The Changing Face of Earth Chapter Notes | Window on the Word Class 7: Solutions, Notes & Worksheets PDF Download

The surface of the Earth is constantly changing—new landforms are created and old ones changed or destroyed. These changes are caused by two types of forces.

Endogenic Forces

• Forces originating deep below the Earth's surface
• Responsible for the formation of continents, ocean basins, fold mountains, volcanoes, and rift valleys
• Endogenic Forces: The rigid and brittle lithosphere, consisting of the crust and the upper mantle, breaks into pieces known as tectonic plates.
• Tectonic Plates: These plates are constantly moved by forces deep within the Earth.
• Effects of Plate Movements: The movements of tectonic plates result in volcanic activity, earthquakes, and the creation of mountains.

Exogenic Forces

• Forces originating and acting on the Earth's surface
• Form river valleys, plains, deserts, and beaches

How Do the Tectonic Plates Move ?

• Tectonic plates move due to heat energy from deep inside the Earth.
• Heat from the outer core creates convection currents in the lower mantle.
• These currents push the plates, causing three types of movements.
• Plates move towards each other.
• Plates pull away from each other.
• Plates slide past each other.

 Landforms and Endogenic Processes

• Fold Mountains: Formed when two plates push towards each other, resulting in the land between them being uplifted in massive folds. Examples include the Himalayas, the Alps, and the Rockies.
• Block Mountains and Rift Valleys: Created when two plates move apart, causing cracks or faults on the Earth's surface. Rift valleys form when land between parallel faults subsides, while block mountains are formed when land between faults is pushed upwards.
• Sudden Events from Endogenic Processes: Plate movements not only shape landforms but also trigger sudden and destructive events like earthquakes and volcanic eruptions.

Earthquakes

• When tectonic plates move past each other, their jagged edges can get stuck, causing pressure to build up.
• When the plates finally break free, the accumulated stress is released as waves, causing the Earth's surface to shake—a phenomenon known as an earthquake.
• Earthquakes typically occur along plate boundaries.
• The point within the Earth where an earthquake starts is called the focus, with shallower earthquakes usually being more destructive.
• The epicenter is the point on the Earth's surface directly above the focus, where the most damage usually occurs.
• Damage diminishes as you move away from the epicenter.
• A seismograph is used to measure earthquake intensity, with the Moment Magnitude Scale (Mw or M) being the modern measurement scale based on total energy released—the higher the reading, the more severe the earthquake.

Case Study on Earthquakes and Tsunamis

In 2001, the city of Bhuj in Gujarat was struck by a severe earthquake with a magnitude exceeding 7.5 Mw. The earthquake's epicenter was situated in the Kutch (Kachchh) district, with a shallow focus only 24 km beneath the earth's surface. This shallow focus intensified the impact of the earthquake, leading to extensive casualties and widespread destruction of homes. Despite the initial devastation, the city underwent reconstruction, establishing better infrastructure to mitigate future earthquake impacts.

Tsunamis Caused by Undersea Earthquakes:

Undersea earthquakes can trigger massive tidal waves known as tsunamis, resulting in catastrophic damage upon reaching land. An illustrative example is the earthquake that occurred in the Indian Ocean on 26 December 2004. This seismic event generated a colossal tsunami that wreaked havoc in several countries, including Indonesia, India, Sri Lanka, Thailand, Myanmar, Somalia, and the Maldives. The earthquake, surpassing a magnitude of 9 Mw, originated in the Indian Ocean near Indonesia.

Volcanoes

• A volcano is a vent or an opening in the Earth's crust through which magma, ash, and smoke erupt.
• Magma that flows onto the Earth's surface is called lava.
• Depending on the chemical composition of the lava, it can either ooze out slowly or burst out explosively.
• When lava flows solidify and accumulate over millions of years, they form volcanic mountains.
• Volcanoes and earthquakes generally occur along tectonic plate boundaries.
• Over two-thirds of the world's volcanoes are located along the edges of the Pacific Ocean, forming the Ring of Fire.

Exogenic Forces

• Natural forces that originate and act on the surface of the Earth are called exogenic forces (from Greek exo, meaning external).
• The primary goal of all exogenic forces is to flatten the Earth's surface, also known as agents of gradation.
• Exogenic forces create landforms through three main processes: weathering, erosion, and deposition.

 Weathering, Erosion and Deposition

• Weathering: The process where rocks break down and disintegrate due to rainwater, temperature changes, and biological activities.
• Erosion: The process where weathered rock debris is transported to different locations by agents like water, wind, and ice.
• Deposition: The process where eroded material or sediment settles in various places, contributing to the formation of landforms such as plains and beaches.

Agents of Gradation - Rivers

• Rivers play a crucial role in reshaping the Earth's surface by wearing down highlands through erosion and building up lowlands through deposition.
• A river's flow can be divided into three main stages: upper course, middle course, and lower course.
• The point where a river begins is called its source, while where it meets the sea or a lake is called its mouth.

Upper Course

• In the upper course, rivers flow swiftly through mountains, eroding the land beneath them and carrying away the eroded material.

Middle Course

• As rivers transition from mountains to plains in their middle course, their speed decreases.
• Rivers form meanders - large bends and loops - as they flow through plains.
• Over time, meanders can form ox-bow lakes when the river cuts through narrow necks, leaving behind 'U'-shaped lakes.
• Periodic river floods deposit fertile silt along the floodplain, enhancing soil fertility.

Lower Course

As the river nears the sea, it enters its lower course. The flow slows down significantly due to carrying a heavy load of sediments. As it reaches the mouth, the river deposits these sediments, reducing its speed. This leads to the formation of distributaries, which are small, narrow channels.

Sea Waves

Coastal landforms are influenced by sea waves through processes like weathering, erosion, and deposition.

Glaciers

Glaciers play a crucial role in shaping landscapes in mountainous and polar regions. They are slow-moving masses of ice resulting from the gradual accumulation and compression of snow over time.

• The ice's weight carves out U-shaped valleys in the terrain.
• As the ice moves, it also scrapes the valley floor, creating deep hollows. When the ice melts, these hollows fill with water, forming glacial lakes.

Wind as an Agent of Erosion and Deposition in Deserts

• Wind: Wind plays an active role in shaping the landscape of deserts through erosion and deposition.
• Desert Environment: The vast open spaces of deserts facilitate the movement of winds with uninterrupted force.