Internal Landform Processes (Tectonic Processes)
Senior 2 · Geography and Environment
Internal Landform Processes (Tectonic Processes)
Lesson 1 of 4
20 min
Stand at the Lake Kivu shore at Rubavu and look around: steep escarpments, volcanic peaks to the north, hot springs near the water. None of these were carved by rain or rivers. They were built from below, by forces inside the Earth. Geographers call these internal (tectonic) processes. They are why Rwanda has volcanoes and earthquakes, and why East Africa has one of the longest valleys on the planet. This unit explains the four main ones: folding, faulting, warping and vulcanicity, plus earthquakes.
The Earth has three main layers: a thin rocky crust, a thick hot mantle, and a core at the centre. The crust is broken into large pieces called tectonic plates. Heat from the interior keeps the mantle slowly moving, and the plates move with it, a few centimetres a year. Where plates or blocks of crust push together, pull apart or slide past each other, the rocks are put under stress:
Movements that act sideways, along the surface, are called horizontal (lateral) movements; those that raise or lower the crust are vertical movements.
Folding is the bending of rock layers when they are compressed. Layers of sedimentary rock (such as sandstone and shale) are laid down flat. If they are squeezed slowly from the sides, they do not snap at once; they buckle like a cloth pushed across a table.
| Fold type | What it looks like | Cause |
|---|---|---|
| Simple (symmetrical) | Both limbs slope at the same angle | Equal pressure from both sides |
| Asymmetrical | One limb steeper than the other | Stronger pressure from one side |
| Overfold (overturned) | One limb pushed over the other, both sloping the same way | Very strong pressure from one side |
| Recumbent | The fold lies almost flat, on its side | Even stronger one-sided pressure |
| Overthrust fold (nappe) | The fold breaks and the upper part slides forward over other rock | Pressure so great that the rock fractures |
A geologist draws a cross-section showing rock layers that rise, peak and fall, and whose left limb dips at 30° while the right limb dips at 70°. Which fold is it, and where did the stronger push come from?
The limbs dip at different angles, so it is asymmetrical. A fold is pushed steeper on the side away from the stronger force, because the rock is shoved towards that side. So the right limb, at 70°, is on the far side, and the stronger push came from the left.
Large-scale folding builds fold mountains: the Atlas Mountains of north-west Africa, the Cape Ranges of South Africa, the Alps, the Himalayas and the Andes. Rwanda's own oldest rocks, quartzites, sandstones and shales more than a billion years old, were folded and invaded by granite in a mountain-building period geologists call the Kibaran. Those ancient mountains have long since been worn down, and many of Rwanda's long hill ridges are made of the hard quartzite that resisted wear.
Tip
Memory hook. Anticline = Arch (up). Syncline = sink (down). For fold types, think of pushing a cloth harder and harder: simple, asymmetrical, overfold, recumbent, and finally it tears: overthrust.
Solution: Compression: the rock layers are squeezed from the sides and buckle.
Solution: upfold = anticline; downfold = syncline.
Solution: a recumbent fold.
Solution: any two of: tourism; relief rainfall on the windward slopes; hydroelectric power from rivers; oil or gas trapped in anticlines.
Summary
Sources and credits
The fold table, worked example and practice items are written by Ganzaa.
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
