VULCANICITY IN EAST AFRICA
Vulcanicity involves all process by which solid, liquid, (molten rocks/materials) and
gaseous materials from the interior of the earth’s crust are intruded/ injected into the
earth’s crust (trapped beneath the earth’s surface within the surrounding country rocks) or
extruded onto the earth’s surface (gaseous and molten materials (lava) are ejected or
poured onto the earth’s surface).
Vulcanicity is caused by heat in the interior of the crust generated by radioactivity and
thermal or convention currents (tectonic movements), any increase in heat triggers
offupward movement of moltern rocks through cracks.
Volcanic activity is closely associated with the major fault zones (faulting) because faulting
created faults or lines of weaknesses which molten materials (magma) exploited leading to
the formation of different volcanic features, through not all fault zones give rise to
vulcanicity.
When molten materials (magma) are intruded or injected into the earth’s crust solidifies to
form intrusive volcanic land forms and when the molten and gaseous materials reach the
surface of earth solidifies to form extrusive land forms or volcanic land forms.
Types of lava
When magma reaches the earth’s surface and loses its gases, it is known as lava.
Lava is classified depending on it composition mainly with respect to silica content.
 Basic lava: this is the type of lava with a low amount of silica [45-51%]; therefore, it is
highly plastic, fluid and mobile able to flow long distances before solidifying.
 Acadic lava: this is the type of lava with very high percentage of silica [more than 66%];
therefore, it is extremely viscous (thick) and largely immobile, solidifying rapidly at high
temperatures of over 8500c.
 Intermediate lava: this is the type of lava with an average percentage of silica content
[51-66%]. It is fairly viscous and unable to flow far before solidifying.
EXTRUSIVE (VOLCANIC) LANDFORMS FOUND IN EAST AFRICA
These are formed when molten and gaseous materials (magma) passes though the vents
(fault lines) and reaches the earth surface, cools, solidifies to form extrusive (volcanic)
features like:
Volcanic cone/volcano:
It is a hill or mountain with a conical shape and circular depression/a funnel-like
depression at the top of the mountain called a crater.
It is formed by the eruption of magma or molten materials rise through the main or central
vent due to great pressure and heat on reaching the earth’s surface it cools and solidifies
around the vent to form a cone or dome. Parasitic cones may be formed if the main vent
becomes blocked and magma escapes through a dyke.
Volcanoes in East Africa include;
Stages of a volcano
Depending on the rate of heat supply to maintain eruptions, a volcano may be:
o Active volcano: is one which is definitely known to have erupted periodically in
historical times. These may show limited activity in the form of fumaroles like Sabinio,
Gahinga and Muhavura, Teleki, Likaiyu, Longonot, Ol-Doinyo Lengai, and Meru.
o Dormant volcano: This has not been known to erupt and yet is not thought to be
extinct. This is where a renewal of eruptive activity is possible for example Kadam,
Lonyili, Moroto, Zulia, Morungole and Elgon in Uganda and Mountain Kilimanjaro in
Tanzania. Most of the East African volcanoes are dormant.
o Extinct volcano: this is a volcano, which has not erupted in historic time for example
Mountain Kenya.
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There are various clasifications of volcanoes depending on the shape, size; nature of
materials ejected and the mode of eruptions as follows:
Composite cone/strato volcanoes: A volcano is made of alternate/successive layers of
lava and ash/pyroclasts. It has more than one conelet.
It is formed by repeated/successive eruptions of lava and ash from the central vent.
The lava and ash solidify around the vent forming a volcanic cone of great height.
The eruption is both violent and effusive (quite), during violent eruptions, the pyroclasts are
ejected, and during effusive eruptions, the lava is ejected.
The ejected lava is acidic. So forms a volcanic with steep sides of great heights.
When the main vent is blocked, the lava escapes to the surface through secondary vents
forming parasitic cones/ secondary cones/ conelets on the sides of the main volcano.
Eruptions leave a crater on the top of volcanoes. Illustration of composite cone
Examples of composite cones are Kilimanjaro, Meru, Muhavura, Ol-Doinyo Lengai,
Longonot and Kenya.
A crater: Is the rounded funnel-shaped hallow at the summit of a volcano formed during
an eruption. Mount Muhavura and Mount Elgon in Uganda both have craters on their
summits. When filled with water, from precipitation, a crater lake is formed like the Crater
Lake on top of Mount Muhavura and Mount Elgon has a crater pool called Jackson’s pool.
Illustration of a crater
Explosion crater/explosive crater/ring crater:
This is a shallow circular flat floor depression surrounded by low rim of pyroclasts (different
layers of ash and dust) and local rock. They are formed when the eruption is very violent
and gases underground vigorously escape to the earth’s surface due to high pressure. They
blow away rocks at the surface leaving behind a depression known as explosion crater.
