Ocean Relief Features

Table of Contents
1. Major and Minor Ocean Relief Features
2. Ocean
3. Ocean Relief Features
4. Major Ocean Relief Features
5. Oceanic Deposits
6. Minor Ocean Relief Features
7. Significance of Studying Oceanic Relief
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Major and Minor Ocean Relief Features

The relief of the ocean floor differs markedly from continental relief because of the contrasting ages and processes affecting the two crustal types. Oceanic crust is relatively young-generally less than 60-70 million years old-whereas continental crust contains much older rock of Proterozoic age (over 1 billion years). The young, tectonically active ocean floor shows features produced by seafloor spreading, volcanism, sedimentation and faulting.

Ocean

There is a single global oceanic body, which covers about 71 percent of the Earth's surface. For geographical, historical and practical purposes the ocean is divided into named regions. Traditionally there were four named oceans-the Atlantic, Pacific, Indian and Arctic. More recently the Southern (Antarctic) Ocean has been recognised as a distinct ocean surrounding Antarctica, making five named oceans in common use.

The Pacific, Atlantic and Indian oceans are often referred to as the three major oceans because of their size and volume.

• Oceans are a primary source of food-fish and other marine products supply a major share of the world's protein.
• Tidal energy and ocean currents offer potential sources of power where harnessing is feasible.
• Oceanography is the scientific study of the physical, chemical and biological properties and processes of the sea.
• Techniques used to map and measure ocean depths have evolved from simple echo-sounding to modern methods such as multibeam sonar, seismic reflection profiling and satellite altimetry, improving the precision of ocean-floor maps.

Ocean Relief Features

Because oceans merge continuously, exact boundaries between them are not always sharp. Geographers and oceanographers describe oceanic relief by identifying major structural divisions and a range of subordinate or minor features. A large portion of the ocean floor lies between about 3 and 6 km below sea level. The features of the ocean floor include broad plains, long mountain ranges, deep trenches and many volcanic and sedimentary forms formed by tectonic, volcanic and depositional processes.

The ocean floor is rugged in many places, containing some of the world's largest mountain ranges and its deepest depressions. Major groups of features are formed by plate motion (e.g., mid-ocean ridges and trenches), volcanism (seamounts, island arcs), and sedimentary processes (continental shelves and abyssal plains).

Major Ocean Relief Features

The principal divisions of the ocean floor are:

• The continental shelf
• The continental slope
• The continental rise and abyssal (deep-sea) plain
• The oceanic deeps or trenches

In addition to these broad divisions, the ocean floor includes ridges, seamounts, guyots, submarine canyons, fracture zones, island arcs, atolls, reefs and other smaller features.

Continental Shelf

The continental shelf is the submerged, gently sloping extension of a continent. It is the shallowest part of the offshore zone and commonly exhibits a very low gradient (often close to 1° or less).

• The outer margin of the shelf is marked by a relatively sharp break known as the shelf break.
• Average continental shelf width is about 80 km, but widths vary widely; some shelves are very narrow (for example off the coasts of Chile and western Sumatra) while others are extremely broad (the Siberian shelf in the Arctic approaches 1,500 km in width).
• Shelf depths also vary; in some regions water over the shelf may be as shallow as 30 m, while in others the shelf edge lies at depths approaching 600 m.
• Continental shelves receive thick sedimentary deposits derived from rivers, glaciers, wind and coastal erosion; these sediments are redistributed by waves and currents and over geological time become important repositories of fossil fuels and other mineral resources.

Three commonly cited origins or contributing processes for continental shelves are:

• Submergence of continental margins due to sea-level rise or crustal subsidence;
• Wave erosion of an earlier coastal plain producing an offshore scarp or terrace;
• Deposition of river-borne and reworked terrestrial material forming an offshore accumulation (shelf sedimentation).

