Study this in the app →

English · हिन्दी · తెలుగు · தமிழ் · മലയാളം · मराठी · ಕನ್ನಡ

The ocean floor, temperature and salinity

मराठीत स्पष्टीकरण

Oceanography: the ocean floor, ocean water and its movements · Part 1 / 3 · Fact-checked on · how we check

Written with AI from the sources listed at the bottom of this page, then fact-checked by a separate AI.

How water is spread over the Earth and the five oceans; the relief of the ocean floor: continental shelf, slope and rise, abyssal plains, trenches, mid-ocean ridges, seamounts and guyots; the temperature of ocean water, its controls and its layers, including the thermocline; and salinity, what raises and lowers it, and how it varies from the subtropics to the poles and from the Red Sea to the Baltic.

Each idea is explained first in Marathi (मराठी), ending with the English and Hindi exam terms, followed by the same point in exam English. UPSC Prelims is in English or Hindi, so understand it in Marathi and revise it in English.

आतापर्यंत आपल्या डोक्यावरच्या हवेबद्दल शिकलो; आता तिच्या खाली असलेले त्याहून कितीतरी मोठे पाण्याचे जग. पृथ्वीच्या पृष्ठभागाचा दोन-तृतीयांशहून जास्त भाग समुद्रच आहे, आणि पृथ्वीवरचे जवळजवळ सगळे पाणी समुद्रांतच. मोठ्या गरम पाण्याच्या टाकीसारखा समुद्र उष्णता साठवून हळूहळू सोडतो, हवेला बाष्प देतो, उष्ण कटिबंधातून ध्रुवांकडे उष्णता वाहून नेतो. म्हणूनच हवामान घडवण्यात वाऱ्यांइतकाच समुद्राचाही वाटा. या प्रकरणाचे तीन भाग: समुद्रतळ, पाण्याचे तापमान आणि खारटपणा; लाटा, भरती-ओहोटी, प्रवाह; एल निनो, प्रवाळ भिंती, सागरी संपत्ती. (→ English: ocean · हिन्दी: महासागर)

In exam EnglishThe previous chapter followed the atmosphere; this one turns to the much larger body of water beneath it. Oceans cover about 71% of the Earth's surface and hold about 97% of all its water, so most of the planet's surface is in fact sea floor. The oceans store heat, release moisture to the air and carry warmth from the tropics towards the poles, which is why they shape climate as much as the winds do. This chapter has three parts: the shape of the ocean floor and the properties of sea water, that is, its temperature and saltiness; the movements of that water in waves, tides and currents; and El Niño, the Indian Ocean Dipole, coral reefs and the resources of the sea.

खरे तर समुद्र एकच आहे, सगळे जोडलेले; सोयीसाठी पाच नावे दिली — एकाच घरातल्या खोल्यांना स्वयंपाकघर, दिवाणखाना अशी नावे देतो तसे. पॅसिफिक सगळ्यात मोठा आणि खोल; पृथ्वीवरची सगळी जमीन एकत्र केली तरी त्याच्यापेक्षा लहानच. अटलांटिक इंग्रजी S अक्षराच्या आकारात लांबट. हिंदी महासागराच्या उत्तरेला जमिनीचा अडथळा — म्हणून त्यावर मान्सूनचे वारे दिशा बदलतात. दक्षिण महासागर अंटार्क्टिकाभोवती, आर्क्टिक सगळ्यात लहान, बर्फाने झाकलेला. जमीन बहुतेक उत्तर गोलार्धात, समुद्र बहुतेक दक्षिण गोलार्धात. (→ English: ocean · हिन्दी: महासागर)

In exam EnglishThe world ocean is one connected body of water, but it is divided into five oceans. The Pacific is by far the largest and deepest, covering roughly a third of the globe, more than all the land put together. The Atlantic, second in size, is a long S-shaped ocean between the Americas and Europe and Africa. The Indian Ocean, third, is closed by land on its northern side, which is why the monsoon winds reverse over it. The Southern Ocean circles Antarctica, and the Arctic, the smallest and shallowest, lies around the North Pole and is partly covered by sea ice. Water and land are not shared evenly: most land lies in the Northern Hemisphere, while the Southern Hemisphere is dominated by ocean, a difference that matters for climate.

