In the structure of the earth, what is the name given to the weak, semi molten upper part of the mantle on which the rigid crust and lithospheric plates float?

Introduction / Context:
Understanding the internal structure of the earth is important for explaining plate tectonics, earthquakes, and volcanic activity. The earth interior is divided into layers based on composition and physical properties. This question focuses on the partially molten, mechanically weak layer beneath the lithosphere that allows tectonic plates to move. Knowing its correct name is a standard requirement in physical geography.

Given Data / Assumptions:

• The question refers to the upper part of the mantle on which the crust or lithosphere floats.
• Options provided are barysphere, mesosphere, Mohorovicic discontinuity, and asthenosphere.
• We assume the typical geophysical definitions of these terms.

Concept / Approach:
The lithosphere, consisting of the crust and uppermost mantle, is rigid and broken into plates. Beneath it lies a mechanically weak, plastic layer in the upper mantle known as the asthenosphere. The asthenosphere is partially molten or close to its melting point, which allows the rigid plates above to move slowly over it. The term barysphere is sometimes used for the core, mesosphere can refer to the lower mantle, and the Mohorovicic discontinuity is the boundary between the crust and the mantle. None of these describe the soft upper mantle layer that supports plate motion.

Step-by-Step Solution:
Step 1: Identify that the crust and rigid uppermost mantle together form the lithosphere.Step 2: Recall that the lithosphere floats on a weaker, more plastic layer called the asthenosphere.Step 3: Recognise that this layer is part of the upper mantle and allows for plate movement.Step 4: Review the meanings of other options: barysphere for the core, mesosphere for deeper mantle, and Mohorovicic discontinuity as a boundary.Step 5: Conclude that asthenosphere is the only option that matches the description given in the question.

Verification / Alternative check:
Diagrams of earth interior in school textbooks often show a thin crust, a thicker upper mantle, and a lower mantle, along with labels for lithosphere and asthenosphere. The asthenosphere is always depicted just below the lithosphere and is shown as a softer, partially molten region. Text explanations explicitly state that tectonic plates glide over this asthenosphere. This visual and textual evidence confirms that asthenosphere is the correct term.

Why Other Options Are Wrong:
Barysphere: Used for the dense core of the earth, not the upper mantle layer under the crust.Mesosphere: In geophysics, sometimes used for the lower mantle, which is deeper than the layer described here.Mohorovicic discontinuity: This is simply the boundary between the crust and mantle, not a layer on which plates float.

Common Pitfalls:
Students may confuse the names because they all sound technical and similar. Some also confuse the atmospheric mesosphere with the geophysical mesosphere. To avoid such mistakes, link each term with a simple image: the lithosphere as rigid plates, asthenosphere as the soft underlying layer, Mohorovicic discontinuity as a sharp boundary, and barysphere as the central core. Repeating these associations during revision makes it much easier to answer exam questions correctly.

Final Answer:
The weak, semi molten upper mantle layer on which the crust and plates float is called the asthenosphere.