Geography and Environment

Activity 7.1

Uncle, where are the rocks?

Sonia recently visited her uncle in Musanze. There used to be many rocks near her uncle’s home.

“Where are the rocks, uncle,” she asked.

Her uncle answered, “The tarmac road needs to be expanded. Large tractors have been leveling the land to enable this.”

“Last week, they were crashing some stones that were on the way,” he continued.

“Uncle, is it true that the foundation of this house was made using some of the stones?”, she asked.

“Yes Sonia,” his uncle replied. “However, we call it gravel, which was mixed with cement and sand. We got the gravel from the nearby quarry,” explained.

“By the way, many of the rocks you see now were very large in the past. Some people say they are slowly sinking. They have really reduced in size!” he exclaimed.

 1.  Using the story above, explain what caused the disappearance of the rocks.

 2.  Explain what may have caused the reduction in the size of the rocks.

From the story above, you will realise that the physical environment changes with time. This may be caused by human activities. It can also be caused by natural processes.

For you to find out

From the knowledge gained from the study of weathering in Unit 5, define the term weathering.

One of the natural processes that causes changes in the environment is weathering.This is because weathering is the process in which rocks break down or decompose near the earth’s surface. The rocks break or disintegrate in situ (that is, the particles do not move from their original position). This is the reason why Sonia’s uncle said they rocks were thought to be sinking. This is because after weathering, the rock materials accumulate around the rock they break from.

The process of weathering may be caused by the exposure of rocks to elements such as temperature changes, rain or human activities. These elements are called agents of weathering.

Activity 7.2

1.  From Sonia’s story, identify the main agent of weathering mentioned. Give reasons for your answer.

2.   In groups of five, find out how the following elements may be agents of weathering:

          • Wind

          • Large and small animals

          • Plants

7.1 Types of weathering and resultant features

There are three types of weathering. These are:

       •   Physical weathering

       •   Chemical weathering

       •   Biological weathering

a)   Physical weathering

This is the breakdown of a rock into smaller fragments, with its mineral or chemical composition remaining the same. Physical weathering is also called mechanical weathering.

The main agent of weathering in physical weathering is temperature.

The main types of physical weathering are:

(i)  Exfoliation

(ii)  Block disintegration

(iii)  Granular disintegration

(iv)  Pressure release

(v)   Frost action

(vi)  Rainwater action

i) Exfoliation

This is the peeling off of a rock surface due to alternate expansion and contraction of the outer layer.

Activity 7.3

In pairs, research about how the process of rock exfoliation takes place from various geographical documents and the Internet. Use an onion to explain your findings.

Photographs such as the one shown below can also be helpful.

ii) Block disintegration

Block disintegration is experienced in areas which experience high temperature during the day and low temperature at night.During the day, the rock gets heated causing it to expand.

During the night, when temperatures fall, the rock cools and contracts. If this process continues for several days, the rock develops cracks. It then breaks from the main rock.Evidence that this type of weathering has taken place is presence of large blocks of rocks over or near the original rock.

iii) Granular disintegration

This occurs to rocks that have different chemical composition. The contraction and expansion of a rock due to temperature changes causes the rock to break. Such rocks break into small grains of individual mineral components of the rock. This is because each mineral has its rate of expansion and contraction. This process is evidenced by presence of numerous small stones spread over the original rock.

iv)  Pressure release

This type of physical weathering is also called loading. Rocks that lie deep underground are under great pressure exerted by the rocks, soils and any other materials piled above them. Removal of these overlying material through processes such as erosion is like removing a load from the rocks that are underneath. Removal of the weight causes the underlying rocks to relax which leads to cracking especially on the upper layers of the rock. The cracked layer slowly wears away. The process is repeated on the next exposed layer

v) Frost action

This is the action of ice on rocks. It occurs due to repeated freezing and thawing of ice that has accumulated in cracks of rocks. This caused the cracks to gradually expand and eventually individual pieces of rock disengage from the original rock. The broken pieces of rock have sharp edges. This weathering process is common on the high mountains at the snow line where freezing and thawing occurs. In Rwanda this is evidence on the slopes of Karisimbi at the snowline.

vi)  Rain water action (slaking)

This process is also known as slacking. It is common in areas that have distinct wet and dry seasons and where rocks contain high amounts of clay. During the wet season, clay absorbs moisture causing it to swell. During the dry season, clay loses water and shrinks. If the swelling and shrinking continues over a long time, individual grains of the rock break off and get detached from the original rock and from each other. Rainwater can also dissolve minerals that bind rocks together. This weakens the rock which then breaks down into fragments.

b) Chemical weathering

Chemical weathering occurs when some or all of the mineral constituents in a rock decompose. This causes the rock to break down, fall apart or disintegrate. Chemical weathering involves chemical reactions between the rock’s minerals and water. Sometimes, atmospheric gases also play a role in the decomposition.