समुद्रतळ भांड्याच्या बुडासारखा सपाट नसतो. खोली कशी मोजली — जहाजावरून आवाज खाली पाठवून तो तळाला आपटून परत येण्याचा वेळ पाहतात. विहिरीत ओरडून प्रतिध्वनी किती उशिरा येतो त्यावरून खोलीचा अंदाज बांधतो तसेच. किनाऱ्यापासून आत गेलो की चार पायऱ्या: खंडीय मंच, उतार, खोल सपाट मैदान, गर्ता. मध्ये पाण्याखाली पर्वतरांगा, एकट्या टेकड्या, माथा सपाट असलेले डोंगर. हे बहुतेक भूपट्टांच्या हालचालीचे परिणाम. (→ English: echo sounding · हिन्दी: प्रतिध्वनि मापन)

In exam EnglishThe ocean floor is not a flat basin. Scientists first mapped it with echo sounding: a ship sends a pulse of sound downwards and times its echo from the bottom, and since sound travels at a known speed in water, the delay gives the depth. Modern sonar and satellites that measure tiny bumps in the sea surface have filled in the picture. Moving outward from a coast, the floor usually shows four main divisions: the continental shelf, the continental slope, the deep-sea or abyssal plain, and the ocean deeps or trenches. Rising from the deep floor are further features: long mid-ocean ridges, isolated seamounts and flat-topped guyots. Many of these features are the visible results of plate movement, taught in the geomorphology chapter.

खंडीय मंच म्हणजे जमीन समुद्रात बुडालेला भाग — जलतरण तलावाच्या उथळ बाजूसारखा, हळूहळू उतरणारा. साधारण दोनशे मीटर खोलीवर तो संपतो, आणि नंतर तळ एकदम खोल जातो. काही ठिकाणी शेकडो किलोमीटर रुंद, डोंगराळ किनाऱ्यांवर जवळजवळ नाहीच. वरचे पाणी उथळ, सूर्यप्रकाश तळापर्यंत पोहोचतो, नद्या पोषक द्रव्ये आणतात — म्हणून मासे भरपूर; आपल्या कोळ्यांच्या होड्या बहुतेक इथेच फिरतात. तळाशी साचलेल्या गाळाच्या थरांत तेल आणि वायू — मुंबई हायसारखे. (→ English: continental shelf · हिन्दी: महाद्वीपीय मग्नतट)

In exam EnglishThe continental shelf is the gently sloping, submerged edge of a continent. It ends at the shelf break, usually somewhere between about 100 m and 200 m deep, beyond which the floor drops more steeply. Its width varies greatly: it is very broad off Siberia and in parts of the North Sea, and almost absent along mountainous coasts such as those of Chile and Peru, where plates converge close to the shore. Because the water above it is shallow and sunlit, and rivers bring nutrients, plankton thrive and the shelf seas hold many of the world's richest fishing grounds. Thick sediments laid down on shelves over millions of years also hold oil and natural gas; India's Mumbai High field lies on the western continental shelf off Maharashtra.

मंच संपला की उतार — डोंगराच्या कडेवरून दरीत उतरल्यासारखा तळ हजारो मीटर खाली जातो; खंडाची खरी कड हीच. या उतारावर मोठ्या नद्यांच्या समोर खोल चरांसारख्या दऱ्या, त्यांतून गाळ घसरत खाली जातो — घसरगुंडीवरून वाळू घसरते तसा. उताराच्या पायथ्याशी तो गाळ ढिगासारखा साचतो. आणखी पुढे खोल सागरी मैदान: तीन ते सहा हजार मीटर खोलीवर, बारीक चिखल आणि मेलेल्या सूक्ष्म जीवांची कवचे पांघरुणासारखी पसरलेली, पृथ्वीवरची सर्वात सपाट जमीन. (→ English: abyssal plain · हिन्दी: वितलीय मैदान)

In exam EnglishBeyond the shelf break, the continental slope falls much more steeply to depths of a few thousand metres; it marks the true edge of the continent. Deep, steep-sided submarine canyons are cut into many slopes, often in line with large rivers, and act as chutes along which sediment slides into deeper water. At the foot of the slope, that sediment collects as a broad, gentle apron called the continental rise. Farther out lies the abyssal plain, at depths of roughly 3,000 to 6,000 m. It is covered by a thick blanket of fine clay and the tiny shells of dead plankton, which bury the rough rock beneath, so abyssal plains are among the flattest surfaces on Earth. Together they make up a large share of the ocean floor.

