OCEAN RELIEF: CHARACTERISTICS, PARTS AND TYPES - GEOGRAPHY - 2025

The oceanic relief is the form of the marine crust that is produced due to the geological processes that act in its production. This oceanic crust is thinner than the continental crust and of different composition, with iron and magnesium predominating.

The crust is divided into plates that are displaced by the differences in density between the lithosphere and the asthenosphere (fluid mantle). It arises in the line of separation of two oceanic plates forming the oceanic ridges.

Ocean relief. Source: wikimedia commons

The emergence of these ridges delimits ocean basins on both sides of them. These basins are made up of extensive rolling abyssal plains, where plateaus and other geological structures also develop.

The abyssal plains reach the foot of the continental slopes or the ocean trenches. If the continental margin coincides with the end of a continental plate colliding with an oceanic plate, a subduction zone is formed.

As a result of this process, a deep trench or oceanic trench is created between the continental margin and the oceanic plate. If two oceanic plates converge, volcanic island chains are generated, common in the Pacific.

The five oceans that exist (Atlantic, Pacific, Indian, Arctic and Antarctic) have a common general structure, but also particularities. For example, the Arctic bottom is part of the North American plate, it is shallow and has a wide continental shelf.

For its part, the Pacific Ocean has very rough ocean margins because it collides with continental plates along almost the entire perimeter, forming ocean trenches. The Atlantic has smoother margins and wide abyssal plains, since it has a long central ridge.

Characteristics of the oceanic relief

Origin of the ocean floor

The oceanic relief is a product of the planet's geology, which began with the condensation by gravity of the mass of cosmic dust. This condensation formed a hot dense mass that subsequently began to cool.

This cooling process, while being subjected to the movements of rotation and translation, originated the characteristic structure of the Earth.

Layers of the Earth

The planetary core is a combination of a solid center with a shell of molten iron, nickel, sulfur and oxygen. Above this core is the terrestrial mantle of siliceous rocks rich in iron and magnesium, and finally the outer crust.

The siliceous material of the mantle flows due to the high pressures and temperatures to which it is subjected. While the crust is the thinnest and most superficial layer on the planet, reaching 6 to 11 km in the oceanic ridges.

In the great continental mountain ranges, the crust reaches 10 to 70 km thick and is made up of rocks with a different composition between the ocean floor and the continents.

Differential composition between the continental and oceanic crust

The continental crust is made up of siliceous rocks in which sodium, potassium and aluminum silicates (felsic rocks) predominate. The oceanic crust is formed by matic rocks with a predominance of iron and magnesium silicates.

Formation of the oceanic crust

This crust is constantly being formed due to the periodic expulsion of molten rock (magma) through undersea volcanoes. This occurs in the mountain ranges that cross the ocean floor between the continents (mid-ocean ridges).

Therefore the crust is hardened lava, volcanic rock and crystalline rock of volcanic origin (garbo and peridotites, basalt). Additionally, on this crust the continental sediments dragged to the oceans by rivers are deposited.

Tectonic plates

The lithosphere, which is the upper layer of the Earth made up of the crust and the outermost part of the upper mantle, is divided into plates. The differential density between the lithosphere and the asthenosphere or fluid part of the upper mantle immediately below, causes them to move towards each other.

In this way, the lithosphere functions as a conveyor belt driven by the formation of new crust in the mid-ocean ridges. This new crust formed on both sides of the submerged ridges horizontally displaces the old crust.

In this expansive process, a collision occurs in the lines of contact between one plate and another of those that form the lithosphere. Thus the oceanic crust is forced to descend below the continental crusts (subduction zone), rejoining the fluid asthenosphere.

Ocean relief

The different processes involved in the tectonics of the lithosphere plates give rise to the structure of the oceanic relief. This relief is expressed in different types, depending on whether it is a point of plate convergence (subduction) or of divergence (crust formation).

Parts (structure)

The relief of the ocean floor is made up of three fundamental parts: the ocean ridges or underwater mountain ranges, the basins and the margins.

Oceanic ridges

They are high and extensive submerged mountain ranges that cross the oceans, which have volcanic activity. These mountain ranges are formed along the emergence line of the magma coming from the earth's mantle.

Oceanic ridge. Source: wikimedia commons

The pressure generated and the outcrop of the magma forms a bulging area of ​​the lithosphere, as well as the formation of the mountain chain.

Ocean basins

On both sides of the ocean ridges an extensive wavy basalt area is formed that constitute the ocean basins. Part of them are covered by sediments dragged into the ocean by rivers and dispersed by marine currents and other rocks emerge from the crust.

