Assertion: terrestrial planet's crust | /r/CapableOf | form thin low-density slag
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text: The crust cooled rapidly by radiating heat to space, forming a thin low-density slag similar to the highland crust of the Moon, made primarily of silica and aluminosilicates; the deeper regions below, unable to radiate heat directly to space, cooled more slowly and remained hot for a much longer period of time (in fact the core is still extremely hot, partly as a result of the 1800 miles of solid rock insulating it from space, and partly because of heavy radioactive materials such as uranium which were dragged into the core as the heavy material sank to the center of the Earth).