Question

What is the mantle composed of? What are the three sublayers within the mantle? Is there any melt within the mantle?

Short answer

The Earth's mantle is primarily composed of solid rock, predominantly peridotite in the upper mantle, denser forms of olivine in the transition zone, and bridgmanite and ferropericlase in the lower mantle. The mantle is divided into three sublayers: the upper mantle (35-410 km depth), the transition zone (410-660 km depth), and the lower mantle (660-2,890 km depth). There is a small amount of melt within the mantle, particularly within the asthenosphere of the upper mantle, which is crucial for tectonic plate movements.

Step-by-step solution

Introduction

The Earth's mantle is a layer of the planet that lies between the crust and the outer core. It is predominantly composed of solid rock, but also contains small amounts of other elements. The mantle is divided into three sublayers: the upper mantle, the transition zone, and the lower mantle. Let's explore each of these sublayers, and address the presence of melt within the mantle.

Upper Mantle

The upper mantle is the topmost sublayer of the mantle and extends from about 35 km to 410 km depth beneath the Earth's surface. It is primarily composed of solid peridotite, which is a dense, coarse-grained rock consisting mostly of the minerals olivine and pyroxene. The upper mantle also features the asthenosphere, a partially molten layer within the mantle. This partial melt allows the rigid lithosphere (the crust and the uppermost mantle) to move over the asthenosphere, resulting in tectonic plate movements.

Transition Zone

The transition zone lies beneath the upper mantle and ranges from about 410 km to 660 km depth. Within this zone, the pressure and temperature conditions lead to changes in the crystal structures of the mantle minerals, resulting in a phase transformation of olivine into more dense and stable forms, such as wadsleyite and ringwoodite.

Lower Mantle

The lower mantle extends from the base of the transition zone (at approximately 660 km depth) to the core-mantle boundary (at about 2,890 km depth). The primary composition of the lower mantle is a solid material consisting of silicate minerals, predominantly bridgmanite (a high-pressure magnesium-iron silicate) and ferropericlase (an iron-magnesium oxide). The lower mantle is characterized by high pressure and temperature conditions that can reach up to 3,500 K.

Melt Presence in the Mantle

The mantle is primarily composed of solid rock, but there is evidence of partial melt within the mantle, particularly within the asthenosphere of the upper mantle. This partial melt allows the movement of the lithosphere, driving tectonic processes. However, the overall melt fraction within the mantle is very low, generally less than 1%. In summary, the Earth's mantle is primarily composed of peridotite in the upper mantle, transitioning to denser forms of olivine in the transition zone, and bridgmanite and ferropericlase in the lower mantle. The three sublayers of the mantle are the upper mantle, the transition zone, and the lower mantle. There is a small amount of melt within the mantle, particularly within the asthenosphere of the upper mantle, which is crucial for the movement of tectonic plates.

Key concepts

Upper Mantle

The upper mantle is a fascinating layer of the Earth, located just below the crust. Extending from 35 km to 410 km below the Earth's surface, it plays a pivotal role in the dynamics of our planet. This part of the mantle is predominantly composed of a dense, coarse-grained rock called peridotite.
Peridotite mainly consists of minerals such as olivine and pyroxene, which contribute to its solidity and structure.

• Composed primarily of solid rock with minerals like olivine and pyroxene.
• Extends from 35 km to 410 km beneath Earth's surface.

The upper mantle is not entirely solid. It contains the asthenosphere, a zone where rocks are partially melted, forming a less rigid solution. This partial melt in the asthenosphere allows the overlying rigid plates, known as tectonic plates, to move over this layer.

Transition Zone

Lying beneath the upper mantle, the transition zone marks a segment of the mantle that ranges from 410 km to 660 km deep. This zone is intriguing because of the significant changes it brings to the rock's structure due to the increasing pressure and temperature.

• Located from 410 km to 660 km below the Earth's surface.
• Exhibits changes in mineral structures due to pressure and temperature variations.

In this zone, minerals like olivine undergo phase transformations, morphing into denser and more stable forms, such as wadsleyite and ringwoodite. These transformations play a critical role in the internal convection currents driving mantle dynamics.

Lower Mantle

The lower mantle plunges from the end of the transition zone down to the core-mantle boundary, at a depth of approximately 2,890 km. Here, the environment is extreme, with immense pressure and temperatures soaring up to 3,500 K.

• Stretches from 660 km to 2,890 km deep.
• Made up of minerals like bridgmanite and ferropericlase.

This section of the mantle is mainly composed of silicate minerals such as bridgmanite—a magnesium-iron silicate—and ferropericlase, an iron-magnesium oxide. These compositions make the lower mantle profoundly different in terms of density and mineral stability compared to the upper layers.

Asthenosphere

Though not one of the main sub-layers, the asthenosphere is a crucial part of the upper mantle. Sitting below the lithosphere, its unique attribute is its partial melting state, which gives it a semi-fluid flexibility. This quality is what supports the lithosphere and facilitates the movement of tectonic plates above it.

• Exists within the upper mantle, beneath the lithosphere.
• Characterized by partial melt supporting tectonic movements.

The asthenosphere's fluidity is pivotal for convection currents within the mantle, contributing to plate tectonics, volcanic activity, and earthquake generation. Without this plastic layer, the solid plates above would have restricted motion.

Tectonic Plates

Tectonic plates are massive slabs of solid rock composed of lithosphere, which includes both the crust and the uppermost part of the mantle. These plates rest on the softer and more malleable asthenosphere of the upper mantle.

• Composed of the crust and uppermost mantle, part of the lithosphere.
• Move over the semi-fluid asthenosphere.

Driven by convection currents in the semi-fluid asthenosphere beneath, tectonic plates constantly move, albeit very slowly. This movement is responsible for a plethora of geological activities, including the formation of mountains, earthquakes, and volcanic eruptions. Understanding tectonic plates and their movement is vital as they shape the Earth's surface over geological time.