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- Social Studies for ECLPE Y2 SB File Uploaded 2/11/21, 14:01
UNIT 3 ROCKS
Key Unit competence: The student-teacher should be able to explain the
mode of formation of rocks and assess the economicimportance of rocks
3.1. Rocks: definition, types and characteristics
3.1.1. Rocks: definition
A rock by definition is any aggregate of one or more minerals existing in
a solid state.it may be consolidated such as granite limestone and coal or
unconsolidated such as sand or clay. A given rock is always composed of more
than one minerals. However, a few varieties consist almost one mineral. Rocks
therefore form a composition of minerals be soft or hard that make up the earth
in a solid state
3.1.2. Types of the rocks and their characteristics
Rocks can be classified according to their mode of formation and according to
their age. The classification of rocks according to their mode of formation
gives three categories: igneous rocks, sedimentary rocks and metamorphic
rocks.
3.1.2.1 Igneous rocks
Igneous rocks are also called fire formed rocks. They are formed when molten
material called magma cools and solidifies. Magma is a molten rock formed in
the interior of the earth. It is formed when rocks deep underground melts due to
intense hot temperatures and pressure inside the earth. The hot temperatures
are generated by radioactivity and geochemical reactions. As magma flows and
cools different types of igneous rocks are formed.
Igneous rocks are further divided into three categories namely, plutonic,
hypabyssal and volcanic rocks.
i. Plutonic if magma cools into large masses of rock deep in the earth crust,
the process is slow and the resultant rocks are compact in texture and highly
crystallized. These are called plutonic rocks examples include granite diorite
and gabbro.
ii. Hypabyssal rocks: These are formed when molten material rises along lines
of weakness and cracks in the country rocks and cools near the earth surface
.In this case, cooling is more rapid than in the case of plutonic rocks. Medium
sized crystals are formed for example quartz and dolerite
iii. Volcanic rocks: When magma is poured on the surface of the earth, volcanic
rocks are formed. They cool rapidly to form small crystalled rocks e.g.
rhyolite, andesite, basalt and obsidian rocks. On the other hand, pyroclasts
consist of igneous materials but fragmented in nature. They were thrown of
volcanic vent and fragments of solid lava, cinders ash and dust.
Characteristics of igneous rocks
• They are usually made of two or more minerals
• They are light or dark coloured
• They are fine grained or glassy so 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 calledvolcanic rocks.
3.1.2.2. Sedimentary rocks
According to the mode of formation, sedimentary rocks comprise three types
namely mechanically, chemically and organically formed sedimentary rocks.
i. Mechanically formed sedimentary rocks: as soon as rocks are exposed to
the ground they begin to be to be broken down by erosion and weathering
processes .The broken minerals are carried by wind, streams or ice and finally
deposited as sediments. This group includes a variety of either course or fine
textured rocks formed by compaction and cementation of sediments such as
sand, sit, clay and gravel. They are further cemented by calcareous, siliceous
or ferruginous materials to consolidate the materials. Examples of such
rocks include, shale, clay, mudstone, sand stones, tillites, and conglomerates.
ii. Chemically formed sedimentary rocks: This type of sedimentary rocks
are formed when dissolved materials precipitate out of salt water after
evaporation. All water falling on the earth surface as rain contains salts
in solution. The salts may be precipitated by direct evaporation of water,
chemical interaction or by release of pressure where underground water
reaches the surface. The deposition of calcium on beds of streams like Lake
Magadi in Kenya, Lake katwe in Uganda is a familiar example. Salt pans also
exist in Narok, Taita, and Samburu. Stalactites and stalagmites in limestone
areas are also examples of chemically formed sedimentary rocks.
iii. Organically formed sedimentary rocks: These are formed from the
accumulation of dead remains of living organisms. As remains of plants
and animals continue to accumulate, pressure in them continues to build
and finally they solidify and form a rock. Good examples of this type of
sedimentary rocks include coral reefs which forms as a result of accumulation
of skeletons of sea creatures called coral polyps. When these creatures die,
they sink down to the sea bed where their shells are broken up, compressed,
cemented and together to form rocks. Another familiar example of organic
sedimentary rock is coal. It consists mainly of carbon derived from masses
of plant matter that accumulate in the forested swamps millions of years
ago. Due to lack of oxygen in swampy water, the plants did not decay but
solidified into coal.
Characteristic of sedimentary rocks
• They have flat or curved surfaces
• Particle size may be the same or may vary
• They usually have pores (spaces) between pieces
• Sedimentary rocks are the product of other rocks that were already
formed.
• They appear in the form of layers or strata.
• 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 ofgreater than one centimetre and are called beds.
