Quick Revision Notes: Class 7 Social Science (Geography)

Chapter: Our Changing Earth

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### 1. Introduction: The Restless Earth

The Earth's surface is constantly changing due to movements taking place inside and outside the Earth.

• Lithospheric Plates: The Earth’s crust consists of several large and small, rigid plates carrying continents and ocean floor. These are called Lithospheric Plates (स्थलमंडल प्लेटें).
• Movement of Plates: These plates move very slowly—just a few millimeters each year—because of the molten magma inside the earth.

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### 2. Forces Shaping the Earth

The forces that cause changes on the earth's surface are divided into two main categories based on where they originate:

                  EARTH MOVEMENTS (पृथ्वी की गतियाँ)
                                |
         -------------------------------------------------
         |                                               |
  ### A. ENDOGENIC FORCES                         ### B. EXOGENIC FORCES
  (अंतർजात बल - Internal)                         (बहिरجات बल - External)
         |                                               |
         -------------------------             -------------------------
         |                       |             |                       |
   Sudden Forces         Diastrophic Forces  Erosional               Depositional
   (आकस्मिक बल)         (पटल विरूपणी बल)      Forces                  Forces
   - Earthquakes         - Mountain Building   - Wind                  - Rivers
   - Volcanoes                                 - Glaciers              - Wind
   - Landslides                                - Sea Waves             - Glaciers

• Endogenic Forces (अंतर्जात बल): Forces that act in the interior of the earth.
  • Sudden Forces: Cause mass destruction in a short time (e.g., Earthquakes, Volcanoes, Landslides).
  • Diastrophic Forces: Slow forces that build mountains over long periods.
• Exogenic Forces (बहिरجات बल): Forces that work on the surface of the earth. They create landforms through erosion and deposition.

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### 3. Sudden Movements: Volcanoes and Earthquakes

A. Volcano (ज्वालामुखी)

• Definition: A vent (opening) in the earth’s crust through which molten material erupts suddenly.
• Magma Chamber: The pool of molten rock beneath the surface.
• Lava: Magma that reaches the Earth's surface.

B. Earthquake (भूकंप)

• Definition: Vibrations caused when the lithospheric plates move, shaking the earth's surface.
• Focus (उद्गम केंद्र): The place inside the earth where the earthquake vibration originates.
• Epicenter (अधिकेंद्र): The place on the surface directly above the focus. Maximum damage is usually felt here.
• Seismic Waves (भूकंपीय तरंगें): The waves of energy radiated outward from the focus in all directions.
• Measurement Instrument: Seismograph (सीस्मोग्राफ).
• Measurement Scale: Richter Scale (रिफ्टर पैमाना).
  • Magnitude 2.0 or less: Can be felt little.
  • Magnitude 5.0: Can cause damage from things falling.
  • Magnitude 6.0 or higher: Very strong; 7.0 is classified as a major earthquake.
• Safety Preparedness (भूकंप से बचाव):
  • Safe Spot: Under a kitchen counter, table, or desk; away from tall/heavy objects, glass, and mirrors.
  • Be Prepared: Inform family and friends, be aware.

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### 4. Major Landforms created by Exogenic Forces

The landscape is continuously worn away by two processes: Weathering (breaking up of rocks on earth's surface) and Erosion (wearing away of the landscape by agents like water, wind, and ice).

A. Work of a River (नदी का कार्य)

As the river flows, it carves out distinct landforms:

• Waterfall (जलप्रपात): Formed when a river falls steeply over very hard rocks down a valley.
• Meanders (विपरीत/विसर्प): Large bends formed as the river enters the plain and twists and turns.
• Ox-bow Lake (चाँप झील / गोखुर झील): Cut-off parts of a meander loop formed over time due to continuous erosion and deposition along the sides.
• Floodplains (बाढ़कृत मैदान): Flat fertile areas formed by layers of fine soil (sediments) deposited during regular floods.
• Levees (तटबंध): Raised banks created by the deposition of sediments along the river banks.
• Delta (डेल्टा): Triangular collections of sediments formed at the mouth of a river just before it enters the sea.

B. Work of Sea Waves (समुद्री तरंगों का कार्य)

Marine processes shape coastal regions:

• Sea Caves (समुद्री गुफाएं): Hollowed-out caves formed by continuous waves striking rocks.
• Sea Arches (समुद्री मेहराब): Caves that become bigger and bigger until only the roofs remain, forming arch-like structures.
• Stack (स्टैक): Pillars of rock left when the roofs of sea arches break due to wave erosion.
• Sea Cliff (समुद्री भृगु): Steep rocky coasts rising almost vertically above sea water.
• Beach (पुलिन / समुद्र तट): Shores formed when sea waves deposit sediments along the shores.

C. Work of Ice (हिमनद का कार्य / Glaciers)

• Glaciers: Rivers of ice that erode the landscape by bulldozing soil and stones to expose solid rock.
• Glacial Moraines (हिम्मोड़): Material carried by glaciers (such as rocks big and small, sand, and silt) gets deposited to form glacial moraines.

D. Work of Wind (पवन का कार्य)

In deserts, wind is the dominant agent of erosion and deposition:

• Mushroom Rocks (छत्रक चट्टानें): Rocks shaped like a mushroom, where winds erode the lower section of the rock more than the upper part.
• Sand Dunes (बालू टिब्बे): Low hill-like structures formed when the wind blows and deposits sand in low areas.
• Loess (लोएस): Fine, light grains of sand carried and deposited over vast areas (e.g., in China).

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### Quick Review Terms (मुख्य शब्दावली)

• Lithosphere: Solid crust of the earth.
• Magma: Molten rock inside the earth.
• Erosion: Wearing away of landscape by different agents like water, wind, and ice.
• Deposition: Accumulation of eroded materials in a new place.
• Epicenter: Point on the surface above the earthquake focus.