Lower mantle

The lower mantle is a massive region located deep within the Earth. It occupies approximately 56% of the planet's total volume. This layer sits below the transition zone and rests directly above the outer core. Historically, scientists have also referred to this region as the mesosphere. Understanding the lower mantle is essential for studying how heat and material move inside our planet.

The boundary between the upper and lower mantle is defined by specific physical changes. At a depth of 660 kilometers, a mineral known as ringwoodite decomposes. This mineral breaks down into two new substances: Mg-Si perovskite and magnesiowüstite. This chemical change causes a sharp increase in density and seismic wave velocities. This transition marks the official start of the lower mantle. The mantle itself moves very slowly, at a rate of about one centimeter per year.

The Preliminary Reference Earth Model, or PREM, divides the lower mantle into three distinct sections. The first section spans from 660 to 770 kilometers deep. In this zone, the transformation of ringwoodite into bridgmanite and ferropericlase causes a 6% to 11% change in compression wave velocity. The second section extends from 770 to 2,700 kilometers. Here, velocity increases gradually due to adiabatic compression, which is the compression of mineral phases without losing heat. The final section, the D-layer, exists between 2,700 and 2,900 kilometers. This layer serves as the transition between the lower mantle and the liquid outer core.

Temperature and pressure change significantly throughout this vast layer. At the topmost boundary, temperatures are around 1,600 degrees Celsius. As you descend toward the base, temperatures rise to approximately 3,700 degrees Celsius. The pressure is also immense, ranging from 24 gigapascals to 135 gigapascals. Heat moves through the lower mantle primarily through convection. This means heat is transported by the movement of the material itself. Other methods, like conduction or radiative heat transfer, are considered negligible in this region.

Scientists are still debating the exact chemical composition of the lower mantle. The layer is primarily composed of three minerals: bridgmanite, ferropericlase, and calcium-silicate perovskite. One theory suggests a pyrolitic composition. This model implies the mantle is homogeneous and contains a magnesium-to-silicon ratio of 1.27. Under this theory, the volume is 75% bridgmanite, 17% ferropericlase, and 8% calcium-silicate perovskite. Another theory suggests a chondritic composition. This model suggests the mantle formed from chondritic meteorites, which would result in a magnesium-to-silicon ratio of approximately 1.

High pressure also causes a unique phenomenon called a spin transition in iron-bearing minerals. In minerals like ferropericlase and bridgmanite, the iron atoms change from a high-spin state to a low-spin state. For example, iron in ferropericlase undergoes this transition between 50 and 90 gigapascals. In bridgmanite, the transition occurs at different pressures depending on the iron's position in the mineral structure. This electronic change can increase the density and incompressibility of the minerals. Researchers are currently using numerical simulations to see how these transitions affect mantle plume dynamics and chemistry.

The study of the mesosphere has a long history in geology. The term was originally coined by Reginald Aldworth Daly, a professor at Harvard University. In 1940, Daly proposed a model of the Earth consisting of the lithosphere, the asthenosphere, and the mesospheric shell. At that time, scientists had not yet established the theory of plate tectonics. Daly's model suggested the mesosphere was a solid shell that extended to the top of the core. Today, while our models have become much more complex, his work helped set the stage for modern deep-Earth science.