समुद्रातील सर्वात खोल जागा म्हणजे सागरी गर्ता: एक भूपट्ट दुसऱ्याखाली घुसतो तिथे तयार होणाऱ्या लांब, अरुंद खाचा. यांपैकी बहुतेक पॅसिफिकभोवती, ज्वालामुखी आणि भूकंपांसोबत. मारियाना गर्तेची सर्वाधिक खोली जवळपास अकरा किलोमीटर — एव्हरेस्ट तिच्यात ठेवला तर शिखरही पाण्याखालीच राहील! याच्या उलट मध्य महासागरीय रांग: भूपट्ट दूर सरकतात तिथे नवी जमीन जन्मून पाण्याखाली लांब डोंगररांग; आइसलंडमध्ये ती पाण्याच्या वर आली आहे. पाण्याखाली एकटे ज्वालामुखी डोंगर आणि लाटांनी माथा तासलेले सपाट डोंगरही असतात. (→ English: ocean trench · हिन्दी: महासागरीय गर्त)

In exam EnglishThe deepest places in the ocean are the ocean trench zones: long, narrow, steep-sided troughs that form where one plate sinks beneath another. Most of them ring the Pacific, alongside chains of volcanoes and frequent earthquakes. The Mariana Trench in the western Pacific contains the Challenger Deep, nearly 11 km below the surface, deeper than Mount Everest is tall; the Sunda or Java Trench is the deepest part of the Indian Ocean. The opposite feature is the mid-ocean ridge, an underwater mountain chain that winds through every ocean where plates move apart and new crust forms. Its crest often has a central rift valley, and in places it rises above the sea, as in Iceland. Scattered across the floor are seamounts, volcanic peaks that stay below the surface, and guyots, seamounts with flat tops planed off by waves before they subsided.
Main relief features of the ocean floor
FeatureDepth or formKey point
Continental shelfGentle; edge usually about 100–200 m deepFishing grounds, oil and gas (Mumbai High)
Continental slopeSteep drop to a few thousand metresTrue edge of the continent; submarine canyons
Abyssal plainAbout 3,000–6,000 m; very flatCovered with fine clay and plankton shells
Ocean trenchDeepest parts; Challenger Deep nearly 11 kmWhere plates sink; mostly around the Pacific
Mid-ocean ridgeUnderwater mountain chain with a rift valleyWhere plates part; rises above sea in Iceland
Seamount and guyotIsolated volcanic peaks below the surfaceGuyots have flat tops cut by waves

समुद्राचे पाणी जमिनीसारखे पटकन तापत नाही, पटकन थंड होत नाही — दुपारी वाळू पायाला चटके देते तेव्हाही समुद्राचे पाणी गारच असते ना. म्हणून वर्षभर समुद्राच्या तापमानात फारसा बदल होत नाही. साधारणपणे विषुववृत्ताजवळ उबदार, ध्रुवांकडे थंड. चारी बाजूंनी जमीन असलेले उष्ण कटिबंधीय समुद्र — तांबड्या समुद्रासारखे — अधिक उष्ण. किनाऱ्याकडून समुद्राकडे वारा वाहिला की वरचे उबदार पाणी ढकलले जाते, खालून थंड पाणी वर येते. सागरी प्रवाह आपली उष्णता किंवा थंडी किनाऱ्यालगत दूरवर नेतात. (→ English: ocean temperature · हिन्दी: महासागरीय तापमान)

In exam EnglishOcean water is heated at the surface by the Sun, but it warms and cools far more slowly than land, so the annual range of sea temperature is small. Surface temperature generally falls from the equator towards the poles, from close to 30 °C in parts of the tropics to below 0 °C in polar seas, where salt water freezes only at about −2 °C. Several factors modify this pattern. Seas partly enclosed by land in the tropics, such as the Red Sea and the Persian Gulf, are warmer than the open ocean at the same latitude. Oceans in the Northern Hemisphere, which touch more land, are on average a little warmer than those in the Southern. Winds blowing from land out to sea push warm surface water away, so colder water rises from below near the coast, and warm and cold currents carry their temperature far along the shore.

