and nickel. Despite extremely high temperatures, the immense pressure at the center of the Earth keeps it in a solid state.