Question

What material makes up the mantle? What are the sublayers of the mantle? Does melt exist within the mantle? (D)

Short answer

The Earth's mantle is primarily composed of solid, dense rocks like peridotite, which are silicate minerals rich in iron and magnesium. It is divided into two main sublayers: the upper mantle and the lower mantle. The upper mantle contains the partially molten asthenosphere and the solid lithosphere, while the lower mantle extends from 660 km to 2900 km below the Earth's surface. Pockets of melt mainly exist in the asthenosphere, allowing the movement of tectonic plates.

Step-by-step solution

Mantle Composition

The Earth's mantle is primarily made up of solid, dense rocks, consisting of silicate minerals rich in iron and magnesium. The major component of the mantle is a type of rock called peridotite.

Mantle Sublayers

The mantle can be divided into two main sublayers: the upper mantle and the lower mantle. The upper mantle extends from the base of the Earth's crust (around 35-40 km deep) to 660 km below the Earth's surface. It is further subdivided into a partially molten layer called the asthenosphere and a solid layer called the lithosphere. The lower mantle lies below the upper mantle and reaches depths from 660 km to about 2900 km below the Earth's surface.

Presence of Melt within the Mantle

Yes, there are pockets of melt within the mantle. Melt primarily exists in the asthenosphere, which is a part of the upper mantle. The asthenosphere contains a small percentage of partially molten rock. This molten material allows the movement of tectonic plates, as the partially molten asthenosphere allows the solid lithosphere to move and slide above it.

Key concepts

Peridotite

The Earth's mantle is largely made up of a dense and solid rock known as peridotite. Peridotite is rich in the minerals olivine and pyroxene, which contain high amounts of iron and magnesium. These minerals give peridotite its characteristic dark color and contribute to its density.
Its composition is crucial in affecting the mantle's behavior, as it can undergo changes under extreme pressure and heat conditions found deep beneath the Earth's surface.
- Olivine: A greenish mineral, often seen in peridotite, stable under high temperatures. - Pyroxene: Another mineral that combines with olivine to form peridotite. Understanding peridotite is essential in grasping how the mantle's properties affect geological phenomena like volcanic eruptions and the formation of tectonic plate boundaries.

Upper Mantle

The Earth's upper mantle spans from just beneath the crust to about 660 kilometers deep. It is one of the two main sublayers of the mantle, the other being the lower mantle. The upper mantle itself is very dynamic.
It plays a crucial role in plate tectonics and geological activity of the Earth.
The upper mantle is further divided into: - **Lithosphere**: The outermost section, which is rigid and solid. The lithosphere includes both the upper mantle's top layer and the crust. It is broken into tectonic plates. - **Asthenosphere**: Located beneath the lithosphere, this zone is partially molten, allowing the rigid lithosphere above to move. The asthenosphere is softer but still solid, helping to facilitate plate tectonics.

Asthenosphere

The asthenosphere is a critical and influential region within the Earth's upper mantle. Extending from about 100 to 200 kilometers beneath the Earth's surface, it is characterized by semi-fluid conditions that allow it to flow very slowly.
This flow is important for the movement of tectonic plates sitting above on the lithosphere.
- **Partially Molten Rock**: The presence of melt makes the asthenosphere more ductile than other parts of the mantle, meaning it can deform and reshape over time. This is significant in helping tectonic plates shift and move. - **Convection Currents**: These currents occur within the asthenosphere due to heat coming from the deeper mantle and core, causing material to rise and fall. This movement helps drive the motion of tectonic plates on the Earth's surface.

Tectonic Plates

Tectonic plates are large slabs of the Earth's lithosphere that fit together like a jigsaw puzzle on the Earth's surface. These plates float on the soft and partially molten asthenosphere below, constantly moving and interacting.
The movement of tectonic plates is a primary cause of many geological phenomena.
- **Plate Boundaries**: Tectonic plates interact at their edges, forming different types of boundaries: - **Convergent Boundaries**: Plates move towards each other, sometimes causing mountains or volcanoes. - **Divergent Boundaries**: Plates move apart, leading to the formation of new crust. - **Transform Boundaries**: Plates slide past each other, which can cause earthquakes. Understanding tectonic plates and their interactions is key to studying the dynamic nature of Earth's surface and predicting geological events.