उन्हात ठेवलेल्या बादलीत हात घातला की वर कोमट, तळाशी गार लागते ना — समुद्रही तसाच, पण हजारो मीटर खोलीचा. वर लाटा आणि वारा ढवळणारा उबदार थर. त्याखाली तापप्रवण थर: इथे खोल जावे तसे तापमान झपाट्याने घटते. त्याखाली तळापर्यंत अंधार आणि थंड पाणी — समुद्राचा बहुतांश भाग हाच. ध्रुवांजवळ वरूनच थंड असल्याने हा थर जवळजवळ नसतो. हा थर झाकणासारखा वरचे आणि खालचे पाणी सहज मिसळू देत नाही. (→ English: thermocline · हिन्दी: ताप प्रवणता परत)

In exam EnglishTemperature also changes with depth. Sunlight warms only the top of the ocean, and waves and wind stir this water into a fairly uniform surface layer, usually a few tens to a few hundred metres thick. Below it lies the thermocline, a zone in which temperature drops quickly with depth, often found somewhere between about 200 m and 1,000 m. Beneath the thermocline the deep ocean is dark and cold, with temperatures only a few degrees above freezing all the way to the floor, so most of the ocean's volume is cold water. In polar seas the surface is already cold, so there is little or no thermocline. The thermocline acts like a lid: it keeps warm surface water and cold deep water from mixing easily, which matters for nutrients, fisheries and the heat that fuels cyclones.

क्षारता म्हणजे समुद्राच्या पाण्यात किती मीठ विरघळलेले आहे ते — एक किलो पाण्यात सरासरी साधारण पस्तीस ग्रॅम. ते मीठ बहुतेक जमिनीवरचे खडक झिजून नद्यांमार्गे आलेले. मिठागरांत काय होते — उन्हाने पाणी उडून जाते, मीठ उरते. तसेच बाष्पीभवन जास्त झाले, किंवा बर्फ गोठताना मीठ बाहेर ढकलले की क्षारता वाढते. मुसळधार पाऊस, मोठ्या नद्यांचे पाणी, वितळणारा बर्फ गोडे पाणी मिसळून क्षारता कमी करतात. आमटीत मीठ जास्त झाले की पाणी घालून सावरतो तसेच! (→ English: salinity · हिन्दी: लवणता)

In exam Englishsalinity is the amount of salt dissolved in sea water, measured as grams of salt in a kilogram of water, that is, parts per thousand. The average for the open ocean is about 35, and common salt, sodium chloride, makes up most of it. The salts come mainly from rocks worn away on land and carried by rivers, with some from volcanic activity on the sea floor. Salinity rises when fresh water is removed: strong evaporation leaves the salt behind, and when sea ice forms it rejects salt into the water around it. Salinity falls when fresh water is added: by heavy rainfall, by large rivers and by melting ice. Currents then mix waters of different saltiness. Like temperature, salinity can change sharply with depth; such a layer is called a halocline.

