Shaping of the Earth’s Surface Class 9 Notes SST Chapter 2

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Class 9 Shaping of the Earth’s Surface Notes

Class 9 SST Chapter 2 Shaping of the Earth’s Surface Notes

Theory of Plate Tectonics and Interior of the Earth

What is Plate Tectonics

• The theory of the movement of the rigid and fragmented Earth’s outer layer over the viscous mantle is called “Plate Tectonics”. The word tectonics has come from the Greek word ‘tektonikos’, meaning ‘building’.
• The Earth’s lithosphere is divided into; 7 major and 20 minor plates.
• Plate tectonics is an important theory given by W. J. Morgan that explains the movement of the Earth’s crust.
• The outermost layer of the Earth is broken into several large and small pieces called tectonic plates.
• The movement of tectonic plates is responsible for the formation of mountains, earthquakes, and volcanoes.
• Based on the above groundbreaking research the three types of Plate-boundaries were constructed:
  

1. Destructive (Converging)
2. Constructive (Diverging)
3. Conservative (Transform).

Destructive (Converging)

When two plates move towards each other it is known as Destructive/ Converging Plate Boundary.

• Old crust is subducted in the oceanic trenches.
• It is accompanied by seismic activity and orogeny, e.g.; the Himalayas and the Andes.

Class 9 SST Chapter 2 Notes – Shaping of the Earth’s Surface Notes Class 9

Constructive (Diverging)

• When two plates move away from each other it is known as Constructive/Diverging Plate Boundary.
• New crust is formed at MOR, e.g., the Mid-Atlantic Ridge.
• It is accompanied by volcanic and seismic activity.

Conservative (Transform)

• When two plates slide past one another, it is known as Conservative/ Transform/ Transcurrent Plate Boundary.
  
• Due to sliding, stress builds up, which results in earthquakes.
• Neither old crust is subducted nor new crust is formed here, e.g., San Andreas Fault.

Forces Responsible for Plate Movement
The movement of tectonic plates is driven by heat generated inside the Earth. This heat creates convection currents in the mantle. These currents slowly move the lithospheric plates.
Major forces are:
Convection Currents: Circular movement of molten material in the mantle.
Ridge Push: New crust formed at mid-ocean ridges pushes older crust away.
Slab Pull: Dense oceanic plates sink into the mantle and pull the rest of the plate behind them.

Interior of the Earth
Based on chemical properties, the Earth is composed of concentric layers, namely – the Crust, Mantle and Core. These are characterised by increasing density, temperature and pressure with increasing depth. Knowledge of the Earth’s interior can be derived by: Direct sources (mining, deep-drilling and volcanic samples) and Indirect sources (seismic waves and meteorites).

Layers of the Earth’s interior:

1. The Crust: It is the outermost solid, thin and brittle part of the Earth. The Oceanic crust is thinner (5-7 km) compared to Continental crust (30-40 km). In the Himalayan region, it thickens significantly reaching up to 100 kms.

The continents are composed of lighter silicates called SiAl (Silica+Aluminium) and the ocean floors are composed of heavier silicates called SiMa (Silica+Magnesium).

2. The Mantle: This layer lies beneath the crust, extending from the Mohorovicic discontinuity (35 km) to a depth of about 2,900 km. The upper mantle includes the asthenosphere (from roughly the base of the crust to —400 km depth), a weak, semi-plastic zone that is the primary source of magma for volcanic eruptions. The lithosphere consists of the crust plus the uppermost (rigid) part of the mantle, with a total thickness varying from 10-200 km.

3. The Core: The core-mantle boundary is located at the depth of 2,900 km (Gutenberg Discontinuity). The outer core is in a liquid state while the inner core is in a solid state. The core is made up of nickel-iron alloy called (NiFe).

Formation of the Himalayas
The oceanic crust of the Tethys sea subducted beneath Asia until the continental crust collided. The collision of the Eurasian and Indian continental plates resulted in the lofty Himalayas.

Relevance and Importance of Plate Tectonics
The theory of plate tectonics explains the dynamic nature of the Earth’s surface. It accounts for continental drift, supercontinent cycles, sea-floor spreading and the recycling of oceanic crust. Major landforms such as mountains, oceans, volcanoes and earthquake zones can be best understood through this theory.

The study of plate tectonics helps in the prediction and reduction of natural hazards. Since most earthquakes and volcanoes occur along plate boundaries, this knowledge is widely used in earthquake and tsunami risk mapping and disaster preparedness.

Plate tectonics plays a crucial role in the formation and distribution of natural resources; like – fossil fuels that develop in sedimentary basins, metal ores like copper and xgold that occur near volcanic arcs and diamonds and rare-
earth that elements are found in ancient collision zones. Geothermal energy is concentrated in tectonically active regions.

