Which of the following best describes the asthenosphere?
a. The asthenosphere is the liquid outer core of Earth.
b. The asthenosphere is a solid but is hot and weak and therefore can flow over long timescales.
c. The asthenosphere is a liquid layer in the mantle.
d. The asthenosphere is just another name for tectonic plates.
The Correct Answer and Explanation is :
Correct Answer: b. The asthenosphere is a solid but is hot and weak and therefore can flow over long timescales.
Explanation:
The asthenosphere is a layer of Earth’s mantle located below the lithosphere (the rigid outer shell of the Earth that includes the crust and upper mantle). While the asthenosphere is solid, it exhibits plasticity, meaning it can deform and flow over long geological timescales due to high temperature and pressure. This characteristic makes it distinct from other parts of Earth’s structure.
Key Features of the Asthenosphere:
- Composition and Physical State:
The asthenosphere is composed mainly of peridotite, a type of rock that remains solid but behaves like a very viscous fluid over time due to the intense heat and pressure it experiences. - Role in Plate Tectonics:
The asthenosphere plays a critical role in plate tectonics as it acts as a ductile layer on which the tectonic plates (lithosphere) move. Its flow accommodates the movement of these plates, driving processes like continental drift, seafloor spreading, and subduction. - Heat and Weakness:
The temperatures in the asthenosphere are high enough (around 1,300°C or 2,372°F) to allow rocks to flow without melting. This weakness makes it an essential layer for Earth’s dynamic behavior. - Depth and Thickness:
The asthenosphere generally lies between 100 km and 200 km (60-120 miles) below Earth’s surface, though its thickness can vary depending on location.
Why Other Options Are Incorrect:
- a. The liquid outer core lies beneath the mantle and is not part of the asthenosphere.
- c. The asthenosphere is not a liquid but a solid with flow capabilities.
- d. Tectonic plates refer to the lithosphere, not the asthenosphere.
Thus, the asthenosphere’s unique properties as a solid yet ductile layer enable it to support plate tectonics and Earth’s geological evolution.