Internal Landform Processes (Tectonic Processes)
Senior 2 · Geography and Environment
Internal Landform Processes (Tectonic Processes)
Lesson 3 of 4
22 min
The Irish potatoes sold in Kigali markets mostly come from the north-west, where the soil is dark and rich. Tourists pay to trek to the mountain gorillas on the forested slopes above Musanze. Families in Rubavu felt the ground shake for days in May 2021 when Nyiragongo erupted across the border. All three facts come from the same cause: the Virunga volcanoes. Volcanic activity is both one of Rwanda's great resources and one of its hazards.
Magma forms where rock in the upper mantle melts, usually where plates pull apart (as in the East African Rift), where one plate sinks under another, or over unusually hot spots in the mantle. The magma is lighter than the rock around it, so it rises through cracks and lines of weakness such as faults. Gas pressure then forces it out.
| Landform | Shape |
|---|---|
| Batholith | A huge dome-shaped mass of cooled magma (often granite) deep in the crust |
| Dyke | A vertical wall of rock that cuts across the rock layers |
| Sill | A horizontal sheet that lies along the rock layers |
| Laccolith | A dome that pushes the layers above it upward, with a flat base |
They are exposed only when the rocks above are worn away.
Most volcanoes lie along plate boundaries: around the Pacific Ocean (the "Ring of Fire"), along mid-ocean ridges (Iceland), and along the East African Rift (Kilimanjaro, Mount Kenya, Mount Elgon, the Virunga). A few lie over hot spots in the middle of plates, such as Hawaii.
"Volcanoes have done Rwanda more good than harm." Discuss.
Good: weathered lava makes deep, fertile soils, so Musanze, Nyabihu and Rubavu are major producers of Irish potatoes, pyrethrum and vegetables; Volcanoes National Park earns large tourism income from gorilla trekking; the lava-dammed Lakes Burera and Ruhondo drive the Ntaruka hydropower station; volcanic rock is used for building; hot springs attract visitors; heat below the ground may one day produce geothermal energy.
Harm: eruptions destroy homes and farms. When Nyiragongo erupted on 22 May 2021, at least 32 people died, and thousands of people from Goma crossed into Rwanda for refuge; earthquakes after the eruption cracked roads and houses in Rubavu. Volcanic gases can be poisonous, and the steep slopes are hard to farm and build roads on.
Judgement: Rwanda's own volcanoes are dormant, so the country mostly enjoys the benefits (soils, tourism, power), while the dangers come mainly from the active volcanoes across the border. A good answer says so, and notes that the risk is real enough to need monitoring and evacuation plans.
Tip
Exam technique. In a "discuss" question, give both sides with Rwandan examples and finish with a one-sentence judgement. Examples that score: Irish potatoes in Musanze, gorilla tourism, Ntaruka hydropower, Rubavu hot springs, Nyiragongo 2021.
Solution: magma is molten rock below the surface; lava is molten rock that has reached the surface.
Solution: a dyke. (A horizontal sheet along the layers is a sill.)
Solution: lava from the Virunga volcanoes dammed river valleys, and water collected behind the lava.
Solution: Nyiragongo and Nyamuragira, both in the DRC.
Solution: any two of: fertile soils for potatoes and pyrethrum; gorilla tourism; hydropower at Ntaruka; building stone; hot-spring tourism.
Summary
Sources and credits
The explanations of landforms, the worked example and the practice items are written by Ganzaa.
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
