1. The thin outermost solid layer of the Earth. It represents less than 1% of the Earth’s volume, and varies in thickness from approximately 5 km beneath the oceans to approximately 60 km beneath mountain chains. Most of the terrestrial planets have a solid surface, generally considered to be of different composition to the underlying, higher-density rocks, and regarded as crust. See also crustal abundance of elements; continental crust; oceanic crust.

2. A surface soil layer, sometimes slightly cemented with calcium carbonate, silica, or iron oxide, which may be from a few millimetres to many tens of millimetres in thickness but which is always harder and more compact than the soil below. Crusts are now being produced by mechanical action or pedogenesis, mainly in arid environments; less commonly they are relict or fossil features exhumed on the soil surface.