Classification of Rocks by origin.

Unlike climate which is classified according to characteristics or similarities, rocks on the other hand are distinguished on three ways:

(a) Origin. This appears to be the most important method because most rocks tend to originate from given compounds, as a result of a given forming processes.

The most common compounds in which rocks are usually formed include iron-oxides, calcite, dolomite clay, quartz and feldspars groups of varying percentages and degrees.'

This life-size elephant is carved from granite.

(b) Age. Despite the facts that different rocks exist simultaneously at given places they still vary in age. There are those which are old while others are young. There are even those which are in the process of making and yet some have to be made!

(c) Characteristics. Rocks can also be classified according to their characteristics. Rock of given age and mode of formation tend to be uniform. This method of classification emphasizes mainly the rock resistance.

From, these two methods rocks can be classified into three main types which include:

Igneous Rocks

Igneous rocks are rocks that have been formed through ignition process. The process of ignition involves fire. This is why they are referred to as fire formed rocks.

The process ignition is two fold and as such there are also two forms of rocks, namely

(a) Volcanic. These are rocks that result from out pouring lava which reaches the surface and congeals. This process is referred to as extrusion or simply volcanism.

(b) Plutonic. These are rocks which have solidified deep in the earth's crust, when lava

fails to reach surface but after sometime they gradually reach the surface through erosion.

Granitic rock type provides a very good example of such rocks. The two types of rocks as described above are jointly formed through the process known as Vulcanism. Through this process the molten magma (hot lava) erupts through the lines of weakness and depending upon the driving force it can either remain underground or reach the surface where it cools and solidifies. These processes are in the order of intrusion and extrusion. There are three distinct forms of Igneous rocks: Plutonic, hypabyssal and volcanic. Each of these rocks is unique in almost every aspect despite the fact they are formed through the same process. For instance:

Plutonic

This form of rocks are formed deep under ground and usually cool much slower. This is why the crystals making them are quite large in size.

Examples are granite, gabbro, diorite, syenite and peridotite.

Hypabyssal

These are rocks which are superficial, i.e. found near the ground surface. They contain crystals which are neither small nor big because they cooled moderately. Examples: granophyre, poryphries, dolerite

Volcanic

These are rocks which are formed at the ground surface. They are the rocks which cooled very fast because of low temperatures existing in the earth, surface.

This rate of cooling is the reason why such rocks have small crystals which are almost fine. Additionally Igneous rocks generally have a high percentage of silica and this is the reason why they can exist as acidic as well as basic. This is shown by the extent of their sizes and trend on the earth surface. Examples: Basalt, Andesite, Rhyolite, Obsidian.

In East Africa there are all three forms of rocks. A majority of them in the areas where volcanic activity has been active in the recent past. The most obvious ones include the East African highlands; Kenya, Elgon and Kilimanjaro mountains.

Metamorphic rocks

The term “meta” means change and morphic means “form”. Thus they are rocks whose original composition has been altered through number of other, processes. Metamorphic rocks can either be sedimentary, another rock type or simply Igneous so long as their chemical as well, as physical composition is changed. Rock metamorphism, is a process, in which a rock is changed from its original composition to the new.

This mica schist is a metamorphic rock. It was changed from a sedimentary rock like shale.

It is applied in the same way as it is used to describe changes in the life history of an insect like a butterfly.

These types of rocks usually have the characteristics:

They are the hardest of the three types.

They contain coarse crystals

They tend to be more resistant to denudation processes and even to high temperatures than the other two. This is because firstly, they are extremely complicated due to the processes in which they could have been formed.

Secondly, are often solid, heavy, massive and above all compact. They do not always have joints or faults.

Metamorphic rocks are formed in many ways. The following are some of them:

By compression.

Compression in the earth crust can be brought in several ways which can include:

(a) When the overlying rocks compress rocks below them

(b) When rocks move against each other through sliding or due to earth movements. In either way heat is always produced. This heat makes the minerals in the rocks to react and as a result lead to their chemical as well as physical weathering.

By heating,

Again there are several ways in which a rock can be heated. For instance .3

a. directly from the sun through insolation.

b. During the processes of folding, faulting, warping, earthquakes, volcanism, etc.

c. When a rock comes in contact with a hot magma;

d. direct fire heating;

The net result of these effects is the production of heat which makes the rocks in contact to react and change their chemical composition.

