Spheres of the Earth
We walk on land, drink river water, breathe air, and there are trees and birds around us. All four belong to four different spheres of the Earth.
The four main spheres of the Earth: lithosphere (land and rocks), hydrosphere (water), atmosphere (air) and biosphere (living things). The whole environment on the Earth is formed from the effects of these spheres on one another.
Key idea 1 : Four spheres — The lithosphere includes land and sea floor; the hydrosphere rivers, lakes, seas and glaciers; the atmosphere air; and the biosphere trees, birds, animals and humans. The biosphere spreads into parts of the other three spheres.
Key idea 2 : What is the lithosphere — About 71 per cent of the Earth is covered with water and 29 per cent with land. This land is called a landmass and is the upper layer of the lithosphere. The part under water, the ocean floor, is the lower layer of the lithosphere.
Key idea 3 : Kinds of lithosphere — The lithosphere is not the same everywhere but made of different rocks and minerals. It has two parts: continental lithosphere (continental crust) and oceanic lithosphere (oceanic crust). The Earth was very hot at first; it slowly cooled and its outer part became solid; that is the lithosphere.
- Easy : Land belongs to which sphere? — Lithosphere.
- Medium : River water belongs to which sphere? — Hydrosphere.
- Exam-type : Birds and trees belong to which sphere? — Biosphere.
In the figure: Four spheres of the Earth.
Li-Hy-At-Bi.
Lithosphere is not only land; the ocean floor under water is also part of the lithosphere.
Memory clue: Shila = rock/land; jal = water; vat = air; jiv = living. Memory clue: "Shi-Ja-Va-Ji".
Spot the difference
| Sphere | What it includes | Example |
|---|---|---|
| Lithosphere | Land, ocean floor | A hill |
| Hydrosphere | Water | A river |
| Atmosphere | Air | Wind |
Practice questions
- 1Which are the four spheres of the Earth?
- 2What is the lithosphere?
- 3What is the proportion of land and water on the Earth?
- 4Which are the two parts of the lithosphere?
- 5How is the biosphere linked with other spheres?
- 6Why is the ocean floor part of the lithosphere?
- 7Write one example of each sphere in your surroundings.
- 8How did the lithosphere form as the Earth cooled?
Show answers (try them yourself first)
- 1Lithosphere, hydrosphere, atmosphere, biosphere.
- 2The solid part of the Earth: land and ocean floor.
- 371 % water and 29 % land.
- 4Continental and oceanic lithosphere.
- 5It spreads in parts of the other spheres.
- 6Because it is part of the Earth’s solid crust.
- 7Student writes own.
- 8The hot early Earth cooled layer by layer and its outer part became solid.
Interior of the Earth
To see the inside of a mango we cut it. But we cannot cut the Earth and see inside; so scientists estimate its interior from earthquake waves and other observations.
From the surface of the Earth to its centre there are three layers: the crust, the mantle and the core. The radius of the Earth is about 6,400 km, so humans have not been able to reach inside directly.
Key idea 1 : Crust — The uppermost solid layer. The continental crust is made of lighter silica and aluminium so it is called ‘SiAl’, and the oceanic crust is thin and made of silica and magnesium so it is called ‘SiMa’. As per the textbook the continental crust has an average thickness of 30 to 70 km and the oceanic crust 3 to 5 km.
Key idea 2 : Mantle — The thick layer between the crust and the core. The outer mantle is semi-fluid and the inner mantle is more solid. Because of the semi-fluid nature of the outer mantle, the plates of the lithosphere move and earthquakes occur.
Key idea 3 : Core — The core begins below the mantle. Scientists believe it to be of nickel and iron (NiFe). The outer core is liquid and the movement of metals in it creates the Earth’s magnetic field. The inner core is solid because of the tremendous pressure there.
- Easy : What is the radius of the Earth? — About 6,400 km.
- Medium : The outermost layer? — Crust.
- Exam-type : NiFe is which part? — Core.
In the figure: Three layers of the Earth.
The textbook gives the temperature of the inner core as about 6,000 °C, while many sources say about 5,200 to 5,500 °C; the oceanic crust too is given as 5 to 10 km (average about 6 km) in many sources, 3 to 5 km in the textbook (as per several sources; check with the book).
Cr-Ma-Co.
It seems the core of the Earth is liquid; but only the outer core is liquid, the inner core is solid.
Memory clue: From top inward: crust → mantle → core. Memory clue: "SiAl up, NiFe down".
Spot the difference
| Layer | State | Main elements |
|---|---|---|
| Crust | Solid | Silica, aluminium, magnesium |
| Mantle | Outer semi-fluid; inner solid | Magnesium, iron |
| Core | Outer liquid; inner solid | Nickel, iron |
Practice questions
- 1Which are the three layers of the Earth’s interior?
- 2How is the interior structure of the Earth known?
- 3State the difference between SiAl and SiMa.
- 4What is the effect of the semi-fluid nature of the outer mantle?
- 5Why is the inner core solid?
- 6How is the Earth’s magnetic field created?
- 7Write the two kinds of crust and their thickness.
- 8Draw a diagram of the Earth’s interior and write names of the layers.
Show answers (try them yourself first)
- 1Crust, mantle, core.
- 2By studying earthquake waves.
- 3SiAl = continental (silica + aluminium); SiMa = oceanic (silica + magnesium).
- 4Plates move and earthquakes occur.
- 5There is tremendous pressure there.
- 6Due to movement of liquid metals in the outer core.
- 7Continental 30–70 km; oceanic 3–5 km (as per book).
- 8Student draws own.
Major landforms
Around our village we see hills, flat farmland, a river valley or a high plateau. These different shapes of land are called landforms.
