Asthenosphere

The asthenosphere (from the Greek asthenos "without force", and sphere) is the most viscous part of the earth's mantle.

For a long time, geophysicists included the asthenosphere in the upper mantle (to a depth of 670 km), the lower mantle being named the mesosphere (from the Greek mesos "middle"); but some researchers now tend to restrict this upper layer to the zone of the mantle where speeds are lowest, where it is the most plastic.

The asthenosphere is the ductile part of the mantle over which the (rigid) plates move.