What causes vulcanicity

                                VULCANICITY


This sub topic describes the processes of Vulcanicity, the features produced and their importance to the people.
              
                                               What causes vulcanicity


Deep inside the earth heat and pressure exerted by the crust cause rocks to melt forming molten rocks known as magma, which is forced to rise along the lines of weakness (fault lines).

Vulcanicity: – involves a process through which molten rock (magma) and gases from interior moves on to the earth surface along lines of weakness (fault lines).

Features formed: – on reaching the surface the magma which becomes lava, cools and solidifies either on the earth surface forming extrusive volcanic features or magma may solidify below the earth ‘s surface forming intrusive volcanic features.

Extrusive and intrusive features. (Bunnet)

Extrusive features

1. Lava

 - that moves to the surface differs in chemical composition, this party explain the different types or shapes of volcanoes and nature of eruption whether explosions or quite.

Acidic lava: very thick, traps a lot of gas , water, rich in silica content violent eruption and lava solidifies very quickly builds up steep cones.

Basic lava: this is a form of fluid poor in silica content flows for a very distance before it solidifies, it builds up gently sloping cones, lava plateau and plains.

Pyroclasts (Heated lava): when lava is heated, it breaks down into small fragments or pieces (pyroclasts) ,which usually falls to the ground and form layers of Ash/cinder. Layers of ash mix with layers of lava to form composite cones.

2. Volcanoes:

 These are hills or mountains formed when magma erupt and piles up around the vent until a cone is formed. These are basically four types of volcanic cones.

                                   Types of volcanic cones

Ash/cinder cone:

These are small but steep sided hills formed when explosive eruptions throw lava high into the air, breaking up lava into small fragments or pieces known as pyroclastics (fire rocks). The erupted materials build up or accumulate around the opening known as vent; layer after layer forming steep sided cones of about 150 metres high. examples in Kenya are Suswa and Menegai hills and Teleki hills south of Lake Turkana and Longonat. The hills are characterised by a large crater /caldera at the top because of violet eruptions.

Crater- circular depression caldera - an enlargement of a crater on top of
at top of volcano. Mountain as a result of violent eruption.

FORMATION OF A CALDERA

Examples are found on Menegai, Suswa, Longnot in Kenya, Napaka, and Elgon in Uganda. Ngorogoro in Tanzania.

There are also low land explosion craters. explosion craters are flat floor depressions formed when explosions eruptions of gases blow off the rocks at the surface leaving a shallow circular depression, depressions are filled with water to form explosion crater lakes. Examples are; L. Katwe, L. Nyamuruka, L. Munyanyang, L. Saka, and Kyegere and others. Found in western Uganda, in Kasese , Kabarole and Bushenyi districts.

Basic lava cone/shield volcano:

 This is a hill with gently slopping sides. Its formed when hot fluid lava, with low silica content, flows from one or two faults in a quiet eruption. It spreads out in flat layers. The layers build up a broad volcano with gently sloping sides, shaped like a shield and hence the name shield volcano. Its common in Uganda- Rwanda boarder along the Nyamulagira ranges.

Acid lava dome: These are doom shaped hills, thick lava rich in silica content, solidifies quickly on reaching the surface, forming a volcanic dome or cone of viscous lava, examples – sernal volcanodomes in Tsavo national park in Kenya.

Volcanic plug: 

The thick lava (viscous) lava, at times solidifies in the vent, after prolonged erosion, the plug is exposed to the surface examples are the Tororo rocks in Eastern Uganda.

A composite cone:

 A composite cones are large mountains formed when volcanic eruptions alternate between quite and violent eruptions. An exposure period releases gas and ash and cinder layers. Then the eruption changes to a quite period erupting lava over the top of ash layer, when the cycle of ash and lave is repeated over and over in alternating layers, a composite volcano is formed.
At times lava is diverted side ways from the vent forming dykes, Corrects or parasite cone .Examples are  Mt. Kenya, Kilimanjaro in Tanzania, Mahambura in  South west Uganda.



Lava plateau:

 formed as a result of quite eruptions, when lava moves out slowly along cracks /faults .the lava fluid lava flows for a very long times on the plateau before it solidifies, forming lava plateau.

Examples are; Kisoro lava plain in Uganda, Yatta, Laikipia and Turkana plateau in Kenya.

Hot springs /geyser:

 These are features produced when heated water in volcanic areas flow out queitely in form of hot springs or erupt periodically shooting out water /steam in the air.

