The outer shell of the Earth may be classified from a standpoint of rigidity. An outer 60-mile thick lithosphere, which includes both the crust and the uppermost layer of the mantle, is cool and hence strong.
The Earth has two types of crust. Basaltic oceanic crust, usually about 4-miles thick underlies the ocean basins. In contrast, the continental crust is composed of granite and typically is about 22-miles thick. Beneath plateaus and mountains it may be as much as 40-miles thick. Continents and ocean basins are topographic landforms of the first order of magnitude. In both cases the sharp composition boundary between the crust and the underlying mantle is termed Moho.
ba‡salt n. 1. A hard, dense, dark volcanic rock composed chiefly of plagioclase, pyroxene, and olivine, and often having a glassy appearance. 2. A kind of hard unglazed pottery. [Latin alteration of touchstone, from Greek from basanos, of Egyptian origin.]
touch‡stone n. 1. A hard black stone, such as jasper or basalt, formerly used to test the quality of gold or silver by comparing the streak left on the stone by one of these metals with that of a standard alloy. 2. An excellent quality or example that is used to test the excellence or genuineness of others: a 1~2~3 standard.