.png)
OCEAN RELIEF
The oceans are confined to the great depressions of the earth’s outer layer.
The nature of the ocean basins of the earth and their topography.
The oceans, unlike the continents, merge so naturally into one another that it is hard to demarcate them.
The geographers have divided the oceanic part of the earth into five oceans
- The Pacific,
- The Atlantic
- The Indian
- Southern ocean
- The Arctic.
The various seas, bays, gulfs, and other inlets are parts of these four large oceans. A major portion of the ocean floor is found between 3-6 km below sea level. The ‘land’ under the waters of the oceans, that is, the ocean floor exhibits complex and varied features as those observed over the land.
The floors of the oceans are rugged with the world’s largest mountain ranges, deepest trenches, and the largest plains. These features are formed, like those of the continents, by the factors of tectonic, volcanic, and depositional processes.
Divisions of the Ocean
The ocean floors can be divided into four major divisions:
- the Continental Shelf;
- the Continental Slope;
- the Deep Sea Plain;
- The Oceanic Deeps.
Besides, these divisions there are also major and minor relief features in the ocean floors like ridges, hills, seamounts, guyots, trenches, canyons, etc. Continental Shelf The continental shelf is the extended margin of each continent occupied by relatively shallow seas and gulfs. It is the shallowest part of the ocean showing an average gradient of 1° or even less. The shelf typically ends at a very steep slope, called the shelf break. The width of the continental shelves varies from one ocean to another.
Deep-Sea Plain
Deep-sea plains are gently sloping areas of the ocean basins. These are the flattest and smoothest regions of the world. The depths vary between 3,000 and 6,000m. These plains are covered with fine-grained sediments like clay and silt.
Oceanic Deeps or Trenches
These areas are the deepest parts of the oceans. The trenches are relatively steep-sided, narrow basins. They are some 3-5 km deeper than the surrounding ocean floor. They occur at the bases of continental slopes and along island arcs and are associated with active volcanoes and strong earthquakes. That is why they are very significant in the study of plate movements.
Seamount
It is a mountain with pointed summits, rising from the seafloor that does not reach the surface of the ocean. Seamounts are volcanic in origin. These can be 3,000-4,500 m tall. The Emperor seamount, an extension of the Hawaiian Islands in the Pacific Ocean, is a good example.
Submarine Canyons
These are deep valleys, some comparable to the Grand Canyon of the Colorado River. They are sometimes found cutting across the continental shelves and slopes, often extending from the mouths of large rivers. The Hudson Canyon is the best-known submarine canyon in the world. Guyots It is a flat-topped seamount. They show evidence of gradual subsidence through stages to become flat-topped submerged mountains. It is estimated that more than 10,000 seamounts and guyots exist in the Pacific Ocean alone.
Atoll
These are low islands found in the tropical oceans consisting of coral reefs surrounding a central depression. It may be a part of the sea (lagoon), or sometimes form enclosing a body of fresh, brackish, or highly saline water.
TEMPERATURE OF OCEAN WATERS
This section deals with the spatial and vertical variations of temperature in various oceans. Ocean waters get heated up by solar energy just as land. The process of heating and cooling the oceanic water is slower than land.
Factors Affecting Temperature Distribution
The factors which affect the distribution of temperature of ocean water are:
- Latitude: the temperature of surface water decreases from the equator towards the poles because the amount of insolation decreases poleward.
- Unequal distribution of land and water: the oceans in the northern hemisphere
SALINITY OF OCEAN WATERS
All waters in nature, whether rainwater or ocean water, contain dissolved mineral salts. Salinity is the term used to define the total content of dissolved salts in seawater.
It is calculated as the amount of salt (in gm) dissolved in 1,000 gm (1 kg) of seawater. It is usually expressed as parts per thousand (o/oo) or ppt.
Salinity is an important property of seawater. The salinity of 24.7 o/oo has been considered as the upper limit to demarcate ‘brackish water’.
