Final answer:
The transition zones in Earth's mantle at depths of 400 km and 670 km are characterized by changes in mineral composition, which result from increased pressure and cause a phase change in mantle minerals.
Step-by-step explanation:
The two transition zones in Earth's mantle, located at depths of 400 km and 670 km, are characterized by changes in mineral composition. These zones mark areas where there is a significant shift in the phases of mantle minerals due to the pressure conditions at those depths, which result in a change of structure of the minerals and an increase in density without changing the overall chemical composition. This process is associated with a sharp increase in seismic velocities, which is how these transitions were initially discovered through seismic studies. These transitions are not directly related to a temperature decrease or the presence of the liquid outer core, which lies beneath the mantle at a depth of 2900 kilometers.