Plate movements influence ocean currents and the global distribution of heat, thereby affecting the Earth’s climate. Mountain building due to plate collisions also impacts rainfall patterns and monsoon systems.

The separation and movement of continents lead to speciation (formation of new species). Plate tectonics helps explain the presence of unique and indigenous species in geographically isolated regions.

Modern technologies such as GPS and satellite systems are used to monitor plate movements. This information is applied in earthquake monitoring, urban planning in seismic zones and climate as well as geological research.

Role of Weathering and Erosion; Agents of Gradation—River, Waves and Currents, Wind, Glaciers and Underground Water

All exogenic forces (forces acting on the surface of Denudation depends upon the rock-type and its structure, the Earth) such as weathering, mass wasting/movement its hardness and softness and mineral constituents, and erosion are covered under the term “denudation”. Weathering and erosion

Weathering is the in-situ (on site) mechanical disintegration and chemical decomposition of rocks caused by atmospheric elements like climate and weather such as temperature changes, water, wind and atmospheric gases.

Significance of weathering – it is the first step of soil formation. It helps in the concentration and enrichment of some valuable ores like Fe, Mn, A1 and Cu.

There are three types of weathering processes- Chemical, Physical/Mechanical and Biological.

Key points of weathering:

Aspect	Physical (Mechanical) Weathering	Chemical Weathering	Biological Weathering
Definition	Breaks rocks into smaller pieces without changing composition	Alters, chemical composition of minerals in the rock	Breakdown caused by living organisms (plants, animals, microbes)
Main process	Physical forces and tempe­rature/pressure changes	Chemical reactions (water, acids, oxygen)	Mechanical force or chemical action by organisms
Examples	Freeze-thaw, thermal expan­sion	Oxidation (rusting),hydro-lysis, carbonation, dissolution	Plant roots wedging, lichen/ moss’acids, burrowing
Effect on rock	Only size reduction (fragments/pebbles/sand)	New minerals form, weaken-ing	Cracks widen or surface dissolves chemically/physically
Dominant conditions	Cold/dry climates, large temperature swings	Warm, wet/humid climates	Vegetated areas, forests, rocky surfaces with life

Erosion is the removing and/or transporting of weathered rock debris (sediments) from one place to another through agents like – running water, groundwater, glaciers, wind, waves and gravity. Unlike weathering, erosion involves the actual movement of loosened materials, leading to the wearing down of elevated landforms and shaping the Earth’s surface.
Importance of Weathering
Weathering is beneficial because:

1. It helps in soil formation.
2. It releases minerals required by plants.
3. It creates raw materials for agriculture.
4. It enriches mineral deposits.
5. It prepares rocks for erosion and transportation.

Gradation: Its agents, depositional and erosional landforms and relevance to India Gradation is the process of levelling the Earth’s surface by degradation (erosion) and aggradation (deposition). Agents of gradation are: running water/rivers, waves and currents, wind, glaciers, underground/groundwater; they perform erosion (degradation), transportation and deposition (aggradation/fUling depressions).

1. River (running water) is the most important agent of gradation in humid regions. They erode in their upper course and deposit in their lower course. They form the following:

Erosional Landforms:
(i) V-shaped valleys: These are narrow valleys having steep sides resembling the letter ‘V’. In their youth stage the river flows swiftly down the steep slopes cutting deeper into the bedrock, where weathering and mass- wasting widens the sides forming a V-shaped valley.

(ii) Gorges and Canyons: Gorges are very deep and narrow valleys with steep sides while Canyons are larger and often have step-like slopes. They are formed^ by intense vertical erosion when a powerful river cuts deeply into the soft-rock layers.

(iii) Waterfalls: These are sudden vertical drop in the course of a river. It occurs where hard rocks resist erosion, while softer rocks below are worn away, creating a sudden drop.

Depositional Landforms:
(i) Floodplains: These are wide and flat plain along the river banks. Floodplains are formed during floods when the river overflows its banks and spreads water over nearby land. Fine sediments (alluvium) are deposited thus forming floodplains.

(ii) Natural levees: Levees are raised embankments along the river banks. They are formed during flooding when heavier sediments are deposited close to the river channel thereby gradually building up raised banks.

(iii) Deltas: These are triangular or fan-shaped landform formed at the mouth of the river. It resembles the Greek letter ‘(∆)’. They are formed when a river meets a sea or a lake, its velocity decreases. It deposits sediments, forming a delta.

2. Sea waves and Currents are important agents in the coastal areas. They form the following landforms:
Erosional Landforms:

(i) Sea cliffs: These are steep vertical rock faces found along the coast, formed due to continuous wave attack which erodes the base of the coastal rocks forming steep cliffs.