Examples – Kitagata in Bushenyi.

                    Sempaya in fortportal

                    Majimoto in Tanzania

                    Majiyamot in Kenya

kitagata hotspring in Bushenyi

Lava dammed lakes: These are formed when lava blocks channels. Water pools behind leading to formation of lakes. Examples L.Bunyonyi and L. Mutande in southwest Uganda.

                            Intrusive features:

These are formed when magma solidifies underground. Magma forms in many different shapes and sizes, the most common are: -

1. Batholiths: 

these are mostly massive rocks. They are dome shaped, formed at great depth and at times they are exposed at the surface by denudational forces as inselbergs. Examples are mumbende hills, parabong and labwa hills in Acholi. Mumias and Kisii batholith in Kenya.

2. Dykes: 

formed when magma solidifies into vertical cracks, cutting across rock layers. When affected by erosion dykes may stand as a ridge. There are ridges in most of Turkana and Sukulu hills South of Tororo town.

3.  Sills:

 formed when lava solidifies in between rock layers, after prolonged erosion, Sills may be exposed as escarpments and while they occur across a river valley they cause water falls and rapids. Examples are Thika and Thompson falls in Kenya, Karuma falls and Bujagali in Uganda.

4. Laccolith:

 this is magma which solidifies in a shape similar to a mushroom. After prolonged erosion, it may form upland.

5. Lappolith:

 This is a sauce shaped magma, after erosion. It can be exposed as a shallow basin. Examples are Arenas in Ankole.

Advantages of Vulcanicity

1. Volcanic mountains and lava plateau provide fertile volcanic soils which support agriculture, e.g. Arabic coffee is grown on mountain Elgon, Mufumbiro, Kenya and Kilimanjaro, coffee earns the country foreign currency and provides employment, other crops grown include wheat, tea, pyrethrum, maize, bananas, vegetables, irish potatoes.

2.  The highland areas are densely settled. This is due to the fertile soils and cool climate, e.g. Bugishu, Kigezi, Kenya and Kilimanjaro highlands. In addition some of these areas have large towns like Kabale, Mbale, Moshi, and Nairobi. This has led to development of commercial activities.

3. The volcanic features especially mountains are tourist attractions. They provide sporting activities like mountain climbing. They generate income in form of foreign exchange and provide employment to local people.

4. Volcanic mountains influence climate, lead to formation of geographic or relief rainfall which is important for agriculture. Some mountains are ice capped. They are a source of many rivers, which provide water for domestic use and generate hydroelectric power.

5. There are forest reserves on the slopes of mountains like Elgon, Mufumbiro Kenya, and Kilimanjaro which are valuable source of timber and firewood. The forests also act as wildlife conservation areas e.g. Bwindi impenetrable forests, has the largest population of gorillas, which promote tourism.

6. Lava or magma is rich in minerals e.g. Tororo rock volcanic plug is a source of limestone for the cement industry. Iron,tin uranium are found in the Mufumbiro ranges.


Lake Katwe which is an explosion crater is a centre for salt mining. The Kimberlite rock in Tanzania is centre for gold mining. Minerals provide revenue and employment.

7. Hot springs or Geysers are potential source of Geothermal power (electricity).in Kenya ,the Olkaria Geothermal Power Station near lake Naivasha in Kenya, generates electricity.

8. There is fishing in lava-damned lakes, which provides food and employment.

The intrusive features Batholiths, dyke, sills, laccoliths and lappoliths once exposed to the surface as inselbergs have the following advantages;

• Good sites for quarrying. They are sources of stones used for construction.
  
• Sills and Dykes once crossed by rivers create waterfalls which are good for hydro power generation.
  
  
• They are tourist attractions.

                     Disadvantages of Vulcanicity

• Volcanic features especially mountains are communication barriers due to steepness.

• It is very expensive and risky to construct roads and railways in the hilly areas.

• Volcanic eruption leads to loss of lives and property.
  
• Heavy rainfall and steepness lead to soil erosion, mass wasting and landslides which are common in Kigezi and Elgon areas.

• Mountains act as barriers to rainfall especially on the leeward side (rain shadow areas) this causes aridity.
  
• Intrusive features like sills and dykes form waterfalls and rapids which hinder navigation of rivers.

• Where there are Inselbergs and Batholiths make agriculture practising difficult.

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