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).
– involves a process through which molten rock (magma) and gases from interior
moves on to the earth surface along lines of weakness (fault lines).
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
and intrusive features. (Bunnet)
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.
lava: very thick, traps a lot of gas , water, rich in silica content
violent eruption and lava solidifies very quickly builds up steep cones.
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
(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.
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
of volcanic cones
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.
circular depression caldera - an enlargement of a crater on top of
of volcano. Mountain as a result of violent eruption.
OF A CALDERA
are found on Menegai, Suswa, Longnot in Kenya, Napaka, and Elgon in Uganda. Ngorogoro
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.
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 volcano
domes in Tsavo national park in Kenya.
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.
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.
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.
– Kitagata in Bushenyi.
Sempaya in fortportal
Majimoto in Tanzania
Majiyamot in Kenya
hotspring in Bushenyi
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
These are formed when magma solidifies underground. Magma forms in many different
shapes and sizes, the most common are: -
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.
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.
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.
Laccolith: this is magma which solidifies in a shape similar
to a mushroom. After prolonged erosion, it may form upland.
Lappolith: This is a sauce shaped magma, after erosion. It can
be exposed as a shallow basin. Examples are Arenas in Ankole.
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
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.
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.
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
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.
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
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.
Hot springs or Geysers are potential source of Geothermal power (electricity).in
Kenya ,the Olkaria Geothermal Power Station near lake Naivasha in Kenya, generates
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.
and Dykes once crossed by rivers create waterfalls which are good for hydro power
are tourist attractions.
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.
rainfall and steepness lead to soil erosion, mass wasting and landslides which
are common in Kigezi and Elgon areas.
act as barriers to rainfall especially on the leeward side (rain shadow areas)
this causes aridity.
features like sills and dykes form waterfalls and rapids which hinder navigation
there are Inselbergs and Batholiths make agriculture practising difficult.