GED Science TestEarth and Space ScienceMedium
A geologist is examining a cross-section of Earth's layers. She observes that the lithosphere is broken into several large, rigid pieces that move over a semi-fluid layer. Which of the following terms correctly identifies this semi-fluid layer?
- AThe outer core.
- BThe mesosphere.
- CThe inner core.
- DThe asthenosphere.
Show answer & explanationAnswer & explanation
Correct answer: D. The asthenosphere.
The asthenosphere is the ductile, semi-fluid layer of the upper mantle upon which the rigid tectonic plates (lithosphere) move. Its plastic-like consistency allows for the movement of plates.
Why the other options are wrong
- A. The outer core is liquid, but it is much deeper than the layer the lithosphere moves upon.
- B. The mesosphere is part of the lower mantle, below the asthenosphere, and is more rigid.
- C. The inner core is solid.
Asthenosphere
A ductile, semi-fluid layer of the upper mantle that lies beneath the lithosphere, allowing tectonic plates to move slowly over it.
- Extends from about 100 km to 700 km depth.
- Characterized by plastic flow, not liquid.
- Crucial for plate tectonics and mantle convection.
Memory trick: Asthenosphere Allows Plates to Amble.