Oceans cover about 71% of the Earth’s surface.
They regulate climate, control weather patterns, support marine life, and influence human activities like fishing, navigation, and trade.
Oceanography is the branch of geography that studies:
- The structure of the ocean floor
- Physical properties of ocean water
- Movement of ocean water
- Ocean currents and tides
This chapter explains how oceans function as a dynamic system.
Table of Contents
Division of the Ocean Floor
The ocean floor is not flat. It has several distinct relief features formed by geological processes.
Continental Shelf
The continental shelf is the shallow, gently sloping part of the ocean floor near the coast.
Characteristics:
- Extends from shoreline to deeper sea
- Rich in marine life
- Important for fishing and oil extraction
Most human activities in oceans occur here.
Continental Slope
The continental slope lies beyond the continental shelf.
Characteristics:
- Steeper than the shelf
- Marks the boundary between continental and oceanic crust
It leads to deeper ocean regions.
Deep Sea Plain
The deep sea plain is the flattest and deepest part of the ocean floor.
Characteristics:
- Covered with fine sediments
- Very few relief features
It covers the largest area of the ocean floor.
Oceanic Ridges
Oceanic ridges are long underwater mountain chains.
Characteristics:
- Formed by volcanic activity
- Sites of sea-floor spreading
Example:
- Mid-Atlantic Ridge
Trenches
Ocean trenches are deep, narrow depressions.
Characteristics:
- Deepest parts of oceans
- Formed due to plate subduction
Example:
- Mariana Trench
Temperature of Ocean Water
Ocean water temperature varies across the globe.
Horizontal Distribution of Temperature
- Highest near the equator
- Decreases towards the poles
This pattern is due to variation in solar heating.
Vertical Distribution of Temperature
Ocean temperature decreases with depth.
Layers:
- Surface layer: warm
- Thermocline: rapid decrease in temperature
- Deep layer: cold and uniform
This layering affects marine life and currents.
Factors Affecting Ocean Temperature
- Latitude
- Ocean currents
- Winds
- Depth
Salinity of Ocean Water
Salinity refers to the amount of dissolved salts in ocean water.
Average salinity is about 35 parts per thousand (ppt).
Factors Affecting Salinity
- Evaporation
- Precipitation
- River inflow
- Ice melting
High evaporation increases salinity, while rainfall and river water reduce it.
Distribution of Salinity
- Higher in subtropical regions
- Lower near equator and poles
Salinity affects density and movement of ocean water.
Movement of Ocean Water
Ocean water is constantly moving.
There are three main types of movements:
- Waves
- Currents
- Tides
Each has different causes and effects.
Ocean Currents
Ocean currents are large streams of moving water on the ocean surface.
They flow in definite directions.
Causes of Ocean Currents
- Wind
- Difference in temperature
- Difference in salinity
- Rotation of Earth
Types of Ocean Currents
- Warm currents: move from equator towards poles
- Cold currents: move from poles towards equator
Effects of Ocean Currents
Ocean currents:
- Influence climate of coastal regions
- Affect rainfall patterns
- Help navigation
- Support marine life
Example:
- Gulf Stream warms Western Europe.
Tides
Tides are the periodic rise and fall of sea level.
They occur due to:
- Gravitational pull of the Moon
- Gravitational pull of the Sun
Types of Tides
- High tide: rise of sea level
- Low tide: fall of sea level
Spring Tides
Occur during:
- New moon
- Full moon
Sun, Moon, and Earth are in a straight line, causing maximum tides.
Neap Tides
Occur during:
- First quarter
- Last quarter of moon
Tidal range is minimum.
Importance of Tides
Tides help in:
- Navigation
- Fishing
- Cleaning of harbours
- Generation of tidal energy
Importance of Oceanography
Oceanography helps us:
- Predict climate changes
- Understand marine resources
- Improve navigation
- Study natural hazards
It is a highly scoring topic if concepts are clear.
FAQs – OCEANOGRAPHY (Exam-Focused)
Which part of ocean floor is richest in marine life?
Continental shelf.
Which is the deepest part of the ocean?
Ocean trench.
What is average salinity of ocean water?
About 35 ppt.
Which layer shows rapid temperature change?
Thermocline.
What causes ocean currents?
Wind, temperature, salinity, Earth’s rotation.
Which current warms Western Europe?
Gulf Stream.
What are tides?
Periodic rise and fall of sea level.
Which celestial body mainly causes tides?
Moon.
When do spring tides occur?
New moon and full moon.
Do tides help navigation?
Yes.
Last-Moment Notes (Cheat Sheet)
OCEANOGRAPHY
- Ocean covers 71% of Earth
- Continental shelf → shallow, rich in life
- Continental slope → steep descent
- Deep sea plain → flat and deep
- Ocean ridges → underwater mountains
- Trenches → deepest parts
- Surface water warm near equator
- Temperature decreases with depth
- Thermocline → rapid temperature fall
- Salinity → dissolved salts
- Average salinity → 35 ppt
- Ocean movements → waves, currents, tides
- Warm currents → equator to poles
- Cold currents → poles to equator
- Tides caused by Moon and Sun
- Spring tides → maximum
- Neap tides → minimum