Which type of Earth's crust is primarily composed of basalt and is found under the ocean?

Oceanic crust is primarily composed of basalt, a dense volcanic rock that forms through the cooling and solidification of lava. This type of crust is found beneath the ocean and is generally thinner and denser than continental crust.

The formation of oceanic crust occurs at mid-ocean ridges, where tectonic plates are diverging, leading to volcanic activity. As magma from the mantle rises to fill the gap created by diverging plates, it cools quickly upon contact with seawater, solidifying into basalt.

In contrast, continental crust is mainly composed of lighter materials such as granite. The lithosphere and asthenosphere refer to different layers of the Earth's structure rather than specific types of crust. The lithosphere includes both oceanic and continental crust along with the uppermost part of the mantle, while the asthenosphere is the semi-molten layer of the mantle beneath the lithosphere that allows for tectonic plate movement.

Thus, the correct identification of the oceanic crust underscores its composition and location as integral characteristics that distinguish it from other geological features.