In some points of the basins there are ancient volcanic formations that formed islands that are now submerged. In the same way, there are elevated areas forming underwater plateaus.

Continental margins

The ocean margins are the transition between the continents and the oceans, and include the coastline, the continental shelf and the slope. The continental shelf extends submerged to a depth of 200 m, then there is a more or less steep slope towards the ocean floor.

There are two types of continental margins, depending on whether it is a convergence or a divergence zone:

Passive margins

It occurs when a plate is continuous between ocean and continent from the oceanic ridge that originates it. For example, in the Atlantic Ocean the continental shelf is low-sloping in a continuous lithospheric plate of granite.

Active margins

It is a collision zone between a continental and an oceanic plate, generating a subduction zone that causes a deep trench. For example, in the Pacific Ocean where there are dissimilar lithospheric plates (granitic versus basaltic) and an oceanic trench is formed.

Formation types

In each of the parts of the oceanic relief, be it the margins, the basins or the ridges, different types of formations are manifested.

Continental shelf and slope

The continental shelf or submerged continental area, presents a relief that is related to the attached continental relief. For example, if there is a mountain range parallel to the coast on the continent, the platform will be narrow and followed by a steep slope.

Continental shelf and slope. Source: Graphic Workshop (fr)

Whereas if the continental surface is flat, this plain will continue widely on the continental shelf, giving rise to a wide shelf. In this case, the slope that follows the platform will have a less steep slope.

The most extensive continental shelf is that of the Arctic, reaching 1,500 km in length, as it is a single tectonic plate (the North American plate).

Continental glacis

At the base of the continental slope, sediments from the dragging of the surface waters of the continent accumulate. In some cases, this accumulation is considerable due to the contribution of large rivers and originates a gentle slope called continental glacis, as occurs on the eastern South American coast.

Abyssal plain

About half of the ocean floor is formed by an undulating plain located between 3,000 and 6,000 m deep. This plain extends from the foot of the continental slope to the oceanic ridges or to an oceanic trench.

It is formed by the large contributions of sediments that are deposited on the ocean floor, being more evident in the Atlantic and Indian oceans. In the Pacific it does not develop because the sediments are captured by the numerous trenches that are on its oceanic margins.

Oceanic ridge

It consists of a very high, wide and long mountain range that crosses the ocean floor between the plates. In these mountain ranges there is volcanic activity and they are the area of ​​origin of the new earth's crust.

These ridges are formed on the line where the oceanic plates separate (divergent boundaries). As the plates separate, the space fills with magma that forms new crust as it cools.

The oceanic ridge located in the middle of the Atlantic (mid-Atlantic), is the longest mountain range on the planet.

Abyssal or ocean trenches

Oceanic trench. Source: USGS

In areas where an oceanic and a continental plate collide, subduction occurs and a deep trench or trench is generated. This is because the oceanic crust descends towards the mantle while the continental crust rises.

The Mariana Trench

It is west of the Pacific Ocean and is the deepest trench that exists, reaching 11,000 m, 2,550 km long and 70 km wide.

Underwater cannons

Underwater cannons. Source: United States Geological Survey

They are deep valleys that cut the continental platforms and slopes in the direction of the slope. They originate from ancient rivers when the continental shelf was emerged or by erosion by sediment currents of current rivers that flow into the area.

Other orographic accidents

Volcanic islands

Mariana Islands. Source: US Geological Survey

They occur in the lines of convergence of two oceanic plates, when subducting one under the other. Active volcanoes in that area can grow from the accumulation of magma and emerge to form islands such as the Marianas and Aleutians in the Pacific Ocean.

Coral reefs and atolls

The oceanic relief is also affected by biological activity, such as the formation of coral reefs and atolls. This is the product of the activity of coral polyps that form large calcareous colonies.

The atolls are coral islands with an interior lagoon, which originate when the volcanic island around which the reef was formed collapsed. While an example of coral reefs are the Great Australian Barrier or the Caribbean coral reef.

Seamounts

They are undersea volcanoes not associated with ocean ridges, that is, they appear in ocean basins on hot spots. Hot spots are areas of the asthenosphere with magma at high temperatures and pressures.

When the moving crust passes over one of these points, these volcanoes emerge, forming mountains and even volcanic islands if they emerge.

Guyots

They are truncated cone formations of more than 900 m in height, which occur isolated or in rows on the seabed. Apparently they are ancient volcanic islands then submerged, whose summit was truncated by landslides and erosion and are abundant in the Pacific Ocean.

Oceanic plateaus

Like continental plateaus, oceanic plateaus are flat areas elevated relative to the ocean floor.

References

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