Most of the oceanic crust is made of the igneous rock basalt , with deeper parts made largely of gabbro; overall it is described as mafic rock rich in iron and magnesium.

Quick Scoop

  • The upper oceanic crust is mainly fineā€‘grained, dark volcanic basalt that forms when lava cools quickly on the seafloor.
  • Beneath that, the lower oceanic crust is mostly coarseā€‘grained gabbro that crystallizes more slowly at depth.
  • Because these rocks are mafic (rich in magnesium and iron), oceanic crust is denser than continental crust.

So if you need one simple answer to ā€œwhat type of rock makes up the oceanic crust,ā€ the goā€‘to word is basalt (with gabbro underneath). Information gathered from public forums or data available on the internet and portrayed here.