The East African Rift and the Effects of Plate Tectonics – The Origin and Distribution of the Continents (Senior 6 Geography) | Ganzaa

The Origin and Distribution of the Continents

Lessons

Senior 6 · Geography

The Origin and Distribution of the Continents

4 lessons

Lesson 4 of 4

The East African Rift and the Effects of Plate Tectonics

25 min

A plate boundary on Rwanda's doorstep

Stand on the shore of Lake Kivu at Rubavu or Karongi and you are standing at the edge of a rift valley, where a continent is slowly being pulled apart. North of the lake rise the Virunga volcanoes; across it, in the Democratic Republic of the Congo, Nyiragongo holds a lake of molten lava. These are not accidents of scenery: they are the effects of plate movement, happening now, a few kilometres from Rwanda's border. This lesson studies the East African Rift System as a case study of a young divergent boundary, and then weighs the effects of plate tectonics on people.

Definition

East African Rift System: a chain of rift valleys, several thousand kilometres long, where the African plate is splitting into the Nubian plate (west) and the Somali plate (east). It has two branches: the Eastern (Gregory) Rift through Ethiopia, Kenya and northern Tanzania, and the Western (Albertine) Rift along the western borders of Uganda, Rwanda, Burundi and Tanzania. Between them lies the Victoria microplate, a small plate with Lake Victoria near its centre.

Figure 9: the East African Rift System. The rift axes are simplified. Rwanda (shaded) lies between the two branches, on the Victoria microplate, whose western edge follows the Albertine Rift through Lake Kivu.

How the rift is forming

Uplift. Hot mantle rising beneath East Africa pushed the crust up into broad domes; the Ethiopian and East African plateaus were raised, and enormous floods of basalt lava erupted.
Stretching and faulting. As the crust stretched it broke along normal faults. Long blocks dropped between them to form rift valleys, flanked by raised shoulders and steep escarpments (Figure 4 in Lesson 3).
Volcanoes and lakes. Magma rose through the thinned crust to build volcanoes such as Kilimanjaro, Mount Kenya and the Virungas, and water collected in the deep troughs to form long, narrow, deep lakes such as Tanganyika and Kivu.
A future ocean? In the north, at Afar, the rift meets the Red Sea and the Gulf of Aden, where the sea has already flooded in between separating plates. If East Africa keeps pulling apart, it could follow the same path, but only over millions of years.

According to the Wikipedia summary of GPS studies, the Somali and Nubian plates are separating at about 8–9 mm per year.

Worked example 1: how fast is Africa splitting?

Problem. Taking a separation rate of 8.5 mm per year, how far apart will the Nubian and Somali plates move in one million years, and how long would they take to open a gap 100 km wide?

Solution.

8.5\ \text{mm} \times 1\,000\,000 = 8\,500\,000\ \text{mm} = 8.5\ \text{km}

100\ \text{km} = 100\,000\,000\ \text{mm}, \qquad \frac{100\,000\,000}{8.5} \approx 11\,800\,000\ \text{years}

About 12 million years for 100 km: the Red Sea, now a few hundred kilometres wide, shows what that looks like. Headlines saying that "Africa is splitting in two" are right about the process but wrong if they suggest anyone alive will see it.

The Albertine Rift beside Rwanda

Rift lakes. Lakes Albert, Edward, Kivu and Tanganyika lie in the floor of the Western Rift. Lake Kivu covers about 2,700 km²; its surface is about 1,460 m above sea level and its greatest depth about 475 m. Some 58% of its waters lie in the DRC and the rest in Rwanda.
Volcanoes. The Virunga mountains cross the rift north of Lake Kivu. Karisimbi, on the border between Rwanda and the DRC, is the highest at 4,507 m. Nyiragongo (3,470 m), in the DRC about 12 km north of Goma and Lake Kivu, is one of Africa's most active volcanoes: with its neighbour Nyamuragira it accounts for about 40% of Africa's recorded eruptions. It erupted in 1977, 2002 and 2021. In 2002 lava flowed into Goma and about 400,000 people were evacuated, many across the border into Gisenyi (Rubavu); in 2021 lava reached the outskirts of the city.
Earthquakes. The U.S. Geological Survey recorded a magnitude 5.9 earthquake on 3 February 2008, centred about 20 km north of Cyangugu (Rusizi) at a depth of about 10 km.
Gas. The deep water of Lake Kivu holds large amounts of dissolved carbon dioxide and methane from volcanic sources. The KivuWatt plant on the Rwandan shore, run by the company ContourGlobal, extracts the methane to generate electricity; the same gases are a hazard, since a sudden release could suffocate people and animals on the shores.
Drainage. Lake Kivu probably once drained north towards Lake Edward. Lava from the Virungas blocked that outlet, and the lake now drains south through the Rusizi River into Lake Tanganyika.

Lake Kivu seen from space. The black line marks the border between Rwanda (right) and the Democratic Republic of the Congo (left). The long, narrow lake with its steep, indented shores fills the floor of the Albertine Rift. Image: NASA, public domain, via Wikimedia Commons.

