Weathering
and
Soil Formation
Query Corner
1. Choose the correct option.
a. Sedimentary rocks have the following characteristics
i) foliations. [ ]
ii) stratified structure [✓]
iii) crystallisation. [ ]
b. Alluvial soil is ideal for growing
i) tea and coffee. [ ]
ii) rice and sugar cane. [✓]
iii) cotton and millets. [ ]
c. Rocks formed due to compaction and cementation of materials
i) slate and graphite. [ ]
ii) basalt and granite. [ ]
iii) sandstone and limestone. [✓]
d. A rock that contains fossils
i) granite. [ ]
ii) diamond. [ ]
iii) limestone. [✓]
2. Match the following,
=
|
Column I |
Column II |
|---|---|
|
a. Shale |
i. Metamorphosed coal. [g] |
|
b. Marble |
ii. Minute-sized mineral grains. [f] |
|
c. Granite |
iii. Intrusive igneous rock. [c] |
|
d. Igneous |
iv. Sedimentary rock [a] |
|
e. Humus |
v. First rocks to be formed. [d] |
|
f. Graphite |
vi. Metamorphosed limestone. [b] |
|
g. Basalt |
vii. Makes the soil fertile. [e] |
3. Give one example of each of the following.
a. Intrusive igneous rock.
= Granite;
b. Chemically formed sedimentary rock.
= Gypsum;
c. Mechanically formed sedimentary rock.
= Sandstone;
d. Decayed plant or animal matter that improves soil fertility.
= Humus;
e. Precious stone associated with extreme metamorphism.
= Diamond;
f. A crop that grows well in black soil.
= Cotton;
g. A type of farming that helps to conserve soil on hill slopes.
= Terrace Farming;
h. An animal that assists in biological weathering.
= Rodents;
4. Give reason to explain each of the following.
a. Soil is called a renewable resource.
= Soil formation is a slow process and includes physical, chemical and biological changes. It may take a thousand years to form a few centimetres of soil. Thus, soil is a renewable resource.
b. Igneous rocks are called primary rocks.
= Igneous rocks were the first rocks to be formed and are thus known as primary rocks.
c. Intrusive igneous rocks have large mineral grains.
= Intrusive igneous rocks have large mineral grains because magma takes a longer period of time to solidify beneath the Earth's surface, as it is warmer there.
d. Sedimentary rocks tell us about evolution of life.
= The layered arrangement can be noticed very clearly in any sedimentary rock. The sedimentary rocks contain fossils of plants and animals. Thus, sedimentary rocks are ideal for for studying the evolution of life through the ages.
e. Rocks containing iron compounds are prone to chemical weathering.
= Rainwater that contains oxygen (from the air) often acts on rocks, especially those that are rich in iron compounds. This leads to the formation of iron oxide that produces rust, which makes the rock weak and eventually causes its disintegration. And causes chemical weathering.
5. Distinguish between the following.
a. Aggradation and degradation.
=
|
Aggradation |
Degradation |
|---|---|
|
i. Aggradation means the raising of the Earth’s surface at a certain
place due to the deposition of sediments transported by the natural agents. |
i. Degradation means the lowering of the relief due to the removal of
rocks and other materials responsible for the formation of land surface. |
|
ii. Often natural hollows and depressions are filled up in this way. |
ii. Weathering and erosion are primarily responsible for the
degradation of land. |
b. Weathering and erosion.
=
|
Weathering |
Erosion |
|---|---|
|
i. Weathering is a static process. |
i. Erosion is a dynamic process. |
|
ii. The process of weathering involves in the crumbling down or
disintegration as well as decomposition of rock masses. |
ii. The process of erosion involves in wearing away or denudation of
the rocks and transformation and finally deposition of the rock sediments. |
|
iii. The different elements of weather responsible for weathering are
– rain, frost, wind and fluctuations in temperature. |
iii. The different natural agents responsible for erosion are –
running water, winds, glaciers and waves. |
c. Dam and levees.
=
|
Dam |
Levees |
|---|---|
|
i. Dam is a wall built across a river that regulates the flow of water
which not only prevents flood but also collects water in artificial lakes
called reservoirs for future use. |
i. Levees are embankments or walls built along the banks of the river
to stop overflow of water and prevent flood. |
d. Physical and Chemical weathering.
=
|
Physical Weathering |
Chemical Weathering |
|---|---|
|
i. The decomposition and disintegration of rocks due to the changes of
temperature and heat is called physical weathering. |
i. The decomposition and disintegration of rocks due to chemical
reactions is called chemical weathering. |
|
ii. This involves physical disintegration of rock without any change
in its chemical composition. |
ii. This involves chemical decomposition of rock-forming minerals. |
|
iii. New minerals never formed in this process. |
iii. New minerals may be formed in the process. |
e. Thermal and dynamic metamorphism.
=
|
Thermal Metamorphism |
Dynamic Metamorphism |
|---|---|
|
i. Thermal Metamorphism takes place due to intense heat. |
i. Dynamic Metamorphism takes place due to intense pressure exerted
especially during Earth’s movement. |
6. State whether the following are igneous, sedimentary or metamorphic rock.
a. Quartzite.
= Metamorphic;
b. Schist.
= Metamorphic;
c. Basalt.
= Igneous;
d. Gypsum.
= Sedimentary;
e. Shale.
= Sedimentary;
f. Granite.
= Igneous;
g. Marble.
= Metamorphic;
h. Rock salt.
