Hydrosphere (Oceanic Basins) – SS2 Geography Lesson Note (Third Term)

An ocean basin is a large, low-lying area on Earth’s surface that is filled with seawater. It includes the entire ocean floor and is often characterised by distinct features such as continental shelves, slopes, abyssal plains, mid-ocean ridges and ocean trenches. Ocean basins are the vast expanses where the world’s oceans are situated, covering about 71% of the Earth’s surface.

Behavioural objectives

By the end of the lesson, students should be able to:

• describe the major parts of an ocean basin;
• define ocean salinity and state the factors that influence it;
• state the causes and types of ocean currents;
• explain the effects of ocean currents on adjacent coastlands;
• list the importance of oceans to man.

What are the major parts of an ocean basin?

The major parts of the ocean basin include:

1. Continental shelf: shallow, nearshore area extending from the coast.
2. Continental slope: steeper region beyond the continental shelf, leading to deeper waters.
3. Abyssal plain: vast, flat expanses of the ocean floor in deeper areas.
4. Mid-ocean ridges: underwater mountain ranges formed by tectonic activity, often associated with volcanic activity.
5. Ocean trenches: deep, elongated depressions in the ocean floor, typically found in subduction zones.

What is ocean salinity?

Ocean salinity refers to the concentration of dissolved salts in seawater, typically measured in parts per thousand (ppt). The average salinity of the world’s oceans is about 35 ppt.

Factors that influence ocean salinity

1. Evaporation: as seawater evaporates, freshwater is removed, leaving behind a higher concentration of salts. Regions with high evaporation rates, like the tropics, tend to have higher salinity.
2. Precipitation: rainfall and snowmelt introduce freshwater into the ocean, reducing salinity. Areas near the equator and those with high precipitation have lower salinity.
3. Ice melting: melting ice from polar regions adds freshwater and decreases salinity in those areas.
4. River input: rivers carry freshwater to the oceans, diluting seawater. Coastal regions near river mouths often have lower salinity.
5. Ocean currents: currents can transport water with different salinities. For example, warm currents from low-latitude regions carry higher-salinity water to higher latitudes.

What are ocean currents?

Ocean currents are continuous, directed movements of seawater. They shape the Earth’s climate, redistribute heat and influence marine ecosystems.

Causes of ocean currents

Ocean currents are primarily driven by wind, temperature, salinity and Earth’s rotation. The major forces include:

1. Wind: surface winds push water, generating surface currents.
2. Temperature and salinity: differences in water density, caused by differences in temperature and salinity, lead to vertical and deep currents.
3. Coriolis effect: Earth’s rotation deflects currents, resulting in the formation of gyres.

Types of ocean currents

1. Surface currents: occur in the upper layer and are mainly driven by wind. An example is the Gulf Stream.
2. Deep ocean currents: flow in deeper layers and are influenced by temperature and salinity. The thermohaline circulation is a significant deep current.

Effects of ocean currents on adjacent coastlands

1. Temperature regulation: warm currents raise coastal temperatures, while cold currents have a cooling effect.
2. Climate influence: currents can modify local climates, affecting precipitation patterns and weather conditions.
3. Erosion and deposition: strong currents contribute to coastal erosion or to the deposition of sediments, shaping the coastline.
4. Biodiversity: currents influence nutrient distribution, affecting marine ecosystems and fisheries along coastlines.
5. Navigation and trade routes: historical trade routes and navigation were often influenced by predictable ocean currents.

Why are oceans important to man?

The oceans are very important to humankind for these reasons:

1. Oceans regulate the Earth’s climate. They absorb and store large amounts of heat, influencing temperature and weather patterns globally.
2. Oceans support a vast array of marine life and contribute significantly to global biodiversity. They are an important source of food, with fisheries providing sustenance for millions of people worldwide.
3. Oceans are rich in resources such as fish, shellfish and algae, which form the basis of a multi-billion dollar fishing industry. They also contain valuable minerals, oil and natural gas.
4. Oceans serve as major highways for international trade, facilitating the transportation of goods between countries. Ports and shipping routes are vital to the global economy.
5. Oceans are popular destinations for recreation and tourism. Beaches, coastal resorts and marine activities contribute significantly to the tourism industry.
6. Phytoplankton in the oceans produce a substantial portion of the Earth’s oxygen through photosynthesis. This oxygen is essential for the survival of terrestrial and marine life.
7. Oceans absorb and store large amounts of carbon dioxide, which helps to reduce climate change. However, this also leads to ocean acidification, which affects marine ecosystems.

Evaluation

1. Define an ocean basin.
2. List the five major parts of an ocean basin.
3. State three factors that influence ocean salinity.
4. Explain how warm and cold currents affect adjacent coastlands.
5. State four ways in which oceans are important to man.

Assignment

Name the three major forces that cause ocean currents. Then state the two types of ocean currents, giving one example of each.