Horizon Academy
Our Changing Earth
Important Questions & Answers
Geography - Class 7
Lithospheric Plates
What are lithospheric plates?
Answer: Lithospheric plates are large and small rigid, irregularly-shaped plates (slabs) or pieces of the Earth's crust that carry continents and oceans.
What do the lithospheric plates carry on them?
Answer: They carry the continents and the ocean floors.
How and why lithospheric plates move?
Answer: Lithospheric plates move very slowly, only a few millimetres each year, because of the movement of molten magma inside the Earth.
What causes the lithospheric plates to move?
Answer: The movement of hot molten magma inside the Earth in a circular way causes the plates to move.
What changes are caused by the movement of lithospheric plates?
Answer: The movement of these plates causes changes on the Earth's surface like earthquakes, volcanoes, and formation of mountains.
Earth Forces
What are the two types of forces that cause earth movements?
Answer:
- Endogenic forces: These act inside the Earth.
- Exogenic forces: These work on the surface of the Earth.
What are Exogenic forces?
Answer: Exogenic forces are the forces that act on the Earth's surface, like wind, water, and ice that cause weathering and erosion.
What types of movements are caused by Endogenic forces?
Answer: Endogenic forces can cause both sudden movements (like earthquakes and volcanoes) and slow movements (like mountain formation).
What are some examples of sudden movements caused by Endogenic forces?
Answer: Earthquakes and volcanoes are examples of sudden movements that can cause great destruction.
How is the movement of molten magma similar to a science activity with water?
Answer: Just like a paper pellet in warm water moves upward with hot water and comes down with cold water, molten magma inside the Earth also moves in a circular way — upward when hot and downward when cooler.
Volcanoes
What is a volcano?
Answer: A volcano is an opening (vent) in the Earth's crust through which molten material (like lava, gases, ash) erupts suddenly.
What causes a volcano to erupt?
Answer: Volcanoes erupt due to pressure from molten magma inside the Earth that forces its way out through weak spots in the crust.
Earthquakes
What is an earthquake?
Answer: An earthquake is a sudden shaking or vibration of the Earth's surface caused by the movement of lithospheric plates.
How do earthquakes occur?
Answer: When lithospheric plates move, the Earth's crust vibrates. These vibrations are called earthquake waves.
What is the focus of an earthquake?
Answer: The focus is the place inside the Earth's crust where the earthquake movement begins.
What is the epicentre of an earthquake?
Answer: The epicentre is the point on the Earth's surface that is directly above the focus of an earthquake.
From where do the vibrations of an earthquake spread?
Answer: The vibrations spread outward from the epicentre as waves.
Where is the damage from an earthquake the greatest?
Answer: The greatest damage occurs closest to the epicentre, and the strength decreases as we move away from it.
What are the three types of earthquake waves?
Answer:
- P waves (Primary or longitudinal waves)
- S waves (Secondary or transverse waves)
- L waves (Surface waves)
Which waves cause the most destruction during an earthquake?
Answer: L waves (surface waves) cause the most destruction because they travel along the Earth's surface.
Can earthquake waves travel all around the Earth?
Answer: Yes, the vibrations from an earthquake can travel all around the Earth.
What are P waves also known as?
Answer: P waves are also called Primary waves or longitudinal waves.
What are S waves also known as?
Answer: S waves are also called Secondary waves or transverse waves.
Additional information about Earthquake waves:
1. P Waves (Primary or Longitudinal Waves)
- They are the first waves to be detected after an earthquake.
- They are fastest among all three types.
- They can travel through solids, liquids, and gases.
- They move in a back-and-forth (push and pull) motion, like a spring.
- Cause less damage compared to other waves.
2. S Waves (Secondary or Transverse Waves)
- They come after P waves.
- They are slower than P waves.
- They can only travel through solids, not through liquids or gases.
- They move in a side-to-side or up-and-down motion.
- They cause more damage than P waves.
3. L Waves (Surface Waves)
- They are the slowest but most destructive.
- They travel only on the Earth's surface.
- They cause the ground to move in circles or waves, like water.
- They are responsible for most of the damage during an earthquake (like buildings falling, cracks, etc.).
Can earthquakes be predicted?
Answer: No, earthquakes cannot be predicted exactly, but their impact can be reduced if we are prepared in advance.
What are some local methods used by people to predict earthquakes?
Answer: People observe animal behavior. For example, fish get agitated in ponds and snakes come to the surface before an earthquake.
What is the name of the machine used to measure earthquakes?
Answer: An earthquake is measured by a machine called a seismograph.
On which scale is the magnitude of an earthquake measured?
Answer: The magnitude of an earthquake is measured on the Richter Scale.
What does an earthquake of 2.0 or less feel like?
Answer: An earthquake of 2.0 or less is very light and can barely be felt.
What can an earthquake of over 5.0 magnitude cause?
Answer: An earthquake over 5.0 can cause things to fall and get damaged.
What is considered a very strong earthquake?
Answer: An earthquake of 6.0 or more is considered very strong.
What magnitude is classified as a major earthquake?
Answer: An earthquake of 7.0 or more is called a major earthquake.
When did the earthquake hit Bhuj (Gujarat)?
Answer: The earthquake hit Bhuj town on 26th January 2001.
What was the magnitude of the Bhuj earthquake?
Answer: The Bhuj earthquake had a magnitude of 6.9 on the Richter Scale.
What damage was caused in Bhuj due to the earthquake?
Answer: At least 971 students and 31 teachers died due to the collapse of school buildings. There were fires, phone lines, water pipelines, and power stations were destroyed.
What safety steps should we follow during an earthquake to protect ourselves?
Answer: Safety steps should we follow are:
- Take shelter under a kitchen counter, table, desk, or in an inside corner of a wall.
