14.3.2 Fault-block mountains Fault-block mountains form when faults (or cracks) in the Earth’s crust force some parts of rock up and other parts to collapse down. Instead of folding, the crust fractures (pulls apart) and breaks into blocks. The exposed parts then begin to erode and shape mountains and valleys (see FIGURE4 ). Fault-block mountains usually have a steep front side and a sloping back. The Sierra Nevada and Grand Tetons in North America, the Great Rift Valley in Africa, and the Harz Mountains in Germany are examples of fault-block mountains. Another name for the uplifted (upthrown) blocks is horst , and the collapsed (downthrown) blocks are graben .
FIGURE4 The formation of fault-block mountains
Upthrown fault block
7
5
Downthrown fault block
1
3
5
4
2
6
Horst
6
Graben
1 Fault zone 2 Steep eastern face 3 Gentle western slope 4 Valley floors filled with sediments of cobbles, gravel and sand 5 Sedimentary rock layers 6 Bedrock 7 Sedimentary rock layers (5) now worn away
FIGURE5 Examples of fault-block mountains overlooking the Great Rift Valley in northern Kenya, Africa
366 Jacaranda Humanities Alive 8 Victorian Curriculum Third Edition
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