The Earth's internal structure consists of an inner core, outer core, lower mantle, upper mantle and the crust. You can identify the different layers of the Earth's interior by their varying chemical compositions, which are determined by temperature, pressure and density. Therefore, each layer of the Earth's interior has different physical properties.
The deeper you go into the Earth, the higher the temperature. The crust is cooler than the mantle, which is cooler than the core. This concept of increased temperature with depth creates a major difference between the upper and lower mantle. The temperature difference between the top of the mantle and the bottom of the lower mantle spans from about 870 to 2,200 degrees Celsius (about 1,600 to 4,000 degrees Fahrenheit).
Depth and Pressure
When space decreases, pressure increases. As you can imagine, the space in a sphere decreases from the outside to the inside. Therefore, with increased depth of the Earth, pressure increases immensely. This concept creates another difference between the upper and lower mantle. The pressure difference between the top of the upper mantle and the bottom of the lower mantle creates two different states. The top of the upper mantle is solid. Then, from the middle of the upper mantle through the lower mantle, the matter is a viscous substance.
The deeper you travel into the Earth, the more dense the matter becomes. Like floatation devices float on top of water, the Earth's crust floats on top of the mantle. Similarly, the liquid part of mantle floats on top of the outer core, which floats on top of the inner core. The density from the mantle to the core increases by about 30 percent.
Physical Properties and Composition
Extending to 670 kilometers (420 miles), the upper mantle consists of peridotite and dunite, which are solid, ultramafic rocks made up of olivine and pyroxene. The rocks become more unstable as you travel deeper into the upper mantle, until they become a thick substance like plastic in the lower mantle. The lower mantle extends from just under the upper mantle to 2,900 kilometers (1,800 miles) below the Earth's surface.