With the aid of diagrams and examples, write explanatory notes on any two of the following: (a) Rift valley (b) Horst (c) Volcanic mountain.

Explanatory notes (any two)

(a) Rift valley

A rift valley is a long, narrow, steep-sided trough formed when a block of land sinks (subsides) between two roughly parallel faults, or when the land on either side is uplifted. It is produced by tensional (pulling apart) forces in the earth's crust. The sunken block is called a graben. Rift valleys often contain long, deep lakes. The best example is the East African (Great) Rift Valley, which runs from the Red Sea through Ethiopia and Kenya, containing lakes such as Lake Turkana and Lake Malawi.

Diagram: a cross-section showing a central down-thrown block (graben) bounded by two normal faults, with the flanks standing higher.

(b) Horst

A horst is an upstanding block of land bounded by two roughly parallel faults, formed when the middle block is uplifted or when the blocks on either side sink. It is the opposite of a graben and results from either tensional or compressional forces. It appears as a block or fault mountain with steep sides and a fairly flat top. Examples include the Ruwenzori and the Vosges and Black Forest highlands of Europe.

Diagram: a cross-section showing a central up-thrown block standing above two down-thrown blocks on either side.

(c) Volcanic mountain

A volcanic mountain is a mountain built up by the accumulation of lava, ash and other materials ejected through a vent from the interior of the earth during volcanic eruptions. As successive eruptions pile up material around the vent, a cone with a crater at the top is formed. Examples include Mount Cameroon, Kilimanjaro and Mount Fuji.

Diagram: a cone showing the central vent (pipe), a magma chamber below, layers of lava and ash, and a crater at the summit.