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Kerala State Syllabus 9th Standard Social Science Solutions Part 2 Chapter 2 The Signature of Time
The Signature of Time Textual Questions and Answers
You have recognized from the map that the earthquake zones, volcanoes and mountain ranges overlap. Why is this so?
- The crust, which is the outermost layer of the earth is solid.
- The crust, together with the upper part of the mantle, is the lithosphere.
- The lithosphere exists as several fragments just like the broken shell of an egg.
- Compared to the thickness of the portion from the crust to the inner core, the lithosphere is very thin.
- The portions of the lithosphere which are several thousand kilometers wide and roughly, 100 kilometers thick are called lithospheric plates.
- Whether big or small, each plate contains oceanic as well as continental areas.
Observe the following figures.
Zones of severe earthquakes:
Can you mark the information in each of the above maps in a single map? Don’t forget to use different colors or symbols for each type of feature.
Look at the following diagrams and find out different movements of the lithospheric plates.
a) Divergent margin: Plates move apart from each other.
b) Convergent margin: Plates move towards each other.
c) Transform margin (sheer margins): Plates slide past each other
What is the average thickness of the lithosphere?
The average thickness of the lithosphere is 100 kilometers.
The following are the pictures of some landforms formed due to plate movements.
Identify their respective plate margins.
a) Convergent margins. Fold mountains are formed along the convergent margins, eg. Himalaya.
b) Divergent margins: Magma comes out through the gap formed due to the divergence of plates and solidifies to form mountains.
Eg. Mid Atlantic Ridge.
Identify the Plate margins where the World’s major fold mountains are formed?
Fold mountains are formed along the convergent margins. The Himalayas, the Alps, the Andes, the Atlas, etc are all folded mountains. The Himalaya fold mountain is formed between the Indian Plate and the Eurasian plate.
Identify and mark the focus and epicenters in the given diagram
A – shows the epicenter B – shows the focus
Collect the details of earthquakes that have occurred since 2005.
Let Us Assess
Identify the different plate margins. Which are the associated landforms:
- There are three types of plate margins. They are Divergent margin, Convergent margin and Trans-form margin.
- Divergent margin: Plates move apart from each other. Oceanic ridges are formed.
- Convergent margin: Plates move towards each other. Fold mountains and ocean trenches are formed.
Answer the following questions based on earthquakes.
a) How are earthquakes formed?
b) Which are the different types of seismic waves?
c) Which seismic wave causes maximum destruction on the earth’s surface?
d) On what scale are earthquakes measured?
a) The processes that take place in the interior of the earth cause in the storage of energy in some portions in the Earth. Earthquakes occur on those instances. When this energy is bring released.
b) i. Primary waves
ii. Secondary waves
iii. Surface waves
c) The surface waves are the most destructive.
d) In Richter scale
What does the term’ the Pacific Ring of Fire’ mean?
Nearly 80 percent of the world’s volcanoes are situated around the Pacific Ocean. This zone containing more than 452 volcanoes is therefore known as ‘the Pacific Ring of Fire’.
Explain the various instances where volcanoes are useful to man.
- Though the volcanic eruptions are threats to human life. The regions where it occurs are useful to man.
- These areas are rich in minerals. The Black soil that formed as a result of volcanic eruption is very suitable for cotton cultivation.
- The hot springs that are found on volcanic mountains are supposed to have healing properties.
- Volcanic ash is a good manure.
- Geysers are formed in many volcanic regions.
- These places have been developed as tourist spots. They are known as spas.
The Signature of Time Model Questions and Answers
Analyze the maps given in textbook and make a list of the conclusions that you have reached.
- Earthquakes are comparatively frequent in certain parts of the earth.
- Volcanoes are more common in certain specific places.
- There are some peculiarities in the distribution of mountains.
- Earthquake zones and distribution of mountains on the earth’s surface more or less coincide.
Define Lithosphere. What is its important feature?
The crust, together with the upper part of the mantle is known as the Lithosphere. The portions of the Lithosphere which are several thousand kilometers wide and roughly 100 kilometers thick enable to exist the life on the earth. Lithosphere exists like a broken eggshell. It consists of 7 major plates and about a dozen of minor plates.
Define Lithospheric plates?
The portions of the Lithosphere, which are several thousand Kilometres wide and roughly 100 Kilometres thick are called Lithospheric plates.
Identify and list the different lithospheric plates from the following map.
- Pacific plate
- Australian plate
- North American plate
- South American plate
- Eurasian plate
- African plate
- Antarctic plate
Complete the following
- Major plates
- Minor plates
Write the names of the landforms that are created along the plate margins by the movements of plates?
- Fold Mountains
- Ocean Trenches
- Oceanic Mountain Ranges
Classify the following into major and minor plates.
The plates move at a speed of cms a year.
2 to 12 cms
Analyze the figure given below summarise your findings
- The lithospheric plates are situated above the asthenosphere which is in a semi-plastic state.
- Magma, which is a part of the mantle, remains molten due to the high temperature at the earth’s interior and undergoes continuous convection.
- The plates move due to convection. As a result of this, new ocean floors and subduction zones are formed.