(ii) Sea caves: These are hollow chambers in coastal cliffs. Waves erode cracks and weak points in rocks, gradually enlarging them into caves.

(iii) Sea arches: They are formed when caves break through a headland. It forms due to continuous erosion that enlarges caves until they join, forming an arch.

Depositional Landforms:
(i) Beaches: They are formed due to accumulation of sand and pebbles along the shoreline. Waves deposit sediments along the coast when their energy decreases.

(ii) Sand bars: These are long ridges of sand formed parallel to the coast. Sand bars are formed when longshore currents deposit sand away from the shore forming bars.

(iii) Lagoons: These are shallow water bodies separated from the sea when sand bars block off a part of the sea, lagoons are formed.

3. Wind is a very active agent in the arid and semi-arid regions where vegetation is sparse. It forms the following landforms:

Erosional Landforms:
(i) Mushroom rocks: These are rocks shaped like mushrooms with narrow bases and are formed when wind carrying sand erodes the lower part of rocks more than the upper part.

(ii) Yardangs: Long narrow ridges of resistant rocks are called yardangs. These are formed when wind erodes the soft rock leaving behind hard rock as ridges.

Depositional Landforms:
(i) Sand dunes: These are mounds or ridges of sand. It forms when wind deposits sand when its speed decreases.

(ii) Loess: Extensive deposits of fine dust particles are > called loess. These are formed when fine particles carried ‘jay wind are deposited over large areas.

4. Glaciers are moving rivers of ice that work in high mountain and polar regions and they form the following landforms:

Erosional Landforms:
(i) U-shaped valleys: These are broad valleys with flat floors and steep sides. They are formed when glaciers erode valley sides and floors through plucking and abrasion.

(ii) Cirques: It is a bowl-shaped depression formed at the head of glacial valleys formed due to erosion by glacier movement at the origin point.

Depositional Landforms:
(i) Moraines: Accumulations of rock debris are called moraines, formed when the glaciers deposit the debris along their sides and/or ends.

(ii) Drumlins: Oval-shaped hills formed by glacial deposits are called drumlins.

5. Underground water is important in limestone areas, forming Karst topography. The landforms created by it are:

Erosional Landforms:
(i) Caves: Underground hollow caves formed due to flowing water in limestone regions. These are formed when carbonation dissolves limestone forming caves.

(ii) Sinkholes: These are circular depressions on the surface of a limestone region. Collapse of cave roofs forms sinkholes.


Depositional Landforms:

(i) Stalactites: These are pointed, icicle-shaped formations hanging down from the cave roofs, formed due to mineral-rich water depositing calcium carbonate. Stalactites and Stalagmites

(ii) Stalagmites: Cone-shaped formations rising from cave floors are called stalagmites. They are formed due to dripping water that accumulates upwards.

Prominent Agents of Gradation in India

Agent of Gradation	Region in India Where Most Active	Reason for Dominance	Major Landforms Formed
Running Water (Rivers)	Northern Plains, Himalayas, Peninsular rivers	Heavy monsoon rainfall and extensive river systems	Floodplains, deltas, gorges, V-shaped valleys
Sea Waves and Currents	Eastern and Western Coastal Plains	Long coastline and constant wave action	Beaches, lagoons, sea cliffs, sand bars
Wind	Thar Desert (Rajasthan), Gujarat	Arid climate, sparse vegetation, loose sand	Sand dunes, loess, mushroom rocks
Glaciers	Himalayas (J&K, Ladakh, Himachal, Uttarakhand)	High altitude and permanent snow cover	U-shaped valleys, moraines, cirques
Underground Water	Limestone regions of Meghalaya, Andhra Pradesh, Chhattisgarh	Presence of soluble limestone rocks	Caves, sinkholes, stalactites, stalagmites

Landforms and Disasters: Earthquakes, Landslides, Avalanches, Glacial Lake Outburst Flood (GLOF) and Dust Storm

Natural processes that shape landforms can result in sudden and destructive events called natural disasters. Many disasters are directly linked to the geomorphic processes such as tectonic activity, glacial movement, wind action and mass wasting.

1. Earthquakes – Its causes and effects An earthquake is the sudden shaking of the earth’s surface caused by the rapid release of energy stored in rocks within the earth’s crust, usually along the fault zones or plate boundaries.

Causes of earthquakes
Most earthquakes occur due to the movement of tectonic plates due to the hot convection currents occurring inside the mantle. Stress builds up in rocks along faults andwhen this stress exceeds the strength of rocks, they slip thereby releasing the energy in the form of seismic waves.

