ETYM Latin crusta: cf. Old Fren. crouste, French croűte.
The outermost part of the structure of Earth, consisting of two distinct parts, the oceanic crust and the continental crust. The oceanic crust is on average about 10 km/6.2 mi thick and consists mostly of basaltic types of rock. By contrast, the continental crust is largely made of granite and is more complex in its structure. Because of the movements of plate tectonics, the oceanic crust is in no place older than about 200 million years. However, parts of the continental crust are over 3 billion years old.
Beneath a layer of surface sediment, the oceanic crust is made up of a layer of basalt, followed by a layer of gabbro. The composition of the oceanic crust overall shows a high proportion of silicon and magnesium oxides, hence named sima by geologists. The continental crust varies in thickness from about 40 km/25 mi to 70km/45 mi, being deeper beneath mountain ranges. The surface layer consists of many kinds of sedimentary and igneous rocks. Beneath lies a zone of metamorphic rocks built on a thick layer of granodiorite. Silicon and aluminium oxides dominate the composition and the name sial is given to continental crustal material.
1. A hard outer layer that covers something; SYN. incrustation, encrustation.
2. The outer layer of the Earth; SYN. Earth's crust.
3. The trait of being rude and impertinent; inclined to take liberties; SYN. gall, impertinence, impudence, insolence, cheekiness, freshness.

To form a crust or form into a crust.