क्षारता सर्वाधिक असते ती विषुववृत्तावर नव्हे — थोडे दूर, उष्ण कटिबंधीय वाळवंटांलगतच्या समुद्रांत; तिथे ऊन जास्त, पाऊस कमी, पाणी उडून मीठ साचते. विषुववृत्ताजवळ रोजचा पाऊस थोडे पातळ करतो, ध्रुवांजवळ वितळणारा बर्फ आणखी पातळ करतो. तांबडा समुद्र वाळवंटांच्या मधोमध, नद्या नाहीत — खूप खारट; बाल्टिक समुद्र थंड, नद्यांनी भरलेला — थोडाच खारट. आपल्याजवळ, बंगालच्या उपसागरात गंगा-ब्रह्मपुत्रेचे पाणी येते म्हणून आपल्या अरबी समुद्रापेक्षा कमी खारट. (→ English: salinity · हिन्दी: लवणता)

In exam EnglishBecause of these controls, surface salinity is highest not at the equator but in the subtropics, roughly 20° to 30° north and south, where sinking air brings clear skies, strong evaporation and little rain. Near the equator, heavy daily rain lowers it slightly, and towards the poles melting ice, low evaporation and river water lower it further. Partly enclosed seas show the extremes. The Red Sea and the Persian Gulf, in hot deserts with almost no rivers, are among the saltiest seas, and the Mediterranean is saltier than the Atlantic it connects to. The Baltic Sea, cold, rainy and fed by many rivers, is only slightly salty. Closer home, the Arabian Sea is saltier than the Bay of Bengal, which receives the huge flow of the Ganga and Brahmaputra and heavy monsoon rain. Inland lakes with no outlet, such as the Dead Sea, Lake Van and the Great Salt Lake, are far saltier than any ocean.
What raises and lowers salinity
FactorEffect on salinityExample
High evaporation, little rainRaisesSubtropical oceans, Red Sea
Heavy rainfallLowersEquatorial belt
Large river inflowLowersBay of Bengal, Baltic Sea
Formation of sea iceRaises (salt rejected into water)Polar seas in winter
Melting of iceLowersPolar seas in summer
Restricted link to the open ocean in a dry regionRaisesMediterranean, Persian Gulf

या भागात भांडे आणि त्यातले पाणी दोन्ही पाहिले. समुद्रतळ पायऱ्यांसारखा उतरतो — उथळ मंच, उतार, सपाट खोल मैदान; मध्ये भूपट्ट खाली जातात त्या गर्ता, दूर जातात त्या रांगा. पाणी उष्ण कटिबंधात वर उबदार आणि खारट; ध्रुवांकडे आणि खोल जावे तसे थंड. उष्णता आणि क्षारता मिळून पाण्याचे वजन (घनता) ठरवतात: थंड खारे पाणी जड होऊन बुडते, उबदार किंवा गोडे पाणी वर तरंगते — ग्लासात तेल पाण्यावर तरंगते तसे. हे वजनाचे फरक आणि वारे मिळून समुद्राला गती देतात — तो दुसरा भाग. (→ English: salinity · हिन्दी: लवणता)

In exam EnglishThis part has described the container and its contents. The ocean floor steps down from the shallow, resource-rich continental shelf over the slope and rise to the flat abyssal plains, broken by the trenches where plates sink and the ridges where they part. The water in this basin is warmest and saltiest near the surface in low latitudes, and becomes colder towards the poles and with depth. Temperature and salinity together decide the density of sea water: cold, salty water is heavy and sinks, while warm or fresh water floats on top. These differences of density, along with the winds, set the ocean's water in motion, which is the subject of Part 2 on waves, tides and currents.

Exam traps

Recap

Practice questions

Q1. Which one of the following parts of the ocean floor is most important for fisheries and offshore oil and gas?

  1. Continental shelf
  2. Abyssal plain
  3. Ocean trench
  4. Mid-ocean ridge
Show answer

Answer: A. Continental shelf

The shallow, sunlit, nutrient-rich water over the continental shelf supports plankton and fish, and its thick sediments hold oil and gas, as at Mumbai High. The abyssal plain and trenches are too deep and dark for rich fisheries, and mid-ocean ridges are deep volcanic zones, not the main source of offshore oil.

Q2. Consider the following statements:
1. Most of the world's ocean trenches are found in the Pacific Ocean.
2. Ocean trenches form where two plates move apart and new crust is created.
Which of the statements given above is/are correct?

  1. 1 only
  2. 2 only
  3. Both 1 and 2
  4. Neither 1 nor 2
Show answer

Answer: A. 1 only

Statement 1 is correct: trenches ring the Pacific along its subduction zones. Statement 2 is wrong: trenches form where one plate sinks beneath another; places where plates move apart produce mid-ocean ridges. So '2 only' and 'both' are wrong, and 'neither' rejects a true statement.

