Lakes Caused by Earth Movement

How Earth Movements Create Lakes

The earth’s crust is not completely still. It moves, bends, and sometimes breaks due to forces deep within the earth. These movements can create hollow areas on the surface. When these depressions fill with water from rivers, rain, or underground springs, they become lakes.

Understanding how earth movements form lakes helps explain why some lakes are very deep while others are shallow. It also shows why certain lakes have specific shapes and locations. In Africa, many of our largest and deepest lakes formed because of earth movements millions of years ago.

The process usually takes a very long time. Forces inside the earth push and pull on the crust for thousands or millions of years. Slowly, the land rises in some places and sinks in others. Eventually, water collects in the lowest areas, creating permanent lakes.

Types of Lakes Formed by Earth Movement

1. Rift Valley Lakes

Rift valley lakes are among the deepest and most spectacular lakes on earth. They form when the earth’s crust pulls apart along parallel faults. The land between the faults sinks down, creating a long, narrow valley. Water then fills the valley floor, forming a lake.

The East African Rift Valley contains many famous lakes formed this way. Lake Tanganyika, the world’s second deepest lake, lies in a rift valley between Tanzania and the Democratic Republic of Congo. It reaches depths of over 1,400 meters because the valley floor sank so far below the surface.

Lake Malawi (also called Lake Nyasa) is another rift valley lake. It formed in a similar way along the East African Rift. The lake is about 560 kilometers long but only 80 kilometers wide, showing the typical long, narrow shape of rift valley lakes.

Lake Turkana in northern Kenya also occupies a rift valley. Local people call it the “Jade Sea” because of its green-blue color. Like other rift valley lakes, it is long and narrow, stretching about 250 kilometers from north to south.

These lakes share common features. They are usually very deep, have steep sides, and follow straight or gently curved lines. The depth makes them important for fishing and water supply. Lake Tanganyika alone contains about 16% of the world’s available fresh water.

2. Tectonic Lakes

Tectonic lakes form when broad areas of the earth’s crust warp, sag, or tilt without breaking into faults. The movement is gentler than faulting but can create large shallow depressions.

Lake Chad in northeastern Nigeria is a good example of a tectonic lake. It formed in a depression created by sagging of the earth’s crust. Unlike rift valley lakes, Lake Chad is very shallow – mostly less than 7 meters deep even in its deepest parts.

The Chad Basin, which holds the lake, formed over millions of years as the weight of sediments caused the land to sink gradually. Rivers like the Chari, Logone, and Yobe bring water into this depression, maintaining the lake.

Lake Chad’s size changes dramatically with the seasons and over longer periods. During the rainy season, it expands. In dry periods, it shrinks. Over the past 50 years, the lake has become much smaller due to climate change and increased water use for irrigation. This would not happen as easily with a deep rift valley lake.

Other tectonic lakes include the Caspian Sea, the world’s largest lake. Despite its name, it is actually a lake formed in a depression created by crustal warping. Lake Victoria in East Africa also formed partly through tectonic processes, though its history involves several factors.

3. Fault Lakes

Fault lakes form along single fault lines where movement displaces the land. Unlike rift valleys that require two parallel faults, a fault lake can form along just one fracture.

When rocks move along a fault, they may create a depression on one side of the fault line. If a river flows across the area, the fault movement may dam the river, creating a lake. Sometimes the fault creates a step in the landscape where water can collect.

The Dead Sea between Israel and Jordan formed partly from faulting. The land dropped along the Jordan Rift Valley, creating a depression that filled with water. At 430 meters below sea level, it is the lowest point on earth’s land surface.

In California, Clear Lake occupies a depression created by faulting along the San Andreas Fault system. The movement of the fault helped create and maintain the basin that holds the lake.

4. Lakes in Grabens

A graben is a block of land that has dropped down between two parallel faults. It is essentially a small rift valley. When a graben fills with water, it creates a lake.

The term comes from a German word meaning “ditch” or “trench,” which describes the shape well. Grabens are usually smaller than major rift valleys but form in the same way – through tensional forces pulling the crust apart.

Many lakes in the East African Rift System occupy grabens. The steep fault scarps on either side of these lakes show where the land dropped down. You can sometimes see these scarps as cliffs rising directly from the lake shore.

Comparison of Lake Types

Characteristics of Earth Movement Lakes

Lakes formed by earth movements have special characteristics that set them apart from other types of lakes.

Great depth: Rift valley lakes are among the deepest in the world. Lake Tanganyika (1,470 m deep) and Lake Malawi (706 m deep) formed when the crust dropped thousands of meters. This depth creates unique ecosystems and affects water chemistry.

Steep sides: Many earth movement lakes have very steep shorelines. The fault scarps rise sharply from the water’s edge. This limits the development of shallow water zones where plants usually grow.

Linear shape: Rift valley lakes follow the line of the rift, creating long, narrow water bodies. Lake Tanganyika is over 670 km long but only about 50 km wide at its widest point.

Stability: These lakes are usually permanent features. Unlike lakes formed by landslides or ice, earth movement lakes last for millions of years. Lake Tanganyika has existed for at least 12 million years.

Unique biology: The long existence and isolation of rift valley lakes has allowed unique species to evolve. Lake Malawi contains over 1,000 species of fish, most found nowhere else on earth.

