<span><span><span>The crust: low density silicate rock, 5-70 km thick. There are two distinct types of crust.<span><span>Continental crust is variable in thickness and composition. Thickness ranges from 5-70 km. The composition ranges from mafic to felsic.</span><span>Oceanic crust is uniform in thickness and composition. It is 5-6 km thick and is mafic in composition.</span><span>The differences in thickness and density between continental and oceanic are responsible for the existence of ocean basins due to isostatic balance as the crust floats on the more dense mantle.</span></span></span><span>The mantle: high density, ultramafic silicate rock which can flow when subjected to long duration stresses. The mantle is over 2900 km thick and makes up over 80% of the volume of the Earth. The mantle is not molten!</span></span><span>The core: iron and nickel, liquid outer region with a solid center. The core is just over half the diameter of the Earth.</span></span>
