Types of vulcanicity

Vulcanicity is divided into two main parts, namely intrusive vulcanicity and extrusive vulcanicity (volcanicity). Both intrusive and extrusive vulcanicity result into the formation of various landforms as described in the following sub-section.

Intrusive vulcanic features
Intrusive features are formed when magma fails to reach the earth’s surface and therefore cools and solidifies within the earth’s crust. In this case, the magma may spread, accumulate or remain in the passage within the crustal rocks where it cools and solidifies to form intrusive igneous features. The intrusive igneous features are categorised into hypabyssal and plutonic features. Hypabyssal features are volcanoes formed beneath the earth but near the surface while plutonic features are formed deeper beneath the earth. Intrusive vulcanic features include dykes, sills, laccoliths, lapoliths, phacoliths and batholiths.

Dyke
It is a wall-like feature formed when a mass of magma cuts across the bedding planes of the rocks. It is formed after magma cools and solidifies along the bedding planes (Figure 2.21). Since it does not follow the nature of the bedding plane of the rock, it is a discordant feature. Examples of dykes are found in the Jos Plateau in Nigeria, and West and South of Blantyre along the Tyolo Scarp in Malawi.

Figure 2.21: Dyke

Sill
This is a sheet of igneous rock which forms when magma solidifies horizontally along the bedding plane as shown in Figure 2.22. It is concordant to structure of rock strata. It takes thickness of different sizes and can cover many kilometres. When exposed to erosion, a sill may form a ridge-like escarpment or waterfall. The natural bridge in Kiwira, Mbeya in Southern Tanzania is an example of a sill. Other examples are found in South Africa along the railway line from Kimberly to Cape town and Kinkon Falls in Guinea.

Figure 2.22: Sill

Laccolith
This is a dome-shaped intrusive feature that has been formed within or between layers of sedimentary rocks as shown in Figure 2.23. It is formed when the pressure of the viscous magma becomes high enough to force the overlying strata to fold and push up making a dome or mushroom like structure.

Figure 2.23: Laccolith

Lopolith
A lopolith is a large saucer-like intrusive igneous rock lying concordant to the rock strata forming a shallow basin, as shown in Figure 2.24. The lopolith is formed as a result of greater weight of the overlying strata and deposits. An example of a lopolith is that which found in Bushveld Basin in the Transvaal, in South Africa.

Figure 2.24: Lopolith

Phacolith
The  is the concordant intrusion of igneous rock, formed after cooling and solidification of magma near the crest of an anticline or the base of a syncline (Figure 2.25). A phacolith can form along the sedimentary bed rock which is concordant thereby exerting great thickness along the synclines or anticlines.

Figure 2.25: Phacolith

Batholith
This is a large body of an igneous rock formed at the base of the earth’s crust. It is formed by the intrusion and solidification of magma very deep in the crust, as shown in Figure 2.26. It is commonly composed of coarse-grained rocks. Most batholiths intrude across mountain folds and are elongated along the dominant axis of range. A batholith forms the root of a mountain.

Figure 2.26: Batholith

Extrusive volcanic features
These features are formed when molten materials reach the earth’s surface through vents or fissures. The molten materials that are ejected onto the surface are called lava. Silent eruption is an ejection of magma which takes place without involving much force. Some extrusive volcanic features take place through fissures, therefore they are called fissure eruptions. Violent eruptions usually occur through a vent, and take place with great force. Lava emerging through a vent can build up a volcano.

Volcanoes
A volcano is formed from the accumulation of molten rock which flows out through a vent onto the earth’s crust. The mountain’s funnel-like depression or vent around which the erupted materials accumulate is called a crater.

Types of volcanoes
Volcanoes are categorized as active, dormant or extinct.

Active volcano
An active volcano is one known to have regular eruptions. Active volcanoes include Mount Oldonyo Lengai in Tanzania, Mount Longonoti in Kenya, Mount Nyiragongo in the Democratic Republic of Congo and Mount Pinatubo in the Philippines.

