VULCANICITY
Vulcanicity is one of the processes that are indirectly produced by faulting. When the earth crust develops a line of weakness (Faultline), the already mobile and molten rocks (magma) under intensive pressure from overlying rocks will over flow through this weakness (Faultline) and move upwards from the mantle into the rocks of the earth crust. The process of movement of this magma from the deeper layers of the earth into or top of the earth crust is what is called vulcanicity.
If this magma cools inside the bedding planes of the earth, it becomes intrusive vulcanicity, and the features it produces are called intrusive features, while if it reaches and cools on the earth surface it becomes extrusive volcanicity and the features produced are called extrusive features. the most common of these features is a volcano. A volcano is a landform produced when molten materials from magma erupt through the surface of the earth and builds around the vent.
Vulcanicity is also called igneous activity because it’s the only process through which igneous rocks are produced. Radioactive materials in the earth generate high amount of heat automatically which keep the earth interior in a semi-fluid form. Hence it is mobile and can easily flow.
Global distribution of vulcanicity.
The major volcanic regions of the world are these areas where seismic activity especially along plate margins exist. (red marks shown in the figure above) This is here that plate convergence or divergence occurs. These are;
1.The circum pacific Belt (the ring of fire regions) stretching from the Andes of South America to the Rockies of North America, to the Eleutian islands, to Kamchatka region of Japan, Papua New Guinea, to the Phillipians, to the Solomon Islands, New celedonia and Newzealand.
2.The North Africa, Middle Eastern, East African and Madagascan Belt.
3.The Pacific Ocean region covering areas of Hawaiian Islands, Galapagos islands and Juan Fernandes islands.
4.The Indian ocean region covering areas of Java, Bali and Sumatra.
5.The Mediterranean and Asian Belt region covering areas of Azores, Canary Islands, through Italy, mountain Ararat.
6.Other minor and isolated volcanic regions of the Caribbean e.g. Parana plateau, in South America, Kimberly plateau of Australia, Drakensberg of South Africa.
Illustration of major volcanic features.
The volcanic material;
The nature and type of volcanic features, their shape and characteristic of magma ejected. Majority, magma is categorized according to the degree of scatter or spreading after exposure on the earth’s surface. This degree of viscosity or fragility is determined by the amount of silica. If silica content is high, the material becomes acidic. If it is average, the material is basic while if it is very low it becomes ultra-basic.
| Silica content (Si02) | Type of lava | Example of rock |
| 66% + Si02Between 52-65% Si02Between 45-52% Si02Between 45%Si02 | AcidicIntermediateBasicUltra-basic | Granite RhyoliteDiorite, AndesiteGabro, BasaltPeridotite, Serpentinite |
TYPES OF LAVA
There are 3 types of lava;-
Basic lava type
Intermediate lava
Acidic lava
BASIC LAVA TYPE
This type of lava is very mobile & less viscous.
When it is exposed onto the surface of the earth, it covers a very wide area /distance before solicitation can take place.
This type of lava always result in the type of lava are the lava plains/plateau, volcanic shield.
INTERMEDIATE LAVA
This type of lava is in between mobile & immobile lava.
it’s not very mobile & in most cases this type of lava results into the formation of volcanoes like mountains, Cinder cones etc. under this type of lava, solidification is moderate ( moderate speed).
ACIDIC LAVA
This type of lava is very immobile. Its eruption is very violent. It’s very viscous, solidification is very fast.
It takes almost the vent.
https://www.youtube.com/embed/21bZx0vBI9s
VULCANICITY IN EAST AFRICA
East Africa is region whose landscape has been seriously affected vulcanicity. vulcanicity has been active throughout geologic time. However, the most important time when East Africa was hard hit by volcanic activity was from Tertiary (Miocene) up to Holocene. The Miocene period was characterized by eruption of mount Elgon, Kadan and Napak (between 25-50 million years ago). The rest erupted in the last geologic period (within 5million years ago) and others are continuing to erupt.
The latest mountains to erupt were Nyamulangira (1981), Nyirangongo (found in Kivu area of Virunga mountains) of Eastern Zaire erupted in 2002 and mount Oldonyo Lengai in 1966. Despite the fact there are no common active volcanoes in East Africa, by Virtue of there recency, volcanicity is not entirely extinct. A series of vulcanic mountains show signs of activity e.g. Mufumbira ranges (south western Uganda), Chyulu range, Simbi and Manengai ranges in Kenya and Oldonyo Lengai in Tanzania. Active vulcanicity is now commonly restricted to geo-thermal regions with features e.g. Hot springs, Geysers and Fumaroles, save for a few mountains e.g. Nyirangongo which has just erupted.