What do you mean by subduction zones?
Due to difference in density between the plates along a convergent margin, the denser plate will submerge under the lighter one. These zones are called subduction zones.
Who put forward the idea of continental drift.
Prepare a flow chart based on plate margins.
Complete the following flow chart.
Which are the Earth Movements that cause a lot of destruction to the mankind?
- Volcanic eruptions
Give examples of fold mountains.
- The Himalayas
- The Alps
How does the Earthquake occur?
Rocks in the deeper interior of the earth undergo displacement and faults due to the plate movements and other causes. Under such situations, severe pressure is exerted on the earth’s lithosphere and seismic waves are generated. These waves create tremors on the earth’s surface: These are experienced by as Earthquakes.
Write down the various reasons for the Earthquake.
- Plates movements and faulting
- Collapse of roofs of mines
- Pressure in reservoirs
- Volcanic eruptions.
What is developed in subduction zones?
Ocean trenches are developed in subduction zones.
Eg. The Challenger Deep in the Pacific Ocean.
Prepare a note on the Continental Drift Hypothesis.
Alfred Wegener, a German meteorologist, put forward the idea of continental drift in 1912. He argued that millions of years ago, all the present-day continents were a single unit forming supercontinent named Pangea which was encircled by an ocean called Panthalassa. Wegener believed that over millions of years, the continental portions drifted over the ocean floor forming the present continents.
Some recent studies indicate that mostly all the continents on earth come together once in every 500 million years. It is believed that the most recent for¬mation of Pangea was about 200 million years back, which means that we will have to wait for another 300 years for the next Pangea
What is seafloor spreading?
New ocean floor is continuously being created as a result of magma that comes out through the divergent margins and solidification along the edges of the plates. This results in the phenomenon called seafloor spreading.
Prepare a table showing earth’s major fold mountains? Where have they formed?
Fold mountains → Plates
Himalayas → Between the Indo-Australian Plate and the Eurasian plate.
The Andes → Between South American and the Nasca plates.
The Alps → Between Eurasia and African plates.
The Atlas → Between Eurasia and African plates.
Give an example of fault zone.
The San Andreas Fault Zone in North America.
The plate margins are generally vulnerable. Why?
The plate margins are generally vulnerable to earthquakes, volcanoes, and faults. This is because the plate margins are weaker than other areas.
Prepare a table showing the features of earthquakes having different intensities.
The features of earthquakes having different intensities
|From 1 to 3||We don’t experience the tremors of the quake, but it is recorded in the seismograph.|
|From 3 to 4||Tremors are felt by us.|
|from 4 to 5||Small objects fall to the ground. Doors and windows shake.|
|From 5 to 6||Weak buildings are damaged.|
|From 6 to 7||Strong tremors are experienced in a large area. Weak buildings collapse.|
|From 7 to 8||Comparatively strong earthquake – large scale destruction takes place over a large area. Even strong building may collapse. May cause a Tsunami.|
|From 8 to 9||Strong earthquake causes devastation at a radius of 100 km from the epicenter.|
|Above 9||Extremely strong earthquake. Such quakes have been rare.|
Give two examples of sudden movements of the earth.
- Volcanic eruption
How are volcanoes formed?
The plate margins are active with volcanoes. The hot molten rock that comes out through the fissures on the crust. Volcanoes are formed by such molten rock material coming out through the fissures along the plate margins.
How are earthquakes formed?
Rocks in the deeper interior of the earth undergo displacement and faults due to plate movements and other causes. Under such situations, severe pressure is exerted on the earth’s lithosphere and seismic waves are generated just like waves in a pond spreading, in all directions when a heavy object falls into it. These waves create tremors on the earth’s surface. These tremors are experienced by us as an earthquake.
Name the most destructive type of seismic wave.
The surface waves
Point out the relief features on the surface of the earth due to the movement of plates.
- Fold mountains
Give reasons for the occurrence of earthquakes.
- Plate movements
- Collapse of roofs of mines
- Pressure in reservoirs
- Volcanic eruptions.
Distinguish between focus and epicenter.
The deep points inside the earth where the earth-quake occurs are known as focus and the point vertically above it on the earth’s surface is known as epicenter.
The seismic waves are recorded by
Which was the most severe earthquake occurred? What was its intensity?
- The earthquake that occurred in Chile.
- It recorded an intensity of 9.5 in the Richter Scale.
Find out and prepare a note on different types of Vol-canoes?
- Active Volcanoes: The frequently erupted Volcanoes. Eg. Mount Fujiyama in Japan
- Extinct Volcanoes: Active at a time. But will not erupt again. Eg. Kilimanjaro in Africa
- Dormant Volcanoes: Volcanoes that have erupted earlier and not active in the recent past. But can become active. Eg. Vesuvius in Italy
What are the three types of waves produced from Focus during the Earthquake? Which is the most destructive among them?
- Primary waves
- Secondary waves
- Surface waves
- The surface waves are the most destructive.
What is Seismograph?
It is an instrument used to record the seismic waves.
The word ‘Tsunami’ means.