Ocean Temperature
Measurement of degree of hotness or coldness of ocean water is called ocean temperature
Normally measures in unit of degree C by thermometers
Major source of heat
Radiation from sun
specific heat of water is greater than land
Water heats and cools slowly relatively
Factors affecting horizontal distribution of ocean temperature
Latitude
Decreases from equator towards poles due to slanting rays of sun
Prevailing wind
Offshore winds raise temperature generally
Winds blowing from snow covered regions in winter lower surface temperature
In trade wind belt, off shore winds initiate upwelling of cooler water from beneath
On shore winds pile up warm water to increase temperature to certain extent
Ocean currents
Warm currents raise temperature and vice versa
Gulf Stream – eastern part of North America and west coast of Europe
Labrador cold current – north eastern coast of North America
Other factors
Submarine ridges, local weather conditions like storms
Cyclones, hurricanes, fog, cloudiness, evaporation and condensation
Highest sea surface temperature
Found few degree north of Equator – 27C to 30C
Lowest temperature
Minus 1.9C near poles
Vertical distribution
Upper Mixed layer (sea level to 200 m)
Uppermost layer of ocean water is warm
On an average this extends up to 200 m in tropical region
Temperature is uniform or slightly changes with depth
Average temperature – 20 to 25C
Thermocline layer
Varies in depth between 200 m to 1000 m
Temperature rapidly decreases with increasing depth
Deep water layer – below 1000 m
Temperature decrease is gradual until 4000 m
Beneath 4000 m, temperature is constant at 4C
Salinity
Defined as ratio between the weights of dissolved salts (in grams) per 1000 grams of water.
expressed as part per thousand (‰) and has no units
30‰ means 30 grams in 1,000 grams of sea water
Average ocean salinity – 35‰
Sources of salt in ocean
derived from weathering and erosion of the earth’s crust by the rivers
dissolved from rocks and sediments below ocean floor
escaped from the earth’s crust through volcanic vents as solid and gaseous materials
Factors affecting salinity
The rate of evaporation
Amount of precipitation,
Addition of fresh water flow from rivers
Ice in Polar Regions – thawing and freezing
Upwelling of deep water initiated by prevailing winds and
Mixing of water by ocean currents.
Horizontal Distribution of salinity
On an average the salinity decreases from equator towards the poles
Highest salinity
between 20° and 40° north latitudes
As it is characterized by high temperature and high evaporation
but less rain than the equatorial region
marginal areas of the oceans bordering the continents
Lower salinity than their interior
Due to addition of freshwater to marginal area by rivers
Very high salinity
Lake Von, Turkey (330‰)
Dead Sea (238‰)
Great Salt Lake, Utah, USA (220‰)
Vertical Distribution of salinity
Salinity increases with depth generally
Halocline – distinct zone where salinity increases sharply
Increasing salinity causes its density to increase
Stratification of salinity
Higher salinity water sinks below the lower salinity water
Salinity at depth
Is fixed almost because there is no way that water is lost or salt is added
Ocean deposits
Sediments derived from various sources deposited at ocean floors are called ocean deposits
Terrigenous deposits or muds
Derived from land and transported to ocean by rivers and wind
Mainly deposited on continental shelves
Gravel, sand, silt, clay, mud etc.,
Blue, green or red mud – colour depends upon their chemical composition
Volcanic deposits
Ash, pumice, cinder, etc.,
Derived from the vents of submarine volcanic mountains
Found near convergent plate boundaries and hotspot volcanoes
Biotic deposits
Neritic deposits, pelagic deposits, oozes
Consists of skeletons, shells of microorganisms and animals
Most common biotic deposits – calcareous ooze and siliceous ooze
Cosmogenous deposits
derived from extraterrestrial sources
Dust, tektites and particles from meteorite impacts
Relief of ocean
Hypsometric curve’ or ‘Hypsographic curve
graph denoting the proportion of a landmass standing above or below the sea level
Major reliefs
Shelf, Slope, Rise, Deep Sea Plain, Trenches
Minor reliefs
Mid oceanic ridges, Sea Mount, submarine canyon, guyot, atoll
Continental shelf
seaward extension of land that lies under the sea water
7% of the sea floor
Covered by shallow seas
with an average depth of 200 fathoms
width of the continental shelf
Depends on nature of the rock beneath the crust
If the crust is dynamic then the shelf would be narrow and vice versa
Formation of continental shelf
fluvial deposits, marine erosion, tectonic forces, and the fluctuations in sea level in the past
Resource rich
oil, natural gas, mineral deposits and coral reefs
Fishing grounds
Grand Bank of New Foundland in Atlantic ocean
Widest continental shelf
along the coast of Siberia, in Russia (1210 km)
Shelf on east coast
formed by deltas of the Ganga, Godavari, Krishna and Cauvery
Shelf on west coast
formed due to faulting and consequent submergence
Continental slope
zone of steep slope
extending from the continental shelf to the deep sea plain or abyssal plain
Slope angle - 5° to 60°.