Landforms of Vulcanicity
- Intrusive features; these are called plutonic land forms and the are formed when magma fails to reach the earth surface and therefore cools within the bedding planes. These includes:
1.Batholith: This is the largest of all intrusive vulcanic landforms. It is a very large mass of magma that often forms the root of the mountain. It is composed of granite and once exposed to the surface by erosion of overlying rocks, it forms large features e.g. Tors, Kopjers and inselbergs and uplands covering hundreds of square kilometers. Examples of batholith are Mubende batholith in Uganda, parts of Acholi have got exposed batholith that produced inselbergs e.g. Labwor, Parabongo, Lalongo, Otuke, Aloi and Akia, Singo batholith in Tanzania, batholiths are found between Mwanza and Iringa and the whole of Sukuma land.
Vulcanicity or vulcanism is a total process by which gases and molten rock from the interior of the Earth are intruded into the Earth’s crust or extruded onto the Earth’s surface. When the molten material, that is magma is intruded into the crust, it solidifies giving rise to intrusive features. when the material reaches the surface of the Earth, it solidifies to form extrusive features. these are the features referred to as volcanic.
The molten rock or magma originates from upper mantle and asthenosphere in the interior of the Earth. This material is kept in a semi-plastic form by pressure exerted by overlying rocks. Through the natural processes of radioactivity, heat is spontaneously generated. This makes the rocks even more plastic and fluid. Any increase in heat can trigger off upward movement of the molten rocks. Additional heat can come from friction along plate boundaries.
When temperatures rise, the lighter elements of the mantle melt and begin to rise towards the surface. The upwelling is made possible if there are lines of weakness like cracks or fault. Vulcanicity will also take place because of reduced pressure above the molten rocks. Gases in the interior escape to the surface creating fissures within the crustal blocks. The magma then forces its way to the surface along these fissures and forms various landforms an reaching the surface.
When magma erupts at the surface and loses its gases, it is known as lava. It is the escaping gases which, because they expand rapidly due to the lower pressure, cause many of the eruptions to be very explosive. Lavas vary considerably in their composition particularly in their silica content. The nature of lava partly affects the type of landforms.
Basic lava, for example is fluid and largely mobile and therefore forms extrusive lava plains and large shield volcanoes. On the other hand, acidic lava is viscous and largely immobile and consequently solidifies quickly to form steep landforms. Sometimes acidic lava solidifies quickly and blocks fissures resulting into explosive eruption thus lava is ejected in form of particles of various sizes known as pyrocdasis.
In Kenya they are found the Maragoli area of western Kenya
2.Laccolith: This is a dome shaped feature produced by intrusion of viscous magma which does not spread but builds up and therefore arches up the overlying rock strata into a dome shape. If it is exposed to the surface by erosion, it produces a rounded hill surrounded by circular scarps. Laccolith can be seen in voi in Kenya, Funjay and Ambereny massifs in Madagascar.
3.Lapolith: this is formed in more less the same way as the Laccolith except that whereas the Laccolith is made up of viscous material that forces the crust into a dome shape, Lapolith is made up of basic materials which after intrusion sags down due to the weight of overlying rocks. Hence as it sags; it produces a saucer shaped feature. This can be seen in Belingwe in South Zimbabwe and in Singo/ Mubende hills.
In Zimbabwe, there are series of Lapoliths intruded into the basement complex. These extend across much of the country. Erosion has however formed a line of low hills out of them. These reach their highest in the Umvukwe range north of Harere. Other good examples are found in the bush veld basin of Transvaal province in south Africa.
4.Dyke: A dyke is formed when a mass of magma cuts across the earth crust and forms a wall like structure. It may be vertical or steeply inclined. They may occur in a group. Some dykes are hard than surrounding rocks and hence after erosion, they may be exposed as ridges while if they are less resistant, they form shallow depressions. Dykes can be seen in Western Lake Turkana (Mbarali), thicker falls, Rangwa complex in Kisumu, and around Tororo rock (Ring complex in Uganda.
Other examples of dykes are found in the Jos plateau in Nigeria, Malawi and in Lesotho.
5.Sill: This is formed in more less the same way as a dyke except that where as in a dyke magma cools in inclined bedding planes, a sill is produced when magma cools in horizontal bedding planes. It is formed from basic magma that can easily penetrate rocks and curve. They can be seen along Pakwach-Arua road in Uganda (Karuma falls), Thika area in Kenya.
The three sister Sills in south and the great Sill in Northern England are also remarkable examples.