Q3. A 'guyot' is best described as:

  1. A flat-topped submarine mountain
  2. A deep, narrow trench at a subduction zone
  3. A canyon cut into the continental slope
  4. A ring-shaped coral island
Show answer

Answer: A. A flat-topped submarine mountain

A guyot is a seamount whose top was planed flat by waves before it subsided. A deep trench at a subduction zone is an ocean trench, a canyon in the slope is a submarine canyon, and a ring-shaped coral island is an atoll.

Q4. With reference to the temperature of ocean water, the 'thermocline' is:

  1. The warm, well-mixed layer at the surface
  2. A layer in which temperature falls rapidly with depth
  3. The cold layer of uniform temperature near the sea floor
  4. A line joining places of equal sea-surface temperature
Show answer

Answer: B. A layer in which temperature falls rapidly with depth

The thermocline is the zone, often between about 200 m and 1,000 m, where temperature drops quickly with depth. The warm surface layer lies above it and the cold deep water below it, and a line of equal temperature on a map is an isotherm, not a layer.

Q5. Why is the surface salinity of the oceans generally higher in the subtropics than at the equator?

  1. The subtropics have higher evaporation and less rainfall
  2. Large rivers bring more salt to the subtropics
  3. Sea ice forms throughout the year in the subtropics
  4. The equator receives no sunlight for part of the year
Show answer

Answer: A. The subtropics have higher evaporation and less rainfall

Sinking air in the subtropical highs gives clear skies, strong evaporation and little rain, so salt is concentrated; heavy daily rain dilutes the equatorial water. River inflow lowers salinity rather than raising it, sea ice does not form in the subtropics, and the equator receives strong sunlight all year.

Q6. Consider the following statements:
1. The formation of sea ice raises the salinity of the surrounding water.
2. The Bay of Bengal has higher surface salinity than the Arabian Sea.
Which of the statements given above is/are correct?

  1. 1 only
  2. 2 only
  3. Both 1 and 2
  4. Neither 1 nor 2
Show answer

Answer: A. 1 only

Statement 1 is correct: when sea water freezes, most of the salt is left in the water below. Statement 2 is wrong: the Bay of Bengal is fresher because of the Ganga–Brahmaputra inflow and heavy monsoon rain, while the drier Arabian Sea is saltier. So '2 only' and 'both' are wrong, and 'neither' rejects a true statement.

Q7. Arrange the following water bodies in increasing order of surface salinity:
1. Baltic Sea
2. Open Atlantic Ocean
3. Red Sea
Select the correct answer using the code given below:

  1. 1-2-3
  2. 2-1-3
  3. 3-2-1
  4. 1-3-2
Show answer

Answer: A. 1-2-3

The cold, rainy, river-fed Baltic is only slightly salty, the open Atlantic is near the ocean average of about 35, and the Red Sea, in hot deserts with almost no rivers, is among the saltiest seas: 1-2-3. The other orders either put the Baltic above the open ocean or the Red Sea below it.

Q8. Match the following:
A. Challenger Deep — 1. Indian Ocean
B. Sunda (Java) Trench — 2. Mariana Trench, Pacific Ocean
C. Iceland — 3. Mid-Atlantic Ridge
Select the correct code:

  1. A-2, B-1, C-3
  2. A-1, B-2, C-3
  3. A-2, B-3, C-1
  4. A-3, B-1, C-2
Show answer

Answer: A. A-2, B-1, C-3

The Challenger Deep is in the Mariana Trench of the Pacific, the Sunda or Java Trench is the deepest part of the Indian Ocean, and Iceland is part of the Mid-Atlantic Ridge rising above the sea: A-2, B-1, C-3. The second option swaps the two trenches, and the others wrongly place Iceland or the Challenger Deep.

All partsNext: Part 2 →

Study this in the Upscore app to keep notes, save it for revision, track your practice scores and follow a plan to the exam date. It's free.

Study this in the app →

Sources