Hazards
Senior 1 · Geography
Hazards
Lesson 2 of 4
25 min
The image below was taken from space eleven days after Nyiragongo erupted on 17 January 2002. Healthy plants show green, clouds white, water dark blue. Find the red lines: they are lava flows. Where did they start, what did they cross, and where did they end?
The lava cut Goma in two and reached the lake. About 250 people died in that eruption. These are geological hazards: they come from forces inside the Earth or from gravity pulling rock and soil down slopes. Unit 4 explained how the rift under Lake Kivu produces volcanoes and earthquakes; this lesson looks at what they do to people, and adds the most common geological hazard in Rwanda, the landslide.
Definition
Earthquake: a sudden shaking of the ground caused by rocks moving along a fault, releasing energy as seismic waves. Its size is measured on a magnitude scale. Example: the magnitude 5.9 earthquake under Lake Kivu on 3 February 2008.
Definition
Volcanic eruption: the escape of lava, ash, rock fragments and gases from a vent in the Earth's crust. Example: the flank eruption of Nyiragongo on 22 May 2021.
Definition
Landslide: the sudden movement of a mass of soil, rock and debris down a slope under gravity, usually along a slip plane. When the material is very wet it flows as a mudslide. Example: the many landslides on the slopes above Lake Kivu on the night of 2–3 May 2023.
Important
Case study 1: the earthquake of 3 February 2008
At 09:34 on a Sunday morning an earthquake of magnitude 5.9 (6.1 in some reports) struck with its epicentre about 20 km north of Bukavu, under the southern part of the Lake Kivu basin. It was caused by movement on the same kind of fault that formed the lake.
Worked example 1: how much stronger?
On the magnitude scale, each step of 1 means about 32 times more energy released. Compare the 2008 earthquake (magnitude 5.9) with the strongest tremor felt in Rubavu in May 2021 (magnitude 4.9), and with a small tremor of magnitude 3.9.
Answer. The 2008 earthquake released about 32 times the energy of the magnitude 4.9 tremor and about 1,000 times that of a magnitude 3.9 tremor. A small rise in magnitude is a huge rise in energy.
Important
Case study 2: Nyiragongo, May 2021
Worked example 2: time to escape
Lava flows about 1 km per hour. How long would it take to travel the 20 km from Nyiragongo to Goma? Compare this with an earthquake.
Answer. About 20 hours, time for a whole city to walk away. An earthquake gives only seconds. That is why volcano plans focus on evacuation and earthquake plans focus on strong buildings and drills.
Figure 1. How a landslide happens on a steep slope (A), and how terraces, trees and drains protect the same slope (B). Drawn by Ganzaa.
Causes. Gravity always pulls soil downhill; friction holds it in place. A landslide happens when something tips the balance: long, heavy rain fills the pores of the soil so it becomes heavy and slippery; slopes are steep; trees and grass whose roots bind the soil have been cleared; a road or house platform has been cut into the foot of the slope; or an earthquake shakes the ground.
Effects. Houses, roads, bridges, water pipes and farms are buried or swept away; soil that took centuries to form is lost in minutes; rivers fill with mud.
Prevention. Terraces and grass strips on steep fields; trees on the steepest slopes and hilltops; ditches that lead water safely off the slope; no houses on slopes that are too steep or at the foot of unstable ones; and moving families out of high-risk zones before the rains.
Warning
Don't blame the rain alone. Rain is the trigger, but a landslide needs a cause too: a steep slope, soaked soil, cleared vegetation or a cut slope. An answer that gives only "heavy rain" misses the causes people can change.
Landslides, by National Geographic (4 min). Watch for the causes listed in Figure 1 and note one that is not shown there.
After the 2021 eruption, many families returned to their homes in Goma and Rubavu within days. Why do people stay in places with known hazards? Give two reasons, and suggest what the government can do to make staying safer.
Tip
Trigger vs cause. For every landslide, name the trigger (what set it off today: heavy rain, an earthquake) and the causes (why this slope was ready to fail: steepness, bare soil, a cut road). Examiners reward both.
Practice
Remember
Sources and credits
The lesson text, landslide 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.
