3. Exogenetic Processes Part- 1
Complete solutions to the Exercise. Maharashtra State Board (Balbharati), Grade 9 Geography.
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Exercise
(a) What is mechanical weathering?
The breaking down or physical weakening of rocks without any change in their chemical composition is called mechanical (or physical) weathering. In this process, solid rocks shatter, disintegrate, or peel off into smaller fragments. It mainly occurs due to variations in temperature, freezing of water (frost), crystal growth, and release of pressure. This type of weathering is most dominant in arid and dry climates where diurnal temperature ranges are high.
(b) What are the main types of chemical weathering?
The main types of chemical weathering are:
- Carbonation: Rainwater absorbs carbon dioxide from the atmosphere to form dilute carbonic acid. Minerals like limestone easily dissolve in this acid, weakening the rock.
- Solution: Some minerals in the rocks get directly dissolved in water. For example, alkalis in the rock dissolve, making the rock brittle and prone to breaking.
- Oxidation: This occurs in rocks that contain iron. The iron reacts with oxygen in the presence of water to form iron oxide (rust). This weakens the rock and forms a reddish layer on it.
(c) How does biological weathering occur?
Biological weathering occurs because of the activities of living organisms (plants and animals):
- Plants and Trees: The roots of trees grow into the crevices and joints of rocks. As the roots grow bigger and thicker, they exert immense tension on the rocks, eventually breaking them apart.
- Animals and Insects: Burrowing animals like rats, mice, rabbits, and ants dig holes, burrows, and anthills in the ground, which loosens rock particles.
- Microorganisms: Flora such as algae, moss, and lichen grow on rock surfaces. They release mild acids that slowly weather the rocks over time.
(d) Distinguish between weathering and mass wasting.
| Weathering | Mass Wasting (Mass Movements) |
|---|---|
| Weathering is the breaking down, weakening, or decaying of rocks in their original place (in situ). | Mass wasting is the downward movement of weathered rock materials along slopes primarily due to gravity. |
| It is caused by environmental factors like temperature, frost, water, and living organisms. | It is caused by the force of gravity acting on weathered materials, often triggered by heavy rainfall or earthquakes. |
| It is classified into mechanical, chemical, and biological weathering. | It is classified into rapid mass movements (rockfalls, landslides) and slower mass movements (soil creep, solifluction). |
| It is a preparatory step that creates loose rock fragments and soil. | It is the transport mechanism that moves those loose fragments down to the foothills. |
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(a) Climate affects earthquakes.
False. Earthquakes are caused by endogenetic (internal) movements in the earth's interior, not by external climatic conditions.
(b) Mechanical weathering is less effective in humid climates.
True. (Humid climates are more conducive to chemical weathering, making mechanical weathering less effective there compared to arid regions).
(c) Mechanical weathering happens on a large scale in dry climates.
True.
(d) The breaking down of rocks into smaller particles is called weathering.
True.
(e) Lateritic rocks are formed through exfoliation.
False. Lateritic rocks are formed through chemical weathering (specifically oxidation and leaching) in areas with high rainfall, not by mechanical exfoliation.
Completed flowchart indicating types of weathering (Answers are highlighted in green).
- (a) Some animals live inside the grounds by making burrows. — Biological weathering
- (b) The rock rusts. — Chemical weathering (Oxidation)
- (c) Water which has accumulated in the crevices of the rocks freezes. Consequently, the rock breaks. — Mechanical weathering (Frost)
- (d) The pipes supplying water in colder regions break. — Mechanical weathering (Frost)
- (e) Sand formation occurs in deserts. — Mechanical weathering (Temperature / Block disintegration)
1. Malin Landslide, Maharashtra (2014):
On 30 July 2014, a massive landslide struck the village of Malin in the Pune district of Maharashtra. Heavy and continuous rainfall soaked the thick layers of weathered soil on the steep slopes of the Sahyadri mountains. The soil lost its friction, leading to a sudden, rapid mass movement (mudslide) that buried the entire village, resulting in the loss of over 150 lives.
2. Kedarnath Landslides, Uttarakhand (2013):
In June 2013, a multi-day cloudburst in the state of Uttarakhand caused devastating flash floods and massive landslides. The sudden surge of water loosened the mountain soil and rocks, causing steep slopes to collapse into the valleys. It destroyed transportation routes, buildings, and resulted in thousands of casualties.
3. Wayanad Landslides, Kerala (2024):
In July 2024, extreme monsoon rainfall triggered massive landslides in the hilly areas of Wayanad, Kerala. Thousands of tons of loose soil, rocks, and uprooted trees slid rapidly down the slopes, burying settlements and tea estates, highlighting the severe consequences of rapid mass wasting in humid, steep terrains.