Final answer:
The oceanic crust beneath a layer of sediments is mainly composed of two types of igneous rocks: basalt and gabbro. Basalts form from lava cooling on the ocean floor, while gabbros are coarser and form from slower cooling magma beneath the seafloor.
Step-by-step explanation:
Beneath a blanket of sediments, oceanic crust is primarily composed of two rocks, basalt and gabbro. Both basalts and gabbros are types of igneous rocks, which are rocks formed from cooled magma or lava. Basalt is a volcanic rock that forms from the cooling of lava on the ocean floor, resulting in a fine-grained rock rich in mafic minerals like pyroxene and plagioclase feldspar.
These minerals are characterized by high amounts of silicon, oxygen, iron, aluminum, and magnesium. On the other hand, gabbro is an intrusive igneous rock that cools slowly beneath the ocean floor, creating a coarser-grained rock when compared to basalt, though both share similar chemical compositions.
The oceanic crust is constructed through the process of plate tectonics where magma erupts along mid-ocean ridges and forms new oceanic crust as the plates spread apart. As this crust moves away from the ridge, it cools and densifies, sinking slightly to form the deep ocean basins.
Besides basalt and gabbro, the oceanic crust also collects thin layers of sediments over time, including deep marine clays and chert. However, the bulk of the oceanic crust below these sediments is composed primarily of basalt and gabbro, which can be exposed on land in ophiolite complexes particularly in mountain ranges formed by tectonic plate collision events.