Geographical significance:

Their shallowness enables sunlight to penetrate through the water, which encourages plants and organisms → now rich in plankton → fishes to thrive on them → . Hence, continental shelves are the richest fishing grounds (for example the Grand Banks off Newfoundland, the North Sea and the Sunda Shelf). Many important ports-Southampton, London, Hamburg, Rotterdam, Hong Kong and Singapore-are sited on continental shelves. Shelves are also important targets for petroleum and gas exploration; a substantial proportion of global offshore oil and gas production comes from shelf and upper-slope settings. Polymetallic manganese nodules and other marine mineral deposits may also occur in nearby deep areas.

Continental Slope

• The continental slope forms the steep transition between the continental shelf and the deep ocean floor or basin; it begins at the shelf break where the seafloor drops more sharply.
• The gradient of the slope commonly ranges from about 2° to 5°.
• Depths in the slope region typically range from a few hundred metres to several thousand metres (commonly from 200 m to 3,000 m beneath sea level).
• The slope marks the geological edge of the continental crust and is frequently cut by submarine canyons and gullies.

Continental Rise

• Below the steepest part of the slope the seafloor gradually becomes less steep. Where gradients reduce to about 0.5°-1° the region is termed the continental rise.
• The continental rise is formed by the accumulation of turbidity currents and other sedimentary processes that transport materials from the shelf and slope into the deep sea.
• At greater depths the rise merges into the abyssal plain.

Deep-Sea (Abyssal) Plain

• An abyssal plain is an extensive, relatively flat region of the deep ocean floor.
• Abyssal plains are typically found at depths between about 3,000 m and 6,000 m.
• They are covered by very fine-grained sediments such as clays and silts and commonly bear isolated volcanic or tectonic features such as seamounts or oceanic islands that rise above the plain.
• Examples of islands rising from abyssal regions include the Azores and Ascension Island.

Oceanic Deeps or Trenches

Ocean trenches are long, narrow, very deep depressions in the seafloor. They are typically formed at convergent plate boundaries where one lithospheric plate is forced beneath another (subduction). Trenches are among the deepest parts of the world's oceans and are closely associated with volcanic arcs and intense seismic activity.

• Trenches are relatively steep-sided, narrow basins and represent the deepest parts of the ocean.
• They are of tectonic origin and form at ocean-ocean or ocean-continent convergent plate margins.
• Trenches are commonly several kilometres deeper than the surrounding seafloor (3-5 km deeper in many cases).
• They occur along the fringes of deep-sea plains, at the bases of continental slopes and adjacent to island arcs; trenches typically run parallel to nearby mountain chains or island arcs.
• Trenches are abundant in the Pacific Ocean, forming an almost continuous ring around the basin's margins.
• The Mariana Trench (near Guam) is the deepest known trench, reaching depths in excess of 11 kilometres.
• Other deep trenches include the Mindanao Deep (~35,000 ft), the Tonga Trench (~31,000 ft) and the Japan Trench (~28,000 ft), all located in the Pacific Ocean.
• Many trenches are associated with deep-focus earthquakes and active volcanism, making them important for the study of plate tectonics and seismic hazards.
• To date a number of deeps have been explored; survey figures quoted in some sources record about 57 deeps explored, with the majority in the Pacific and others in the Atlantic and Indian Oceans.

Oceanic Deposits

The ocean floor receives material from continental sources, from chemical and biological processes in the sea, and from extraterrestrial input. These deposits are important for understanding past climates, ocean circulation and for economic resources.

Classification by origin

• Terrigenous deposits: Derived from the erosion of continental rocks and transported to the ocean by rivers, glaciers, wind and gravity flows. Terrigenous sediments typically dominate on continental shelves and upper slopes.
• Hydrogenous deposits: Precipitated directly from seawater by chemical processes; these include mineral precipitates such as some manganese and iron oxides and other authigenic minerals.
• Cosmic deposits: Extra-terrestrial materials (micrometeorites, tektites) incorporated into ocean sediments; these account for a very small fraction of total sediment but are widely dispersed over the deep ocean floor.

Pelagic sediments and oozes

The biological or biogenic component of deep-sea sediments is often collectively termed ooze. Pelagic deposits accumulate slowly far from shore as the remains of marine organisms settle through the water column.

