The Earth Hydrosphere

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The Earth Hydrosphere

Earth Hydrosphere is the name given to the mass of water that covers about 71% of the earth’s surface.

The average depth of oceans is about 4 km.

Earth Ocean Floor

It is very irregular as the surface of the continents.

Four major units of Earth ocean floor are:

Continental Shelf of India

It is the coastal part of the ocean which is not very deep and the slope of the bottom is very gentle.

Extends to a depth of 100 fathoms (1 fathom = 1.8 m).

In regions where the mountains extend along the coast, the shelf is narrower.

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About 20% petrol and gas found here. They also provide the richest fishing ground in the world. Marine life exists entirely here.

They occupy about 7% of the total ocean area.

Continental Slopes

Extends seawards from the Continental Shelf. The continent blocks are supposed to end at the site of continental slope.

The boundary between shelf and slope is known as Andesite Line, named after the andesite rock.

Depth is up to 2000 fathoms.

They cover about 8.5% of the total ocean area.

Continental Rises

At the foot of slope is found an area slightly rising due to the accumulation of debris transported over the slope.

Oil deposits occur here.

Abyssal or the Deep Sea Plains

It is the deepest and the most extensive part of the ocean floor and accounts for about 40% of the total ocean floor.

Parts of the abyssal plains are occupied by raised ridges or submarine mountains and by very deep trenches or canyons.

Ridges are the raised areas in sea. E.g., Mid-Adantic ridge (S-shaped), Indian Ocean ridge (inverted Y-shaped).

A ridge rising more than 1000m above the ocean floor is called Seamount. Flat topped seamounts are called Guyots (maximum in Pacific Ocean)

Some parts of the ridge or volcanic peaks reach the surface of the oceans and form islands (E.g. Hawaii Islands).

Trenches are narrow and steep sided depressions. They occur where two plates of the earth’s crust are moving together and one is being pushed down below the other. Deepest is. Challenger Deep, a part of Mariana Trench in Pacific Ocean, near Philippines, is more than 11 km deep.

Submarine canyons are the deep gorges on the ocean floor and are restricted to the continental shelves, slopes and rises.

Salinity of Water

The proportion of dissolved salts to pure waster is called salinity. The average salinity in the oceans and seas is 350/00, i.e., 35 grams of salt in one litre of water.

Salinity in decreasing order is: NaCl, MgCl, MgSO4, CaSO4, KSO4, etc. Chlorine is the most abundant element.

Max salinity: Lake Van (Turkey) – 3330/00. Then Dead Sea – 2400/00. Most saline sea is Red Sea.

The main source of salinity is dissolution of the rocks of oceanic crust, which contains salts.

It is maximum at the tropics, because here temperature is high. Equatorial regions come second because although they have high temperatures, they have high rainfall also. Poles have minimum salinity because of addition of fresh water in the form of icebergs and excessive snowfall.

It causes vertical circulation of water.

Earth Waves

They are caused due to the friction with the winds.

There is no forward movement of water in a wave. When a wave enters shallow water, it breaks. The top of it is thrown forward and this is when water moves forward. Water from the breaking wave runs up the shore as swash and back down the shore as backwash

The maximum height of waves in most oceans is about 12 m but they may be as high as 15 m. Seismic waves or tsunamis are the waves caused by earthquakes in volcanic eruptions in the sea bottom. The tsunamis which hit the coasts in the SE Asia on Dec 26, 2004, caused havoc in that region.

Coral Reefs in India

  • Corals are a kind of calcareous rocks chiefly made of the skeletons of minute sea organisms called ‘polyps’. They are formed due to accumulation and compaction of skeletons of these lime secreting organisms.
  • Corals are found mainly in the tropical oceans and seas because they require high mean annual temperature ranging around 20° c. They cannot survive at a greater depth than 60-77m below sea level. Muddy or very saline water is injurious for their growth.
  • The coral reefs are classified on the basis of nature, shape and mode of occurrence into the following three:
  1. Fringing Reef: Coral reefs that develop along the continental margins or along the islands are called fringing reefs. The seaward slope is steep and vertical while the landward slope is gentle. Sometimes there is a lagoon or shallow channel between the fringing reef and the land. Such reefs are found near Rameshwaram in the Gulf of Mannar.
  2. Barrier Reef: They are the largest, most extensive, highest and widest reefs of all. They are formed off the coastal platforms and parallel to them. There is an extensive but shallow lagoon between the coastal land and the barrier reef. The Great Barrier Reef of Australia is the largest barrier reef in the world.
  3. Atoll: A reef of narrow growing corals of horse shoe shape and crowned with palm trees is called an atoll. It is generally formed around an island or in an elliptical form on a submarine platform. There is a lagoon in the middle of the coral ring. E.g. Fiji Atoll.

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