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 because the high pressure offsets the high temperature, except in specific areas of partial melting.

Step-by-step solution

Introduction

To understand why Earth's mantle is solid under most conditions, we need to consider its composition, temperature, and pressure.

Composition of the Mantle

The Earth's mantle is composed primarily of silicate rocks that are rich in magnesium and iron. These rocks are solid under normal conditions.

Temperature and Pressure in the Mantle

While the temperature increases with depth in the mantle, so does the pressure. High pressure prevents the mantle rocks from melting, keeping them in a solid state. Although parts of the mantle are extremely hot, the pressure exerted upon these rocks is sufficient to maintain their solid state.

Role of Partial Melting

In some regions, like beneath tectonic plates, partial melting occurs due to lower pressure or the addition of volatiles like water, which can lower the melting point of the rocks. However, this is limited to specific areas and does not imply the entire mantle is molten.

Key concepts

Mantle Composition

The Earth's mantle is a thick layer made predominantly of silicate rocks. These silicate rocks are primarily composed of minerals rich in magnesium and iron. This combination makes them very dense and heavy. The mantle lies between Earth's crust and core and spans a depth of about 2,900 kilometers (1,800 miles).

The outer part of the mantle is more rigid, while the deep mantle is semi-solid due to variations in temperature and pressure. Despite the intense heat, these rocks remain solid because of the immense pressures acting upon them, which we will discuss further in the next sections.

Temperature and Pressure

As we delve deeper into Earth's mantle, the temperature clearly rises. It can reach up to 4,000 degrees Celsius (about 7,200 degrees Fahrenheit) near the boundary with the core. You might think these temperatures should be enough to cause melting, but that's not the whole story.

Pressure plays a crucial role in keeping mantle rocks solid. The deeper you go, the greater the pressure, which counteracts the high temperatures. This pressure prevents the rocks from expanding and melting. In most parts of the mantle, pressure is so high that despite extreme heat, the rocks stay solid. This is why Earth's mantle is largely solid rather than molten.

Partial Melting

In certain parts of the Earth's mantle, partial melting can occur. This happens mainly under specific conditions like beneath tectonic plates or where mantle rocks are disrupted. Partial melting is not a full melting process; only some of the minerals in the rocks melt.

Several factors contribute to partial melting:

• Decreased pressure as mantle rocks rise closer to the surface.
• The addition of volatiles, such as water, which significantly lowers the melting point of rocks.
• Temperatures reaching the melting point of some minerals.

These melted portions can lead to volcanic activity or contribute to the creation of new crust material. Importantly, partial melting only occurs in specific areas, leaving the bulk of the mantle in a solid state.

Tectonic Plates

Tectonic plates are immense slabs of Earth's crust and upper mantle that float on the semi-fluid asthenosphere below. The movement of these plates is responsible for many geological phenomena, including earthquakes and volcanoes, and is a key aspect of Earth's geology.

At times, when these plates diverge or converge, they create conditions for partial melting in the mantle below. For example, when an oceanic plate subducts beneath a continental plate, it drags water down, which can induce partial melting in the overlying mantle. This is one way volcanoes and new crust are formed at convergent plate boundaries.

Silicate Rocks

Silicate rocks form the bulk of Earth's mantle. They are primarily made up of silicate minerals, which are compounds containing silicon and oxygen, along with other elements like magnesium, iron, and aluminum.

Silicate rocks are unique because they can remain solid even under extreme conditions of heat and pressure, such as those found in the mantle. Their ability to stay solid is due to their chemical structure and the pressure conditions in the mantle. These rocks are crucial for understanding Earth's layered structure and the behavior of mantle materials under different conditions.