Asthenosphere
The asthenosphere is softer than the central most layers of the mantle. The Asthenosphere is “estimated as being from fifty to several hundred miles thick” (Asthenosphere, n.d.). Heat from the Earth’s core layers keeps the asthenosphere in a liquid like state which results in “…lubricating the undersides of Earth’s tectonic plates and allowing them to move” (Asthenosphere, 2015, para. 2). Due to the asthenosphere being in a softer solid state the layers above the asthenosphere are able to move slowly and this brings about tectonic activity. The movement of the lithosphere and the asthenosphere together is where Earth gets geologic events like volcanoes, ocean trenches and earthquakes all from the movement of tectonic plates (National Geographic, 2015c).
Learn more about the asthenosphere here.