occupies 9% of sea floor
Dynamic region
landslides, turbid currents, large sediment slumps, under water canyons, gorges cut by the currents and rivers
Origin of continental slope
erosional, tectonic and aggregational processes
deposit from the continental shelves immediately falls down here
Continental rise
area between the continental slope and the sea floor
Occupies 5% of ocean area
accumulation of sediments
similar to the alluvial fans near the foot hills in the land
represents the boundary
Between continents and abyssal plain
Abyssal plain
vast area of flat terrain in the bottom of the oceans
largest part of ocean relief
Covering more than 50% of the total area
accumulation of very fine sediments on the floor
combinations of fine particles of clay and microorganisms
Sediments are in layers like sedimentary rocks
used to trace geological events in the past
Ocean trench
long, narrow, steep-sided depressions
formed by tectonic forces beneath the abyssal plain
extend 3 to 4 km
below the level of the abyssal plain.
26 oceanic trenches in the world:
22 in the Pacific Ocean, 3 in the Atlantic Ocean and only one in the Indian Ocean
Challenger Deep in the Mariana Trench in Pacific Ocean
deepest part of the earth – 10994 km
convergent boundary where one plate subducts below the other
Mid oceanic ridges
Submarine mountains continuous and connected
Forming a single global mid oceanic ridge system
Longest mountain range in world
Origin
Tectonic forces acting from within the earth
Location - divergent plate boundaries
Magma flows through the fissure to form new oceanic crust
Island
landmass surrounded by water on all sides.
may be formed on the continental shelf or as oceanic islands
Most of the oceanic islands are volcanic in origin – A&N
Archipelago
Group of islands formed by subduction of one place
Islands of Japan
Coral islands
coral polyps colonize the tropical warm water and form islands
Lakshadweep Islands are coral islands
Guyots
Flat topped volcanic hills submerged under the sea water
part of an underwater chain of volcanic mountains produced by slow plate movement
Seamounts
conical, volcanic hills submerged under ocean water
Do not reach water surface
Isolated rise with limited summit area – elevation of 1000 m or more
Occupies 4.39% of ocean region
Seamounts and guyots are most abundant in the North Pacific Ocean
Atolls
Submarine Canyons
Indian ocean relief
Fb
Indian Ocean is the third largest of the world’s oceanic divisions
Boundaries
Asia in the north, Africa in the west, Asia in the east, Australia in the south-east and Antarctica in the south.
Average depth – 3890 m
Indian subcontinent in the north divides the Indian Ocean into Arabian Sea and Bay of Bengal.
Divided into three zones
Western zone
African coast and the mid-Indian Oceanic Ridge - large number of islands
Eastern zone
deepest of all the zone - continental shelves are narrow but have steep slopes.
Central zone
represents the mid-oceanic ridge where many tiny islands are located.
Continental shelf
Ranging from 192km to 280km
Extensive shelfs in India, Africa, Madagascar
Narrow along Java and Sumatra
Mid oceanic ridges
Mid-Indian Oceanic Ridge
from the southern tip of Indian Peninsula to Antarctica in north-south direction
Laccadive-Chagos Rigde (also known as Maldives Ridge).
Chagos- St. Paul Ridge between equator
Other ridges - East Indian ridge, West Australian ridge, South Madagascar Ridge
Branches of Central ridge
Socotra-Chagos Ridge, Seychelles – Mauritius, Madagascar Ridge, South Western Ridge, Ninety East Ridge
Ocean basins
Oceanic ridges divide ocean into basins
Comoro basin, north Australian basin, South Indian basin, Arab basin
Deep sea plains
Somali Abyssal plain, Ceylon (Sri Lanka) Abyssal plain, Indian Abyssal Plain
Trenches
Sunda Trench – 7450 m – deepest point - lies to the south of the island of Java
Diamantina Trench; separates Australia from the Antarctica continent.
Hotspot volcano – Reunion Island
Formed by mantle plumes emanating from plates away from plate boundaries
Islands in Indian ocean
Big islands – Sri Lanka and Madagascar
Coral Islands - Lakshadweep and Maldives
Volcanic origin islands – Andaman Nicobar, Mauritius and the Reunion Islands
Presence of Islands
Present in northern and western Indian oceans
eastern section of the Indian Ocean is almost free from islands
Straits
Major choke points - Bab al Mandeb, Strait of Hormuz, the Strait of Malacca and the Palk Strait.
Marginal seas
Arabian sea, Bay of Bengal, Persian Gulf, Red Sea, Gulf of Oman, Gulf of Aden, Gulf of Kutch
Coral reefs in Indian ocean
Gulf of Kutch, Gulf of Mannar, Palk Bay, A&N and Lakshadweep