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Why are ocean basins lower than continents?

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Map graphic showing the boundaries of the five oceans-Pacific, Atlantic, Indian, Arctic, and Southern-using color-coded regions. Four different map projections are included: North Pole view, South Pole view, International Date Line view, and Greenwich Meridian view, each displaying the oceans' extents around the continents. — ocean Earth's oceans and their marginal seas cover nearly 71 percent of the planet's surface.

Ocean basins are lower than continents primarily due to differences in the density and thickness of the Earth's crustal layers, a concept governed by isostasy.123

The continental crust is composed of lighter, granitic rock with a density of approximately 2.7 grams per cubic centimeter and is typically about 40 km (25 miles) thick. In contrast, the oceanic crust is made of denser, basaltic rock—richer in iron and magnesium—with a density of about 2.9 to 3 grams per cubic centimeter, and it is much thinner, averaging only about 6 km (4 miles) in thickness.2

Because the less-dense continental crust has greater buoyancy, it floats higher in the mantle, whereas the denser, thinner oceanic crust sits at a lower elevation. This physical relationship is often compared to a thick piece of styrofoam and a thin piece of wood floating in water; the thicker, less-dense material naturally rises higher above the surface than the thinner, denser material.12