The mantle is a thick, rocky shell situated between the crust and the outer core, accounting for about 84% of the planet’s total volume. This vast region is physically and chemically divided, with the lower mantle representing the deepest and most voluminous part of this layer. Understanding the physical state of the lower mantle requires exploring the extreme conditions and composition that define it.
Defining the Lower Mantle
The lower mantle is the layer of rock situated immediately beneath the mantle’s transition zone. Its upper boundary is defined by a seismic discontinuity at approximately 660 kilometers below the surface. This boundary marks a profound change in mineral structure, separating the upper mantle’s mineral phases from the denser, high-pressure phases below.
The layer extends downward for over 2,200 kilometers, terminating at the Core-Mantle Boundary (CMB) at a depth of about 2,890 kilometers. The lower mantle occupies about 56% of Earth’s entire volume, and its conditions are governed by the immense column of rock above it.
Chemical and Mineral Composition
The elemental makeup of the lower mantle is primarily silicate rock, similar to the upper mantle, but intense pressure forces these materials into highly compact crystal structures. The dominant mineral is magnesium-silicate perovskite, now called bridgmanite. Bridgmanite is the most abundant mineral on Earth, making up an estimated 70 to 80% of the lower mantle’s volume.
The remaining composition consists mainly of two other high-pressure phases: ferropericlase (a magnesium-iron oxide) and calcium-silicate perovskite. The presence of iron in both bridgmanite and ferropericlase is important, as its behavior under extreme pressure affects the layer’s physical properties and its ability to conduct heat.
Physical State: Temperature and Pressure Extremes
The physical state of the lower mantle is defined by a combination of high temperature and crushing pressure. Pressure increases steeply with depth, ranging from about 24 gigapascals (GPa) at the 660 km boundary to 136 GPa at the Core-Mantle Boundary. This immense pressure is the result of the weight of the entire overlying crust and mantle.
Temperatures also increase dramatically, estimated to range from approximately 1,900°C at the top to as high as 4,000°C near the core. Despite these temperatures, which would instantly melt rock at the surface, the lower mantle remains predominantly a solid. The tremendous pressure raises the melting point of the silicate minerals so high that the rock remains solid.
The physical state is best described as a highly viscous or plastic solid, often referred to as the mesosphere. Although it is solid, the material can deform and flow very slowly over geological timescales. This behavior is directly related to the movement of heat within the planet, which drives the dynamics of the entire mantle system.
Dynamics and Flow
The slow, plastic flow of the solid lower mantle drives mantle convection, a process that transfers heat from the core to the surface and drives plate tectonics. This movement constitutes a massive global circulation pattern, occurring at rates of only a few centimeters per year. Hot material slowly rises toward the surface, while cooler, denser material, often in the form of subducting tectonic plates, sinks toward the core.
The D” layer, a complex zone 200 to 300 kilometers thick situated just above the Core-Mantle Boundary (CMB), concentrates much of this dynamic activity. This region is thought to be the termination point for sinking subducted slabs and the birthplace of massive, rising columns of hot material known as mantle plumes. These plumes ascend through the lower mantle, potentially contributing to volcanic activity at the surface.
The D” layer is also characterized by a phase change from bridgmanite to a denser mineral called post-perovskite. This lowermost boundary layer is the most thermally and structurally complex part of the lower mantle, influencing heat exchange between the solid mantle and the liquid outer core. Understanding this deep flow is essential to modeling the planet’s long-term evolution and heat budget.