The Earth’s structure is described in layers, including the crust, the vast mantle, and the metallic core. The mantle is divided into upper and lower portions. The lower mantle is the single largest reservoir of material inside the planet, accounting for over half of Earth’s volume. Scientists cannot physically sample this depth, yet they have identified a single mineral phase that dominates this massive layer, dictating the planet’s internal dynamics and the behavior of seismic waves.
Location and Conditions of the Lower Mantle
The lower mantle begins approximately 660 kilometers beneath the surface and extends down to the core-mantle boundary at about 2,890 kilometers. This immense depth subjects minerals to extraordinary pressure and temperature. Pressure starts at roughly 24 gigapascals (GPa) at the upper boundary, increasing to about 136 GPa near the core.
Temperatures range from approximately 1,900 Kelvin (1,630 degrees Celsius) to over 2,600 Kelvin (2,330 degrees Celsius). This combination of heat and extreme compression forces surface silicate rocks to convert into ultra-dense, compact crystalline structures. These physical conditions determine which minerals can exist and remain stable in the deep interior.
Identifying the Main Mineral Component
The dominant mineral in this environment is a magnesium-iron silicate known as Bridgmanite. This phase is considered the most abundant mineral on Earth, making up an estimated 70 to 80% of the lower mantle’s mineral assemblage. Its chemical formula is represented as \((\text{Mg}, \text{Fe})\text{SiO}_3\), indicating it is primarily a silicate of magnesium with iron atoms integrated into its structure.
Before its official naming, the mineral was known as silicate perovskite due to its crystal structure. In 2014, the International Mineralogical Association approved the name Bridgmanite, honoring physicist Percy Bridgman for his pioneering work in high-pressure physics. Its stability comes from its dense perovskite structure, which allows it to withstand the incredible pressure of the lower mantle.
The crystal structure is orthorhombic, defined by the way the silicon atoms are surrounded by six oxygen atoms in an octahedral arrangement. The larger magnesium and iron atoms fill the spaces between these octahedra. This close-packed configuration gives Bridgmanite the necessary density to be the stable form of silicate at these extreme depths.
Stability Under Extreme Pressure
The transition into Bridgmanite occurs precisely at the 660-kilometer seismic discontinuity, marking the boundary between the upper and lower mantle. This boundary is a dramatic phase change driven by pressure, not just a compositional shift. Minerals dominating the transition zone, specifically Ringwoodite, a high-pressure form of olivine, become unstable as pressure increases further.
Upon reaching lower mantle conditions, the Ringwoodite structure breaks down in an endothermic reaction. This decomposition yields a mixture of two new, denser phases: Bridgmanite and a magnesium-iron oxide called ferropericlase, \((\text{Mg}, \text{Fe})\text{O}\). The rearrangement of atoms into these new crystal structures allows the material to occupy less volume, significantly increasing its density.
Bridgmanite is dominant because its structural collapse is the most space-efficient way for the mantle’s bulk chemical composition to exist under massive compression. The resulting change in density and rigidity causes the sharp increase in seismic wave velocity observed at the 660-kilometer boundary. This phase transition is a major control on the Earth’s internal convection, as the resistance it presents influences the movement of the mantle.
Uncovering Deep Earth Composition
The existence and properties of Bridgmanite are confirmed through two primary, indirect scientific methods. Since no drill has ever reached the lower mantle, scientists rely on these techniques.
Seismic Tomography
The first method involves analyzing the behavior of earthquake waves as they travel through the Earth’s interior, a technique called seismic tomography. Scientists measure how quickly these waves move, and the observed seismic velocities are consistent with the predicted speeds through a rock assemblage dominated by Bridgmanite.
Laboratory Experiments
The second method involves laboratory experiments that recreate the lower mantle’s extreme conditions on tiny samples. The Diamond Anvil Cell (DAC) is the primary tool, employing two opposing diamonds to squeeze a minute mineral sample. This process generates pressures exceeding 130 GPa, and laser heating raises the temperature to thousands of degrees. By observing samples inside the DAC using X-rays, researchers see minerals like Ringwoodite transform directly into the denser Bridgmanite phase. These controlled laboratory results link the seismic observations to the specific mineral composition of the deepest part of the Earth’s mantle.