Ocean Relief : Key Features and Types of Ocean Relief

April 2, 2016 April 2, 2016

Surface area of earth is 510,072,000 km². It comprises of 148,940,000 km² land (29.2 %) & 361,132,000 km² water (70.8 %). Relief features of oceans are quite different from those of the continents. Please note that much of the oceanic crust is less than 60 million years old, while the great bulk of the continental crust is of Proterozoic age—mostly over 1 billion years old. Thus, the young age of the oceanic crust is quite remarkable.

Ocean is blue because water shows slightly blue color and that is because of the fact that it absorbs the Red photons of the light. Because the absorption which gives water its color is in the red end of the visible spectrum, one sees blue, the complementary color of red, when observing light that has passed through several meters of water.

Some Extreme Points in Oceans
The deepest point in the ocean is the Mariana Trench, located in the Pacific Ocean near the Northern Mariana Islands.
Its maximum depth has been estimated to be 10,971 meters (plus or minus 11 meters).
British naval vessel, Challenger II surveyed the trench in 1951 and named the deepest part of the trench, the “Challenger Deep”.
In 1960, the Trieste successfully reached the bottom of the trench, manned by a crew of two men.

The ocean relief can be divided into various parts such as Continental Shelf, Continental Slope, Continental Rise or Foot, Deep Ocean basins, Abyssal plains & Abyssal Hills, Oceanic Trenches, Seamounts and Guyots.

Continental Shelf

Continental Shelf is the submerged edge of a continent. It is a gently sloping plain that extends into the Ocean. The typical gradient is less than 1°. Taken together, total area of the continental shelves is 18% of earth’s dry land area. The width of the continental shelf varies considerably; there are many places on earth where there is virtually no shelf at all. The largest continental shelf is the Siberian Shelf in the Arctic Ocean, which stretches to 1,500 kilometers in width. The average width of continental shelves is about 80 km. The depth of the shelf also varies, but is generally limited to water shallower than 150 m. Continental shelf is made up of Granite rock overlain by the sediments. Because of the gentle slope, the continental shelf is influenced by the changes in the sea level.

Continental Slope & Continental Rise

Continental slope is relatively steep descent from the shelf break to the deep sea floor. Inclination of the typical continental slope is around 4° and usually between 2° to 5°. Shelf break is almost constant all over the globe and is around 150 meters, except the Antarctica and Greenland continental slopes. The slope plunges down at least 1 kilometer and usually 2-3 kilometers.

The Continental Rise connects the Continental slope to the deep sea or abyssal plain. Its width is around 100-1000 kilometers. Slope is gradual and around l/8th of the continental slope. The transition from continental to oceanic crust commonly occurs within the continental rise.

Deep Ocean basin & Oceanic Ridges

Deep Ocean Basin is the lowest layer in the ocean. The sea floor is like a covering of sediments over a basalt rock which may be up to 5 kilometers thick. Oceanic ridges or Mid-oceanic Ridges refer to the boundary between the diverging plates. A mid-ocean ridge (MOR) refers to an underwater mountain system that consists of various mountain ranges (chains), typically having a valley known as a rift that runs along its spine, formed by plate tectonics. The midoceanic ridge of submarine hills divides the basin in about half. Precisely in the center of the ridge, at its highest point, is a narrow trenchlike feature called the axial rift. The location and form of this rift suggest that the crust is being pulled apart along the line of the rift.

The oceanic ridges present a mountainous chain of young rocks which stretch around 65000 kilometers, i.e. 1.5 times of the earth’s circumference. Oceanic ridges are made up of basalt rocks,, are geologically active as the new magma constantly emerging onto the ocean floor accumulates in the crust at and near rifts along the ridge axes. The adjacent graphic shows distribution of some Oceanic Ridges around the world.

Location of the important ridges are as follows:

Aden Ridge: Gulf of Aden and Indian Ocean along the south-eastern coastline of the Arabian Peninsula.
Explorer Ridge: Located 240 km west of Vancouver Island, British Columbia, Canada.
Gorda Ridge: off the coast of Oregon and northern California north of Cape Mendocino
Juan de Fuca Ridge: off the coasts of the state of Washington in the United States
Cocos Ridge: Its is a Volcanic hotspot. Also known as Galapagos hotspot located in East Pacific Ocean responsible for the creation of the Galapagos Islands as well as three major seismic ridge systems. Carnegie, Cocos and Malpelso.
Gakkel Ridge: located in the Arctic Ocean between Greenland and Siberia, and has a length of about 1,800 kilometers. It is slowest known spreading ridge on the earth.
Pacific-Antarctic Ridge: located in South Pacific Ocean
Southeast Indian Ridge: It is located in the Indian Ocean and separates the Indo Ocean plate from the Antarctic plate.
Carlsberg Ridge: Located in the Indian Ocean.

