The Earth’s interior is layered, with the mantle situated between the thin crust and the dense core. The mantle is the thickest layer, extending from roughly 410 miles (660 km) down to the core-mantle boundary at about 1,800 miles (2,900 km). This region is subjected to extreme heat from the core and crushing pressure from the rock layers above. These intense conditions raise questions about its state of matter, especially since it neighbors the liquid outer core.
The Definitive Answer: Solid, Not Liquid
The lower mantle is definitively in a solid state. While temperatures deep within the mantle reach thousands of degrees Celsius, the immense pressure keeps the rock from fully melting. This high-pressure environment forces the material to maintain a rigid, crystalline structure.
The confusion arises because the lower mantle does exhibit movement, known as mantle convection, which is typically associated with liquids. This movement, however, is not like water boiling but is an extremely slow flow occurring over geologic timescales. The material behaves as a solid when subjected to sudden, short-term forces, yet it can deform and flow plastically when stress is applied over millions of years.
Evidence from Seismic Waves
Scientists determine the state of the Earth’s interior by analyzing seismic waves generated by earthquakes. These waves travel through the Earth, changing speed and direction as they pass through different materials. Two main types of body waves are used for this analysis: P-waves and S-waves.
P-waves, or compressional waves, are the fastest and can travel through all states of matter: solids, liquids, and gases. S-waves, or shear waves, are slower and can only propagate through solids because liquids cannot support the shearing motion required for these waves to move. The successful transmission of S-waves through the entire lower mantle provides direct evidence that this layer possesses the rigidity of a solid.
This finding contrasts sharply with the layer immediately beneath the mantle, the outer core, which scientists know is liquid because S-waves are completely blocked from traveling through it. By observing the paths and arrival times of these two distinct wave types, seismologists can effectively “x-ray” the Earth’s interior and confirm the solid nature of the lower mantle rock.
Flow and Rigidity Under High Pressure
The apparent contradiction of a solid material that can still flow is explained by the physical properties of the lower mantle under extreme conditions. The material acts as a viscoelastic solid, meaning it exhibits both elastic characteristics, like a solid, and viscous characteristics, like a fluid, depending on the timescale. Over short periods, such as during an earthquake, the mantle is rigid and transmits S-waves effectively.
The rock’s ability to flow slowly is quantified by its extremely high viscosity, a measure of its resistance to flow. Estimates range from \(10^{21}\) to \(10^{23}\) Pascal-seconds, making it many orders of magnitude thicker than materials like glass or asphalt. This high viscosity allows for creep, or plastic deformation, where the crystalline structure slowly deforms under constant stress.
This continuous, sluggish movement creates convection currents that slowly cycle heat and material from the core-mantle boundary toward the surface, driving plate tectonics. Despite the intense heat, the pressure gradient elevates the rock’s melting point, ensuring it maintains its solid, crystalline state while undergoing this slow-motion convection.
The Mineral Structure
The solid state of the lower mantle is maintained by the high-pressure, dense atomic packing of its constituent minerals. This layer is predominantly composed of magnesium, iron, and silicon oxides. The two most abundant minerals are Bridgmanite, a magnesium-iron silicate perovskite, and Ferropericlase, a magnesium-iron oxide.
Bridgmanite is the most abundant mineral on Earth, making up an estimated 70% of the lower mantle. It has a perovskite-type crystalline structure that is incredibly dense and stable under the immense pressures found deep within the planet. The high-density structure of these minerals is what provides the necessary rigidity for the lower mantle to act as a solid and transmit shear waves.
Ferropericlase is the second most common mineral, making up about 15% to 20% of the layer. Both of these mineral phases have rigid crystal lattices that resist deformation on short timescales. The combination of these dense, crystalline structures under pressure ensures that the lower mantle remains solid.