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

The mantle is solid due to high pressure preventing complete melting, surrounded by intense heat.

Step-by-step solution

Understanding Earth's Layers

The Earth is composed of several layers: the crust, the mantle, and the core. The mantle is located between the Earth's crust and its core and is divided into the upper and lower mantle. Each section has distinct physical and chemical properties.

Characteristics of the Upper Mantle

The upper mantle extends from the boundary with the crust down to about 410 kilometers below the Earth's surface. It is composed primarily of solid rock rich in silicate minerals, which remain solid due to immense pressure despite high temperatures.

Investigating Temperature and Pressure

Temperatures in the upper mantle range from about 500 to 900 degrees Celsius. Despite these high temperatures, the rocks remain mostly solid because the pressure in the mantle is incredibly high, typically ranging from 10,000 to 140,000 times atmospheric pressure at sea level.

The Concept of Partial Melting

While the rocks in the mantle are primarily solid, there are areas called 'asthenosphere,' where the rock is partially molten. However, these areas are not completely liquid but rather have a small fraction of melt mixed with solid rock, influenced by convection currents.

Conclusion - Solid Yet Flowing

The mantle is solid because of the high pressure that prevents the rocks from fully melting. Nonetheless, the rocks are ductile, allowing for slow flow over geological time scales, which is crucial for the movement of tectonic plates.

Key concepts

Upper Mantle

The upper mantle is a crucial part of Earth's structure, located right below the crust. It spans approximately from the surface to about 410 kilometers deep. Despite being subjected to extreme conditions, such as high temperatures, it remains predominantly solid. This solidity can largely be attributed to the minerals that comprise the upper mantle, mainly silicate minerals, which are capable of withstanding intense pressure.

• The upper mantle is distinct from the Earth's crust and core, exhibiting unique physical and chemical properties.
• It plays a significant role in tectonic activities due to its ability to flow slowly.
• Understanding its nature is vital for comprehending Earth's dynamic processes.

Pressure and Temperature

In the Earth's mantle, both pressure and temperature play pivotal roles. However, it's the interplay between these factors that determines the state of matter within it. In the upper mantle, temperatures can range from about 500 to 900 degrees Celsius.

One might expect such high temperatures to liquefy the rocks. However, the immense pressure, which can scale from 10,000 to 140,000 times the Earth's atmospheric pressure, typically keeps these rocks in a solid state. The relationship between pressure and temperature is quite fascinating as it contributes to maintaining the mantle's stability, preventing the rocks from melting entirely.

• High pressure increases the melting point of rocks, ensuring they remain solid even at high temperatures.
• This solid-form under pressure allows for a slow, ductile flow rather than a complete melt.

Asthenosphere

Beneath the upper mantle lies a region known as the asthenosphere. It is not entirely solid nor completely liquid. Instead, it comprises a semi-solid material that exhibits both solid and fluid-like properties. The asthenosphere is paramount in the movement of tectonic plates. It generates an environment where rocks are partially molten, approximately 1-10% melt, mixed with mostly solid material. This partial melting is sufficient to weaken the rocks and allow them to move over geological timescales.

• The asthenosphere's partial melt properties facilitate plate tectonics, shaping Earth's surface.
• Its unique characteristics help absorb and distribute the tremendous forces from tectonic movements.

Mantle Convection

Mantle convection is a fundamental process influencing the movement within Earth's upper mantle and the asthenosphere. This convection process is akin to the slow, churning motion seen in liquids that are heated from below. Even though the mantle is mostly solid, the material can flow slowly due to the intense heat emanating from the Earth's core.

Mantle convection causes warmer, less dense material to rise while cooler, denser material sinks. This cyclical motion results in the horizontal movement of the Earth's crust—what we perceive as tectonic plate movement. Essentially, convection drives the dynamic geological phenomena responsible for the creation and renewal of Earth's surface over immense timescales.

• Mantle convection is essential for renewing the Earth's surface.
• It plays a critical role in earthquakes, volcanic activity, and continental drift.