Landforms are different natural shapes of the Earth’s crust. They are formed by slow or sudden movements inside the Earth, stress and other natural factors. Their combined structure is called the physical structure.
Key idea 1 : Mountain, hill and hillock — A mountain is much higher than the surroundings, becomes narrow towards the peak and as per the textbook its height is more than 1000 m (Sahyadri, Satpura, Himalaya, Andes, Rockies). A hill is lower than a mountain with gentler slopes (as per the textbook more than about 600 m). A hillock is still lower, with a rounded top and gentle slope (as per the textbook 300 to 600 m).
Key idea 2 : Plateau and plain — A plateau is lower than a mountain but higher than the surroundings, with a flat top and often steep edges. As per the textbook height is about 100 to 1000 m. A plain is a flat and extensive land at low elevation from sea level; as per the textbook height is less than 300 m. Most plains are in river basins and deltas and some are fertile.
Key idea 3 : Valley and gorge — The low-lying, elongated landform between mountains, hills and hillocks is called a valley. Its shape is like the English letter V or U and streams flow through most valleys. A narrow and deep valley is called a gorge.
- Easy : Sahyadri is which landform? — Mountain range.
- Medium : Saswad, Kas are what? — Plateau.
- Exam-type : A V-shaped landform with a stream? — Valley.
In the figure: Major landforms and their height.
Mo-Hi-Hk-Pl-Pn-Va.
Plateau and plain are both flat but not same; a plateau is higher than the surroundings while a plain is low.
Memory clue: Mountain highest, plain lowest. Memory clue: "Mountain tall, plain down, plateau like a table".
Spot the difference
| Landform | Height (as per book) | Feature |
|---|---|---|
| Mountain | More than 1000 m | Narrow peak |
| Plateau | About 100 – 1000 m | Flat top |
| Plain | Less than 300 m | Flat, extensive |
Practice questions
- 1What are landforms?
- 2State the difference between a mountain and a plateau.
- 3In which areas are plains formed?
- 4What is a valley? What are V and U shapes?
- 5What is a gorge?
- 6Write three landforms in your area and their features.
- 7State the difference between a hillock and a hill.
- 8State the difference between a mountain range and a mountain chain.
Show answers (try them yourself first)
- 1Different natural shapes of the Earth’s crust.
- 2Mountain over 1000 m, narrow; plateau lower, flat top.
- 3In river basins and deltas.
- 4The low part between mountains; shaped like the letter V or U.
- 5A narrow and deep valley.
- 6Student writes own.
- 7Hillock 300–600 m, round; hill higher (as per book).
- 8Range = continuous high mountains; chain = many ranges spread in any direction.
Elevation and depth from sea level
When telling the height of our house we tell it ‘from the ground’; but when telling the height of a hill we tell it ‘from sea level’, as ground level differs everywhere while the sea level can be taken as the same.
Sea level is the average level of sea water; it is taken as zero metres. The elevation of a place on land is how high it is above sea level; and depth in the sea is how far below sea level it is.
Key idea 1 : Elevation and depth — The elevation of land is measured upwards from sea level (e.g. 600 m), while the depth of the ocean floor is measured downwards from sea level (e.g. 2000 m). This makes it possible to compare different places.
Key idea 2 : Temperature and depth — As per the textbook the average temperature at the surface is 15 °C and going deep into the ground it increases by about 1 °C every 32 metres. That is why more heat is felt in a deep mine.
Key idea 3 : Landforms of Maharashtra — Sahyadri and Satpura are mountain ranges. Saswad (Pune), Kas (Satara), Malegaon and Toranmal are plateaus. Jarandeshwar (Satara) and Jyotiba hill (Kolhapur) are hills. Parvati and Vetal (Pune) are hillocks. Kundalika (Raigad) is a valley and the Konkan coast a plain.
- Easy : What height is sea level taken as? — Zero.
- Medium : How much does temperature rise at 80 m depth? — About 2.5 °C (80 ÷ 32).
- Exam-type : How high is Kalsubai peak? — 1,646 m.
In the figure: Sea level: the zero of elevation and depth.
As per several sources: the elevation of Mount Everest is 8,848.86 m above sea level (joint Nepal-China survey, 2020) and the depth of Challenger Deep in the Mariana Trench is about 10,935 m (a few metres’ difference by measurement); the highest point of Maharashtra is Kalsubai peak (1,646 m, Sahyadri) (as per several sources; check with the book).
Sea level = 0.
Elevation is not always measured from the ground; in Geography it is measured from sea level.
Memory clue: Elevation = above sea level; depth = below sea level. Memory clue: "Sea at 0 metres".
Spot the difference
| Point | Elevation | Depth |
|---|---|---|
| From where | From sea level | From sea level |
| Direction | Upward | Downward |
| Example | Kalsubai 1,646 m | Challenger Deep about 10,935 m |
Practice questions
- 1What is sea level?
- 2Why is elevation measured from sea level?
- 3State the difference between elevation and depth.
- 4By how much will temperature rise at 160 m depth (as per book 1 °C per 32 m)?
- 5How much lower is Kalsubai than Everest (take 8,849 m)?
- 6Name three plateaus and two mountain ranges of Maharashtra.
- 7Why is more heat felt in a deep mine?
- 8Find from a map the elevation of your village from sea level.
Show answers (try them yourself first)
- 1Average level of sea water, zero metre.
- 2Because it can be taken as the same everywhere.
- 3Elevation = above sea level; depth = below it.
- 4160 ÷ 32 = 5 °C.
- 58,849 − 1,646 = 7,203 m.
- 6Plateaus: Saswad, Kas, Malegaon; ranges: Sahyadri, Satpura.
- 7Temperature rises with depth.
- 8Student finds from a map.