The weak, partly molten upper portion of the mantle, lying just below the lithosphere, is called the:

The asthenosphere is the upper part of the mantle, lying just below the rigid lithosphere, roughly from about 100 km to 400 km below the surface. It is hot and weak, and its rocks can flow very slowly, like soft plastic.

Meaning of the word:

"Asthenosphere" comes from the Greek "asthenes", meaning weak.

Features of the asthenosphere:

• It is mostly solid, but a small fraction (perhaps 1 to 5 percent) of it is molten, which makes it soft and able to deform.
• Seismic waves slow down when they pass through it, so it is also called the low-velocity zone.
• It is the main source of magma that reaches the surface during volcanic eruptions.

Relationship with the lithosphere:

The lithosphere is the rigid outer shell of the Earth, made up of the crust and the uppermost, solid part of the mantle. It is broken into tectonic plates.

The lithospheric plates "float" and move slowly over the softer asthenosphere.

Convection currents in the mantle and other forces help move the plates over this layer.

Isostasy:

• The ability of the asthenosphere to flow allows the lithosphere to rise or sink until it is in balance, a concept called isostasy.
• For example, when ice sheets melt, the land beneath slowly rises (rebounds), as seen in Scandinavia and Canada since the last Ice Age.

Layers of the Earth (mechanical division):

• Lithosphere: rigid; crust and uppermost mantle, up to about 100 km thick.
• Asthenosphere: weak, plastic; upper mantle, to about 400 km.
• Mesosphere (lower mantle): solid and strong because of high pressure.
• Outer core: liquid.
• Inner core: solid.

Discovery:

The idea of a weak layer below the lithosphere was proposed by the American geologist Joseph Barrell in 1914, who coined the term.

Why not the other options?

• ✗The mesosphere, in geology, is the strong lower mantle; in meteorology, it is a layer of the atmosphere.
• ✗The barysphere is an older term for the dense interior of the Earth, especially the core.
• ✗The hydrosphere is all the water on the Earth.