Class 11 Geography Notes Chapter 13 (Water (oceans)) – Fundamental of Physical Geography Book
Alright class, let's delve into Chapter 13, 'Water (Oceans)', from our Physical Geography textbook. This is a crucial chapter, not just for understanding our planet, but also frequently tested in various government examinations. Pay close attention to the details.
Chapter 13: Water (Oceans) - Detailed Notes
1. Introduction: The Hydrological Cycle
- Water is essential for all life forms. The Earth is often called the 'Blue Planet' as about 71% of its surface is covered by water.
- Oceans hold approximately 97% of all Earth's water. The remaining 3% is freshwater (ice caps, glaciers, groundwater, lakes, rivers, atmosphere).
- Hydrological Cycle: The continuous circulation of water between the atmosphere, oceans, and land (lithosphere). It involves processes like:
- Evaporation: Water turns into vapour and rises.
- Transpiration: Water vapour released from plants.
- Condensation: Water vapour turns into liquid water (clouds).
- Precipitation: Water falls back to Earth (rain, snow, sleet, hail).
- Runoff: Water flows over the land surface into rivers, lakes, and oceans.
- Infiltration: Water seeps into the ground.
- Oceans are the primary source of atmospheric moisture through evaporation and play a vital role in regulating global climate.
2. Relief of the Ocean Floor
The ocean floor is not flat; it has complex and varied features similar to continents. Major divisions include:
- (a) Continental Shelf:
- Shallow, submerged extension of the continent.
- Gentle slope (average gradient 1° or less).
- Average depth: around 100 fathoms (approx. 180 meters).
- Width varies greatly (narrow along mountainous coasts like the Andes, wide along low-lying coasts like Siberia).
- Importance: Rich fishing grounds, mineral resources (oil, natural gas, sand, gravel). Receives sediments from land.
- (b) Continental Slope:
- Marks the seaward edge of the continental shelf.
- Steep gradient (2°-5°).
- Connects the shelf to the deep ocean floor.
- Depth ranges from about 200m to 3,000m.
- Features: Submarine canyons often cut through the slope.
- (c) Continental Rise:
- Located at the base of the continental slope where the gradient decreases.
- Accumulation of sediments deposited from the slope (like an alluvial fan on land).
- Gradually merges into the abyssal plain.
- (d) Abyssal Plains:
- Extensive, flat areas of the deep ocean floor (between 3,000m and 6,000m).
- Covered with fine-grained sediments (clays, silts) called 'oozes' (pelagic deposits).
- Considered among the flattest and smoothest regions on Earth.
- (e) Mid-Oceanic Ridges (MORs):
- Vast underwater mountain ranges formed by tectonic plate divergence (seafloor spreading).
- Characterized by a central rift valley, volcanic activity, and earthquakes.
- Example: Mid-Atlantic Ridge, East Pacific Rise.
- Covers extensive areas of the ocean basin.
- (f) Ocean Deeps / Trenches:
- The deepest parts of the oceans.
- Long, narrow, steep-sided depressions formed at convergent plate boundaries (subduction zones).
- Located parallel to continents or island arcs.
- Example: Mariana Trench (deepest point, Challenger Deep), Tonga Trench, Peru-Chile Trench.
- (g) Seamounts and Guyots:
- Seamounts: Underwater mountains of volcanic origin rising from the seafloor but not reaching the surface.
- Guyots: Flat-topped seamounts, likely eroded by wave action when they were near the sea surface before subsiding.
3. Temperature of Ocean Waters
- Source of Heat: Primarily solar radiation (insolation). Absorption occurs mainly at the surface.
- Factors Affecting Temperature Distribution:
- Latitude: Temperature generally decreases from the equator towards the poles due to decreasing insolation.
- Unequal Distribution of Land and Water: Oceans in the Northern Hemisphere receive more heat due to greater land contact than in the Southern Hemisphere.
- Prevailing Winds: Winds blowing from land towards oceans can affect surface water temperatures (offshore winds cause upwelling of cold water).
- Ocean Currents: Warm currents raise temperatures in colder areas; cold currents lower temperatures in warmer areas. (e.g., Gulf Stream warms Western Europe; Labrador Current cools Northeast North America).
- Local Factors: Enclosed seas in low latitudes have higher temperatures; seas in high latitudes have lower temperatures. Salinity also has a minor effect.
- Vertical Distribution:
- Temperature generally decreases with increasing depth.
- Three Layers:
- Surface Layer (Epipelagic Zone): ~500m thick. Warmest layer, well-mixed by wind and waves. Receives sunlight. Temperature relatively uniform vertically. Seasonal variations occur.