= Igneous;
7. Answer the following questions about rocks and weathering processes.
a. What do we mean by crust? What is it made up of?
= The surface layer of the Earth is known as crust.
It is made up of rocks, soil and inorganic substances like - silica, quartz and feldspar.
b. What are the chief agents of denudation?
= The chief agents of denudation are moving water, ice, wind and waves.
c. Define rocks. Which are the most commonly occurring rock-forming minerals?
= Rocks may be defined as an aggregate of minerals that make up the Earth's crust or surface.
The most commonly occurring rock-forming minerals are - silica, quartz and feldspar.
d. What is soil? Why is it considered an important resource?
= Soil is the most important layer of the Earth's crust. It is formed due to the weathering processes. Rocks are weathered, eroded and transported by wind, rain, glacier or sea waves and are deposited on the crust as a fine film called soil.
It is considered an important resources because a thick layer of soil rich in humus is ideal for agriculture. And soil is home to many creatures on the Earth.
e. How are igneous rocks formed? Why ate they known as 'primary rocks'?
= Igneous rocks are formed due to cooling and solidification of hot molten magma.
They were the first rocks to be formed and are thus known as 'primary rocks'.
f. Describe the formation of sedimentary rocks and mention the feature that makes this type of rock easily distinguishable.
= Sedimentary rocks are formed from sediments deposited by rivers in river beds or along their banks and also in the beds of the seas or lakes.
Sedimentary rocks have a clearly visible stratified structure. It may contain fossils, for example coal and limestone.
g. How are metamorphic rocks formed? State one distinguishable feature of metamorphic rocks.
= Metamorphic rocks are formed due to application of great heat and pressure on pre-existing rocks under surface of the Earth.
Metamorphic rocks are crystallised and foliated.
h. Explain the rock cycle with a suitable illustration of the same.
= Rock formation is a cyclical process. It all begins with magma that reaches the Earth's surface as lava, and gets cooled and solidified (crystallised) into igneous rocks. All rock masses get eroded and weathered to form sediments, which get transported, deposited and compacted to form sedimentary rocks.
Pre-existing rocks under heat and pressure may get transformed into metamorphic rocks and finally turn to molten magma again due to the intense heat of the Earth's interior. Subsequently, lava again solidifies into igneous rocks and the cycle continues. All these changes may take millions of years. Study the diagram that explains the rock cycle on the next page.
Rock Cycle Diagram
i. Which factors affect the process of weathering? How does human activity contribute to the process of weathering?
= The factors that affect the process of weathering are - Climate; Type of rocks; Topography; Presence of vegetation and Human activities.
Human activities : Building of roads, deforestation, construction of buildings exposes rocks to elements of the weather, thus making them more susceptible to weathering.
j. What do we mean by block disintegration?
= Sometimes, due to the expansion and contraction of minerals, cracks appear on the rocks and blocks of the rocks get dismantled leading to block disintegration.
k. Explain the process of formation of exfoliation domes.
= Continuous expansion and contraction may lead to the weakening, loosening and subsequent removal of a thin outer layer of a rock mass as this layer absorbs and loses heat the most.
Subsequently, several outer layers of rock may be removed concentrically, like a peeling onion, reducing the size of the rock. This process is known as exfoliation.
In desert areas marked with daily extremes of temperature, small rounded features called exfoliation domes are common.
8. Questions about soil and soil conservation.
a. Draw a neat, labelled diagram of the soil profile. Mention the most important soil layer responsible for life on Earth.
=
Soil Profile
b. Define soil erosion. Why is soil conservation a necessity?
= Soil erosion is defined as the wearing away of topsoil by natural agents such as running water, wind and ice.
Soil is prone to erosion by wind, rivers, glaciers, waves and human activities. Faulty agricultural techniques, deforestation, pressure of population on land, urbanisation and many more such factors have led to destruction and degradation of this valuable resource. Soil takes thousands of years to develop. Hence, it needs to be conserved.
c. Explain the soil conservation measure that is applicable in each of the following:
i. Desert regions.
= Planting rows of trees over long distances, perpendicular to wind direction, creates a shelter belt. It acts as a windbreak and prevents soil from being carried away.
ii. Hill slopes.
= Hill slopes must be terraced so that soil is not washed away from the slopes during rains.
iii. Near rivers that are prone to floods.
= Dams built across rivers and levees along the river, can check inundation by floodwater.
d. Explain the importance of the following as measures of soil conservation:
i. Contour ploughing.
= Contour ploughing, which means planting crops along same height instead of up and down the hill slope, checks the flow of water and prevents erosion.
ii. Crop rotation.
= Multiple crops must be cultivated and crop rotation must be done to replenish soil fertility.
iii. Plugging of gullies.
= Plugging of gullies with big boulders or by planting trees across them help to prevent gully formation.
iv. Afforestation.
= Afforestation i.e. planting of trees in rows helps in holding the soil in place.
e. Which are the principal soil types of India? Name two important crops that grow well in each of the type of soil you have mentioned.
= The principal soil types of India are - Alluvial soil; Black soil; Red soil; Laterite soil; Mountain soil; and Desert soil.
Two important crops that grow well in each of the type of soils mentioned above are-
Alluvial soil - rice; wheat;
Black soil - cotton; sugar cane;
Red soil - millets; tobacco;
Laterite soil - coffee; cashewnuts;
Mountain soil - Alpine grass; Pine;
Desert soil - dates; groundnuts;