Continental crust thinner, under mountain ranges you find thickest crust. Earth"s crust: continental crust crust: ~ 35 km thick relatively low density (lighter 2. 65 g/cm3) Oceanic crust crust: ~10 km thick higher density (heavier 3. 2 g/cm3) Relatively rigid: asthenosphere (upper mantle) around 200 km thick. Lithosphere is thin, cool and hard: asthenosphere is weak and hot. Lithosphere broken into large fragments called plates: plates float on the asthenosphere, plates move around and interact with each other. Isostasy plates float at an elevation depending on thickness and density. What happens at the edges: plates boundaries can be: Alfred wegner"s observations (~1915: actually a meteorologist not a geologist. He observed: fit of the continents- all the continents could be joined with few overlaps and gaps. America, sothern europe, and northwestern africa would have straddled the equator and would have had tropical and subtropical climates: matching glaciations- glaciers= rivers or sheets of ice that flow across the land surface.