Here's why:
* Oceanic crust is thinner and denser than continental crust. This means it sits lower in the Earth's mantle.
* The weight of water presses down on the oceanic crust, contributing to its submerged state.
* However, tectonic forces can uplift sections of the ocean floor. This is how we get mid-ocean ridges, which are underwater mountain ranges formed by the spreading of tectonic plates. In some cases, these ridges can rise above sea level, forming islands.
* Volcanic activity also plays a role. Underwater volcanoes can erupt and create islands, which are technically part of the oceanic crust but sit above sea level.
So, while most oceanic crust is below sea level, some portions are above due to tectonic uplift and volcanic activity.