Factors affecting ocean salinity are
- The salinity of water in the surface layer of oceans depends mainly on evaporation and precipitation.
- Surface salinity is greatly influenced in coastal regions by the freshwater flow from rivers, and in Polar Regions by the processes of freezing and thawing of ice.
- Wind also influences the salinity of an area by transferring water to other areas.
- The ocean currents contribute to the salinity variations. Salinity, temperature, and density of water are interrelated. Hence, any change in the temperature or density influences the salinity of water in an area.
Ocean Deposits:
The unconsolidated sediments, derived from various sources, deposited at the sea floors are included in ocean deposits. The study of ocean (marine) deposits includes the consideration of types of sediments, their sources, methods of their transportation, horizontal distribution, lithological successions or vertical variations in their distribution and composition, etc.
The sediments derived from weathering and erosion of continental rocks are transported to the oceans by rivers, winds, etc. Volcanic eruptions also provide sediments. Besides, the decay and decomposition of marine organisms (both plants and animals) also contribute sediments to ocean deposits.
Ocean deposits are classified on different bases e.g.:
(1) On the basis of location,
(2) On the basis of depth,
(3) On the basis of the origin of sediments etc.
Distribution of Ocean Deposits:
Terrigenous Deposits:
Terrigenous deposits include gravels, sands, muds, and volcanic materials, etc. which are derived through weathering and erosion of continental rocks by various denotational processes. There is marked gradation of these sediments when they are deposited in the oceans. The sequence of these materials from the coast towards the sea is gravel, sand, silt, clay, and mud. The ocean currents and waves very often disturb the gradation and sequence of sediments.
Pelagic Deposits:
Pelagic deposits consisting of remains of marine plants and animals in the form of different types of oozes cover about 75.5 percent of the ocean areas. Pteropod, diatom, and radiolarian ooze cover 0.4, 6.4, and 3.4 percent areas of all the oceanic deposits respectively. Red clay constitutes 31.1 percent of the total ocean deposits.
Ocean Currents
Ocean current is a continuous general movement of Ocean Water in a specific direction. As an analogy, you can perceive it as a river flowing over the ocean surface. About 10% of the water in the World’s Ocean is involved in the Surface Currents.
Most surface currents move water in the horizontal and vertical direction in the top layer above the thermocline (The layer where temperature changes steeply). The water beneath the thermocline also circulates but the movement is very slow.
Forces Responsible For Ocean Currents
Primary Forces
Heating by solar energy causes the water to expand. That is why near the equator, the ocean water is about 8 cm higher in level than in the middle latitudes.
This causes a very slight gradient and water tends to flow down the slope. The flow is normally from east to west.
Secondary Forces
Temperature differences and salinity differences are secondary forces.
Differences in water density affect the vertical mobility of ocean currents (vertical currents).
Water with high salinity is denser than water with low salinity and in the same way cold water is denser than warm water.
Denser water tends to sink, while relatively lighter water tends to rise.
Cold-water ocean currents occur when the cold water at the poles sinks and slowly moves towards the equator.
Types of Ocean Currents
There are 2 types of Ocean Currents
Surface Currents also called Wind-Driven Currents
The Surface Currents are a result of the Wind-stress and are modified by Coriolis force. Hence Surface currents mimic Atmospheric Wind Circulations. The diagram below shows how Easterlies are in line with the equatorial current, the Westerlies are in line with Kuroshio and Gulf Stream.
Ocean currents are constrained by the continental masses that border the three major oceans. This causes slight variations in their movement.
Deep-water Currents also called Thermohaline Currents –
While wind effects drive ocean currents in the upper 100 meters of the ocean’s surface, ocean currents also flow thousands of meters below the surface. These deep-ocean currents are driven by differences in the water’s density, which is controlled by temperature (thermo) and salinity (haline). This process is known as thermohaline circulation.
Cold, salty water is dense and sinks to the bottom of the ocean, and eventually returns to the surface through mixing. On a global scale, this sinking and rising of ocean water create what scientists call the ‘great ocean conveyor belt.