Abyssal Plain

Abyssal plain is flat, cold and sediment covered ocean floor. Abyssal plains are more extensive in Atlantic and Indian Oceans and less extensive in Pacific Ocean. Abyssal plain is found at an average depth between 3000 and 6000 meters. They are among the flattest, smoothest and least explored regions on earth.

Oceanic Trench

An elongated through or deep in the ocean floor is called ocean trench. It is more or less a U shaped valley. Most of world’s trenches are in Pacific Ocean. Trenches are most active geological features on earth where great earthquakes are Tsunamis are born. Here is a brief info about important trenches:

Mariana Trench

Mariana Trench is the deepest part of the world’s oceans. It is located in the western Pacific Ocean, east of the Mariana Islands. The trench is about 2,550 kilometers long but has a mean width of only 69 kilometers. The maximum known depth is 11.03 kilometers at the Vityaz-l Deep and about 10 91 kilometers at the Challenger Deep.

Tonga Trench

Tonga Trench us located in South Pacific Ocean and is second deepest trench. Its deepest point is called Horizon Deep. It is Steepest Trench of the World.

Factbox: Tonga Trench and Apollo 13
Apollo 13 was the third Apollo mission which was launched to land on the Moon. It was successfully launched toward the Moon, but the landing had to be aborted after an oxygen tank ruptured. It was launched on April 11, 1970 and subsequently failed. Its lunar module re-entered earth’s surface on April 17. 1970 and was targeted over the pacific Ocean to reduce the contamination from the Radioisotope Thermoelectric Generator (RTG) on board, which would have provided energy to the mission. This RTG was landed in the Tonga Trench. The RTG will remain active for next 2000 years. It has 3.9 kilogram of radioactive plutonium.

Puerto Rico Trench

Puerto Rico Trench is located on the boundary between the Caribbean Sea and the Atlantic Ocean. The trench is 800 kilometers long and has a maximum depth of 8,605 meters at Milwaukee Deep. which is the deepest point in the Atlantic Ocean.

Seamounts & Guyots

Seamounts are elliptical projections from the sea floor which look like mountains and have a steep slope of around 22° to 24°.Half of the world’s total seamounts arc in Pacific Ocean. Guyots are basically inactive volcanoes which are flat topped. Some of them are tall enough to approach or even penetrate the sea surface. Guyots are confined to Central Pacific Ocean.

Rift valleys

A rift valley is linear-shaped lowland between highlands or mountain ranges created by the action of a geologic rift or fault in opposite or parallel. The result is the formation of a long steep sided, flat floored valley. World’s largest Fresh water lakes are typical rift valleys. Examples are Lake Baikal in Siberia, Lake Tanganyika, Lake Superior, Lake Vostok, Lake Nipissing and Lake Timiskaming. Jordan Rift Valley, which is lowest land elevation on earth is located in the Dead Sea and is 760 meters below the surface of the Mediterranean Sea. Gulf of Aqaba in the Red Sea is also a rift valley.

Factbox: Important Lakes in Rift Valleys
Lake Baikal
Lake Baikal, also known as “Pearl of Siberia” is located in Siberia and is second most voluminous lake in the world after the Caspian Sea. It is also world’s oldest and deepest lake. It’s a Rift valley, created by the Baikal Rift Zone, and a World Heritage site declared in 1996. Lake Baikal is home to Buryats , the largest ethnic minority group and a tribe in Siberia. It was referred as North Sea by ancient Chinese writers.
Lake Tanganyika
After lake Baikal, Lake Tanganyika is second deepest lake in the world. It is world’s longest Lake spanning in 4 countries of Africa viz. Burundi, Tanzania, Congo and Zambia. This lake is a Rift Valley and largest rift lake in Africa.
Lake Superior
Lake Superior is largest lake of North America, shared by Canada as well as USA. It is largest freshwater lake in the world by surface area if lake Michigan and lake Huron are NOT considered one.
Lake Vostok
Lake Vostok is in Antarctica and is a Sub Glacial lake. It is located below the Vostok Station of Russia in Antarctica.
Lake Nipissing
Lake Nipissing is located in Canada. It’s one of the shallowest lakes of Canada

Abyssal Fans

Abyssal fans are also known as deep-sea fans, underwater deltas, and submarine fans. They are delta like structures formed at the deep sea surfaces. Abyssal plain is found at the depths between 3000 and 6000 meters. Abyssal plains cover more than 50% of Earth’s total surface. They are considered to be major reservoir of biodiversity.