In volcanic and MORs, earthquakes may also be triggered by the movement of magma and the expansion of gases such as carbon dioxide and water vapour. As these gases separate from magma and expand, they increase pressure in surrounding rocks.

Some earthquakes can be human-induced too. Activities such as mining, construction of dams and underground explosions can cause earthquakes of the magnitude of 5.5 and above.

Earthquakes cause shaking and cracking of the ground. Collapse of buildings, bridges and infrastructure, landslides and undersea earthquakes (tsunamis). It also causes loss of life and property.

Seismic waves: Seismic waves are waves of energy that cause shaking of the Earth’s tectonic plates.
There are three types of earthquake/ seismic waves:

1. P waves or longitudinal waves: P waves or primary waves are the fastest and can travel through solids, liquids and gases.
2. S waves or transverse waves: S waves or secondary waves are slower than P-waves and can travel only through solids.
3. L waves or surface waves: L waves travel along the Earth’s surface and cause maximum damage to life and property.

Focus: The point of origin of the seismicity inside the earth’s crust, where the movement starts is called focus (hypocentre).

Epicentre: It is the point on the surface directly above the focus. It is the point from which the L-waves (Surface waves) spread outwards.

2. Landslides: The downward movement of rocks, soil and debris under the force of gravity is called a landslide. , Heavy rainfall, earthquakes, deforestation, steep slopes and/or heavy construction activities in hilly and sensitive gohes can be the causes of landslides. It is a common in the Himalayan region, where slopes are steep and unstable. Landslides can be in the form of rockfall, debris flow or mudflow.

3. Avalanches: An avalanche is the rapid downward movement of snow and ice down the mountain slope. It is common in high-altitude areas such as the Himalayas. It is caused due to very heavy snowfall, sudden temperature rise or vibration caused by earthquakes or human activities. The effects of an avalanche can be very devastating like -destruction of vegetation, burial of roads and settlements and loss of human and animal life.

4. Glacial Lake Outburst Flood (GLOF): A GLOF is a sudden release of water from a glacial lake due to the failure of natural ice or moraine dams. Gladal lakes form behind moraines. If the dam breaks, large volumes of water rush downstream causing floods. It is also common in the Himalayan region and other glaciated regions. It causes flash floods, destruction of villages and bridges and deposits heavy sediment. It takes place due to melting of glaciers due to climate change, weak moraine dams and when earthquakes trigger dam failure.

5. Dust Storms: It is a strong wind that lifts and carries large amounts of dust and sand. It is common in desert and semi-arid regions like Rajasthan where wind is a major agent of erosion. Dust storms are caused by dry and loose soil, strong winds, sparse vegetation and drought conditions. It can result in reduced visibility, respiratory problems, damage to crops and soil erosion.

Mitigation measures:
1. Earthquakes:

1. Construct earthquake-resistant buildings and follow building codes.
2. Avoid construction near fault zones.
3. Conduct mock drills and awareness programmes.
4. Develop early warning systems and hazard mapping.

2. Landslides:

1. Promote afforestation and vegetation cover on slopes.
2. Build retaining walls and proper drainage systems.
3. Avoid construction and mining on steep slopes.
4. Monitor rainfall and issue early warnings.

3. Avalanches:

1. Install snow fences and barriers in vulnerable areas.
2. Monitor snowfall and issue weather warnings.
3. Restrict movement during heavy snowfall.
4. Conduct rescue training and preparedness programmes.

4. Glacial Lake Outburst Flood (GLOF):

1. Regular monitoring of glacial lakes using satellite data.
2. Controlled drainage of excess lake water.
3. Avoid settlements near glacial lake zones.
4. Construct protective embankments downstream.

5. Dust Storms:

1. Develop shelter belts and afforestation in desert areas.
2. Stabilise sand dunes with vegetation.
3. Promote soil conservation and controlled grazing.
4. Use weather forecasting and public warning systems.

Shaping of the Earth’s Surface Class 9 Short Notes

→ Mohorovicic Discontinuity: Also called Moho separates the crust from the mantle. It lies 35 km below the Earth’s surface.

→ Gutenberg Discontinuity: It Ees between the mantle and the outer core, 2900 km below the Earth’s surface.

→ Orogeny: It has been taken from two Greek words – “Oro” meaning mountain and “Genesis” meaning origin. It deals with the study of the origin of mountains.

→ Fault: A fault is a plane of weakness on the Earth’s crust; generaEy a crack or fracture, along which rocks on either side move relative to each other due to endogenic forces.

→ Sea-floor Spreading: It is the process by which new crust is formed at the mid-oceanic ridges as magma rises from the mantle, then cools and solidifies.

→ Island Arc: It is a chain of volcanic islands formed along a subduction zone when one plate moves beneath another and gets subducted into the mantle.