By chemical and water action

In addition to heat and pressure, the effect of acids and water is equally very significant in bringing about rock metamorphism. This is because both of them are capable of reacting with a rock by dissolving weak ones and even carrying away in solution. This is usually very common in areas underlain by limestone and dolomite.

In east Africa metamorphic rocks are found in the areas with basement rock complex such as:

1. The Ankole surface

2. Outcrop rocks e.g. the inselbergs such as Akisim, Iriri south Karamoja; Soroti hill; the rock outcrop on the Koja peninsula at the lake shore Basin.

3. The Parabongo hill complex in northern Uganda.

4. The flat-topped hills of Buganda Province of Uganda.

5. The escarpment or steep slopes found in the Rift valley

6. The exposed granitic intrusion of the sukumaland near Mwanza, Tanzania including the Inselberg corridor of southern Tanzania are among the few examples.

3. Sedimentary rocks.

These are deposited or laid down rocks by the agents of deposition. The agents doing this remain, water, ice, wind, waves, etc.

Sedimentary rocks result from the rock waste which is eroded from somewhere and later deposited by the same agents in a most suitable place which is usually low or flat. As such rocks of this type are never uniform in content.

Like the two other rock types, sedimentary rocks have also defined characteristics. These include:

§ They contain a lot of fossilised life.

§ They usually occur in the form of layers which have been squeezed, cemented and hardened by the pressure of the overlying rocks.

§ The rocks are not uniform in character;

§ They are most youngest of the three because the process of deposition is continuous.

§ Less resistant of the three types of rocks. This is why agriculturally they are easier to cultivate, and in addition are fertile.

Sedimentary rocks are mainly/ formed in three ways. Through these methods, it has been possible to classify these rocks clearly and in, a manner that it is easy to explain and describe. These include:

Physically or Mechanically formed

These are rocks which are formed mechanically through the effect of:

Wind deposition. As wind blows it erodes rock, transport it and later deposits. In areas where wind activity is much, like deserts rock accumulations are produced. The loess found in the desert areas provide very good example on this. Other examples include:

a) Arenaceous. The material that constitutes this type is usually sand, sandstones and Grit.

b) Argillaceous. Under this category the following are found: Mud, Clay, Mudstone and shape.

c) Rudaceous. Under this category we find the following: breccia, Conglomerate, tillite, gravel scree, boulder: clay (principles of physical Geography by F.S.Monkhouse).

Riverine sedimentation. Rivers are one of the world's most important agent of erosion and deposition. Along river valleys the following are usually found clay, gravels, pebbles, boulders/and alluvium.

This sandstone is an example of a sedimentary rock. It formed when many small pieces of sand were cemented together to form a rock.

Glacial deposition. "Like rivers, glaciers are also often associated with the following deposits; moraines, sand, shingle, gravel and- boulder clay.

Organically formed

These rocks are mainly composed of the accumulated remains of the ancient living organisms. There are two sources in which such rocks have been formed.

From sea animals. The most important of these are the polyps which live in warm ocean water of tropical lands.

The following rocks are products of them: chalk and coral, the coral reefs at the East African coast provide a good example of this type of rocks.

From plants. When plants die and get buried underground, they form deposits of rocks which at times are proved valuable. Some of such rocks include coal and lignite. Peat, marsh or bog are other forms of these rocks.

Chemically formed

These are rocks which are chemically formed. Chemically formed rocks result mainly from the following processes;

(i) Leaching

(ii) Evaporation and

(iii) Precipitation from the solutions of salts.

In tropical areas such rocks are common especially in the areas underlain by calcareous rocks such as limestone.

They form mainly through leaching in the following ways:

When rain falls the rock gets dissolved and form a solution which is leached into the ground. The mineral precipitates formed are deposited and hardened with time.

During the process of evaporation at the surface these minerals are again reversed to the surface where they are left as the moisture in them evaporates. In East Africa the most important form of such rocks is the murram which is quite spread in most areas. There are many other rocks which are often formed. The most common include rock salt, borax, gypsum, nitrates, potash and many other limestone.