Chemical weathering mainly occurs in areas with high temperatures and high humidity.

Chemical weathering occurs in the following ways:

      a)   Hydrolysis

      b)   Oxidation

     c)  Solution

     d)  Carbonation

     e)  Hydration

Each of these terms is as explained below.

(a) Hydrolysis

This is a reaction between the hydrogen ions in water and the ions in the mineral. This reaction breaks down. The best example is the decomposition of feldspar rocks to form clay. Hydrolysis is common in rocks that contain silica. This process is common in the weathering of granite.

(b)  Oxidation

Oxidation is where rocks containing iron compounds come into contact with oxygen to form iron oxide. Rocks which contain iron, when exposed to water or oxygen, readily breaks down to form iron oxides. The oxide is a brown crust which is usually seen on rocks that have undergone this process.

(c)  Solution

This is a weathering process that affects rocks that have minerals that can dissolve in water. When these rocks come into contact with water, the soluble minerals dissolve, causing the rocks to crumble.

(d)  Carbonation

This is where water combines with carbon dioxide to form weak carbonic acid that reacts with rocks. These are rocks that mainly have calcium or magnesium minerals such as limestone. The acid removes the mineral in solution, leaving the insoluble rock residues.

(e)  Hydration

This involves the absorption of water by the minerals contained in the rock. The rock minerals then expand, causing stress to develop within the rock. This ends up crumbling the rock.

NOTE: All the processes of chemical weathering are closely interrelated and depend on one another. These processes break down the rock by:

• Converting the original minerals to newer minerals that are easily removable.

• Weakening the bonds within the rock by removing what cements the rock together. This breaks down the rock.

c) Biological weathering

This type of weathering is due to the action of living organisms on rocks. People, plants and animals can act to cause the breakup of rocks.

Activity 7.4

In pairs, explain how the following living activities can cause biological weathering:

    i)  The penetration of plant roots into cracks in the rocks.

    ii)  Activities of burrowing animals such as moles and squirrels.

    iii)  Action of worms which mix the soil from time to time.

    iv)  Hoofed animals which walk and run on rocks.

    v)  Human activities such as quarrying, digging and mining.

7.2 Factors influencing weathering

These are things that either promote or prevent weathering from taking place. These factors include:

     •  Climate

     •  Rock types

     •  Vegetation

     •   Relief

Activity 7.5

In groups of five, research on how each of the above mentioned factors can influence weathering. Use the following hints in your explanation.

7.3 Relationship between weathering and different rock types

Different rock types have different structures. It is these structures that determine the rate of weathering. The structure can be explained in terms of:

     i)  Rock permeability

     ii)  Mineral composition

     iii)  Joints in the rock

      iv)   Rock pH

      v)  Colour of the rock

Rock permeability: This is the ability of a rock to absorb water. Rocks with limestone promote rain percolation. This causes decomposition of rocks due to its reaction with water. Where permeable rocks alternate with impermeable rocks, weathering by solution method occurs on dissolvable parts of the rock. This leaves some parts of the rock intact as other parts are ‘eaten’ away.

Mineral composition: Rocks with uniform minerals expand and contract at the same rate. This may result into exfoliation due to the peeling off of the outer layers uniformly.

Rocks with iron on the other hand can easily be weathered by oxidation while those containing limestone can be weathered by carbonation.

Rocks with different minerals get weathered differently. The minerals may have different abilities to absorb water or heat. This causes them to disintegrate at different rates.

Rock joints: Unjointed rocks are difficult to be weathered as neither water nor plant roots can penetrate them easily. This is unlike jointed rocks which allow water and plant roots as well as small animals to pass through them.

Rock pH: Acidic rocks are more resistant to weathering than other rocks. However, such rocks easily undergo physical weathering. An example is a rock made up of granite. Alkaline rocks on the other hand are easily weathered chemically but are resistant to physical weathering. An example is basalt.

Colour of the rock: The type of mineral determines its colour as well. Some minerals are dark coloured than others. An example of such a mineral composition is Augite. Rocks with such minerals tend to absorb heat faster than those which are generally bright coloured, such as olivine. This makes them to be easily weathered through block disintegration.

END OF UNIT REVISION EXERCISE

In groups:

        1.  State and explain different types of weathering.

        2.  Explain the factors that influence weathering.

        3. Determine the relationship that exists between weathering and rock types.