Question

A common misconception about Earth's upper mantle is that it is a thick shell of molten rock. Explain why Earth's mantle is actually solid under most conditions.

Short answer

Earth's mantle is mostly solid due to high pressure despite high temperatures, with minor areas of partial melting.

Step-by-step solution

Understanding the Composition

The Earth's upper mantle primarily consists of silicate minerals, including olivine and pyroxene, which are solid at Earth-like conditions of pressure and temperature.

Exploring Temperature and Pressure

The temperature in the mantle can reach up to 4000 degrees Celsius, but the pressure is extremely high as well. This combination actually keeps the minerals solid even though they are very hot.

Identifying Partial Melting Zones

While the mantle is mostly solid, some areas experience partial melting due to specific conditions like elevated temperatures and lower pressures, as seen in magma production zones.

Explaining Plasticity

The mantle exhibits plastic behavior, allowing it to flow slowly over time. This plasticity is due to the minute movements within its solid state, rather than being completely molten.

Examining the Role of Convection

Mantle convection, a process of heat transfer, happens due to the solid but ductile nature of the mantle, allowing for the slow movement of solid rock.

Key concepts

Silicate Minerals

The Earth's mantle is primarily composed of silicate minerals. These minerals form a pivotal part of the mantle's structure and include components like olivine and pyroxene. Silicate minerals are rich in silicon and oxygen, which are some of the most plentiful elements in the Earth's crust and mantle.
Under the immense pressures found in the Earth's mantle, these minerals remain solid, despite the high temperatures.

• **Olivine**: A prominent green mineral, crucial to the mantle's structure.
• **Pyroxene**: Another key mineral, usually dark and forms part of many igneous rocks.

These minerals contribute significantly to the mantle's solid state. The structure of these silicates maintains its form under high pressures, resisting melting, which is why the mantle remains solid under typical conditions.

Mantle Convection

Mantle convection is akin to a slow-moving circulation system within the Earth's mantle. Even though the mantle is solid, it behaves in a ductile manner, enabling it to flow over geological timeframes. This flow results from differences in temperature and density within the mantle:

• **Hotter, less dense material** rises.
• **Cooler, denser material** sinks.

This continuous cycle of rising and sinking material facilitates the transfer of heat from the deep mantle to the Earth's crust. Mantle convection is responsible for driving plate tectonics, as it moves the overlying crustal plates. The process illustrates the dynamic nature of the Earth's interior, contrary to the static impression one might expect from a solid mantle.

Partial Melting

While the mantle is largely solid, certain conditions can lead to partial melting. This process occurs primarily at tectonic boundaries or hotspots where there's a localized increase in temperature or a reduction in pressure.

• **Subduction Zones**: Here, oceanic plates dive beneath continental plates, leading to melting due to increased temperatures and pressures.
• **Mid-Ocean Ridges**: Magma forms as plates diverge, reducing pressure and triggering partial melting.

In these regions, only a portion of the mantle rocks melts to create magma. Partial melting is crucial for the generation of basaltic magma, which eventually contributes to new crust formation as the magma reaches the surface.

Plastic Behavior

Despite being predominantly solid, the mantle exhibits plastic behavior, allowing it to deform without breaking. This property lets it respond to stress over long periods, contributing to the mantle's flow, even though it is not liquid. Plastic behavior in geological terms means the ability to undergo slow, continuous deformation. This is facilitated by the movement of the mineral grains that make up the mantle, allowing the solid structure to adapt to external forces: - **Creep**: The slow imperceptible movement of solid that behaves like a very viscous fluid. - **Ductility**: The ability of a material to deform under stress without cracking. These characteristics enable the mantle to support convective currents and plate movements, effectively reshaping the Earth's surface over millions of years.