Oceanic crust forms mafic magma.

Oceanic crust forms mafic magma.

• A. felsic
• B. mafic
• C. intermediate
• D. ultramafic

Answer: (B)

The main idea here is that oceanic crust is built from basaltic magma, which is mafic. Mafic magma is richer in magnesium and iron and has lower silica content than felsic magma. At mid-ocean ridges, partial melting of mantle rocks (peridotite) produces basaltic magma. When this magma erupts at the seafloor it forms basalt, and it can crystallize deeper in the crust as gabbro. Felsic magma, with high silica and light-colored minerals like quartz and feldspar, is typical of continental crust and is not what mainly forms oceanic crust. Intermediate magmas (andesite/dacite) are commonly associated with subduction zones, not spreading centers. Ultramafic magmas are even richer in Mg and Fe and are not the usual product of oceanic crust formation. So the statement is correct because oceanic crust forms from mafic, basaltic magma derived from the mantle.

The main idea here is that oceanic crust is built from basaltic magma, which is mafic. Mafic magma is richer in magnesium and iron and has lower silica content than felsic magma. At mid-ocean ridges, partial melting of mantle rocks (peridotite) produces basaltic magma. When this magma erupts at the seafloor it forms basalt, and it can crystallize deeper in the crust as gabbro. Felsic magma, with high silica and light-colored minerals like quartz and feldspar, is typical of continental crust and is not what mainly forms oceanic crust. Intermediate magmas (andesite/dacite) are commonly associated with subduction zones, not spreading centers. Ultramafic magmas are even richer in Mg and Fe and are not the usual product of oceanic crust formation. So the statement is correct because oceanic crust forms from mafic, basaltic magma derived from the mantle.