The East African Rift System

The East African Rift System provides the best examples of lakes formed by earth movement. This system stretches over 6,000 kilometers from Mozambique in the south to the Red Sea in the north.

The Western Rift contains Lakes Tanganyika, Edward, Albert, and Kivu. These are all deep rift valley lakes with steep sides. The Eastern Rift includes Lakes Turkana, Naivasha, and several smaller lakes in Kenya and Tanzania.

Scientists believe the African continent is slowly splitting apart along this rift. In millions of years, the rift may widen enough to allow ocean water to flood in, creating a new sea. The rift lakes show us this process in action.

The rift system demonstrates how earth movements continue today. Earthquakes occur frequently along the rift. Volcanoes like Mount Kilimanjaro and Mount Kenya formed from magma rising through the fractures. Hot springs and geysers dot the area, showing that heat and movement continue below the surface.

Importance of Earth Movement Lakes

These lakes are important for several reasons:

Water supply: They hold huge amounts of fresh water. Lake Tanganyika alone contains more water than all the Great Lakes of North America combined.

Fishing: Millions of people depend on fish from these lakes for protein. Lake Chad supports fishing communities in Nigeria, Chad, Niger, and Cameroon.

Transportation: Large lakes provide water routes. Boats and ferries carry people and goods across lakes like Tanganyika and Malawi.

Biodiversity: The unique species in rift valley lakes are important for science and conservation. They show how evolution works in isolated environments.

Tourism: Beautiful lakes attract tourists, bringing income to local communities. The scenery of rift valley lakes with mountains rising from the shore is spectacular.

Changes in Lake Chad

Lake Chad shows how tectonic lakes can change over time. In the 1960s, the lake covered about 25,000 square kilometers. Today, it has shrunk to less than 2,000 square kilometers in the dry season.

Several factors caused this shrinking. Climate change has reduced rainfall in the Sahel region. Rivers that feed the lake carry less water. Irrigation projects take water for farming. The lake’s shallow depth makes it vulnerable – a small drop in water level causes a large reduction in surface area.

This shrinking affects millions of people who depend on the lake. Fishing communities have lost their livelihoods. Farmland around the lake has dried up. Countries sharing the lake are working on plans to bring more water from the Congo River basin.

Common Exam Mistakes

WAEC examiners note these frequent errors:

• Confusing rift valley lakes with tectonic lakes: Students think all earth movement lakes are the same. Remember: rift valley lakes form from faulting (breaking), while tectonic lakes form from warping (bending without breaking).
• Wrong examples: Using Lake Chad as an example of a rift valley lake. Lake Chad is a tectonic lake formed by sagging, not a rift valley lake.
• Not explaining the process: Simply naming lakes without explaining how earth movements created them. Always describe the movement process.
• Mixing up characteristics: Writing that tectonic lakes are very deep or that rift valley lakes are shallow. Know the typical features of each type.
• Poor distinction: When asked to “distinguish between” types, students just define each type separately instead of comparing them directly.

Practice Questions

Multiple Choice Questions

1. Which of these lakes is a rift valley lake?
a) Lake Chad
b) Lake Tanganyika ✓
c) Caspian Sea
d) Lake Victoria

2. Lake Chad is an example of a:
a) Rift valley lake
b) Volcanic lake
c) Tectonic lake ✓
d) Glacial lake

3. What is the main difference between rift valley lakes and tectonic lakes?
a) Size of the lake
b) Rift valley lakes form from faulting while tectonic lakes form from warping ✓
c) Location on earth
d) Temperature of the water

4. Rift valley lakes are typically:
a) Broad and shallow
b) Long, narrow, and deep ✓
c) Circular and shallow
d) Irregular in shape

Essay Questions

1. Explain how rift valley lakes are formed. (6 marks)

Examiner’s tip: Describe the faulting process step by step. Mention tensional forces, parallel faults, the sinking of land between faults, and water filling the depression. Use a diagram if helpful.

2. Distinguish between tectonic lakes and rift valley lakes, giving one example of each. (8 marks)

Examiner’s tip: “Distinguish” means compare directly. Discuss formation process, typical depth, shape, and give correct examples. Don’t just define each separately.

3. Describe three characteristics of lakes formed by earth movements. (9 marks)

Examiner’s tip: Choose three clear characteristics like depth, shape, or stability. Describe each one fully with examples. Three marks per characteristic means you need good detail.

4. State four reasons why Lake Chad is important to Nigeria. (4 marks)

Examiner’s tip: “State” means list briefly. Each reason should be clear and specific to Lake Chad’s importance for Nigeria – fishing, water supply, grazing, irrigation.

Memory Aids

Remember the three main types with “RTF”:

• Rift valley lakes – Rips apart (faulting), deep and narrow
• Tectonic lakes – Tilts and sags (warping), shallow and broad
• Fault lakes – Fracture line, single fault

East African rift valley lakes – “TAMEK”:

• Tanganyika (deepest)
• Albert
• Malawi
• Edward
• Kivu

Lake Chad characteristics – “SNIF”:

• Shallow (less than 7m)
• Nigeria’s shared lake
• In Chad Basin
• Flood plain around it

Related Topics

• Types of faults and their effects on landforms
• Plate tectonics and continental drift
• River systems feeding African lakes
• Climate change effects on lake levels
• Volcanic lakes and crater lakes