What is the mechanical layer of Earth that has the most active convection currents?
The Earth's core.
Convection currents inside the Earth happens. The part where the convection currents go up, it brings up materials from the mantle like rocks. This forms new crust. When the convection currents go down, it brings down some older, colder oceanic crust to the mantle, destroying it. In conclusion, the crust is formed and destroyed when convection currents inside the Earth happen.
The lithosphere is generally thought of as the crust and outer mantle of the earth. They're solid, and there are no convection currents in solids as we normally think of them. As we move inward toward the core of the earth, we begin to encounter molten rock (magma), and convection currents exist in this superheated fluid.