Crust
Crust
Earth's mass is divided into an inner core, outer core, mantle, and crust. The crust is outermost layer of the earth, 3–44 miles (5–70 km) thick and representing less than 1% of the earth's total volume. Thin compared Earth's diameter, the outermost crustal layer is further subdivided into two basic types of crust—each unique in composition, origin and fate. Although the earth is dynamic, with new crust constantly being created and destroyed, the fact that size of the earth remains constant argues that there is no net creation or destruction of force and that these two processes are in equilibrium.
Although there are thousands of minerals , about 40 minerals represent more than 99% of the mass of Earth's crust. In terms of percentage of Earth's crust by weight, oxygen and silicon account for nearly 75% of Earth's crust. Oxygen is the most abundant element (approximately 46.5% followed by silicon, approximately 28%). In order of percentage by weight, other important elements include aluminum , iron , calcium, sodium, potassium, and magnesium. All other elements (e.g., gold, silver, copper, etc.) compose the remaining one to two percent of the crust.
Crust is classified as oceanic crust or continental crust. Oceanic crust is thin (3–4.3 mi [5–7 km]), basaltic (<50% SiO2) and dense. Compositional chemical studies also establish that oceanic crust is substantially younger than the continental crust. No rock specimen dated to more than 250 million years old have ever been identified in oceanic crust.
Continental crust is thick (18.6–40 mi [30–65 km]), granitic (>60% SiO2), light, and old (250–3,700 million years old).
The outer crust is further subdivided into lithospheric plates , that contain varying sections of oceanic and continental crust.
At the deepest crustal border there exists a compositional change from crust material to mantle pyriditite called the Mohorovicic discontinuity , and the lithospheric plates carrying both oceanic and continental crust move on top of mantle's asthenosphere .
See also Dating methods; Earth, interior structure; Geologic time; Hawaiian Island formation; Isostasy; Lithospheric plates; Mid-ocean ridges and rifts; Mohorovicic discontinuity (Moho); Ocean trenches; Plate tectonics; Rifting and rift valleys; Soil and soil horizons; Subduction zone
crust
crust / krəst/ • n. the tough outer part of a loaf of bread. ∎ a hard, dry scrap of bread: a kindly old woman might give her a crust. ∎ a slice of bread from the end of the loaf. ∎ a layer of pastry covering a pie. ∎ a hardened layer, coating, or deposit on the surface of something, esp. something soft: a crust of snow. ∎ the outermost layer of rock of which a planet consists, esp. the part of the earth above the mantle. ∎ a deposit of tartrates and other substances formed in wine aged in the bottle, esp. port.• v. [intr.] form into a hard outer layer: the blisters eventually crust over. ∎ [tr.] cover with a hard outer layer.DERIVATIVES: crus·tal / ˈkrəstəl/ adj. (in the geological sense).
crust
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; and 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.