The effects of plate tectonics

Effect	Benefit or hazard	Example
Fertile volcanic soils	Benefit: intensive farming and dense settlement	Slopes of the Virungas
Rift lakes	Benefit: fishing, water, transport, tourism	Lakes Kivu and Tanganyika
Gas and geothermal heat	Benefit: energy	Methane from Lake Kivu; geothermal power in the Kenyan Rift
Scenery and wildlife habitats	Benefit: tourism	Volcanoes National Park; the rift escarpments of Kenya
Minerals	Benefit: mining	Minerals concentrated by old tectonic and igneous activity
Earthquakes	Hazard: collapsed buildings, landslides	3 February 2008, near Rusizi
Volcanic eruptions	Hazard: lava flows, gas, evacuation	Nyiragongo, 2002 and 2021
Dissolved lake gases	Hazard: possible gas release	Lake Kivu
Steep escarpments	Hazard: landslides; costly roads	Rift shoulders along Lake Kivu

Worked example 2: an evaluation question

Question. "The people of western Rwanda gain more than they lose from living beside the Albertine Rift." Evaluate this statement.

Model answer outline.

For: fertile volcanic soils support farming; Lake Kivu provides fish, transport and water; its methane is used to generate electricity; the volcanoes and lake attract tourists. These benefits are continuous, every year.
Against: earthquakes such as that of 2008, eruptions of Nyiragongo such as those of 2002 and 2021, possible gas release from the lake, and landslides on the steep slopes can cause sudden, severe losses.
Judgement: the benefits are steady and large, while the hazards are rare but potentially severe. The balance depends on preparation: monitoring of the volcanoes and the lake, earthquake-resistant building, controlled gas extraction and evacuation plans turn a risky setting into a mainly beneficial one. A strong answer reaches a judgement and says what it depends on.

Warning

Rift valleys are not river valleys. A river valley is carved by running water and usually has a V-shape or a floodplain; a rift valley is a block of crust that has dropped between faults, with straight, steep walls, whatever rivers do later. Also avoid saying that "the Virunga volcanoes are at a subduction zone": they are rift volcanoes at a divergent boundary.

Tip

Exam technique. Learn one case study in detail with named places and dates. For the Albertine Rift: Lake Kivu (rift lake, gases, methane use), the Virungas (Karisimbi 4,507 m; Nyiragongo's eruptions of 2002 and 2021) and the 2008 earthquake near Rusizi. Specific evidence lifts an answer from "describe" to "evaluate".

Earth Science Classroom: "East African Rift Valley System: Geography & Tectonics". An overview of the rift system and the plates that are separating along it. As you watch, find the two branches and Lake Kivu on Figure 9.

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Practice

Name the two branches of the East African Rift and two lakes along each.
With the help of a labelled diagram, explain how a rift valley forms.
At 8.5 mm per year, how long would the Nubian and Somali plates take to separate by 50 km?
On which plate does Rwanda lie, and which feature marks that plate's western edge?
Give two pieces of evidence that the Albertine Rift is an active plate boundary today.
Explain why Lake Kivu is both a resource and a hazard.
"The East African Rift will become an ocean." Evaluate this statement.

Show answersHide answers

Western (Albertine): Albert, Edward, Kivu, Tanganyika (any two). Eastern (Gregory): Turkana, Manyara (and other smaller lakes).
Your diagram should show crust being stretched, two normal faults, a central block dropped between them (the rift valley floor), raised shoulders, and arrows showing the sides pulling apart; volcanoes and a lake may be added on the floor.
$50\ \text{km} = 50\,000\,000\ \text{mm}$ ; $\frac{50\,000\,000}{8.5} \approx 5\,900\,000$ years, about 5.9 million years.
The Victoria microplate; its western edge follows the Albertine Rift through Lake Kivu.
Active volcanoes (Nyiragongo erupted in 2002 and 2021); earthquakes (magnitude 5.9 near Rusizi in 2008); gases from volcanic sources dissolved in Lake Kivu.
Resource: water, fish, transport, tourism and methane used to generate electricity. Hazard: large amounts of dissolved carbon dioxide and methane that could be released suddenly, suffocating people and animals near the shore.
Possibly, but not certainly. The Red Sea shows that a continental rift can develop into a narrow sea, and the Afar region is already close to that stage. But rifting at 8–9 mm per year would take millions of years, and some rifts stop before they split a continent. A good answer states the evidence for and against and concludes with "possibly, over millions of years".

Summary

The East African Rift is a young divergent boundary where the Nubian and Somali plates separate at about 8–9 mm a year; Rwanda sits on the Victoria microplate between its two branches.
The Albertine Rift gives Rwanda's west its rift lakes, volcanoes, earthquakes and lake gases.
Plate tectonics brings benefits (soils, lakes, energy, tourism, minerals) and hazards (earthquakes, eruptions, gas, landslides).

1A plate boundary on Rwanda's doorstep

2How the rift is forming

3The Albertine Rift beside Rwanda

4The effects of plate tectonics

A plate boundary on Rwanda's doorstep

How the rift is forming

The Albertine Rift beside Rwanda

The effects of plate tectonics