The Geology of Ocean Basins - Common Features
Prominent Features of Ocean Basins
The geology of ocean basins is better clarified by studying the different features that compose them. The ocean basins present the following topographic features:
1. Continental Shelf. In general, continental shelves are covered with a layer of sand, silts, and silty muds. About 8% of the area covered by oceans represents continental shelves. The inclination of the shelf is usually very slight with an average slope of about 0.1 degree. A continental shelf has an extension of about 65 Km in average. Its surface is covered with different kinds of sediments, what is determined by the tectonic setting, the quantity of sediments that is carried in by the rivers that empty into it, and the wave energy that affects it. Sand is mainly constituted by quartz, though feldspar, calcareous material, and volcanic glass may be important constituents of sand in some localities. In general, sands also contain small quantities of other minerals, such as garnet, tourmaline, zircon, rutile, and topaz. Coarse and medium sand and gravel are deposited on the beach, since the shoreline is affected by breaking waves with energy enough to remove all the fine sediment.
2. Continental Slope. A continental slope is the border of the continental shelf on the sea side, where the predominant sediments are muds with smaller amounts of sand or gravel. It is indented by submarine canyons and mounds. Some of these canyons have been carved by bottom currents that carry a great amount of sediments. The beginning of the continental slope is called the continental shelf break, which usually takes place at around 130 meters depth. The set of continental shelf and slope is called the continental margin.
3. Continental Rise. The continental rise is formed by accumulated sediments carried by currents, mass wasting, and the settling of clastic and pelagic (biogenic) sediment. Mass wasting represents deposition events, such as landslides and turbidity currents. Biogenic sediments present more than 30% of their constitution in skeletal material. These sediments can be made up of calcareous or siliceous. Siliceous oozes present amorphous silica on its constitution.
4. Abyssal Plains. Abyssal plains, which can be considered the flattest places on Earth, are vast areas of the deep ocean floor that consists of volcanic rock covered with sediments. These areas are situated at depths of over 3 Km below sea level. The volcanic rock is predominantly basalt and the components of the abyssal plain sediment include nodules of manganese which contain iron, nickel, cobalt, and copper.
5. Mid-Oceanic Ridges. Mid-oceanic ridges are long volcanic mountain ranges which constitute the boundary between two tectonic plates. At this point, two plates are moving apart from each other causing the rocks to break and the magma to rise through the cracks. As magma cools and solidifies, it piles up and forms the long chain of mountains.
6. Ocean Trenches. Ocean trenches are long and narrow depressions on the sea floor. They typically extend from 3 to 4 Km below the ocean floor level. Geophysical data state that the trenches present negative gravity anomalies, since gravitational force is usually lower than normal at these points. These regions present only a small amount of sediments.
Science Clarified. Ocean Basin
Continental Shelf. Britannica Online Encyclopedia
Abyssal Hill. Britannica Online Encyclopedia
CRAMP, A. et al. Geological Evolution of Ocean Basins: Results from the Ocean Drilling Program. Geological Society Special Publication, v. 131.
HESS, J. History of Ocean Basins. Petrologic Studies. pp. 599-620, November 1962.