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Oceanic crust is composed largely of dark colored, basic (45–52% SiO₂) rocks (Chapter 7) enriched in oxides of magnesium, iron, and calcium (MgO, FeO, and CaO) relative to average crust. The elevated iron (Fe) content is responsible for the both the dark color and elevated density of oceanic crust. Oceanic crust is thin; the depth to the Moho averages 5–7 km. Under some oceanic islands, its thickness reaches 18 km. The elevated density and small thickness of oceanic crust cause it to be less buoyant than continental crust, so that it occupies areas of lower elevation on Earth's surface. As a result, most oceanic crust of normal thickness is below sea level and covered by sea water to a depth of several thousand meters. Oceanic crust consists principally of basic igneous rocks such as basalt and gabbro composed largely of the minerals pyroxene and calcic plagioclase. These dark‐colored, mafic igneous rocks comprise layers 2 and 3 of oceanic crust and are commonly topped with sediments that comprise layer 1 (Table 1.1). An idealized profile of typical ocean crust consists of these three main layers, each of which can be subdivided into sublayers which are briefly discussed later in this chapter.

Oceanic crust is young relative to the age of the Earth (~4.55 Ga = 4550 Ma). The oldest ocean crust in the major ocean basins, less than 190 million years old (190 Ma), occurs along the western and eastern borders of the Atlantic Ocean and in the Western Pacific Ocean. Recently, still older oceanic crust that may be 340 Ma has been discovered in the eastern Mediterranean Sea (Granot 2016). Still older oceanic crust has largely been destroyed by subduction, but fragments of such crust are preserved on land in the form of ophiolites. Ophiolites contain slices of ocean crust thrust onto continental margins and provide evidence for the existence of Precambrian oceanic crust. The age of the oldest true ophiolites of Precambrian age remains controversial (Chapter 18).

Table 1.1 Characteristics of oceanic and continental crust: a comparison.

Properties	Oceanic crust	Continental crust
Composition	Dark colored, mafic rocks enriched in MgO, FeO, and CaO	Complex; many lighter colored felsic rocks
		Enriched in K2O, Na2O, and SiO2
	Averages ~50% SiO2	Averages ~60% SiO2
Density	Higher; less buoyant	Lower; more buoyant
	Average 2.9–3.1 g/cm3	Average 2.6–2.9 g/cm3
Thickness	Thinner; average 5–7 km thickness	Thicker; average 30 km thickness
	Up to 15 km under islands	Up to 80 km under mountains
Elevation	Low surface elevation; mostly submerged below sea level	Higher surface elevations; mostly emergent above sea level
Age	Up to 190 Ma for in‐place crust	Up to more than 4000 Ma
	~3.5% of Earth history	85–90% of Earth history