Question

What properties of the asthenosphere allow the lithospheric plates to glide over it?

Short answer

The partially melted, semi-fluid nature and low viscosity of the asthenosphere allow lithospheric plates to glide over it.

Step-by-step solution

Understanding the Asthenosphere

The asthenosphere is a layer of the Earth's mantle located below the lithosphere. It extends approximately 100-300 km below the Earth's surface. The asthenosphere is semi-fluid, which means it has the ability to flow slowly.

Compositional Properties

The asthenosphere is primarily composed of hot, partially melted rock known as peridotite. This partially melted composition allows it to behave plastically. Despite being solid, this plasticity allows the asthenosphere to deform and flow over geological time scales.

Temperature and Pressure

The high temperatures and pressures found in the asthenosphere result in its rocks being very close to their melting point. This generates a ductile, or semi-fluid behavior that can support the movement of the lithospheric plates above it.

Viscosity

The viscosity of the asthenosphere is significantly lower than that of the lithosphere. This low viscosity enables the lithospheric plates to glide or move over it easily as it can slowly flow and adjust to accommodate the movement of the overlying plates.

Key concepts

Lithosphere

The lithosphere is the rigid outermost shell of the Earth, encompassing both the crust and the very upper part of the mantle. This layer is crucial because it comprises tectonic plates—large pieces that fit together over the softer asthenosphere. The lithosphere is typically about 100 km thick, but its thickness can vary depending on factors such as location and underlying structures.
Lithospheric plates - These plates can be oceanic or continental. - Oceanic plates are denser and thinner, whereas continental plates are thicker and less dense. - The motion and interaction of these plates are responsible for many geological processes, like earthquakes and volcanic activity. Understanding the lithosphere's characteristics is essential to comprehending how Earth's outer shell interacts with the more plastic asthenosphere below.

Plate Tectonics

Plate tectonics is a scientific theory explaining the movement of Earth's lithospheric plates on the more fluid asthenosphere. This dynamic process is responsible for shaping the Earth's surface over millions of years, leading to the creation of mountains, oceans, and volcanic activity.
Key processes - Divergent boundaries where plates move apart, allowing magma to rise and create new crust. - Convergent boundaries where plates collide, leading to mountain building or subduction (one plate moving under another). - Transform boundaries where plates slide past one another, causing earthquakes. These movements occur because plates float on the semi-fluid asthenosphere, their varying densities and thermal interactions determine their specific paths.

Mantle

The mantle is the vast layer of the Earth lying between the crust and the core, extending to a depth of about 2,900 km. It comprises about 84% of Earth's volume and functions as a thermostat for plate tectonics, with convection currents driving the motion of the tectonic plates above.
Composition and behavior - Primarily made of silicate rocks rich in magnesium and iron. - Although mostly solid, it behaves like an extremely viscous fluid over long time scales. - Heat from the core causes the mantle to exhibit convection currents, which power plate motions. The mantle's properties make it integral to understanding the Earth’s dynamics, as changes deep within can have profound surface impacts.

Viscosity

Viscosity is a measure of a fluid's resistance to flow. In the context of geology, it significantly impacts the movement of tectonic plates. The asthenosphere, unlike the rigid lithosphere, has a much lower viscosity, enabling it to flow slowly. This characteristic allows the lithospheric plates to move over it with relative ease.
Understanding viscosity - Low viscosity means the asthenosphere deforms under pressure, behaving like a thick but flowing liquid. - High temperatures in the asthenosphere lower its viscosity, making it more ductile. - The difference in viscosity between the lithosphere and asthenosphere is crucial for the movement of plates. Viscosity is key to Earth's geodynamics, giving the rigidity of plates contrast against the plasticity of the underlying asthenosphere.