The mantle alone accounts for approximately 84% of the Earth’s total volume, extending from the base of the crust down to a depth of about 2,900 kilometers. The answer to whether this layer is liquid or solid involves a complex interplay of heat and pressure, revealing a state of matter that defies simple categorization.
The Mantle’s Predominant Physical State
The vast majority of the Earth’s mantle, extending for thousands of kilometers, exists in a solid state. This conclusion is based on observations of how the material behaves under the extreme pressures found deep within the planet. Despite temperatures reaching thousands of degrees Celsius, the immense pressure keeps the silicate rock from fully liquefying.
The movement of tectonic plates is possible because the mantle material is not a brittle, static solid like the crust. Instead, it behaves as a viscoelastic or ductile solid over geological timescales, a property known as solid-state flow. This means it can deform and flow extremely slowly over millions of years, driving plate tectonics at the surface.
The viscosity, or resistance to flow, in the mantle is extraordinarily high, estimated to be around \(10^{21}\) Pascal-seconds. This extreme resistance causes the mantle to act as a rigid solid when subjected to sudden, short-term forces, such as the passage of earthquake waves. However, continuous, long-term stresses allow it to deform and circulate material throughout the layer.
Defining the Internal Physical Layers
The mantle is not uniform in its physical properties, and scientists divide it into mechanical layers based on rigidity and ability to flow. The uppermost layer is the lithosphere, which includes the crust and the rigid, uppermost part of the mantle. This layer is relatively cool and strong, behaving as a brittle solid that breaks under stress.
Beneath the lithosphere lies the asthenosphere, a region of the upper mantle characterized by greater ductility. Extending to about 400 kilometers, this layer is closer to its melting point than the lithosphere above it. The balance of heat and pressure here allows the rock to soften and flow more easily, facilitating the movement of the rigid tectonic plates above.
The lower mantle, often called the mesosphere, extends from the asthenosphere down to the core-mantle boundary at 2,900 kilometers. Here, temperature continues to rise, but the confining pressure increases even more dramatically. This extreme pressure compresses the mantle material, making the mesosphere substantially more rigid than the asthenosphere above it. Its overall strength and resistance to deformation are much higher due to the pressure.
Determining Mantle State Using Seismic Waves
Scientific understanding of the mantle’s state comes primarily from studying how seismic waves travel through the Earth’s interior. Earthquakes generate two main types of body waves: Primary waves (P-waves) and Secondary waves (S-waves). These waves propagate through materials in distinct ways, providing a powerful remote sensing tool.
P-waves are compressional waves that travel through solids, liquids, and gases. S-waves, however, are shear waves that move material perpendicular to the direction of travel. A defining characteristic of S-waves is their inability to propagate through a true liquid medium because liquids cannot sustain a shear stress.
The consistent observation of S-waves traveling successfully through the entire mantle confirms that this massive layer is overwhelmingly solid. If the mantle were liquid, S-waves would be stopped entirely. Scientists observe S-waves stopping abruptly at the boundary with the liquid outer core, which provides definitive evidence that the outer core is the only truly liquid layer within the Earth.
The speed of both P-waves and S-waves decreases slightly in the asthenosphere, which indicates a small reduction in rigidity. This slight slowing is consistent with the material being partially molten in some regions or simply being closer to its melting point, which explains its greater flow capacity.
The Process of Magma Generation
The existence of volcanoes and erupting lava might seem to contradict the conclusion that the mantle is solid, but magma is the exception, not the rule. Only a tiny fraction of the mantle is molten at any given time, confined to specific tectonic settings. Magma forms through partial melting, where only a portion of the rock melts, creating a silica-rich liquid that rises toward the surface.
One major mechanism is decompression melting, which occurs where hot mantle rock rises and experiences a significant drop in pressure. This commonly happens at mid-ocean ridges or at mantle plumes. Since the temperature of the ascending rock remains high, the decrease in pressure lowers the rock’s melting point, causing it to melt without any additional heat.
Another element is flux melting, which occurs at subduction zones where one tectonic plate slides beneath another. The subducting oceanic plate carries water trapped within its minerals. This water is released into the overlying mantle wedge as the plate heats up, lowering the melting temperature of the surrounding rock. This allows the rock to melt partially and form the magma that feeds volcanic arcs.