The mantle is subdivided into the upper mantle, the transition zone, and the lower mantle. The lower mantle is the deepest and most voluminous part of the planet’s silicate interior, extending from a depth of approximately 660 kilometers down to the core-mantle boundary at about 2,890 kilometers below the surface. This region makes up about 55% of the Earth’s total volume. Its composition is fundamental to understanding the planet’s internal dynamics and long-term evolution, as the extreme conditions force constituent elements to combine into dense, unique mineral structures.
Defining the Lower Mantle’s Physical Characteristics
The physical environment of the lower mantle is characterized by an immense pressure and temperature gradient that fundamentally controls its mineral phases and behavior. Pressure begins at about 24 gigapascals (GPa) at the upper boundary and increases dramatically to approximately 136 GPa at the core-mantle boundary.
Temperatures also rise significantly with depth, starting at around 1,900 Kelvin (approximately 1,630 degrees Celsius) and potentially reaching 4,000 Kelvin (around 3,730 degrees Celsius) near the core boundary. Despite these extreme temperatures, the lower mantle remains in a solid state due to the crushing pressure, although it behaves as a highly viscous, slowly flowing material over geological timescales.
The upper limit of the lower mantle is defined by the 660-kilometer seismic discontinuity, which marks the breakdown of the high-pressure mineral ringwoodite into denser phases. The base of the lower mantle is marked by a complex region known as the D” layer, which is the lowermost 200 to 300 kilometers just above the core-mantle boundary. This layer exhibits distinct seismic anomalies that suggest a change in mineral structure, driven by the final increase in pressure and temperature.
Dominant Mineralogy: Silicate Perovskites and Post-Perovskites
The primary constituent is magnesium silicate perovskite, formally named Bridgmanite. With the approximate chemical formula \((\text{Mg, Fe})\text{SiO}_3\), Bridgmanite is the most abundant mineral on Earth, making up around 80% of the lower mantle’s volume.
This mineral is stable throughout most of the lower mantle, crystallizing in an orthorhombic structure that allows it to maintain stability under extreme pressure. It forms from the decomposition of minerals like ringwoodite at the 660-kilometer boundary. The presence of iron (Fe) in the formula indicates that magnesium and iron atoms can substitute for each other within the crystal structure, influencing the mineral’s density and seismic properties.
Near the core-mantle boundary, within the D” layer, Bridgmanite undergoes a transformation to post-perovskite. This phase transition occurs at pressures exceeding 125 GPa and temperatures around 2,500 K. The post-perovskite phase, which has a layered structure, accounts for the sudden changes in seismic wave velocities observed in the D” region. The phase boundary between Bridgmanite and post-perovskite has a positive slope, meaning the transition occurs at greater depth in hotter regions, which has profound implications for how heat is transferred out of the core.
Secondary Components: Iron and Oxide Phases
The second most abundant phase in the lower mantle is ferropericlase, a magnesium-iron oxide with the formula \((\text{Mg, Fe})\text{O}\). Ferropericlase typically constitutes about 15% to 20% of the lower mantle’s mineral assemblage by volume. This mineral is a primary reservoir for iron that is not incorporated into the Bridgmanite structure, and its iron content is higher than that of the silicate perovskite.
The distribution of iron between ferropericlase and Bridgmanite influences the overall density and magnetic properties of the deep mantle. Under the high pressures of the lower mantle, the iron in both minerals undergoes a spin transition, which is a change in the electronic state of the iron atoms. This electronic change alters the physical properties of the minerals, such as their density and compressibility.
Other components are also present, including calcium silicate perovskite, formally known as Davemaoite \((\text{CaSiO}_3)\). Aluminum is a common impurity, often incorporated into the Bridgmanite structure, forming Al-bearing Bridgmanite. These minor phases and impurities add complexity to the mineral assemblage, creating heterogeneity.
The Role of Lower Mantle Composition in Earth Dynamics
The mineral assemblage of the lower mantle, dominated by dense Bridgmanite and ferropericlase, controls the large-scale processes that govern the Earth’s dynamics. The high-pressure phases affect the viscosity of the rock, which is a measure of its resistance to flow. This viscosity is directly linked to mantle convection, the slow churning motion that transfers heat from the planet’s interior toward the surface.
The phase transition from Bridgmanite to post-perovskite in the D” layer has a significant impact on this heat transfer. The properties of post-perovskite influence the thermal boundary layer just above the core, potentially destabilizing it and inducing the formation of mantle plumes. These upwellings of hot material are thought to originate near the core-mantle boundary, carrying heat and material up toward the surface and linking deep Earth processes to surface geology.
The chemical composition, including the iron content and the presence of aluminum, further affects the density and seismic velocity of the lower mantle. These variations create a complex, three-dimensional structure that influences the speed and path of seismic waves, which scientists use to map the deep interior. Ultimately, the unique mineral makeup of the lower mantle ties the planet’s deep thermal engine to the surface phenomena of plate tectonics and volcanism.