UNIT 4:ROCKS IN RWANDA
Key Unit competence: The student-teacher should be able to compare major types of rocks in Rwanda and evaluate their importance.
Introductory Activity
1. Identify the types of rocks given above.
2. In which category can they be classified?
3. Give the characteristics of rocks.
4. Explain the economic importance of rocks in Rwanda.
4.1. Types of rocks in Rwanda and their characteristics
Learning activity 4. 1
Use observation of rock samples provided to you and make research to identify the types of rocks found in Rwanda and their characteristics.
The largest parts of Rwanda are made up of rocks that changed their mineralogical and chemical compositions due to the metamorphism. The examples include schist and quartzite that were exposed forming granites which occupy the greatest part of Rwanda.
The rock types of Rwanda are grouped into three distinct categories:
1. Igneous rocks
They are formed by volcanic activities in the north western and the south western regions of Rwanda. Igneous rocks are rocks formed by cooling of molten material from a volcano or from deep inside the earth. This molten material from inside the earth is known as magma. Igneous rocks are also called magmatic rocks or volcanic rocks. They are in two types:
i. Extrusive rocks
They are formed when lava solidified on the surface. These extrusive rocks are basalt, obsidian, ashes and cinders, etc. They are abundant in Northern Province (Musanze and Burera) and in Western Province (Nyabihu, Rubavu, Rutsiro, Rusizi and Nyamasheke districts). They are dark because they come from basic and fluid lava.
ii. Intrusive or plutonic rocks
They are formed when magma solidifies in deep rock layers before reaching the surface land. E.g. granite, diorite, gabbros, etc. They are found under the highlands of Rwanda and they appear on the steep slopes. There are some igneous intrusions in some parts of Rusizi and the shores of Lake Kivu. They are used as quarries to get stones, sand, etc.
Characteristics of igneous rocks
Igneous rocks have the following characteristics:
• They have a lot of minerals.
• They do not have strata or layers.
They do not contain fossils (fossils are remains of plants and animals fixed in rocks).
• The number of joints increases upwards in any igneous rock.
• Igneous rocks are mostly associated with volcanic activities and are mainly found in the volcanic zones. That is why they are also called volcanic rocks.
2. Sedimentary rocks
Sedimentary rocks are the result of the accumulation of small pieces broken off from pre-existing rocks (igneous rocks, metamorphic rocks and sedimentary rocks) or precipitation of dissolved minerals. Sedimentary rocks form when sediments become pressed or cemented together or when sediments precipitate out of solution.
Most of sedimentary rocks found in Rwanda are mechanically formed because they are deposited by running water in the valleys and in depression where they are known as alluvium. E.g. silt, sand, sandstones, mudstones, etc. There are also organically formed sedimentary rocks by decomposition of organic matter.
Examples: Limestone in Bugarama plain and close to Nyabarongo valley near Kigali, peat in many swamps of Rwanda such as Akanyuru, Akagera, and Methane gas in Lake Kivu are extracted from sedimentary rocks.
Characteristics of sedimentary rocks
Sedimentary rocks have the following characteristics:
• Sedimentary rocks are the product of other rocks that were already formed;
• They appear in the form of layers or strata;
• They are formed from materials from the older rocks, plant and animal remains;
• Sedimentary rocks are found over the largest surface area of the earth;
• Sedimentary rocks have various minerals because they are a product of different sources;
• Most of the sedimentary rocks allow liquids and gases to pass through them (permeable and porous);
• Sedimentary rocks are characterized by different sizes of joints;
• Sedimentation units in the sedimentary rocks having a thickness of greater than one centimetre are called beds.
3. Metamorphic rocks
They are formed from either igneous or sedimentary rocks under the influence of greater pressure and heat. Most of metamorphic rocks are found under the highlands of Rwanda (Congo-Nile Crest, Central plateau, etc.) because they are caused by pressure and heat from compressional tectonic forces and molten rocks in motion under the crust (Birunga region). E.g. quartzite, slate, gneiss, schist, etc.
In Rwanda, there are several quarries where we exploit metamorphic rocks. Almost 80% rocks of Rwanda are metamorphic and igneous in origin. These rocks found in Rwanda contain minerals necessary for economic development of Rwanda.
Characteristics of Metamorphic Rocks
Metamorphic rocks present two distinctive physical characteristics: Foliated metamorphic rocks and Non-foliated metamorphic rocks. Foliated metamorphic rocks such as gneiss, phyllite, schist and slate have a layered or banded appearance that is produced by exposure to the heat and pressure. Non-foliated metamorphic rocks such as hornfels, marble, quartzite do not have a layered or banded appearance. Metamorphic Rocks have also the following characteristics:
• They are harder than the original rocks and are not easily eroded;
• They do not split easily;
• They contain minerals;
• Some are made up of just one mineral, for example, marble;
• They have a different texture from the original rock.
Application activity 4.1
1. Identify an area in Rwanda where igneous rocks are mostly found.
2. Observe rocks found in your environment and explain their characteristics.
4.2. Importance of rocks in Rwanda
Learning activity 4.2
Use geographical documents and internet to identify the importance of rocks.
Rocks are very important to man in the following ways:
• Formation of soils. Rocks are broken down into tiny particles through the process of weathering. This leads to the formation of soil that supports plant growth. For example, the igneous rocks around the volcanic mountains in the Northern and Western provinces of Rwanda have been weathered leading to the formation of fertile volcanic soils. These soils have supported crop production in these areas. These mountains include Mt Muhabura, Karisimbi, Bisoke and Sabyinyo.
• Construction materials. Rocks are used in various ways in the building of infrastructure. For example, igneous and Sedimentary rocks are obtained from quarries to provide stones that are needed in building. Some coloured stones are used to decorate houses and to construct fences. Some igneous rocks especially in Musanze area are used in the construction of fences around homesteads to provide security
• Tourism development. There are many rocks in Rwanda that attract tourists from all over the world. For example, there are wonderful cliffs and rocks such as “ Ibere rya Bigogwe (Bigogwe Breast), Urutare rwa Ndaba (Ndaba Rock) and Urutare rwa Kamegeri (Kamegeri Rock).
• Fertilizers: rocks are very useful in the manufacturing of fertilizers. For example, phosphate bearing rocks are used to make phosphate fertilizers.
• Minerals. Rocks provide humans with valuable minerals that are used in various ways. For example, micro- diamonds in Gicumbi and Tin in Muhanga are Igneous rocks. These are exported hence earning foreign exchange.
• Sources of energy. Peat coal is used as a source of energy in some homes in Rwanda. Hot rocks found beneath the Earth’s surface are responsible for the generation of geothermal energy. This project is still underway in Rwanda but has already picked up in places like Eburru in Kenya
There are some disadvantages of rocks such as:
• Areas with many rocks such as outcrops make it difficult to develop transport and communication infrastructure, especially roads. This is clearly witnessed in some parts of Northern and Western provinces of Rwanda where some areas are inaccessible due to their rocky nature.
• In steep areas, falling rocks lead to serious accidents where houses or homes are destroyed. Sometimes people and animals lose their lives.
• It is difficult and expensive to construct houses in rocky areas. Sometimes people give up on such important developments.
• The formation of sedimentary rocks leads to creation of young soils which barely support agriculture; etc
Application activity 4. 2
Discuss the disadvantages of rocks.
Skills lab
Make a short study on rocks and select the best rock suitable for house construction.
End unit assessment
1. Rocks are good and bad’’ discuss this statement in relation to the context of Rwanda.
2. With detailed explanation, show how the following rocks were formed in Rwanda:
a) Igneous rocks
b) Sedimentary rocks
c) Metamorphic rocks