Discovery Science: Earth – Water – Oceanic Crust

Earth Science: Earth – Water – Oceanic Crust

Oceanic crust covers two-thirds of the Earth’s surface. Crustal rocks are relatively recent, with none older than 200 million years, and they are constantly in motion. This movement expands the oceans and shifts entire continents.

The oceanic crust begins beyond the flooded part of the continental shelf. It originates at the edges of the large continental plates in the area of the mid-oceanic ridges, large mountain systems that are at a maximum 930 miles (1,500 km) wide and more than 37,000 miles (60,000 km) long traversing all the oceans The mountain ridges rise up to almost 10,000 feet (3,000 m) above the ocean floor and occasionally reach the water surface as volcanic islands.

Basaltic lava from the Earth’s mantle constantly rises through central ridges 12.5 to 30 miles (20 to 50 km) wide. New crust material forms from the lava, and the seafloor expands several centimeters a year due to lateral pressure and convection currents of the Earth’s mantle. This process is referred to as seafloor spreading. Oceanic trenches located close to the continental edges are the counterpart to the mid-oceanic ridges.

In these areas, old oceanic crust moves under the continental crust where it is literally swallowed (subduction). The drag of the sinking material causes trenches to develop on the seafloor. These trenches are the deepest places on the Earth’s surface. The Mariana trench in the Pacific Ocean has a depth of 36,200 feet (11,034 m)and is the deepest point on Earth.


Most parts of the ocean floor are on average 12,234 feet (3,729 m) below sea level. They are covered by a continuously growing layer of sediment, usually a few hundred yards thick and in some places even several miles. Part of the sediment comes from the mainland, distributed by rivers, wind, or glaciers. Suspended sediments and volcanic ash are the main components of the deep-sea red clay, which covers more than a quarter of the seafloor.

The majority of ocean deposits, however, are decomposed sea organisms. For example, calcareous globigerina ooze is composed of shells from unicellular planktic foraminifers, while siliceous diatom ooze consists of cell walls from dead diatoms. Radiolarian ooze is made up of the skeletons of dead radiolarians.


The oceans are continuously changing due to the formation of new oceanic crust. The Mediterranean is predicted to shrink, while the Atlantic is growing. The Red Sea will become a new ocean, dividing the African continent.

The Afar Depression, also known as the Afar Triangle, is where the East African Rift meets the ridges of the Red Sea and Gulf of Aden It is also where the African and Arabic tectonic plates drift a part at about 0.4 inch (one cm) per year.