Explain the process responsible for the formation of igneous rocks in East Africa. 

Candidates are expected to:-

• Define igneous rocks
• Identify process of formation.
• Give origin of process
• Identify/describe the types of igneous rocks.

 Igneous rocks are fire formed rocks. The main process of formation is vulcanicity. This is the process by which molten rock (magma/lava) cools down, crystallizes and solidifies either within the earth’s crust to form intrusive rocks or onto the surface of the earth to form extrusive rocks.

Vulcanicity originates from the interior of the earth due to radio activity, geochemical reactions and connectivity within the mantle – a lot of heat is generated that melts the rocks in the mantle creating convective currents. These currents exert pressure on crustal rocks creating lines of weakness/ fissures through which molten rocks is either intruded or extruded.

Igneous rocks are categorized according to rate of cooling/depth and chemical composition.

According to rate of cooling and depth they include;

Plutonic rocks/ Abyssal rocks: These are formed from cooling and solidification of magma at great depth hence they are deep seated rocks.

• The intruded magma cools and solidifies extremely due to lack of contact with air leading to formation of rocks with large crystals such as granite, gabbro, diorite, synite etc.
• These rocks are found in features such as exposed batholith at Mubende, Singo etc.

Hypabyssal/intermediate rocks: These are formed from magma that cools and solidifies near the surface of the earth/shallow depth. The rate of cooling is moderate leading to formation of medium sized crystals e.g. Quartz, dolerite, porphyry, etc. They are found in features such as dykes in Turkana land etc.

Volcanic rocks/extrusive rocks are formed from fast cooling of lava extruded onto the surface of the earth due to exposure to air leading to the formation of rocks that have very small crystal e.g. obsidian, basalt, andesite, pumice etc.

• Some of the volcanic rocks are spongy in nature such as pumice due to enclosure of gases at cooling time.
• Some rocks are glassy in appearance such as obsidian.
• Some rocks are dark coloured such as Basalt.
• Volcanic rocks are mainly found in features such as volcanic mountain e.g. Elgon, lava plateaus such as Yatta etc.

 Igneous rocks according to chemical composition: Magma/ lava vary in chemical composition and so has an effect on its viscosity/thickness which in turn influences the rate of cooling and the process of crystallization.

In this category the rocks include;

Acidic/felsic rocks: these have a high content of silica (65%) and 35% or less basic oxides. They are light coloured, have low density e.g. granite, diorite, Rhyolite, Andesite etc.

Intermediate rocks have moderate silica content between 55- 65% and 45-35% basic oxides. They are a mixture of mineral structure of acidic and basic e.g. porypbyites.

Bask rocks contain much mica and olivine. They are rich in iron and magnesium. They are dark coloured, dense or heavy, have low silica content between 45-55% and 55- 45% basic oxide