Summarily chemically formed rocks are usually ^:formed through the following ways:

Through chemical precipitation. The most important of this processes is leaching especially; in the areas underlain by calcareous rocks such as limestone. In contrary, some rocks particularly calcite have "been deposited directly on the beds of streams underground in the system referred to as caverns. Tlxe _nend result has been the formation of chemical deposits in the form of stalactites, stalagmites and pillars. This is especially important in the areas having rocks that contain calcium carbonates,

Through soil creatures.

Rocks can also be derived from organisms such as ants, termites and polyps. Polyps are probably the most important for they produce extensive accumulations of rocks called coral reefs. The other animals also .produce but smaller relief features called earthmounds or anthills.

Through mechanical processes:

Though chemically produced, sedimentary rocks are also indirectly produced mechanically through the processes of erosion and mass wasting by agents of erosion; As a rock is weathered it is exposed to the effect of weathering. Through this process the rock is further broken into much more small particles as they get deposited somewhere.

Cobbles pebbles and sands are the sediments that are seen on this beach.

Through cementation.

This is a process also significant in forming sedimentary rocks. It involves squeezing and compaction of the mineral using mainly silica, carbonates and Iron oxides. These cementing agents are generally referred to as sesguioxides. The most common rocks produced are called duricrusts or hardpans. In East Africa such rocks are common. They are usually found capping most hills like those in Buganda and generally around the lake shore region.

Compression.

Continued layering of the rocks by agents of deposition often also result to the formation of this type of rocks. One good example of this type is tufa which is often formed if the calcareous substance is spongy. The most common of this is the deposits that contain fossilised lives of algae or mosses.

In East Africa, chemically formed rocks are mainly found in

§ the lake shores where, sediments are brought by rivers flowing into the lakes,

§ Swamps: This can be brought through river flooding.

§ Valleys: The main agents can be:

(i) Mechanical weathering i.e. exfoliation.

(ii) Mass wasting

(iii) Rivers and streams flowing towards the valley floor from the highlands.

§ Coastal areas: Mainly by river discharge from inland. Waves, currents and tides are the main agents of deposition in these regions.

(d) Intrusive into the Palaeozoic and Mesozoic rocks.

Intrusive made up of ijolites, meltegites and nepheline syenites with associated dykes cut through rocks of Karoo age at Jombo, while carbonatite intrusions occur nearby at Mrima, Nguluka and Kiruku (all in Kenya near the Tanzania border). Small alkaline dykes also occur in the Galana (Sabaki) valley about 80 Km north of Mombasa. -In Mbeya, (Tanzania) is an alkaline plutonic intrusion (batholith).

Other intrusives in Tanzania include basic and ultra basic rocks of Matumbi hills south of Utete (Rufiji-Ruhuhu valleys), a dolerite dyke at Mbarali, and in the Rufiji river area are plugs and dykes of phonolite, andesite and basalt. Cimberlite intrusions occur in northern Tanzania south of Lake Victoria. These are located in an arc extending about 100 Km. South - South east from Mwadui. The Tanzania Kimberlite intrusions are diamond bearing, hence the Mwadui Diamond Mines. These non bearing Kimberlite intrusions exist in South eastern Kenya near the Tanzania border.

Cainozoic Rocks.

Cainozoic period is divided into:

(a)Tertiary period.

(b) Quartenary period.

During these periods, the greater part of East Africa remained above sea level with submergence taking place only along the coast. Three major activities occurred:

(a)There was protracted sub aerial erosion (denudation)

(ii)There was widespread tectonism (earth movements).

(iii)There was extensive igneous activity (Vulcanism).

Denudation during Tertiary period produced continental deposits and peneplains. In all the East African Countries, Sands, Sandstones, Limestones, conglomerates, mudstones, silts were deposited. Alkaline lavas and pyrocclasts were also produced by volcanic activity. Pleistocene, Kenya experienced the formation of coral reefs, deposition of terrestrial Mestones and lignite beds. In Tanzania, sands, silts and limestones were deposited, and in Swamp and alluvial deposits and in Uganda, Swamp and alluvial deposits. There was also arching, warping, faulting and uplifts.

