The Structure of the Earth
Senior 1 · Geography
The Structure of the Earth
Lesson 3 of 3
22 min
Just across Rwanda's border, in the crater of Nyiragongo volcano, lies one of the few lakes of molten lava in the world. On a clear night its red glow can be seen from Rubavu. That liquid rock has come up from below the crust. Western Rwanda is one of the best places in Africa to see the Earth's inner heat at work.
Western Rwanda lies on the Albertine Rift, the western branch of the East African Rift. Here the continental crust is being slowly pulled apart. As it stretches it cracks along faults, and a long block of land has sunk between them, making a deep valley in which Lake Kivu lies. The stretched crust is also thinner, so hot mantle rock is closer to the surface. That one fact explains three features of the region (Figure 1).
Figure 1. Why volcanoes, hot springs and earthquakes are found together in western Rwanda. The features are placed side by side for clarity; in reality the Virunga volcanoes stand at the northern end of Lake Kivu.
Definition
Volcano: an opening in the crust through which molten rock, gases and ash erupt, and the mountain built up around it. Molten rock underground is called magma; once it reaches the surface it is called lava. Example: Nyiragongo and Nyamuragira (in the DR Congo) are active; Karisimbi has not erupted in recorded history, and the other Rwandan Virunga volcanoes are quiet: the last eruption linked to one of them was a small one on the northern flank of Bisoke in 1957.
Definition
Earthquake: a sudden shaking of the ground caused by rocks moving along a fault. The point underground where the movement starts is the focus; the point on the surface directly above it is the epicentre. Its strength is given as a magnitude.
Definition
Fault: a crack in the crust along which the rocks on either side have moved. Example: the faults on either side of the Lake Kivu basin.
Definition
Hot spring: a place where water that has been heated by hot rock deep underground comes out at the surface. Example: the hot springs at Nyamyumba on the shore of Lake Kivu near Rubavu.
Worked example 1: how far away was the earthquake?
An earthquake sends out two main kinds of wave. The faster P-waves arrive first; the slower S-waves arrive later. For nearby earthquakes, each second of delay between them means the earthquake was about 8 km away:
A seismometer in Rubavu records the P-waves, then the S-waves 5 seconds later. How far away was the focus?
With readings from three stations, scientists can draw three circles and find the epicentre where they cross.
Worked example 2: how deep did the hot water come from?
Water flows out of a hot spring at where the average surface temperature is . If rock warms by per km, how deep must the water have travelled?
But under the rift, hot magma lies closer to the surface, so the rock warms much faster than 25 °C per km. The water there may have needed to go far less deep. The normal rule only works where the crust is normal.
Important
Case study: the Lake Kivu earthquake of 3 February 2008
At 09:34 Rwanda time on Sunday 3 February 2008, an earthquake of magnitude 5.9 struck with its epicentre about 20 km north of Bukavu, in the Lake Kivu basin. It was caused by movement on the same kind of fault that formed the lake. In Rwanda 38 people were killed and 556 were injured or traumatised (UN figures), most of them in Rusizi and Nyamasheke districts; in Rusizi a church collapsed. Buildings of unreinforced brick and mud suffered most.
Lessons learned: in an earthquake, drop, cover and hold on under a strong table, away from windows and heavy objects; build with reinforced frames on firm ground; and plan schools and health centres as safe shelters. The same rift gives the region fertile soils, methane in Lake Kivu, hot springs for tourists and possible geothermal energy: Rwanda has explored areas near Karisimbi, Rubavu and Bugarama as sources of geothermal power.
The 'Killer Lake' Powering Rwanda, by BBC Click (4 min). Watch for where the gas in Lake Kivu comes from, and how the plant turns a hazard into electricity.
The west of Rwanda has volcanoes, earthquakes and dangerous lake gases, yet it is one of the most crowded parts of the country. Why do people choose to live there? Should the government encourage them to move? Give arguments on both sides.
Tip
Focus is Far down; Epicentre is on the Exterior (the surface). And remember the chain: pulled crust → faults and thin crust → magma rises → volcanoes, hot springs and earthquakes.
Practice
Remember
Further reading
Books and resources for this unit
Sources and credits
The lesson text, cross-section diagram, worked examples and practice are written by Ganzaa.
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
