Final answer:
Oceanic crust is made of basalt and gabbro and becomes more dense as it ages, leading to subduction where it's recycled into the mantle. The continental crust, mainly composed of granite, is less dense than oceanic crust, causing it to float higher and form landmasses. Subduction helps to balance the creation and destruction of Earth's crust.
Step-by-step explanation:
The oceanic crust covers about 55% of the Earth's surface and is primarily submerged under the oceans. Composed of mafic magma, it erupts on the seafloor to form basalt lava flows or cools as the intrusive igneous rock gabbro.
Oceanic lithosphere, the rigid outermost shell of a rocky planet, is more dense than the underlying asthenosphere due to its cooler temperature and the composition of the rocks.
As the oceanic lithosphere ages, it becomes even more dense and can be subducted into the mantle at subduction zones, creating an ocean trench and consuming the ocean basin as it is recycled into the mantle.
Continental crust, on the other hand, consists of a variety of rock types with an average composition similar to granite and is generally less dense than that of the oceanic crust.
Because the continental crust has relatively low density and is thicker, it floats higher on the mantle in comparison to the oceanic crust, which, being denser, sinks to form basins that, when filled with water, become our oceans.
The process of subduction, where one tectonic plate moves under another and sinks into the mantle due to gravity, is a key part of the plate tectonics theory.
This dynamic process contributes to the balance between the creation of new crust at rift zones (such as mid-ocean ridges) and the destruction of old crust at subduction zones. The oceanic crust is continuously rejuvenated through this cycle and plays a critical role in the geological and topographical features on Earth.