Archipelago

Archipelago refers to a cluster of islands which are formed tectonically. This term was initially used for Aegean Islands located in the Aegean Sea between Greece and Turkey. Indonesia is often referred to as the world’s largest archipelago; however, this means that it is largest by area and not by number of islands. Indonesia has 17,500 islands which span more than 5000 km². World’s largest archipelago by number of Islands is Archipelago Sea which is located Baltic Sea between the Gulf of Bothnia and the Gulf of Finland. It has 50,000 Islands. Top 5 archipelagos in the world by number of Islands are as follows:

Archipelago Sea (Finland) 50,000
Canadian Arctic Archipelago 36.563
Stockholm Archipelago 24,000
Indonesian Archipelago 17.508
Philippine Archipelago 7.107

Lagoon

Lagoon is a shallow body of sea water or brackish water separated from the sea by some form of barrier. The biggest lagoon in the world is located in New Caledonia, in southwest pacific. In India, Chilika Lake in Orissa and the Vembanad Lake in Kerala are both connected to the Bay of Bengal and the Arabian Sea respectively through a narrow channel and they are typical lagoons.

Coral Reefs

Coral reefs, which are also called as “rainforests of the sea”, are underwater reefs made by calcium carbonate secreted by Corals. Coral is the hard exoskeleton of the polyps. Coral Reefs grow best in warm, shallow, clear, sunny and agitated waters.

Coral reefs are some of the most diverse ecosystems on earth which despite of covering less than 10% of world ocean surface (284,300 km²) provide home to 25% of marine species including fishes, molluscs etc. Coral Reefs are very fragile ecosystem and are susceptible to “Surface Temperature” of the oceans. They are threatened by the climate change, ocean acidification, blast fishing, cyanide fishing for aquarium fish, overuse of reef resources, and harmful land-use practices.

Some Observations on Coral Reliefs

The Indian Ocean and Pacific Ocean region including the Red Sea, Indian Ocean, Southeast Asia and the Pacific account for 91.9% or total Coral reefs in the world.
Southeast Asia accounts for 32.3%, while the Pacific including Australia accounts for 40.8%. Atlantic and Caribbean coral reefs only account for 7.6%.
Largest Coral reef in the world is Great Barrier Reef. It is located in the Coral Sea, off the coast of Queensland in north-east Australia. It is composed of over 2,900 individual reefs and 900 islands stretching for over 2,600 kilometers. This reef can be seen from outer space and is the world’s biggest single structure made by living organisms. It is a World Heritage Site (1981). It is also a state icon of Queensland, made by Queensland National Trust. A large part is protected by the Great Barrier Reef Marine Park, established by Government of Australia through Great Barrier Reef Marine Park Act 1975.
Belize Barrier Reef is world’s second largest Coral Reef which is a part of 900 kilometer Mesoamerican Barrier Reef System. It was described by Charles Darwin in 1842 as “the most remarkable reef in the West Indies”.
Pulley Ridge is located off the coast of Florida, United States. It is deepest photosynthetic coral reef known so far.
Raja Ampat Islands, largest marine national park in Indonesia are located in Indonesia and New Guinea and comprise 1,500 small islands .It is known for highest recorded marine biodiversity on Earth. It makes the Coral Triangle which is a triangular shaped area of the tropical marine waters of Indonesia, Malaysia, Papua New Guinea, Philippines, Solomon Islands and Timor-Leste. These waters contain at least 500 species of reef-building corals in each ecoregion. Coral Triangle as well as Raja Ampat Islands is considered to be the global epicenters of marine biodiversity. INWF considers the region as a top priority for marine conservation and has launched the Coral Triangle Program in 2007

Beach

Beach is the shoreline of an ocean, sea or lake which consists of loose particles such as sand, gravel. pebbles etc. They are formed as a result of wave action by which waves or currents move sand or other loose sediments.

Cox’s Bazaar

Cox’s Bazaar sandy beach in Bangladesh’s Chittagong is considered to be world’s longest natural sandy beach. It has an unbroken length of 120 kilometers.