Dormant volcano
A dormant volcano is one that erupts rarely but still shows signs of eruption. Such signs include rumbling, gaseous emission and lava flow which indicate the likelihood of a volcano to erupt again. Mount Kilimanjaro and Meru in Tanzania, and Mount Menengai in Kenya are good examples of dormant volcanoes.

Extinct volcano
An extinct volcano is one which shows no signs of erupting again, although it was formed through volcanic activity many years ago. This type of volcano is no longer characterised with signs as rumbling, emission of smoke, lava flow or ash. Much of its original structures may have been destroyed by denudation. Examples of extinct volcanoes include Mpoli in Tanzania, Kulod in Kenya as well as Mikeno, Karisimbi and Sabinyo in Uganda. Volcanic activities may form several features which are described in the following sub-sections.

Ash and cinder cones
These are cone shaped accumulation of rock fragments (pyroclasts) around the vent. They are formed when lava is blown and ejected violently to great height and fall back to the earth and builds up a cone-like feature (Figure 2.27). The slopes of the cone are always concave due to the spreading tendency of lava at the base of the cone. Examples of ash and cinder cones are Sarabwe Fileko in Rungwe (Southern Tanzania), Busoka and Bitale in (South West Uganda), South of Lake Turkana (Kenya) and Jos Plateau of Nigeria.

Figure 2.27: Ash and cinder cone

Composite cone (strata-volcano)
It is a large cone with alternative layers of lava and ash. Lava often escapes from the sides of the cone where it builds up small conelets, as shown in Figure 2.28. The cone has steep sided slopes and it is the most common volcano. Examples of composite cones are Kilimanjaro and Meru mountains in Tanzania, Nyiragongo Mountain in the Democratic Republic of Congo and Muhavura Composite in the East of Virunga Ranges lying in the south-west of Uganda.

Figure 2.28: Composite cone

Volcanic plug
A volcanic plug is also referred to as a plug dome or spine volcano. It is formed when a mass of very viscous acid magma is forced out through a vent from the ground (Figure 2.29). The plug is extruded amid clouds of hot blowing ash and cinders. An example of a plug dome is Mount Hoggar in Algeria.

Figure 2.29: Volcanic plug

Crater
A crater is a depression on top of a volcanic cone. It is formed by violent volcanic eruption when the upper part of a volcanic plug is blown off. The depression may turn into a crater lake when filled with water from either rainfall or melting ice, as shown in Figure 2.30. Examples of craters are Embakai, Olmoti and Ngozi in Tanzania.

Figure 2.30: Crater

Caldera
It is a large rounded depression formed when the upper part of a volcano is either blown away by violent eruptions or subsides into the crust or in the volcanic cone, as shown in Figure 2.31. Caldera is a broadened crater. Examples are Ngorongoro Caldera in Tanzania, Eboga Caldera in Cameroon and Katmei Caldera in Alaska.

Figure 2.31: Caldera

Acid lava cone (Cumulo dome volcano)
It is a dome shaped volcano with (convex) steeply- sloping sides formed when acidic lava cools and solidifies around the vent, as shown in Figure 2.32. Lava does not flow away, rather it piles up near the vent due to its viscocity. The viscosity of the lava that forms a cumulo dome is a result of high content of silica and its high melting point. An example of cumulo dome is Ntumbi dome found 30 km East of Mbeya Region, in Tanzania.

Figure 2.32: Acid lava cone

Shield volcano
A shield volcano is an extensive cone with gentle slope sides formed when basic (basalt) lava is poured onto the surface and spreads to occupy a large area as shown in Figure 2.33. Lava from the earth’s interior flows out through a vent. A basic or shield volcano can also form when lava flows out through a single or many fissures.

Figure 2.33: Shield volcano

Other associated volcanic features
These features are associated with vulcanicity as described in the following sub-sections.

Solfatara
It is a volcanic hole emitting sulphurous gases as dominant and water vapour. Other materials emitted by solfatara are hot muds. Solfataras are generally found in places with young volcanic activities. An example of solfatara is Naple Solfatara emissions in Italy.

Fumarole
It is a volcano which emits steam, mud and gases like sulphur. Examples of fumarole are found in the valley of Thousand Smokes in Alaska. Also, Kibo in Kilimanjaro can be put under this category, though it emits only gasses.