- Thermocline: Boundary region below the surface layer where temperature decreases rapidly with depth. Typically between 500m - 1000m. Acts as a barrier between surface and deep waters. About 90% of ocean water volume is below the thermocline.
- Deep Layer (Hypopelagic/Bathypelagic Zones & below): Below the thermocline. Very cold (near freezing) and relatively uniform temperature.
- Horizontal Distribution: Shown using Isotherms (lines connecting points of equal temperature). Generally parallel to latitudes, but modified by currents and landmasses.
4. Salinity of Ocean Waters
- Definition: The amount of dissolved salts in seawater, usually expressed as parts per thousand (‰) or ppt.
- Average ocean salinity is about 35‰ (35 parts of salt per 1000 parts of water).
- Major Salts: Sodium Chloride (common salt - NaCl) is the most abundant, followed by magnesium chloride, magnesium sulphate, calcium sulphate, potassium sulphate, etc.
- Factors Affecting Salinity:
- Evaporation: Increases salinity by removing freshwater. Higher in hot, dry areas.
- Precipitation: Decreases salinity by adding freshwater. Lower near the equator (high rainfall).
- Freshwater Inflow: Rivers, melting ice (glaciers, icebergs) decrease salinity near coasts and in polar regions.
- Wind: Can transfer water to other areas, affecting local salinity.
- Ocean Currents: Mix waters of different salinities.
- Horizontal Distribution:
- Highest salinity generally found near the Tropics of Cancer and Capricorn (~37‰) due to high evaporation and low precipitation.
- Lower salinity near the equator (~35‰) due to high rainfall and cloudiness.
- Lowest salinity in polar regions (<32‰) due to ice melt.
- Enclosed seas show variations: Red Sea (high salinity >40‰), Baltic Sea (low salinity <10‰).
- Vertical Distribution:
- Salinity generally increases with depth but can vary.
- Halocline: A zone where salinity increases sharply with depth. Often coincides with the thermocline.
- In high latitudes, salinity may decrease with depth (surface freezing increases salinity, while deeper water is less affected) or increase with depth (melting ice creates a low-salinity surface layer).
5. Movements of Ocean Water
Ocean water is constantly in motion. Major movements are Waves, Tides, and Currents.
- (a) Waves:
- Energy moving through water, causing water particles to oscillate (move in circles/ellipses) but not travel horizontally over long distances (except near shore - breakers).
- Generated primarily by wind blowing over the water surface. Friction transfers energy.
- Characteristics:
- Crest: Highest point of a wave.
- Trough: Lowest point of a wave.
- Wave Height: Vertical distance between crest and trough.
- Wave Length: Horizontal distance between two successive crests or troughs.
- Wave Period: Time taken for two successive crests/troughs to pass a fixed point.
- Wave Frequency: Number of waves passing a fixed point per unit time.
- Wave Speed: Rate at which the wave form travels. Depends on wavelength and water depth.
- Wave size depends on wind strength, duration, and fetch (distance over which wind blows).
- Tsunami: Very long wavelength waves caused by underwater earthquakes, volcanic eruptions, or landslides. NOT caused by wind. Extremely destructive in coastal areas.
- (b) Tides:
- Periodic rise and fall of sea level, typically twice a day.
- Causes: Primarily the gravitational pull of the Moon (stronger effect due to proximity) and the Sun, combined with the centrifugal force generated by the Earth-Moon system's rotation.
- Types based on Sun-Moon Alignment:
- Spring Tides: Occur during New Moon and Full Moon when the Sun, Moon, and Earth are aligned (syzygy). Gravitational pulls combine, resulting in unusually high high tides and low low tides (greater tidal range).
- Neap Tides: Occur during the First and Third Quarter Moon phases when the Sun and Moon are at right angles relative to Earth. Gravitational pulls partially counteract each other, resulting in lower high tides and higher low tides (smaller tidal range).
- Types based on Frequency and Height:
- Semi-diurnal Tide: Two high tides and two low tides each day, of approximately equal height. (Common along eastern North America).
- Diurnal Tide: One high tide and one low tide each day. (Common in Gulf of Mexico).
- Mixed Tide: Two high tides and two low tides each day, but with significant differences in height. (Common along the Pacific coast of North America).
- Importance: Navigation (entering/leaving shallow harbours), fishing (tidal currents concentrate fish), desilting estuaries, potential for tidal energy generation.
- (c) Ocean Currents:
- Continuous, directed movement of large masses of ocean water, flowing like rivers in the oceans.
- Causes:
- Primary Forces (initiate movement):
- Solar Heating: Causes water expansion; water piles up slightly higher at the equator, creating a gentle slope towards poles.
- Wind: Drags surface water through friction.
- Gravity: Pulls piled-up water down the slope.