6.Ring Complexes: these are circular intrusions of one or many ring dykes. Ring dykes are concentric ridges of igneous rock that are formed when the land around the circular dyke system subsides (cauldron subsidence) exposing circular and ring like features called ring complexes.
Extrusive features
These are landforms of vulcanicity that are formed when magma reaches the earth surface. These are features of volcanicity i.e. a process through which magma escapes from the interior of the earth up to the surface.
Volcanoes. A volcano is a hill or mountain produced by eruption of magma and piling up of the pyrocrasts of magma around the vent until a highland is built.
Types of Volcanoes
An acidic lava cone. Acidic lava as already seen above has got 66%+ of Silica and therefore is more viscous. It is this viscous magma which after erupting that forms acidic cones. This cone has got steep and convex sides (sides that bulges outside). The base of this cone is small but is very high in altitude. Examples of acidic cones are mount Mgahinga (Western Uganda) and mount Rungwa in Tanzania. they have small craters.
Strato volcanoes: This type of volcano is composed of alternating layers of lava and ash. It is formed by violent eruption such that lava is blown to great heights and it breaks into small fragments that fall back and build a layer of ash. As eruption becomes less violent, the gentle lava settles on top of ash to form a layer of lava. It is called strato because of being composed of strata of ash and lava. Examples of composite cones are examples of strato volcanoes in East Africa. They include Mgahinga, Mount Kilimanjaro, Muhavura, Elgon, Longonot, Oldonyo.
Ash and cinder cones: These are small and symmetrical cones composed of pyrocrasts. When lava is violently ejected it is blown to great heights and it breaks into small fragments. These fall back to the earth and builds up a cone. This sides of this cone is greatly concave due to the spreading of the material around the base.
A basalt dome: This is made up of basic magma that therefore is able to flow far and spread widely before solidification. It is also called a shield volcano. As the material erupts, it spreads and therefore produces a volcano that is low in altitude but with abroad base. The sides are concave, examples are Mt Virunga, Muhavura (Mufumbira), Mt Kenya, Mt Longonot and Nyamulangira (E. Zaire).
A composite cone; This is a volcano that is composed of more than one cone. After the formation of the main cone, lava may solidify in the vent and therefore block all subsequent eruptions. When it wants to erupt again magma will find the main vent blocked and it will find alternative ways through the sides and hence create other secondary cones called parasitic cones. This cone has got characteristics of both acidic and basalt domes with the lower slopes gentle and the upper slopes steep. Examples of composite cones are mt. Kilimanjaro (with mawenzi and shire peaks are conlets), mt Muhabura has Mgahinga as a conlet, mt Elgon, mt Longonot, Oldonyo Lengai and Vrunga.
A dissected volcano: This is a volcano whose sloping sides are deeply dissected by radial valleys. Erosion begins first on the upper slopes and slowly but surely incises these slopes finally producing a volcano whose slopes are made up of ridges and valleys. Examples of dissected volcanoes include; Mt Kenya and Mufumbira.
Volcanic plug/neck: When magma is very viscous, it will form a rigid volcanic plug. The material is so viscous that is just pushed out of the ground without flow but as a rigid and consolidated plug. Eruptions are very explosive and they are intruded together with clouds of hot ash and cinders, examples are Toro rock in Eastern Uganda, Alekilek in Napak Caldera, in South Karamoja, Batian and Nelion peaks on mt. Kenya, Tinderet, Loldiani and Timborora in Kano plains of Western Kenya.
Craters. A crater is a depression on top or on the side of volcanic cones. It is bow or funnel shaped or circular shaped in nature with in facing escarpments. Its depression is generally less than 1 mile (1.2km) in diameter. It is formed as a result of successive eruptions such that after the first eruption, the vent is properly blocked. Any successive eruption will find it impossible to flow out. As a result, pressure will build up inside and finally it blows off the top forming a crater. Such craters include; Manengai, Suswa, Shomble, Kilimanjaro, Elgon, Sabinyo.
A ring craters. A ring crater unlike other ordinary craters is formed as a result of less violent eruption. If explosion consists of mainly gases and some few pyrocrasts, the explosive eruption of gases drills a hole in the ground and the pyrocrasts then gently pile around the hole (cent) to form a low rim around the vent forming a ring depression called a ring crater or explosion vent. This process of crater formation is called fluidization. Examples of ring craters are Lake Katwe, Nyamunuka, Nkugute and Kasenyi (in Fort Por98tal Uganda), Lake Saka, Kyegere. In Tanzania are Galama and Ndoto craters N.E of Singida. Ring craters will not necessarily form on top of mountains. if these craters are filled by water, they form lakes called crater lakes. Examples given above for craters have lakes.