Mofette
It is a volcano which emits carbon dioxide gas. Examples of mofette are Kyejo in Rungwe, Southern Tanzania, Auverge in France and Java in Indonesia.

Hot spring
It is a quiet outflow of superheated water from the ground. The outflowing hot water contains some mineral substances in solution or in suspension. Hot springs are common in Iceland as well as in some African countries such as Tanzania, Kenya and Ethiopia. Hot springs may be used to provide geothermal energy for different purposes especially for electricity generation. Examples of hot springs are Amboni in Tanga and Nanyala in Songwe region, Tanzania.

Geyser
This is a hot spring that throws out water into the air with great force and sometimes accompanied by steam. It occurs when heated water in the crustal rock is ejected explosively through a plumb-like narrow channel higher above surface. (Figure 2.34). Examples of geysers are Allalobed and Dallol found in Afar region in Ethiopia.

Figure 2.34: Geyser

Mud volcano
Mud volcano is also known as mud dome. It is a landform formed from accumulated mud or slurries, water and gases erupted from the earth’s interior. Mud volcanoes do not produce lava and are not necessarily triggered by magmatic activity.

The world distribution of major volcanic zones
Volcanoes are related to earth movements. They occur in the following zones as shown in Figure 2.35.

(a) Zones of recent mountain building such as the fold mountain zones of South East Asia;

(b) Along divergent boundaries where volcanic materials outflows through the boundaries to
form such features as mid-Atlantic ridge in Atlantic ocean;

(c) Continental coastlines such as the Western coast of North and South America;

(d) Convergent boundaries such as the Island of Japan; and

(e) Faulting regions such as those within the zone of the Great Rift Valley in Tanzania, Kenya,
Uganda, Ethiopia and Malawi.

Figure 2.35: World map showing major volcanic zones

The economic importance of volcanic activities
Volcanoes have enormous economic importance. Volcanic materials produce fertile soil that supports agriculture activities both cash and food crop production. Volcanic activities result in the formation of precious stones and minerals, like gold from Geita in Tanzania and silver from Kakamega, Kenya. Minerals bring foreign currency used for the development of country’s economy. Consistently, volcanic activities facilitate geothermal energy production because when magma rises close to the earth’s surface, it heats the groundwater to boiling point. When a well is drilled in these regions, hot water is pumped out as steam due to the extreme heat. The steam can then be used to drive turbines and produce electricity useful for different domestic, industrial and office activities. A good example is in Iceland where over 90% of homes are heated through geothermal energy. Also, it is a source of hydroelectric power generation through geysers and hot springs. Tanzania extracts carbon gas from Kyejo and other parts of Rungwe District.

Moreover, volcanic activities contribute towards provision of building materials, for instance, igneous rocks are used for building and construction of roads. Basalt, diabase and pumice are good examples. Features that result from volcanic activities attract tourists. For example, the snow-capped Mount Kilimanjaro, Mount Oldonyo Lengai and the Ngorongoro caldera in Arusha attract tourists who bring in foreign currency and create employment opportunities among the citizens. Apart from that, volcanic mountains influence the formation of orographic rainfall. For example, the windward side of Mount Kilimanjaro receives adequate rainfall that supports agriculture activities; and some volcanic features are sources of rivers which can be harnessed through dams to produce electricity or for irrigation purposes. For example, the source of River Pangani is Mount Kilimanjaro whose water is used to generate hydro-electric energy at Hale and Nyumba ya Mungu Electrical Power Stations. Volcanic soils are fertile and suitable for agricultural production.

Negative effects of volcanic erruptions
Large volcanic eruptions can cause deaths as well as destruction of properties. Lava and mudflows caused by volcanic eruption kill people and destroy properties as it erupts and flows rapidly down the volcano sides. The lava flow can also destroy agricultural land since the solidified lava hardens and makes it difficult for farmers to cultivate the land. Moreover, volcanic eruptions may release poisonous gases such as sulphur dioxide and carbon dioxide which may have adverse impacts to human beings as well as the atmosphere.