- Stay away from fireplaces, chimneys, windows, mirrors, and picture frames.
- Do not panic and stay as calm as possible.
- Spread awareness among friends and family so everyone knows what to do during an earthquake.
Earth Surface Processes
What are the two processes that wear away the landscape?
Answer: The landscape is worn away by:
- Weathering – breaking of rocks on the Earth's surface.
- Erosion – wearing away of the land by agents like water, wind, and ice.
What happens to the eroded material?
Answer: The eroded material is carried away (transported) by water, wind, or ice and later deposited somewhere else. This process creates different landforms.
How does running water shape the land?
Answer: Running water in rivers erodes, transports, and deposits materials, changing the shape of the land and creating landforms.
What is a waterfall and how is it formed?
Answer: A waterfall is formed when a river tumbles down steeply over hard rocks or a steep valley side.
What is the highest waterfall of Jharkhand?
Answer: The highest waterfall of Jharkhand is Lodh Falls, also called Budhaghagh Falls, with a height of about 143 meters. It is formed by the Budha (बूढ़ा) River, a tributary of the North Koel River.
Which is the highest waterfall in the world?
Answer: The highest waterfall in the world is Angel Falls in Venezuela, South America.
Name two other famous waterfalls and where they are located.
Answer:
- Niagara Falls – on the border of Canada and USA in North America.
- Victoria Falls – on the border of Zambia and Zimbabwe in Africa.
What is a meander?
Answer: A meander is a large bend or loop formed in a river when it flows through the plains.
How is an ox-bow lake formed?
Answer: Due to erosion and deposition, the ends of a meander loop come closer and eventually cut off from the river, forming a cut-off lake, also called an ox-bow lake.
What happens when a river overflows its banks?
Answer: When a river overflows, it floods the nearby areas and deposits fine soil and sediments, forming a floodplain.
What is a floodplain?
Answer: A floodplain is a flat and fertile area formed by sediments deposited by a river during floods.
What are levees?
Answer: Levees are raised banks formed along the sides of a river due to the deposit of sediments during floods.
What are distributaries?
Answer: As a river reaches the sea, it slows down and breaks into smaller streams, called distributaries.
What is a delta and how is it formed?
Answer: A delta is a triangular landform formed where the river deposits sediments at its mouth. It is created when distributaries deposit sediments into the sea.
What causes coastal landforms?
Answer: Sea waves cause erosion and deposition which leads to the formation of coastal landforms.
What happens when sea waves continuously strike rocks?
Answer: Cracks are formed in the rocks, which become larger and form sea caves.
What are sea caves?
Answer: Sea caves are hollow spaces formed in rocks by the action of sea waves.
How are sea arches formed?
Answer: When sea caves become larger, only the roof remains, forming a sea arch.
What are stacks?
Answer: When the roof of a sea arch breaks, only the walls remain, and these are called stacks.
What is a sea cliff?
Answer: A sea cliff is a steep rocky coast that rises almost vertically above sea water.
How are beaches formed?
Answer: Sea waves deposit sediments along the shore, which forms beaches.
What are glaciers?
Answer: Glaciers are rivers of ice that move slowly and erode the land.
How do glaciers erode the landscape?
Answer: Glaciers bulldoze soil and stones, scraping off the surface and exposing the solid rock.
How are mountain lakes formed by glaciers?
Answer: Glaciers carve deep hollows, and when the ice melts, these hollows get filled with water and form beautiful lakes.
What is glacial moraine?
Answer: Glacial moraine is the deposit left behind by glaciers, made of rocks, sand, and silt.
What is the main agent of erosion and deposition in deserts?
Answer: The main agent of erosion and deposition in deserts is the wind.
What is a mushroom rock?
Answer: A mushroom rock is a rock shaped like a mushroom, with a narrow base and wide top, formed by wind erosion.
Why do mushroom rocks have a narrow base?
Answer: Because the wind erodes the lower part of the rock more than the upper part.
What are sand dunes?
Answer: Sand dunes are small hills of sand formed when wind lifts and drops sand in low areas.
What is loess?
Answer: Loess is fine and light sand that is carried by wind and deposited over large areas.
Where are large deposits of loess found?
Answer: Large deposits of loess are found in China.
Key Comparisons
Difference between Weathering and Erosion
| Weathering | Erosion |
|---|---|
| It is the breaking of rocks into smaller pieces on the Earth's surface. | It is the wearing away and moving of rocks and soil from one place to another. |
| Rocks are broken at the same place. | The broken material is carried away by agents like water, wind, or ice. |
| It does not involve movement of materials. | It involves movement and transportation of materials. |
| Caused by things like temperature, water, or plants. | Caused by wind, water, glaciers, and sea waves. |
Difference between Endogenic and Exogenic Forces
| Endogenic Forces | Exogenic Forces |
|---|---|
| These forces act inside the Earth. | These forces act on the surface of the Earth. |
| They cause sudden and slow movements like earthquakes and volcanoes. | They cause slow changes like weathering and erosion. |
| They can build landforms like mountains. | They mostly break down landforms. |
| Example: Earthquake, volcano, mountain building. | Example: River, wind, sea waves, glacier shaping the land. |
Give reasons:
Some rocks have the shape of a mushroom.
In deserts, wind carries sand that hits the rocks. The lower part gets eroded more, making the rock look like a mushroom.
Flood plains are very fertile.
During floods, rivers deposit fine soil (called silt) on the plains, which makes the land very fertile.
Sea caves are turned into stacks.
Sea waves keep hitting the sea caves and slowly break the roof. Only walls are left. These are called stacks.
Buildings collapse due to earthquakes.
Earthquakes cause the ground to shake strongly. If buildings are not strong, they break and fall.
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