During Quaternary period, there was major faulting in the Western rift valley, while in the rift valley, renewed faulting took place. Faulting was associated with volcanism especially the Bufumbira volcanism), explosion craters in Fort Portal and the intrusion of carbonatite complexes along a line stretching from Mt. Moroto through Mt. Elgon, and angwa to Homa in Kenya, and another one at Jombo and Mrima south of Mombasa, and migu and Majiya Wela in Tanzania. Other volcanics include alkaline intrusions (and cones such as Elgon, Kenya, Kilimanjaro, Ol Doinyo L'Engai, Teleki and Rungwe.

Quaternary deposits includes the Kaiso-Kasenyi beds in the Lake George - Lake Edward basins and the Semlike deposits in Uganda; these are arenaceous and argillaceous deposits which contain gypsum. The Kaiso beds also have a thin seam of lignite. Salt, Sulphur and Oil are associated with these deposits (Sulphur is plenty in Lake Nyamununka; hence the hydrogen sulphide which causes the lake to smell, giving the name 'Nyamununka').

The Semliki deposits in the Albert rift is thought to be over 2400 Metres thick while the Kaiso beds are about 1200 Metres thick. In Kenya, and Tanzania, numerous Lake and River deposits occur along the eastern rift floor e.g. the evaporites of lakes Magadi and Natron. Others are in Lake Turkana, Naivasha, Nakuru, Eyasi and Manyara. Deposits of Clays, limestones, tuffs, ashes and gravels occur around the intrusives of Homa and Gwasi in the Kavirondo Gulf area. Similar deposits exist in Rukwa, Buhoro and Serengeti areas of Tanzania. Gypsum deposits occur in Garissa, Wajir and El Wak. Lignite exists in Kitui District.

The Quaternary period is also characterised by glaciation on the higher peaks (Mt. Kilimanjaro, Kenya and Rwenzori).

Soils and alluvial deposits are widespread in eastern) Kenya; between Pangani and Rufiji rivers and north of river Ruvuma in Tanzania. In Uganda, the Kyoga and Aswa swamp deposits are the most important.

The formation of hot springs and many geothermal activities also took place during this time e.g. Kitagata, Kananarok and Ihimbo in Uganda, Eldoret Spa in Kenya and Amboni in Tanzania.

Understanding the rock cycle

Processes of the Rock Cycle

Any type of rock can undergo changes and become any new type of rock. Several processes are involved in the rock cycle that make this possible. The key processes of the rock cycle are crystallization, erosion and sedimentation, and metamorphism. Let’s take a closer look at each of these:

Crystallization. Crystallization occurs when molten material hardens into a rock. An existing rock may be buried deep within the earth, melt into magma and then crystallize into an igneous rock. The rock may then be brought to Earth’s surface by natural movements of the Earth. Crystallization can occur either underground when magma cools, or on the earth’s surface when lava hardens.

Erosion and Sedimentation. Pieces of rock at Earth’s surface are constantly worn down into smaller and smaller pieces. The impacts of running water, gravity, ice, plants, and animals all act to wear down rocks over time. The small fragments of rock produced are called sediments. Running water and wind transport these sediments from one place to another. They are eventually deposited, or dropped somewhere. This process is called erosion and sedimentation. The accumulated sediment may become compacted and cemented

together into a sedimentary rock. This whole process of eroding rocks, transporting and depositing them, and then forming a sedimentary rock can take hundreds or thousands of years.

Metamorphism. Sometimes an existing rock is exposed to extreme heat and pressure deep within the Earth. Metamorphism happens if the rock does not completely melt but still changes as a result of the extreme heat and pressure. A metamorphic rock may have a new mineral composition and/or texture.

Note that the rock cycle really has no beginning and no end: therefore, it’s a never-ending cycle. The concept of the rock cycle was first developed by James Hutton, an eighteenth century scientist often called the “father of geology.” Hutton spoke of the cyclic nature of rock formation and other geologic processes and said that they have “no [sign] of a beginning, and no prospect of an end.” The processes involved in the rock cycle take place over hundreds or even thousands of years, and so in our lifetime, rocks appear to be fairly “rock solid” and unchanging. However, a study of the rock cycle shows us that change is always taking place.

The next three lessons of this chapter will discuss each type of rock in more detail.