The Onion Earth

I knew the internal structure of the Earth and its layers- crust, mantle, outer core, and inner core since I was 9 years old. I also knew about plate tectonics and how continents change over time. But few days ago when I was thinking about the plate movements and volcanoes I came across a contradiction... If the crust lies on top of the mantle and if the upper part of the mantle is solid, how do the plates move about? where does the lava come from?
After reading about it I saw that I had not known the existence of a layer within the mantle until then..-The "Asthenosphere". Upon further reading I found that there are so many layers of the earth in addition to the above mentioned primary layers.. like an Onion.
So the Earths tectonic plates comprise of the crust and the uppermost, solid part of the mantle. This is called the "Lithosphere" below the lithosphere lies the asthenosphere which is also part of the mantle. The asthenosphere is of higher temperature and partly molten. It on this layer that the lithospheric plates lie and slowly move giving rise to the continental drift. Beneath the mantle lies the usual liquid outer core and the inner core.

Image taken from Wikipedia

 The onion like structure of the earth even extends beyond the surface of the earth into the atmosphere. The layers of the atmosphere starting from the bottommost layer are, Troposphere, Stratosphere, Mesosphere, Thermosphere and Exosphere.