6 INTRUSIVE VULCANIC FEATURES




Vulcanism refers to movement of molten magma toward or onto the earth’s surface.

Igneous intrusive features are features formed when magma cool and solidify within the earth’s crust.

The following are feature resulting from intrusive igneous activities :

Dyke

this is a sheet of rock that is formed in the fracture of the pre-existing rock.

Dyke forms when a mass of magma cuts across the bedding planes and forms a wall-like structure.




An example of dyke is one that is found in Jos plateau in Nigeria this has been exposed to the surface through erosion.

Sill

this is formed when a sheet of magma lies along the bedding planes of the earth’s surface.

A sill is therefore a horizontal sheet of rock that solidifies from magma that has been injected concordantly between bedding planes; they may be of any thickness and extend for many squares of kilometers.

An example of a sill is a great Whill sill in great Britain.




Batholith

this is a large body of intrusive igneous rock believed to have been crystallized at the considerable depth below the earth’s surface.

They are the largest type of pluton.

An example of a batholith is Idaho batholith which covers a surface area of over 15500 kilometers square.

Lopoliths

these are saucer-shaped features formed beneath the earth’s surface through magma intrusion.

Lopolith forms a great shallow basin when magma solidifies within the crust a good example is the bushveld igneous complex of South Africa which is composed of both granite and basic rock.




Laccolith

this is a sheet intrusion that has been injected between two layers of sedimentary rock.

The pressure of magma is powerful enough that the overlying strata are forced upward giving laccolith a dome or mushroom-like form with a generally planar base.

Some laccolith can be found in Madagascar and Utah USA where they have been exposed by erosion.

Phacolith




this is a lens-shaped pluton that occupies either the crest of an anticline or trough of the syncline. A good example of phacolith can be seen in Corndon hills in the Shropshire United Kingdom.

EFFECTS OF CONTINENTAL DRIFT




Most of the relief features are related with continental drift due to the fact that forces such as tensional and compression forces affected many parts of the world.

Features that have been formed as a result of continental drift include; rift valleys, basins, plateau, highland ranges, coastal plains and associated features, block mountains etc.

A rift valley is one of the spectacular features associated with continental drift. This is an elongated depression bordered by two in-facing escarpments. It is formed due to either compressional forces or tensional forces or through differential uplift theory, basin and swell theory, and the seafloor spreading theory.




Plate tectonics also caused the vertical movement of the crust which led to the formation of Block Mountains.

A block mountain is also known as a Horst. It is formed when the middle block bounded by more or less parallel faults is made to rise or is uplifted by the compressional forces.

The best example of a block mountain in East Africa is mount Rwenzori, also known as the mountain of the moon. Others include Mathews and Nyiru, Ndoto in Northern Kenya.




Plate tectonism also led to the formation of down warped depressions which are indirect effects of faulting for example warping of central Uganda thus the formation of lakes Victoria and kyoga.

Plate tectonism also led to the formation of volcanic landforms due to radio activity and geochemical reactions which create a lot of heat that melt the mantle and asthenosphere rocks.  

Landforms due volcanic activity are;

Volcanoes

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




Volcanoes: – These are hills or mountains formed when magma erupts and piles up around the vent until a cone is formed. There are basically four 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 Pyroclasts (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 Menengai hills and Teleki hills south of Lake Turkana and Longnot.




The hills are characterised by a large crater /caldera at the top because of violet eruptions.

Crater

This is a circular depression on top of at top of volcano.

Caldera

This is an enlargement of a crater as a result of a violent eruption. Examples are found on menengai, suswa, longnot in Kenya, Napaka, and elgon in Uganda, Ngorongoro in Tanzania.

Caldera: – This is an enlargement of a crater as a result of violent eruption. Examples are found on menengai, suswa, longnot in Kenya, Napaka, and elgon in Uganda, Ngorongoro in Tanzania




Explosion craters

These are found in lowland areas 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. Nyamunuka, L. Munyanyang, L. Saka, and Kyegere and others. Found in western Uganda, in Kasese, Kabalore and Bushenyi districts.  

Basic lava cone/shield volcano

This is a hill with gently sloping sides. It’s 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 nameshield volcano. It’s common in Uganda- Rwanda border along the Nyamuragira ranges.

 Basic lava cone/shield volcano - This is a hill with gently sloping sides. It’s 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

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, an example is sernal volcano domes 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 of the earth due to erosion. Examples are the Tororo rocks in Eastern Uganda.

A composite cone

A composite cones are large mountains formed when volcanic eruptions alternate between quiet and violent eruptions. An exposure period releases gas and ash and cinder layers.




Thenthe eruption changes to a quiet period erupting lava over the top of ash layer, when the cycle of ash and lava is repeated over and over in alternating layers, a composite volcano is formed.

At times lava is diverted sideways from the vent forming dykes, Corrects, or parasite cones. Examples are Mt. Kenya, Kilimanjaro in Tanzania, Muhabura in Southwest 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. Thenthe eruption changes to a quite period erupting lava over the top of ash layer, when the cycle of ash and lava is repeated over and over in alternating layers

Lava plateau

This is formed as a result of quiet eruptions when lava moves out slowly along cracks/faults. the lava fluid lava flows for a very long time on the plateau before it solidifies, forming a lava plateau. Examples are; Kisoro lava plain in Uganda, Yatta, Laikipia, and Turkana plateau in Kenya.




Lava plateau: – This is 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 flows out quietly in form of hot springs or erupt periodically shooting out water /steam in the air. Examples – Katagata in Bushenyi, Sempaya in fort portal, Majimoto in Tanzania, Majiyamoto in Kenya.

Lava dammed lakes

these are formed when lava blocks channels. Water pools behind leading to the formation of lakes. Examples are L.Bunyonyi and L. Mutanda 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 denudation forces as inselbergs. Examples are Mubende hills, parabong and labwor hills in Acholi, Mumias and Kisi batholiths in Kenya.  




2. Dykes: – These are 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.  

Dykes: - These are 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: – These are 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 waterfalls and rapids. Examples are Thika and Thompson falls in Kenya, Karuma falls and Bujagali in Uganda.  

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

5. Lapoliths: – This is saucer-shaped magma, after erosion. It can be exposed as a shallow basin. Examples are Arenas in Ankole.




Continental drift has also been responsible for the formation of different types of lakes such as down warped lakes like Kyoga and Victoria, rift valley lakes like Lake Albert, Tanganyika, Malawi etc

Continental drift has also been responsible for the formation of different types of lakes such as down warped lakes like Kyoga and Victoria, rift valley lakes like Lake Albert, Tanganyika, Malawi etc

It has also influenced the formation of swamps especially in areas associated with gentle gradients due to down warping.

Fault-guided valleys have also been formed like Aswa valley in Northern Uganda.

Plateaus have also been formed and these include; Northern Uganda plateau, Central and southwestern Tanzania plateau, The west-central Tanzania plateau, Lake Victoria plateau, and central-eastern Tanzania