- Coriolis Effect: Deflects moving water to the right in the Northern Hemisphere and to the left in the Southern Hemisphere due to Earth's rotation.
- Secondary Forces (modify flow):
- Differences in Water Density: Caused by variations in temperature and salinity. Colder, saltier water is denser and sinks, while warmer, less saline water rises. Drives deep ocean circulation (thermohaline circulation).
- Shape of Coastlines and Ocean Basins: Influence the direction and intensity of currents.
- Primary Forces (initiate movement):
- Types:
- Warm Currents: Flow from lower latitudes (equator) towards higher latitudes (poles). Bring warmer water to colder regions. (e.g., Gulf Stream, Kuroshio). Generally flow along the east coasts of continents in lower/middle latitudes.
- Cold Currents: Flow from higher latitudes (poles) towards lower latitudes (equator). Bring colder water to warmer regions. (e.g., Labrador Current, Benguela Current, Oyashio). Generally flow along the west coasts of continents in lower/middle latitudes.
- Major Ocean Currents: (Familiarize yourself with maps)
- Atlantic Ocean: North Equatorial C., South Equatorial C., Gulf Stream (Warm), North Atlantic Drift (Warm), Canary C. (Cold), Labrador C. (Cold), Benguela C. (Cold), Brazil C. (Warm), Falkland C. (Cold).
- Pacific Ocean: North Equatorial C., South Equatorial C., Kuroshio C. (Warm), North Pacific Drift (Warm), Oyashio C. (Cold), California C. (Cold), Peru/Humboldt C. (Cold), East Australian C. (Warm).
- Indian Ocean: Currents influenced by Monsoons. North Equatorial C. (changes direction seasonally - SW Monsoon Drift, NE Monsoon Drift), South Equatorial C., Mozambique C. (Warm), Agulhas C. (Warm), West Australian C. (Cold). Antarctic Circumpolar Current (West Wind Drift) flows clockwise around Antarctica.
- Effects of Currents:
- Climate Moderation: Warm currents make adjacent coasts warmer and wetter; cold currents make them cooler and drier (often associated with coastal deserts like Atacama, Namib).
- Navigation: Ships use currents to save fuel and time.
- Fishing Grounds: Mixing zones of warm and cold currents (e.g., off Newfoundland, Japan) are rich in plankton, attracting large fish populations.
- Fog Formation: Warm air moving over cold currents causes dense fog (e.g., Grand Banks off Newfoundland).
Multiple Choice Questions (MCQs)
-
Which part of the ocean floor represents the submerged edges of continents and is generally shallow?
a) Abyssal Plain
b) Continental Slope
c) Continental Shelf
d) Oceanic Trench -
The rapid decrease in ocean temperature with increasing depth is known as the:
a) Halocline
b) Isotherm
c) Thermocline
d) Pycnocline -
What is the average salinity of ocean water?
a) 25‰
b) 30‰
c) 35‰
d) 40‰ -
Spring Tides occur when:
a) The Sun and Moon are at right angles to the Earth.
b) The Moon is closest to the Earth (perigee).
c) The Sun, Moon, and Earth are aligned in a straight line.
d) Only during the spring season. -
The primary driving force behind surface ocean currents is:
a) Salinity differences
b) Temperature differences
c) Wind
d) Gravitational pull of the Moon -
Which of the following is a major Cold Ocean Current in the Atlantic Ocean?
a) Gulf Stream
b) Brazil Current
c) Canary Current
d) North Atlantic Drift -
Mid-Oceanic Ridges are formed primarily due to:
a) Subduction of oceanic plates
b) Divergence of tectonic plates (seafloor spreading)
c) Folding of the ocean crust
d) Deposition of sediments -
The Coriolis effect causes moving water in the Southern Hemisphere to be deflected towards the:
a) Right
b) Left
c) Equator
d) Poles -
Guyots are best described as:
a) Deepest parts of the ocean
b) Active underwater volcanoes
c) Flat-topped submerged mountains
d) Gently sloping areas near continents -
Which factor generally leads to lower salinity in ocean surface waters?
a) High rate of evaporation
b) Formation of sea ice
c) Low precipitation
d) High river water inflow
Answer Key:
- c) Continental Shelf
- c) Thermocline
- c) 35‰
- c) The Sun, Moon, and Earth are aligned in a straight line.
- c) Wind
- c) Canary Current
- b) Divergence of tectonic plates (seafloor spreading)
- b) Left
- c) Flat-topped submerged mountains
- d) High river water inflow
Ensure you revise these notes thoroughly. Understanding the interplay between different oceanic features and processes is key. Refer to the diagrams in your textbook, especially for ocean floor relief and major currents. Good luck with your preparation!