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
How the Snake Lost Its Legs the butterfly, the snake, and the cheetah. Each consists of three to eight sections named in the style of Rudyard Kipling's "Just So Stories", with titles like "How the butterfly got its spots" or "How the snake elongated its body", though a couple of sections use "Why?" rather than "How?", as in "Why the fly twirls his penis" (it rotates during embryonic development under the control of different genes). In these chapters, Held explains the mechanics of evolutionary developmental biology, complete with accounts of what genes such as hox, hedgehog, and engrailed do to shape bodies. The third part is