Question

Describe the asthenosphere. Why is it also known as the plastic layer? What are the consequences of its convection currents?

Short answer

The asthenosphere is a highly viscous layer of the Earth's upper mantle, called the 'plastic layer' due to its ability to slowly deform under stress. Convection currents within the asthenosphere are responsible for the movement of the lithospheric plates on the Earth's surface, causing phenomena such as earthquakes and volcanic activity.

Step-by-step solution

Define the Asthenosphere

The asthenosphere is the highly viscous, mechanically weak and deformable layer of the Earth's upper mantle located just below the lithosphere, generally at depths of approximately 80 to 200 km beneath the Earth's surface.

Identify Why It's Called the 'Plastic Layer'

The asthenosphere is often referred to as the 'plastic layer' because it behaves plastically. That means while it is solid, it can flow and deform under the stress imposed by the weight of the overlying lithosphere. Its unique properties allow it to slowly deform over long timescales, which is why it's likened to plastic.

Consequences of Convection Currents in the Asthenosphere

The primary consequence of convection currents in the asthenosphere is the movement of lithospheric plates on the Earth's surface. Heat from the core of the Earth causes material in the asthenosphere to rise. As it nears the top, it cools and sinks back down. This process causes the lithosphere to move, leading to various geological phenomena like earthquakes, volcanic activity, and the creation of mountain ranges.

Key concepts

Plastic Layer

The asthenosphere is a remarkable layer situated beneath the Earth's lithosphere, often described as the 'plastic layer'. This nomenclature stems from its unique ability to behave plastically, meaning it can flow and deform like clay or soft plastic under immense pressure. Although it is solid, the asthenosphere can move very slowly due to its high temperature and viscosity. This ability allows it to stretch and support the movement of the lithosphere above it. Because of this plastic behavior, tectonic plates resting on this layer can gradually shift, accounting for various geological events over millions of years.

Convection Currents

Convection currents are the circulatory motion within the asthenosphere driven by thermal differences. This process resembles a fluid cycle where hot material, heated by the Earth's core, becomes less dense and rises towards the surface. Upon reaching the cooler top, it loses heat and becomes more dense, causing it to sink back down. These movements create a convection cycle crucial for the dynamics of our planet. As these currents move the material within the asthenosphere, they exert lateral pressure on the lithosphere, leading to its gradual shift. This dynamic is a fundamental driver of plate tectonics and contributes to the formation of earthquakes, volcanic eruptions, and mountain ranges.

Lithosphere

The lithosphere is the rigid outer layer of the Earth, comprising the crust and the uppermost part of the mantle. Unlike the ductile asthenosphere below, the lithosphere is brittle, breaking rather than bending. It is fragmented into tectonic plates of varying sizes which float and move atop the plastic-like layer of the asthenosphere. These movements are responsible for many geological features and activities on Earth, such as the formation of continents, ocean basins, and the occurrence of seismic activities. The lithosphere's interaction with the underlying asthenosphere is essential for understanding the surface geology of our planet.

Plate Tectonics

Plate tectonics is the theory explaining the movement of the Earth's lithospheric plates. This concept is vital for understanding the distribution of geological phenomena. As the asthenosphere's convection currents shift beneath them, the rigid lithospheric plates drift, collide, or slide past each other. These interactions cause the development of various geological features:

• Mountain ranges form where plates crash together.
• Volcanoes arise at both divergent and convergent boundaries.
• Earthquakes occur where plates slide past one another.

This theory unifies a multitude of geological phenomena, explaining the dynamic nature of earthquakes, volcanic activity, and the creation of new crust. Understanding plate tectonics helps predict these natural events, which is crucial for mitigating their impacts on human life and structures.