Caldera: This is more-or-less a crater except that unlike a crater, a caldera has got a more extensive depression more than 1 mile in diameter. Like in a crater above, the secondary eruption may blow off the top of a volcano hence creating an extensive depression called a caldera.
Alternatively, a caldera can be formed by a process called cauldron subsidence or basalt wreck. According to this, a caldera is formed when during eruption, a lot of magma is poured out due to high pressure such that there is a big empty chasm left under the volcano. After eruption, this chasm is left unoccupied and later on the weight of the volcano on top sinks (subsides) down into the chasma leaving a wide depression on top.
Examples of calderas include; Ngorongoro caldera (Tanzania), Napak caldera (Uganda), Enbagai caldera (Tanzania), Longonot (Kenya) and Suswa in Kenya.
Cumulo dome: This volcanic feature is formed by volcanic eruption of viscous magma. This viscous magma does not flow as other features form but instead the pressure forces the material to bulge outwards forming the dome shaped feature. After the formation of an acidic cone, the surface quickly hardens while the interior is still fluid. Further uprising within already hardened layer forces these layers outwards (as a pumped ball expands).
Consequently, forming a more-or-less rounded feature without a crater called a cumulo dome-or memelon. If it forms into a crater, it is called a thalloid. Examples of cumulo dome are Ntumbi cumulo dome in Tanzania and Tsavo National park. Athalloid is also found in a caldera on top of mt Rungwe.
Lava plateau: A plateau is a flat-topped highland area. A lava plateau therefore is a flat-topped highland area made up of lava. When basic magma flows out of the linear fissures or a series of them, extensive sheets of magma will be produced covering large areas. Such a flow is quiet and not explosive called fissure eruption. It may be formed as a result of inter connected cones over an extensive area.
Examples of lava plateaus are Yatta plateau in Kenya, Loita, Athi plains, Kapti phonlite, Kencho, Laikipia, Kano and Uasin Gishu in Kenya and Bunyonyi in Uganda. Also, the chain of extinct volcanoes West of mt. Meru in Tanzania is thought to be a result of fissure eruption.
3.Other forms of volcanic activity: These are other minor igneous features produced by geo thermal activity. Within volcanic active areas there are hot springs, gysers, fumaroles.
- Hot spring; This is a feature produced by constant flow of hot water from a rock. As it rains, water seeps into the rock and finally reaches a super-heated rock. If this water accumulates on this rock, it will be heated up. If this heated water reservoir is connected to fissure, this hot water will flow out as a hot spring.
Examples of hot springs are Ihimbo and Kisizi (Rukungiri) and Kitagata (Bushenyi), Sempaya, Lututura and Kananorok all in Uganda, Njorwa, Maji yamoto and Lake Hannington in Kenya, Ambooni and Maji moto in Tanzania.
ii.Geysers; A geyser is formed almost in the same way as a hot spring except that in a geyser hot water with stream are ejected out with a great force and periodically. They periodically emit powerful jets of hot water. Water collects in an underground rock cavity with a sump that boarders a very hot rock and traps steam behind it. As the steam increases its pressure builds up to a point where it pushes water out with violence.
Alternatively, geysers can be formed when water collects into under ground cavervans that are bordered by very hot rocks. The opening of the cavern is very small such that it restricts the amount of steam to escape. High pressure in a cavern will force water with steam out with force. Examples of geysers in East Africa are in Lake Hannington area (at lake Bogoria) in Kenya.
iii.Fumeroles: The formation of a fumeroles is not any different from the above two. The only difference however, is that with a fumeroles, steam only escapes continuously under low pressure. If this steam and other gases are sulphurous it becomes a solfatara. Fumaroles are also found in Lake Hannington area, in Longonot craters and between Lake Elementaita and Lake Naivasha, Manengai and Eburi area in Kenya. In Tanzania, fumaroles are found at Amboni and Kilimanjaro.
https://www.youtube.com/embed/Aqti5yTuCeA
EFFECTS OF VULCANICITY ON DRAINAGE
Vulcanicity resulted into the formation of crater lakes.
A crater lake is a depression filled with water formed on top of a volcano.it was formed by sealing off of the vent of the volcano & the depression was filled with water during the period of pulliviation ( hen there was a lot of rain fall) examples of craters include Nyabikyere, Nyakugere.
It led to the formation of the hot springs.
Hot springs is the hot water which is springing from within the earth’s interior & these hot springs reach the surface through line of weakness which develops within earth’s crust.
We have hot springs within Semliki trough around smpaya & kitagata.
It led to the formation of the lava dammed lakes.
