Which crust is thin and dense and lies beneath ocean waters?

• A. Oceanic crust
• B. Continental crust
• C. Mantle
• D. Core

Answer: (A)

The main idea is that the Earth’s crust comes in two types with different thicknesses and densities, and the ocean is carried by the thinner, denser type. Oceanic crust sits under the oceans and is thin—about 5 to 7 kilometers thick—and relatively dense, roughly 3.0 g/cm^3. It’s made mostly of basalt, formed from magma that rises at mid-ocean ridges and then cools to solid rock. Because it’s denser, this crust forms the ocean floor and can be subducted under lighter crust at convergent boundaries.

Continental crust, by contrast, is much thicker—tens of kilometers to up to around 70 kilometers in places—and less dense, made mainly of granitic rocks. That combination of greater thickness and lower density allows continents to "sit higher" on the mantle, while the ocean basins lie lower because of the denser oceanic crust.

The mantle lies beneath the crust, and the core is much deeper inside Earth, so neither the mantle nor the core is the crust beneath ocean waters. The ocean floor is formed by oceanic crust, which is why the correct answer describes that specific layer.