What Shapes the Relief of the Ocean Floors?

Map of the Indian Ocean from 1967

The expanse of the ocean floor (also known as the seafloorsea floor, or seabed) deviates significantly from being merely flat and lacking in diversity. Within this underwater realm, one encounters a dynamic interplay of geological features such as volcanoes, trenches, valleys, seamounts, and abyssal plains, interspersed with varying degrees of elevation, at times even exhibiting remarkable prominence.

General Morphology of the Ocean Floor

Depiction of the abyssal zone in relation to other major oceanic zones
Depiction of the abyssal zone in relation to other major oceanic zones. Image: Wikimedia.

In a general context, it is feasible to delineate distinct oceanic sectors characterized by specific morphological attributes. Starting from the coastal periphery, one encounters a shallow continental shelf (with depths ranging from 150 to 200 meters or approximately 492 to 656 feet), which then undergoes a gradual descent, forming the continental slope. A multitude of subaqueous canyons or channels, mainly situated at the mouths of rivers, incise this steep declivity. Particulate matter originating from the continent or the shelf is transported into these canyons, creating turbidity currents similar to cascades of unconsolidated sediments.

These dynamic currents deposit sedimentary materials at the base of the slope, resulting in the formation of sedimentary fans. The sector at the foot of the slope, situated at an approximate depth of 3,000 meters or about 9,843 feet, where sediments accumulate, is referred to as the continental rise. This region then transitions into the abyssal plain, a relatively level and profoundly deep expanse typically ranging between 5,000 and 6,000 meters (16,404 to 19,685 feet) in depth.

Moving towards the core of the oceanic basin, one encounters one of the foremost topographic attributes of the oceans: the oceanic ridge, also known as a mid-ocean ridge or rift. The genesis of these ridges can be attributed to volcanic activity associated with the creation of nascent oceanic crust, effectively resulting in an underwater mountain range. Extending over a staggering 64,000 kilometers (39,768 miles) throughout the central extents of all global oceans, these ridges exhibit diverse morphological profiles contingent upon the degree of volcanic activity. These crests rise approximately 1,500 meters (4,921 feet) above the adjoining abyssal plain. A central axial valley, characterized by variable dimensions in terms of depth and width, typically demarcates its central axis.

The Romanche Trench with red arrows indicating directions of movements of tectonic plates
The Romanche Trench with red arrows indicating directions of movements of tectonic plates. Image: NOAA.

Numerous transform faults, which facilitate tectonic shifts linked to the formation of oceanic crust, frequently intersect the axial trajectories of these ridges. Exhibiting pronounced morphological manifestations, these faults are conspicuously evident in all bathymetric maps. They create narrow valleys encircled by towering vertical precipices that can attain elevations of up to 2,000 meters (6,562 feet). While primarily active at the ridge axis, these faults can propagate for extensive distances, extending into more aged oceanic crust and occasionally even reaching continental margins. Such features are designated as fracture zones.

For example, the Romanche fracture zone in the Atlantic Ocean extends for a remarkable distance of 5,000 kilometers (3,107 miles), characterized by a horizontal displacement of 900 kilometers (560 miles). Additionally, the abyssal plain may abruptly terminate at a subduction trench. These trenches stretch for hundreds or even thousands of kilometers along continental peripheries or volcanic island arcs. They epitomize the deepest realms on Earth, plunging to depths nearing 10,928 meters (35,856 feet) within the Mariana Trench.

Main Landforms Along the Oceans

Transform Faults
Transform Faults. Image: Plate Tectonics.

In this general diagram, various types of reliefs appear punctually. Prominent examples include volcanoes, volcanic plateaus, and transform faults. Volcanic occurrences are prevalent across the oceanic lithosphere. Despite their varying sizes, some of these volcanic structures are among the highest points on Earth. Volcanic formations related to thermal anomalies, like Mauna Kea in the Hawaiian archipelago, rise several kilometers (miles) above the abyssal plain, similar to Mount Everest‘s elevation.

Bathymetric studies also reveal many volcanic plateaus formed from extensive effusive events on the ocean floor. These formations cover an area of over 200,000 km² and rise approximately 2,000 to 3,000 meters (6,561 to 9,842 feet) above the seabed. These are part of the Large Igneous Provinces, including the Kerguelen Plateau in the Indian Ocean or the Ontong Java Plateau in the Pacific.

The ocean experiences widespread extensional forces, leading to various fault formations. These fractures create obliquely inclined crustal blocks, contributing to the complexity of the seafloor’s texture. Some normal faults can uplift significant tectonic blocks, often near continental boundaries. Other fault configurations, resulting from the behavior of normal faults, lead to significant undulations on the seafloor known as oceanic core complexes. These dome-shaped features result from the tectonic uplift of ultramafic rocks (serpentinized peridotites) from the mantle.

They can be moderately extensive, with widths around ten kilometers (6.2 miles) and lengths ranging from 10 to 100 kilometers (6.2 to 62.1 miles). These formations can vary in verticality, occasionally reaching heights of several kilometers (miles). They are often associated with increased hydrothermal activity and the presence of hydrothermal vents, such as the Atlantis Massif in the North Atlantic, which rises over 4,000 meters (13,123 feet) above the ocean floor and houses a notable hydrothermal site known as the Lost City.

Modern satellite-based imaging methodologies have ushered in an enhanced comprehension of the subaqueous panorama, which is marked by an eclectic multiplicity of elevational reliefs and profound incisions akin in scale to their continental analogs.