Grade 10 Geography Natural Systems and Processes – Rocks Notes
Geography — Natural Systems & Processes
Subtopic: Rocks (for age ~15, Kenya focus)
- a) Examine the classification of rocks according to mode of formation and age.
- b) Describe the distribution of rocks in Kenya.
- c) Analyse the significance (economic, environmental, social) of rocks in Kenya.
- d) Sample rock types in your locality (fieldwork skills).
- e) Appreciate the significance of rocks in Kenya.
- Mode of formation → igneous, sedimentary, metamorphic.
- Age → old (Precambrian basement), younger (Tertiary/Quaternary sediments and volcanics).
- Distribution → Basement Complex, Rift Valley volcanics, coastal sediments, sedimentary basins (e.g., Turkana).
- Uses and importance → building materials, minerals, energy, soils, tourism, water.
1. Classification by mode of formation
Igneous rocks
Formed from cooled molten rock (magma or lava). Two types:
- Intrusive (plutonic) — magma cools slowly under the ground → coarse-grained: e.g., granite.
- Extrusive (volcanic) — lava cools quickly on the surface → fine-grained: e.g., basalt, obsidian.
Formed by deposition and cementing of material. Types:
- Clastic — from rock fragments (sandstone).
- Chemical/organic — formed by precipitation or shells (limestone).
Formed when pre-existing rocks are changed by heat and pressure (e.g., gneiss, schist, marble).
Rocks change form: melting → igneous; weathering/erosion → sediments → sedimentary; heat/pressure → metamorphic.
2. Classification by age
Relative age — layers: older rocks are usually below younger rocks (principle of superposition). Look at contacts, faults and intrusions.
Absolute age — measured in years using radiometric dating. In Kenya:
- Basement Complex (Precambrian): very old, often >540 million years (gniess, schist, migmatite).
- Rift Valley volcanics: mainly Neogene–Quaternary (last ~23 million years to present) — basalts, trachytes, phonolites.
- Coastal and basin sediments: Tertiary to recent (younger than basement rocks).
3. Distribution of rocks in Kenya (simple overview)
- Basement Complex (central, western, parts of north): ancient metamorphic and intrusive igneous rocks (gneiss, schist, migmatite, granite).
- Kenyan Rift Valley: extensive young volcanics — basalts, trachyte, phonolite (areas: Olkaria, Menengai, Mount Suswa, Mount Longonot).
- Highlands (Mount Kenya, Aberdares): older igneous intrusions and volcanic rocks.
- Coastal strip: limestone, coral and sand deposits (Kwale, Mombasa, Lamu).
- Turkana & Tertiary basins: sedimentary rocks rich in fossils (Koobi Fora, Lake Turkana region).
Local examples to remember: Olkaria (geothermal & volcanic rocks), Menengai caldera (volcanics), Kwale (coastal heavy-mineral sands), Lake Magadi (soda ash), Turkana (fossil-bearing sediments).
Simplified Kenya — rock zones
(This is a simple sketch — consult the Geological Survey of Kenya map for accuracy.)
4. Significance of rocks in Kenya
Economic
- Minerals and mining: gold (parts of western Kenya and Migori), soda ash (Lake Magadi), industrial minerals, heavy mineral sands (titanium minerals at Kwale).
- Energy: geothermal power in the Rift Valley (Olkaria, Menengai).
- Building materials: granite, basalt, limestone, sand, ballast, and dimension stones used for construction and roads.
- Soil formation and fertility depend on rock type (e.g., volcanic soils are often fertile).
- Groundwater aquifers often relate to rock types and structures — important for wells.
- Landscape, tourism: volcanoes, hot springs, escarpments, and fossil sites (Turkana) attract visitors and researchers.
- Hazards: earthquakes and volcanic activity mainly along the Rift; erosion and landslides in some rock/soil combinations.
5. How to sample rock types in your locality (student fieldwork)
Preparation
- Ask permission for visits (landowner, teacher). Carry ID and first-aid kit.
- Tools: notebook, pencil, rock hammer (teacher/experienced person handles hammering), hand lens (10x), safety goggles, gloves, sample bags, phone/GPS, camera, ruler, and dilute vinegar (for carbonate test).
- Record location (village name, GPS or phone map), date, weather, and type of exposure (road-cut, quarry, stream bank).
- Describe rock: color, grain size (coarse/fine), texture (layered/foliated), hardness (scratch with fingernail/glass/steel), reaction to dilute vinegar (fizzes = carbonate like limestone).
- Take small labeled samples (no vandalism). Note sample code in your notebook (e.g., R1, R2).
- Sketch a small outcrop diagram and note relationships (which layer is on top?).
- Hardness test: fingernail (~2.5), copper coin (~3), knife (~5–5.5), glass (~5.5–6). Basalt is hard; limestone softer and fizzes with vinegar.
- Grain observation: igneous coarse = intrusive (granite), fine = extrusive (basalt).
- Layering: visible layers suggest sedimentary; foliation suggests metamorphic.
Sample ID: ______ | Location: ______ | Date: ______
Rock type (student guess): ______ | Colour: ______ | Grain size: ______
Texture/notes: ______ | Hardness test result: ______ | Vinegar: yes/no ______
Rock type (student guess): ______ | Colour: ______ | Grain size: ______
Texture/notes: ______ | Hardness test result: ______ | Vinegar: yes/no ______
Suggested learning experiences (activities)
- Field trip to a local quarry, river cut, or roadside outcrop: collect 3–5 small samples, record observations, classify them in groups.
- Class lab: perform simple tests (hardness, acid test, hand lens observations) and sort samples into igneous/sedimentary/metamorphic.
- Map activity: mark local rock exposures on a sketch map of the school area or village; relate rock type to soil and land use (farmland, building).
- Group research: prepare a short report on one Kenyan rock resource (e.g., Olkaria geothermal, Kwale heavy minerals, Magadi soda ash) covering formation, use and impacts.
- Presentation and reflection: explain why rocks are important to local community (jobs, energy, materials) and environmental concerns (mining impacts, water).
Assessment tasks & reflection
- Describe three ways rocks are useful to people in your county. (short answer)
- Bring two rock samples from home/locality. Classify them and explain how you determined the type. (practical)
- Explain why Rift Valley areas in Kenya are good for geothermal energy. (explain link between rock type and heat source)
- Reflect: which rock type is most common near your school and how does it affect farming or building?
- Geological Survey of Kenya – regional geological maps and reports (consult school library or teacher for selected pages).
- Local museum or university geology department (for fossils and rock collections).
- Field guides: basic rock identification books and school laboratory manuals.