Figure 3. 2: Sedimentary rocks
3.1.2.2. Metamorphic rocks
Metamorphic rocks were once igneous or sedimentary rocks, but have been
changed (metamorphosed) as a result of intense heat and/or pressure within
the Earth’s crust. They are formed as a result of exposure to conditions different
from those under which they were formed. Heat within the earth interior is
generated through radioactivity and geo-chemical reactions. Magma moving
from deep in the earth crust towards the earth surface also generates heat
which leads to metamorphism. When this heat comes into contact with rocks,
serious effects occur on rocks eventually changing their chemical and physical
properties. ‘meta’ means change and ‘morphic’ means form. Hence metamorphic
rocks are those whose form has changed. There are there forms metamorphism
namely thermal, dynamic and thermal dynamic metamorphism.
i. Thermal metamorphism which occurs due to heat changes the rock
structure for example sandstone changes into quartzite, limestone changes
into marble
ii. Dynamic metamorphism occurs when pressure is subjected to rocks. This
pressure occurs due to earth movements leading to compressional and
tensional forces. This led to the formation of shale and phyllite.
iii. Thermal dynamic metamorphism. This where changes are brought about
by a combination of both heat and pressure. In this case coal or organic
matter is changed into graphite by both heat and pressure. Slate turns into
schist when subjected to both heat and pressure.
Characteristics of metamorphic rocks
• They are formed from igneous rocks, sedimentary rocks or any other
metamorphic rocks
• They have alternate bands of light and dark minerals
• They rarely have openings
• They do not split easily
• Some are made up of just one mineral, for example, marble• They have a different texture from the original rock.
3.2.1. Positive impact of rocks
i. Soil formation: when rocks weather down, they produce soils of varying
fertility. Igneous rocks especially volcanic soil when weathered produce
fertile soils that promote agriculture. Sedimentary rocks along river valleys
and lowlands have weathered to produce fertile soils supporting rice growing
in swampy areas.
ii. Minerals: rocks are mined to provide minerals used in various purposes. For
example gold and diamonds are deposited rocks that are mined in igneous
rocks, marble is mined from metamorphic rocks, sand and limestone from
sedimentary rocks. These minerals have supported economic development
through foreign exchange earned through their export.
iii. Building and construction materials: rocks both sedimentary,
metamorphic and igneous provide building materials like sand, and building
stones. Cement used in construction of buildings and roads is extracted from
limestone which is a sedimentary rock. Bricks, tiles and other decorative
stones are got from rocks. Besides, some rocks are also quarried to construct
roads, and other infrastructure.
development. The slopes of these mountains are cool and suitable for
settlement.
v. Tourism development: rocks have formed relief landforms which provide a
fascinating scenery and as a result, they have attracted tourism in these areas.
Good examples are inselbergs, volcanic plugs, stalactite and stalagmites,
exfoliation domes and coral reefs
vi. Source of Energy : some energy sources like lignite coal, bituminous coal,
anthracite coal and natural gas are extracted in sedimentary rocks around
the world. These minerals have been instrumental in spurring development
where they are discovered.
vii. Chemical industries: some rocks have got chemicals like salts, nitrate,
phosphates and potash and Sulphur. These chemical are used in making
the manufacturing of dyes, fertilizers and medicines
3.2.2. Negative impact of rocks
• The fertility of soils depends on the nature of parent rock from which
it is weathered. When certain rocks are weathered, they produce
poor infertile soils which are porous in nature and cannot enhance
agriculture. A good example is quartz that weathers to give rise to
sandy soils.
• Weathered limestone rock produce poor soils with limited water
retention capacity which limits farming
• Some rocks are very hard and brittle and they cover extensive areas.
These rocks discourage mechanical agriculture because they destroy
farm implements like tractors. Some massive lava flows are also
difficult to break using ordinary farming tools hence rendering farming
activities difficult.
• Engineering and construction works are also rendered difficult by
some hard igneous rocks like batholiths, dykes and sills. In some places,
breaking these rocks to pave way for road construction may require
use of explosives which is very expensive.
• Rocks also limit surface drainage in some areas. When a rock is
permeable, it leads to the absence of drainage features like rivers,
swamps or wetlands. This is because as it rains, water percolates
through the permeable rocks and sinks in deeper layers of the crust.
Such areas always lack streams or rivers. This is rampant in porous
volcanic areas.
• In other places especially limestone areas, some rivers disappear
leading to the emergence of dry valleys.
iv. Climate modification: igneous rocks form highlands like Mt. Kilimanjaro,
Birunga, Drakensberg etc. These highlands receive heavy orographic rainfall
which has supported a dense population through agriculture activities on
slope of these mountains. Heavy rains in these highlands has supportedgrowth of forests. These forests are very important in stimulating economic
• It is also important that the quality of water depends on the nature of
the rock through which a stream flows. In some places water tends to
be hard because of the presence of soluble calcium carbonate.
• High mountains formed from volcanic rocks act as barriers to rainfall
on the leeward sides.
• These dry conditions on the leeward side of the highland make
agricultural activities difficult.
• In some places, rocks have created a negative impact especially where
different types of landslides have occurred. Rock fall involving the
falling of large boulders and rock slide have caused accidents, blockedroads and sometimes have led to loss of life and property.
