Question

What is the difference between lithosphere and asthenosphere? Which layer is softer and flows easily? At what depth does the lithosphere-asthenosphere boundary occur? Is this above or below the Moho?

Short answer

The lithosphere is the outermost rigid layer of the Earth, made up of the crust and the upper part of the mantle, while the asthenosphere is the ductile, semi-solid layer beneath it. The asthenosphere is softer and flows more easily than the lithosphere due to its partially molten state. The lithosphere-asthenosphere boundary (LAB) occurs at a depth of around 100 kilometers, although this may vary. The Mohorovičić discontinuity (Moho) lies above the LAB, marking the boundary between the Earth's crust and the upper mantle.

Step-by-step solution

Define Lithosphere

The lithosphere is the outermost rigid layer of the Earth, comprising of the crust and the uppermost part of the mantle. This layer is made up of tectonic plates that move and interact with each other.

Define Asthenosphere

The asthenosphere is the layer below the lithosphere, which is a more ductile and pliable region of the mantle that extends from around 100 to about 700 kilometers below the surface. It consists of partially molten, semi-solid rock material that allows for the movement of tectonic plates.

Compare Lithosphere and Asthenosphere

The main difference between lithosphere and asthenosphere is their consistency and behavior. The lithosphere is a rigid layer composed of solid rocks, while the asthenosphere is a ductile layer with semi-solid rocks that exhibit plastic properties, which means it can flow.

Identify Softer Layer and Flow Characteristics

The asthenosphere is the softer layer of the two and flows more easily than the lithosphere, due to its partially molten state and the pressure from the overlying lithosphere.

Determine Depth of Lithosphere-Asthenosphere Boundary

The lithosphere-asthenosphere boundary (LAB) occurs at a depth of around 100 kilometers below the Earth's surface. However, the depth may vary depending on various factors like the age of oceanic lithosphere, location, and tectonic activity.

Relate to the Mohorovičić Discontinuity

The Mohorovičić discontinuity (Moho) marks the boundary between the Earth's crust and the upper mantle; it lies above the lithosphere-asthenosphere boundary. The depth of the Moho varies between 5 kilometers beneath oceanic crust to up to 70 kilometers beneath continental crust. Since the lithosphere also includes the uppermost part of the mantle, the lithosphere-asthenosphere boundary is deeper than the Moho.

Key concepts

Asthenosphere

The asthenosphere is a crucial layer within the structure of our Earth. Located directly below the lithosphere, it plays a vital role in the movement of tectonic plates. This layer spans from approximately 100 kilometers to about 700 kilometers beneath the surface. The asthenosphere is composed of rock material that is not entirely solid. Instead, it is partially molten and exhibits a ductile, or plastic-like, behavior. This unique property allows it to flow slowly, similar to how hot tar might.

Because of its flexibility and ability to flow, the asthenosphere enables the lithosphere's tectonic plates to move over it. This movement is fundamental to the processes that lead to phenomena such as earthquakes and the formation of mountains.

Tectonic Plates

Tectonic plates are the massive slabs that form the Earth's lithosphere. These plates are like giant puzzle pieces covering the Earth's surface, and they float on the more pliable asthenosphere below. Though they rest on a softer layer, these plates themselves are rigid and comprised of the crust and the uppermost mantle. The movement and interaction of tectonic plates are responsible for many geological features and events:

• Mountain ranges are formed by the collision of two plates.
• Earthquakes occur due to the sudden movement and shifting of plates.
• Volcanoes are often found where plates are coming apart or colliding.

The boundaries where plates meet are sites of significant geological activity, and understanding these interactions helps explain the dynamic nature of Earth's surface.

Mohorovičić Discontinuity

The Mohorovičić discontinuity, commonly known as the Moho, is a significant boundary located within the Earth's layers. It represents the transition between the Earth's crust and the mantle. This discontinuity is named after the Croatian seismologist Andrija Mohorovičić, who discovered it in 1909 while examining seismic waves from earthquakes. The Moho is characterized by a noticeable change in seismic velocity; it lies at varying depths, ranging from about 5 kilometers beneath oceanic crust to about 70 kilometers under the continental crust.

Thus, the Moho is situated above the lithosphere-asthenosphere boundary. This knowledge is important in helping us understand the composition and structure of our planet's interior.

Mantle

The Earth's mantle is a massive layer situated between the crust and the core, composing a significant part of the planet's volume. It extends to a depth of about 2,900 kilometers beneath the Earth's surface. This layer is primarily made up of silicate rocks that are rich in iron and magnesium. The mantle itself is divided into the upper and lower mantle, with the asthenosphere being a part of the upper mantle. The topmost part of the mantle combines with the crust to form the lithosphere.

• Understanding the mantle is crucial because it impacts tectonic plate movement.
• The mantle's material is capable of slow convective circulation, aiding in the heat transfer from the core to the Earth's surface.
• The study of the mantle assists scientists in predicting certain geological events and understanding Earth's evolution over millions of years.

Despite being solid, the mantle behaves plastically over long timescales, allowing it to deform and flow, which is essential for dynamics within the Earth.