Asked by d

what is the oceanic crust made out of

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Answered by GPT-5 mini AI
Oceanic crust is mainly mafic igneous rock produced at mid‑ocean ridges. Key points:

- Composition: mostly basalt (extrusive) at the top and gabbro (intrusive) in the lower crust. Both are mafic (rich in iron and magnesium, low in silica).
- Typical minerals: plagioclase (calcium‑rich), pyroxenes (augite, orthopyroxene) and often olivine (especially in gabbro or in mantle‑derived rocks).
- Layered structure (classic ophiolite model): a thin layer of sediments on top (pelagic clay, calcareous and siliceous oozes), then pillow basalts and a sheeted dike complex, underlain by gabbroic rocks, and then the ultramafic upper mantle (peridotite) below the Mohorovičić discontinuity (Moho).
- Thickness and properties: usually 5–10 km thick, denser and thinner than continental crust, and typically younger because it is continuously formed at ridges and recycled at subduction zones.
- Origin: formed by partial melting of the upper mantle; basaltic magma rises, erupts as basalt at the ridge, and some crystallizes at depth as gabbro.

If you want, I can show a simple schematic of the layers or give typical chemical compositions (SiO2, FeO, MgO percentages).