This is accompanied by volcanic particles of all sizes (pyroclasts) falling besides the
depression forming a raised rim. Illustration of explosion crater
When filled with water, explosion crater lakes are formed like Lake Katwe near Lake
Edward, Lake Nyungu, Lake Nyamunuka, Lake Nyamusingire, Lake Kyamwiga, Lake
Kitogato, Lake Kitinda, Lake Mirambi, Lake Rutoto all found in western Uganda.
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Ash and cinder cones (scoria cones):
These are small and symmetrical cones of pyroclasts (Lava and ash) usually less than
150m with fairly steep sides and large craters, which cover all summit areas. Cinder cones
however, have steeper slopes than ash cones. Ash and cinder cones are small usually less
than 300metres (100feet) in heights and often form parasitic cones. Both types often erupt
in groups and on the flanks of larger volcanoes. They are formed when lava is violently
ejected. It is blown to great heights and breaks into smaller fragments, which fall back ans
build up a cone. Illustration of Ash and cinder cones
Examples of ash and cinder cones are found in Kisoro district like Muganza Shozi, Sagitwe
and Bisalo in Uganda and Likaiyu cinder cones in south of lake Turkana; Mount Morthioni,
Sambu, Chyulu hills of Machakos in Kenya.
Shield Volcano: is a large flat topped convex dome of basic lava (fluid lava) with very gentle
slopes. It is formed when lava (magma) is forced out through a central vent due to great
pressure from the earth’s crust to the surface of the earth and the lava flow in all directions
for a long distance before cooling, solidifying and forming a volcano with gentle slopes.
Examples are Nyamlagira found along Uganda-Congo border.
Illustration of Shield Volcano
Caldera: This is a large rounded depression resulting from the destruction of the upper
part of the volcano in a volcanic eruption.
Sometimes, the summit cone is blown off and explodes into incandescent rocks and ashes
leaving a depression on top. Calderas are also due to subsidence major by reducing the
(magma supply or leave a huge space beneath a volcano). The weight of the overlying cone
becomes too great, faults develop and it collapses into the space (Chasm). Examples are
Napak in Nakapiripirit, Menangai near Nakuru, Ngorongoro, Longonot and Suswa.
When a caldera is filled with water from heavy precipitation, it forms a caldera lake for
example Ngorongoro Caldera Lake is the largest of this kind in Africa.
Illustration of caldera
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Lava plateau/lava plain :
A lava plain is upland with a flat/gently sloping surface made of several layers of lava
hundred metres thick. It is formed when basic lava, which is non-viscous, wells up quietly
from different tiny fissure vents in the earth’s crust. The basic lava flows for a long time
before solidify.
The former relief surfaces are covered by lava layers to form a lava plain. Lava fills the
depressions or valleys to make it flat as illustrated below: Illustration of Lava plateau
Example of lava plateaux are Yatta plateua, Laikipia plateau, Loita, Athi plains, Kapiti
phoblite, Uasin-Gishu plateau in Kenya; Kisoro lava plateau on the side of Virunga ranges.
Lava dammed lakes
These are formed when basic lava flows from a volcanic cone for long distance and in the
process blocks river valleys such that water collects behind the lava to form a lake of the
same shape as its former river valley like Lake Bunyonyi in Kabale district, Chahafi, Mulehe
and Mutanda in Kisoro district.
Cumulo dome: is a steep sided convex dome of acid or intermediate lava. It is formed by
volcanic eruption of viscous magma. This lava is unbable to flow for long distances and on
eruption, piles up around the vent, it harderns quickly while the interior is still fluid
leading to the formation of more or less rounded feature without a crater called a cumulo
dome for example Ntumbi cumulo dome in Tanzania. Illustration of the cumulo dome
Volcanic plug:
Sometimes lava is so viscous and is forced out as rigid and cylindrical plug. The basement
of the plug is surrounded by exploded debris. Eruptions are very explosive and plugs are
extruded amidst clouds of hot ash and cinder for example Tororo Rock in Tororo in eastern
Uganda.
Volcanic neck
A central core of solidified lava in the vent of a volcano is exposed by the erosion of the
surrounding cone. Besides the volcanic neck, differential erosion may also reveal resistant
dykes which have solidified in the fissures of the original cone.
