Mid Ocean Ridges:
Mid Ocean Ridges (MOR) is a seafloor mountainous system, which is formed as a result of plate tectonic movement of the crustal plate. Its depth is 2,600 meters, and it rises about 2,000 meters above the deepest portion of an ocean basin. Mid-oceanic ridges are found in all of the major oceans. They are the most prominent linear topographic features of the seafloor. The axes of the ridges are marked by frequent earthquakes and are characterized by a much higher heat flow through the crust. Deep rifts occur at the crest of the ridges. It is a site where two crustal plates are being pulled apart and new oceanic crust is being created. It is, therefore said that the seafloor is spreading gradually.
also read: Depth Zones of Sea
Deep Ocean Trenches:
A deep ocean trench is a long and narrow deep trough, which represents the deepest part of the ocean. Several trenches in the western pacific ocean have a depth of about 10,000 meters. Trenches are the sites where the crustal plates plunge down into the mantle. Volcanic island arcs are found associated with trenches in the open ocean, while volcanic mountains, like the Andes, may be found along trenches that are adjacent to the continents. Mariana Trench is the deepest place in the world.
They are the underwater plains on the deep seafloor, usually found at the depth between 3,000 and 6,000 meters. They are an extremely flat area of the deep seafloor. They are formed due to the deposition and accumulation of thick layers of sediments. Abyssal plains are so wide, that they contain more than 50% of the earth’s surface.
- Abyssal Hills: The alleviated hilly region within an abyssal plain is known as the abyssal hills. Abyssal hills rise from several meters to several hundred meters.
- Abyssal Mountains: These are the mountains on the seafloor at abyssal depths having a vertical relief of greater than 1,000 m within a search radius of ~25 km are termed “abyssl mountains”. Some of the abyssal plains along with their abyssal hills and mountains are detailed in the following;
Isolated Volcanic Islands dot the ocean floors. Those volcanic mountains, which rise at least 1,000 meters above the surrounding topography are known as the ”Sea Mounts”. Seamounts can be found in every ocean basin in the world. Seamounts generally do not rise above sea level. They are underwater geologic landforms that rise from sea basins but do appear above the surface of seawater, therefore, they are not included in the list of islands, islets, or cliff rocks. Seamounts are typically formed from extinct volcanoes that rise abruptly and are usually found rising from the seafloor to 1,000–4,000 m (3,300–13,100 ft) in height. They are defined by oceanographers as independent features that rise to at least 1,000 m (3,281 ft) above the seafloor, characteristically of conical form. The peaks are often found hundreds to thousands of meters below the surface and are therefore considered to be within the deep sea. During their evolution over geologic time, the largest seamounts may reach the sea surface where wave action erodes the summit to form a flat surface. After they have subsided and sunk below the sea surface such flat-top seamounts are called “guyots” or “table mounts”.
So far, there are more than 14,500 know sea-mounts all through the earth’s ocean, but only few of them have been studied by earth scientists. A seamount with a flat top is known as ”Guyot”.
Seamounts and guyots are most abundant in the North Pacific Ocean, and follow a distinctive evolutionary pattern of eruption, build-up, subsidence and erosion. In recent years, several active seamounts have been observed, for example Loihi in the Hawaiian Islands.
Because of their abundance, seamounts are one of the most common marine ecosystems in the world. Interactions between seamounts and underwater currents, as well as their elevated position in the water, attract plankton, corals, fish, and marine mammals alike. Their aggregational effect has been noted by the commercial fishing industry, and many seamounts support extensive fisheries. There are ongoing concerns on the negative impact of fishing on seamount ecosystems, and well-documented cases of stock decline, for example with the orange roughy (Hoplostethus atlanticus). 95% of ecological damage is done by bottom trawling, which scrapes whole ecosystems off seamounts.