Here's why:
* The asthenosphere is located below the lithosphere, the rigid outer layer of the Earth. It starts about 100 kilometers (62 miles) below the surface and extends down to about 700 kilometers (435 miles).
* The asthenosphere is characterized by its semi-molten state. This means it's not entirely liquid but has a high viscosity, allowing it to flow slowly over geological timescales.
* This flow is driven by convection currents. Heat from the Earth's core rises through the asthenosphere, causing it to expand and become less dense. This hot material then rises, while cooler material sinks, creating a circular flow pattern.
* The movement of the asthenosphere is what drives plate tectonics. The lithosphere, which is broken into tectonic plates, floats on top of the asthenosphere and is dragged along by its movement.
So, while the asthenosphere isn't actually liquid, it behaves like a thick liquid due to its high viscosity and the movement of convection currents within it.