For example a volcanic plug is formed when magma rises and solidifies in the vent but after
reaching the earth’s surface. The lava is usually acidic and therefore viscous for example
Tororo plugs and plug on Mt Kenya. Illustration of the volcanic neck
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Hot springs or thermal springs
A hot spring is a stream of hot water flowing from the ground continuously. It is formed
when water flows/sinks far enough into the earth and gets in contact with heated magma
rocks. Convectional currents will take place resulting into a continuous flow of water at the
surface as a hot spring. They are common in volcanic regions where eruptions have caused
for example Sempaya near Fort portal, Kitagata in Bushenyi, Kisiizi north of the Muhavura,
Kibiro East of Lake Albert. In Kenya hot springs occur in the rift valley region like Majimoto
near Nakuru and around Lake Hannington and Eldoret.
INTRUSIVE VOLCANIC LANDFORMS FOUND IN EAST AFRICA
Are formed when solid, liquid (molten rocks) and gaseous materials from the interior of the
earth are intruded into the earth’s crust (trapped beneath it earth surface within the
surrounding country rocks). Some of these landforms are exposed to the earth’s surface by
the denudation processes. They include sill, Dyke, Batholith, Laccolith and Lapoliths
Sill: is a tabular sheet of magma that lies along a bedding plane, which is intruded
horizontally into rock fissures/lines of weakness. They may also occur singly or in groups.
They are formed when molten materials (magma) rises to the crust and spreads horizontly
along the bedding planes of sedimentary rocks, cools and solidify between the layers of
rocks. Examples are Sukuku hills near Tororo town.
When exposed by denudation, they form ridge like escarpments or flat topped hills. When
they outcrop a river bed, they cause waterfalls to develop for example Sezibwa River, Thika
River where they have caused falls and rapids as illustrated below:
Dyke: is a vertical or steeply inclined rock sheet intruded into rock fissures or lines of
weakness/crack to form walls of solidified magma.
Alternatively, a dyke is an igneous rock structure cutting across the bedding planes of the
rock strata. Sometimes dykes occur in swarms (great numbers) and have parallel or radial
pattern. They are exposed to the surface by denudation processes. Weak dykes are eroded
easily to form shallow trenches while resistant ones form ridges as illustrated below:
Examples are around Tororo rock in eastern Uganda where there is a ring complex of
dykes, west of Lake Turkana and Rangwa area south east of Kisumu-Kenya.
Batholith: is a large extensive intrusion granitic rock or dome shaped feature that is
buried deep in the earth’s crust. It is formed when mass of magma cools very slowly
beneath the earth’s crust at great depth. It is the largest landforms of intrusive volcanicity.
After exposure by the prolonged denudation processes, Batholith usually form uplands
covering many square kilometers.
Examples are Mubende Batholiths in Kikandwa village, Kawungere granite, Singo
Batholiths, Labwor and parabong ranges, Tididyla granites in Ngora, Kachumbala in
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Bukedea in Uganda, Maragoli area in western Kenya and the area between Mwanza and
Iringa in Tanzania.
Illustration
Laccolith
It is a dome shaped intrusion with a flat floor. It is formed when viscous magma rises and
solidifies within the crust before reaching the surface.
Viscous magma cannot spread far hence accumulates in a large mass forcing overlying
rocks upwards. If it is more resistant than adjacent rocks, it forms uplands after
denudation. They are found near Voi town in Kenya.
NB: Lapoliths are not found in East Africa.
IMPORTANCE or CONTRIBUTIONS or BENEFITS OF VULCANICITY TO EAST AFRICA:
 Vulcanicity has been responsible for the formation of volcanic limestone at Tororo,
Hima, which is used to manufacture cement. Salt lakes such as Katwe provide salt,
Lime in Muhokya, Vermiculite in Manafwa and diamond in Mwadui in Tanzania.
 Volcanicity has led to the formation of high relief landforms such as Mount Elgon,
Mount Muhavura, Moroto, Kilimanjaro, Kenya, Meru and Longonot. These are important
tourist attractions in East Africa which bring in foreign exchange.
 Areas affected by vulcanicity well drained deep fertile soils which have facilitated
agricultural activities like along the slopes of Mount Elgon and Kilimanjaro(Chagga area)
Arabic coffee is grown, pyrethrum on the gentle slopes of mount Kenya, on slopes of
highlands of Kabale and Kisoro, Irish potatoes, beans, apples, vegetables are grown.
 Geothermal energy from the underground heat that is released from areas that have
active volcanic activity is being harvested maily in Kenya.
 Volcanicity is responsible for the formation of high mountains, which trap winds thereby
inducing relief rainfall in areas where they are found on the windward side. This helps
to support agricultural actvities and settlement.
 Granitic rocks that are formed through volcanicity provide gravels, which, are used in
road construction, and providing a firm base upon which lay the railway trucks in
different parts of East Africa.
 Layers of lava act as traps for water in the sub - soil and therefore useful sources of
domestic water for example around Kabale and Kisoro.
 The slopes of volcanic mountains are ideal for extensive development of forests, which
are exploited to get timber, herbs, collection of nuts and fruits, honey harvesting.
