Chapter 13. Water [Oceans]

Hydrological Cycle
• Water is a cyclic resource. It can be used and re-used. It undergoes a cycle from ocean to land and land to ocean.
• hydrological cycle describes movement of water on, in, & above earth.

Water on Earth
• About 71% of earth’s surface constitutes water and only 29% constitutes land area of Earth.
• Nearly 97% of water area constitutes Oceans and Seas and 2.5% is Fresh Water.
• Around 75% of Fresh Water is in Glaciers and Icecaps.
• Approximately 7% of Freshwater is present in as Groundwater.
• Fresh Water ◊ Glaciers ◊ Groundwater ◊ Ice and Snow ◊ Lakes ◊ Soil moisture ◊ Atmosphere ◊ Rivers.

Relief of Ocean Floor
• Ocean relief is largely due to Tectonic, Volcanic, Erosion & Depositional processes and their interactions.
• Ocean relief controls motion of Seawater. bottom relief of oceans influences navigation and fishing.

Major Ocean Relief Features of Ocean Floors Divisions of Ocean Floor
• major ocean relief features of ocean floors are given below.

Continental Shelf
• This is gently sloping [gradient of 1% or less] seaward, extension of a Continental plate. Continental shelves cover 7.5% of total area of oceans.
• Shallow Seas and Gulfs are found along continental shelves. shelf typically ends at a very steep slope, known as Shelf Break.

• shelf is formed mainly due to: Submergence of a part of a continent, Relative rise in Sea level and Sedimentary deposits brought down by Rivers, Glaciers
• There are various types of shelves based on different sediments of Terrestrial origin: Glaciated Shelf, Coral Reef Shelf, Shelf of a large River, Shelf with Dendritic Valley and Shelf along with young Mountain ranges.

Width and Depth of Continental Shelves
• Continental shelves have an average width of 70–80 km. shelves are almost absent or very narrow along a convergent boundary. Continental shelves may be as shallow as 30 m in some areas while in some areas it is as deep as 600 m.
• Siberian shelf in Arctic Ocean is largest in world and stretches up to 500 km from Coast.

Importance of Continental Shelves
• Almost 20% of world’s production of petroleum and gas comes from shelves. They are sites for placer deposits and phosphorites.
• Continental shelves forms best fishing grounds. Marine food comes almost entirely from Continental shelves.

Continental Slope
• gradient of slope region varies between 2-5°. depth of slope region is between 200 and 300 m.
• Continental slope connects Continental shelf and ocean basins. Continental slope boundary indicates end of Continents.
• seaward edge of Continental slope loses gradient at this depth and gives rise to Continental rise. Canyons and Trenches are observed in this region.

Continental Rise
• Continental slope gradually loses its steepness with depth. With increasing depth, rise becomes virtually flat and merges with Abyssal Plain. When slope reaches a level of between 0.5 and 1%, it is referred to as Continental Rise.

Oceanic Deeps and Trenches
• trenches are relatively steep-sided, narrow basins [Depression]. These areas are deepest parts of oceans. They are some 3–5 km deeper than surrounding ocean floor. They are of tectonic origin and are formed during ocean-ocean convergence and ocean-continent convergence.
• trenches lie along fringes of deepsea plain at bases of continental slopes along island arcs and they run parallel to bordering fold Mountains or Island chains. They are very common in Pacific Ocean and form an almost continuous ring along western and eastern margins of Pacific.

Deep-Sea Plain or Abyssal Plain
• Deep sea planes are gently sloping areas of ocean basins. These covers nearly 40% of ocean floor. Their depths vary between 3000 and 6000 m.
• These are flattest and smoothest regions of world because of Terrigenous and shallow water – sediments that bury irregular topography. These plains are covered with fine-grained sediments like clay and silt.

Minor Ocean Relief Features
• Ridges [along a divergent boundary],
• Abyssal Hills [submerged volcanic mountains]: Seamounts and Guyots,
• Trenches [along a convergent boundary],
• Canyons [erosional landforms],
• Island arcs [formed due to volcanism along a convergent boundary or hotspot volcanism]; and
• Atolls and Coral Reefs

Mid-Oceanic Ridges or Submarine Ridges
• A mid-oceanic ridge is composed of two chains of mountains separated by a large depression [divergent boundary]. ridges are either broad, like a plateau, gently sloping or in form of steep-sided narrow mountains.
• mountain ranges can have peaks as higher as 2,500 m & some even reach above ocean’s surface. Running for a total length of 75,000 km, these ridges are largest mountain system on Earth.

Submarine Canyons
• Canyon: This is a deep gorge, especially with a river flowing through it.
• Gorge: This is a steep, narrow valley or ravine.
• Valley: This is a low area between hills or mountains typically with a river or stream flowing through it.
• Submarine Canyons are deep valleys often extending from mouths of rivers to Abyssal plains. They are formed due to erosion by sediments brought down by rivers that cut across Continental shelves. Those which have a dendritic appearance and are deeply cut into edge of shelf and slope.

Abyssal Hills
• Seamounts: This is a mountain with a pointed summit, rising from seafloor that does not reach surface of ocean. Seamounts are volcanic in origin. These can be 3000 – 4000 m tall.

• flat-topped mountain seamounts are called Guyots. Seamounts and Guyots are very common in Pacific Ocean.

• A reef is a predominantly organic deposit made by living or dead organisms that form a mount or rocky elevation like a ridge.
• Coral reefs are a characteristic feature of Pacific Ocean. Since reefs may extend above surface, they are usually dangerous for navigation.

• These are low islands found in tropical oceans consisting of coral reefs surrounding a central depression. It may be a part of sea [lagoon], or sometimes form enclosing a body of fresh, brackish, or highly saline water.

Temperature of Ocean Water Factors Affecting Temperature Distribution
• Like landmasses, ocean water varies in temperature from place to place both at surface and great depths.
• Since water warms up and cools down much more slowly than land, annual range of temperature in any part of ocean is very smaller. Factors are Latitude, unequal distribution of land and water, prevailing wind and ocean currents.

Horizontal and Vertical Distribution of Temperature
• Thermocline: This is a boundary region in oceans, from where a rapid decrease in temperature starts usually at 0°C and about 90% of water is below this region. Generally, there are 3 layers in an ocean. At Arctic and Antarctic Circle, we have only 1 such layer as surface temperature, which itself is 0°C with a very slight temperature change.
• Generally, mean annual temperature of oceans decreases from equator areas toward poles, but reduction of temperature with latitude is never constant, because of interference by warm and cold currents, winds & air masses.

Salinity of Ocean
• All waters in nature, whether rain water or ocean water, contain dissolved mineral salts. Sodium and Chlorine are major salts present. other important minerals include magnesium, calcium & potassium along with others. Due to free movement of ocean water, proportion of different salts remains remarkably constant in all oceans even to great depths.
Salinity is a measure of how much salt is in a solution and how much it varies from place to place. amount of salt in different oceans and seas is mostly determined by how fast water evaporates, how much freshwater is added by rain, streams, and icebergs, and how much water is mixed by currents. salinity is highest at Tropic of Cancer and Tropic of Capricorn. It is because clear skies, high temperatures, and steady trade winds cause a lot of water to evaporate.

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