Question 5
With the aid of diagrams, describe the characteristics and mode of formation of the following landforms:
- rift valley;
- horst.
Observation
This question was answered by very few candidates and most of those who attempted the question performed poorly. The characteristics of rift valley are:
- valley/depression produced by tensional forces
- valley produced by compressional forces also
- it has steep sides
- width varies from 30 – 100 km
- length varies and could be up to 7,200 km
- usually very deep and could be up to 800 metres
- usually consists of series of connected troughs
- water collects in them to form rift valley lakes
- it has parallel faults
- the side blocks are uplifted
- the floor may be flat
- also called graben
- examples include the Great Rift Valley of East Africa, River Rhine rift Valley, River Jordan rift valley
The mode of formation of rift valley goes thus:
- found in area of intense earth movement or crustal instability
- due to earth movement or tectonic activity, faults are developed in the earths crust
- this faults divide the earth crust into blocks
- tension or compressional force set in
- tensional forces tear the blocks apart from the central block
- central block sinks or subsides
- the sinken block forms a depression called rift valley
- compressional force can also form a rift valley
- due to compressional forces the two blocks on either side of the fault line ere pushed over the central block
- this results in the outer block over thrusting the central block
- this forces the central block to sink and form a depression known as rift valley.
Characteristics of horst are:
- it is also called block mountain
- associated with rift valleys
- represents the uplifted portion of land
- has steep sides
- has great heights
- the top may be flat or tilted
- also called block mountains
- examples include Ruwenzori Mts in East Africa, Vosges, Black Forests in France, etc
Mode of formation of horst goes thus:
- caused by internal disturbance within the crust
- caused by tensional or compressional forces
- parallel vertical faults develop in the crust
- land is divided into blocks
- subsidence may occur along fault lines forcing the flanking blocks to sink
- the central block standing becomes a horst or block mountain
- the flanking blocks may be forced up by compressional forces over the central block
- the central block is held down as rift valley while upthrusted blocks forms horsts or block mountain