Structure of the Earth easily than does the upper mantle (chemical changes within the mantle may also be important). The viscosity of the mantle ranges between 10 and 10 Pa·s, depending on depth. In comparison, the viscosity of water is approximately 10 Pa·s and that of pitch is 10 Pa·s. The source of heat that drives plate tectonics is the primordial heat left over from the planet’s formation as well as the radioactive decay of uranium, thorium, and potassium in Earth’s crust and mantle. The average density of Earth is . Because the average density of surface material is only around , we must
What Shall We Do Now? the form of a fist that becomes a hammer (a hammer that would reappear in the animated sequence of "Waiting for the Worms"). "What Shall We Do Now?" would be performed at all concerts for "The Wall", and so appears on "Is There Anybody Out There? The Wall Live 1980–81". It was also included in Waters' 1990 "The Wall" Concert in Berlin, therefore appears on the album "The Wall – Live in Berlin". Waters also performed it on his 2010-2013 tour. What Shall We Do Now? "What Shall We Do Now?" (working title "Backs to the Wall") is a song