• Pelagic deposits are the fine-grained sediments laid down on the open-ocean floor.
• They are largely composed of the shelly and skeletal remains of microscopic marine organisms.
• Pelagic deposits have a very fine, flour-like texture and may occur as widespread layers on the abyssal plain.
• Two commonly described types of ooze are:
• Calcareous oozes - rich in calcium carbonate (from foraminifera, coccolithophores, pteropods); commonly described as occurring to depths of about 2,500 m where dissolution begins to limit carbonate preservation in some texts.
• Siliceous oozes - rich in silica (from radiolaria and diatoms); commonly found at greater depths, beyond the depths where calcareous material is preserved.

Deep-sea clays

• Very fine clays are characteristic sediments of the deep ocean basins. Red clay, abundant in parts of the Pacific, is often attributed to fine volcanic dust and slow accumulation of very small mineral particles blown from volcanoes and continental sources.

Minor Ocean Relief Features

In addition to the major divisions, the ocean floor displays many minor but important features that affect bathymetry, ecology and human uses of the sea. These include:

• Ridges
• Hills
• Seamounts
• Guyots
• Trenches
• Canyons
• Fracture zones
• Island arcs
• Atolls
• Coral reefs
• Submerged volcanoes
• Sea-scarps

Mid-Oceanic Ridges

A mid-oceanic ridge is an extensive submarine mountain chain that runs through the ocean basins. It typically consists of two parallel chains of mountains separated by a central rift valley or axial depression where new oceanic crust is formed by seafloor spreading. Peaks along ridges can rise to about 2,500 m above the adjacent seafloor; parts of some ridges rise above sea level (for example Iceland, which lies on the Mid-Atlantic Ridge).

Seamounts

Seamounts are isolated submarine volcanic mountains with pointed summits that do not reach the sea surface. They are volcanic in origin and can be very high-commonly in the range of 3,000-4,500 m above the surrounding seafloor. The Emperor Seamount chain is an extension of the Hawaiian volcanic chain and is a classic example of seamounts formed by hotspot volcanism.

Submarine Canyons

Submarine canyons are steep-sided valleys carved into the continental shelf and slope; some rival terrestrial canyons in size and depth. They are often located offshore of major rivers and are believed to form by a combination of riverine, turbidity current and erosional processes. The Hudson Canyon (off the eastern coast of the United States) is a well-known example.

Guyots

Guyots are flat-topped seamounts that once reached near sea level and were flattened by wave action before subsiding. They record stages of volcanic island growth followed by erosion and subsidence. Estimates suggest there are many thousands of seamounts and guyots in the Pacific Ocean alone.

Atolls

An atoll is a ring-shaped reef, island or chain of islets that encircle a central lagoon. Atolls are characteristic of tropical oceans and form by the growth of coral reefs around sinking volcanic islands; as the volcanic core subsides, the reef continues to grow upward, ultimately forming a ring. Lagoons within atolls may contain marine water or, in rare cases, brackish or slightly freshened water.

Banks, Shoals and Reefs

• Bank: A relatively flat-topped submarine elevation on continental margins where water depth is shallower than surrounding areas but usually sufficient for navigation. Examples include the Dogger Bank in the North Sea and the Grand Banks off Newfoundland, both noted for rich fisheries.
• Shoal: A detached elevation with shallow depths that can pose hazards to navigation when it projects near the sea surface.
• Reef: A ridge or mound formed primarily by organic activity (most famously by corals) or by rock. Coral reefs are especially abundant in the tropical Pacific and Indian Oceans and may be associated with seamounts and submerged volcanic structures. The world's largest coral reef system is the Great Barrier Reef off the Queensland coast of Australia. Reefs that extend to or above the sea surface are navigational hazards.

Significance of Studying Oceanic Relief

• Oceanic relief controls the motion of seawater; seafloor topography influences current paths, upwelling and mixing, which in turn affect climate and marine ecosystems.
• Knowledge of underwater relief is essential for navigation, submarine cable and pipeline routing, port design and coastal engineering.
• Relief controls the distribution of sediments and biological habitats and therefore affects fisheries, mineral resource distribution and geohazards (for example submarine landslides and earthquake/tsunami generation).