Here's why it's considered "soft":

* Partial melting: The asthenosphere is partially molten, meaning that some of the rock is in a liquid state. This makes the asthenosphere more ductile and able to flow.

* High pressure: While the asthenosphere is "soft" compared to the lithosphere above it, it's still under immense pressure from the weight of the Earth's crust and the rest of the mantle.

* Temperature: The asthenosphere is hot, which contributes to the partial melting and its plasticity.

Key features of the asthenosphere:

* Source of magma: Volcanic eruptions are often fueled by magma that originates from the asthenosphere.

* Plate tectonics: The asthenosphere is the layer on which the Earth's tectonic plates "float" and move.

* Convection currents: Convection currents within the asthenosphere drive plate movement and are a major force shaping the Earth's surface.

Let me know if you'd like more information about the asthenosphere or plate tectonics!