Alava dammed lake is the one formed when lava flows across a river valley & after solidification has taken place the lava blocks the river from flowing & the river floods its valley on the side where the source is & a lake will be formed on that side which is known as the lava dammed lake.
NB
The remaining part of the river remain dry or will contain depending on the climatic conditions prevailing in that area examples of such lakes are Bunyonyi, Mutanda, Ashaka etc.
It led to the formation of a caldera lake. (Caldera lakes are formed in the same way as a caldera as formed by later the caldera is filled with water). E.g. many calderas in south western Uganda like kagodi Caldera Lake.
Indirectly, vulcanicity led to the formation of sources of rivers. through volcanicity, mountains were formed & on top of these mountains we have ice ,when it melts it flows down ward inform of rivers and therea are very many rivers on mountain Kilimanjaro.
Directly, vulcanicity led to the source or water rivers eg on mountain Elgon, eg R.manafa
Vulcanicity led to the emission of gases & steam on the sides of mt. Rwenzori there is a small volcano.
Negative effects;
Faulting poses problems of transport & communication both in terms of construction of communication lines & in terms of fuel consumption. Roads have to meander along the along the slopes e.g. in Bundibugyo.
Faulting has also hampered the navigability of rivers at certain sections e.g. the Nile at Murchison falls can’t be crossed by boats etc.
Faulting can cause destruction to life and properly.
Faulting is responsible from creation of rain shadow areas especially along the kasese –Mbarara corridor. This is the reason why at Mubuku, irrigation is practised. Also rift valley areas of Narok in Kenya experience very dry climate conditions.
Generally similium fly (Mbwa) which causes river blindness breeds in fast flowing water & this is associated with where rapids & waterfalls exists. So such areas are potential sites where these vectors could easily live & breed.
The steep block mountain sides & fault scarps (escarpments) promote soil erosion & this damage the crops.
There is flooding within the rift valley in some parts.
Sometimes those areas are affected with earth quakes i.e. in kabarole near the Semliki.
It led to the formation of the caldera lakes. Caldera lakes are formed in same way as a caldera is formed y later the caldera is filled with water e.g. may calderas I south western Ug like Kagodi caldera lake.
Indirectly, Vulcanicity led to the formation of sources of rivers. Through volcaiciuty, mountains were formed & on top of these mountains we have ice, when it melts it flows down ward information of rivers & there are very may rivers of mountain Kilimanjaro.
Directly, Vulcanicity led to the sources of water rivers eg Mt.Elgon eg R.Manafa.
Vulcanicity led to the emulsion of gasses & steam on the sides of Mt.Rwenzori there is a small volcano.
EFFECTS OF VULCANICITY ON HUMAN GEOGRAPHY OF EAST AFRICA
The formation of volcanic mountains led to the modification of climate and as a result a lot of rainfall is formed on the windward of those mountains ad a lot of agriculture is carried out.
It led to the formation of beautiful scenery like, Mountains, calderas and they have attracted tourists hence boosting the tourism industry.
It led to the formation of fertile soils directly or indirectly & this also favors agriculture.
It led to the extraction of minerals which has led to the boosting of the mining industry.
It provides employment to people e.g. drivers, miners, farmers, guides.
It boosted fishing especially within crater and lava dammed lakes.
It has led to the improvement of infrastructure where the volcanic features are hence attracting may people e. g roads
It led to formation of hot springs which are going to e used I the formation of geothermal electricity e .g Sempaya.
Directly, it led to improvement I the department of forests especially on the windward side i.e. people have started growing forests on those mountains.
It has led to the provision of water for irrigation e .g Doho irrigation scheme in eastern Uganda using water from Manafa.
It has led to formation of raw materials like limestone or cement.
There are animals I the volcanic features which they
There are animals in volcanic features which they harbour and these animals boost wild animals.
Rocks which are not minerals are used for construction purposes of roads
Negative effects
Vulcancity has led to the occurrence of landslides especially on the high volcanic mountains and this has resulted into the destruction of crops and property.
Volcanic eruptions can be destructive to life and property especially where human activities are heavily concentrated.
Volcanic land forms create problems of constructing communication networks especially where high mountains are formed
Volcanic hills and mountains are areas of serious soil erosion for example alongside slops of mountain Elgon where serious deforestation has taken place.
Generally the holes in between boulders encourage posts and vennus to lode easily. This makes these varmints such as rodents to destroy crops for example in kisoro Uganda.
Some sides of volcanic mountains experience rain shadow affect for example the north western slopes of Mountain Kilimanjaro.
Poor soils due to young volcanic rock which haven’t under gone sufficient weathering for example around Rukungiri.
Kakuru_Benny's Site 