Most of the ocean floor is made of the igneous rock basalt.

Quick Scoop

  • The oceanic crust is dominated by dark, dense, mafic rock called basalt.
  • This basalt forms mainly at mid‑ocean ridges, where magma rises and cools to create new seafloor.
  • Beneath the surface lavas, related igneous rocks like gabbro also occur, but the main rock making up most of the ocean floor is basalt.

So if you see a multiple‑choice question asking “What type of igneous rock makes up most of the ocean floor?”, the correct answer is: Basalt.

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