 The volcanic lakes provide water for domestic and industrial consumption as well as
fishing grounds for example Lake Bunyonyi in Kabale provides water used in the villages
around the lake as well as Kabale town.
 Volcanic highlands and mountains are water catchment areas and sources of rivers to
the East Africa for example Mount Elgon has many rivers flowing from its source thereby
forming radial drainage pattern. The rivers include River Sironko, Siipi, Malaba,
Lwakhakha and Manafwa which provide water for irrigation at Doho in Butaleja.
Mountain Kenya has many rivers flowing from the mountain such as Tana and Galana
Rivers.
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Volcanic hot springs act as source of warm water in cold areas for example Nyamasiizi
hot spring in Kisoro.
 The hot springs have medicinal values. In East Africa, water of the hot spring is used to
cure different type of diseases like skin diseases and as well as joint pains.
 Volcanicity led to the formation of mountains, which are used to mark international
boundaries in East Africa for example Mount Elgon is at the border between Uganda and
Kenya.
PROBLEMS ASSOCIATED WITH VOLCANICITY:
 Volcanic highlands and mountains act as a barrier to the construction of transport
facilities or routes like roads, railways and airfields.
 Land slides and mass wasting due heavy rainfall are common in areas where volcanic
ash constitutes much of the rocks for example on the slopes of Mount Elgon. Lava
deposits of mud, sand and volcanic boulders/lava rocks exist on the north and eastern
parts of Mount Sabinio or Sabinyo Mountain in Kisoro.
 Volcanic hills and mountains are also areas of serious soil erosion
 Ash and lava layers are permeable therefore, the water drains easily through so that
water becomes very difficult to obtain for example in Kisoro, and there is limited water
supply due to porosity of soils there.
 The holes that are formed between the boulders/lave rocks encourage and act as
habitats for pests and vermins. This makes the vermins such as rodents to destroy
crops for example in Kisoro.
 High volcanic mountains lead to the formation of the rain shadow effects on the leeward
side of the mountain thereby bringing about aridity
 Infertile soils are formed due to the young volcanic rocks, which have not undergone
sufficient weathering do not support agriculture.
 Steep slopes of volcanic mountains hinder agricultural modernization in East Africa.
 The volcanic mountains are a barrier between the communities on either side of the
mountains
 Some of the lakes formed because of Vulcanicity are saline like Lake Katwe is saline
(salty) and this has discouraged the development of aquatic life.
 Volcanic eruptions can be destructive to settlement and agriculture especially in areas
where human activities are heavily concentrated.
The influence of vulcanicity on climate of East Africa
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Has created highlands/volcanic mountains, which have different climatic zones
Lower mountains slopes have tropical and equatorial climates
Middle slopes of the mountains have temperate climatic conditions
The upper parts/mountain tops have a tundra type of climate
Rainfall is received on the wind ward slopes of the volcanic mountains.
The lee ward sides of the volcanic mountains lie in the rain shadow/west facing slopes
Valleys receive sunshine only a few hours when the sun is overhead. East facing slopes
are warm in the morning and cold in the evenings. West sloping slopes are warm in the
evenings; there is usually temperature inversion in the valleys.
Difference in pressure and local windw like Anabatic and Katabatic winds, Anabatic
winds flow upslopes during day and katabatic winds flow down the slope at night.
Pressure decreases with increasing altitude on the mountains
Temperature decreases with increasing altitude on the mountain
Fog in the valley in the morning is very common
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The influence of vulcanicity on agriculture
Positive influence:
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Promotes agriculture due the existence of well drained deep fertile volcanic soils
Avariety of crops are grown both temperate like apples and tropical crops.
Heavy well distributed relief rainfall is ample for the growth of perennial and annual
crops like banana and coffee on the slopes of mountains Elgon, Kilimanjaro and Kenya
and animal rearing.
Bee keeping and agro forestry
Negative influence:
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Steep slopes limit distribution and marketing of agricultural products in mountainous
areas.
Dry conditions on the leeward sides of the volcanic mountains requires irrigation
Porous volcanic soils are too fragile for continuous agricultural production.
The young volcanic soils are prone to soil erosion and land slides because of the steep
gradient. They destroy crops.
Steps being taken to promote agriculture in volcanic areas
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Practice terracing on steep slopes to minimise soil erosion
Intercropping to provide cover crops for the fragile soils
Application of farm yard manure/mulching to increase the water retention capacity of
the soil
Irrigation of the dry areas found in the rain shadow
Agro-forestry practice to stablise soils on the steep slopes to check landslides
Provision of adequate capital to the farmers
Improvement of transport and communication networks
Looking for markets for the agricultural products
Establishment of food processing industries
Control of pests and diseases
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