The Inner Structure of Earth: Why Isn't the Mantle Entirely Liquid?

The premise of your question—that the Earth's entire mantle is liquid—is fundamentally incorrect. The mantle is not molten due to the combination of radiative cooling and radioisotope heating, which keeps it at temperatures too low for its rocks to fully liquefy. Instead, the mantle is plastic, meaning it can deform and flow easily under extreme pressure and temperature conditions. Unlike the mantle, the Earth's core, which is roughly half its radius, is composed mainly of molten iron and nickel.

Why the Mantle Isn't Molten

The key factor that keeps the mantle from being fully liquid is pressure. Even though the rocks in the mantle experience high temperatures, the extreme pressure at depth prevents them from reaching their melting point. For every kilogram of rock that melts, at least a kilogram of magma must solidify to maintain overall mass balance. This balance ensures that the mantle remains in a state of plasticity rather than becoming a fully liquid layer.

Differences in Temperature, Pressure, and Magma Formation

The differences in temperature, pressure, and structural formations within the mantle and the crust play a crucial role in the formation of magma. Areas of lower pressure have a lower melting point, allowing the surrounding mantle rock to melt if the pressure is reduced or during decomposition. This process can be further influenced by flux-induced melting, which occurs when the local chemical composition changes due to interactions with other materials.

Plasticity and Viscosity of the Mantle

While the mantle is not entirely liquid, it is not solid either. It is highly viscous, like forcing frozen clay to move under pressure. Over millions of years, the mantle behaves like some form of plastic, allowing the solid crust to flow on its surface. This process is analogous to a hands-on experiment using thick molasses and talc powder. An inclined surface exhibits the movement and deformity of the material, revealing wavy inclines, shearing, and ripples where sections behave differently due to stress.

Experimental Evidence: Molasses and Talc Powder

A practical demonstration of the plasticity of the mantle can be replicated using thick molasses and talc powder. Place these materials in several layers on a backing plate and let them rest for some time. Over time, the material will begin to sag on an incline, and you can use a wooden spoon or block to create more interesting ripples. Additionally, gently heating some spots on the backing plate or running water at a single spot can simulate changes in temperature and pressure. After slicing the material into sections, you will observe patterns that mirror the complex deformations seen in real mantle rocks.