5. Lithosphere

(1) Complete the sentences by choosing the correct option.

Heights and depths are measured from the average sea level, which is taken as 0 metres.

The radius of the Earth is about 6400 km, so its diameter is about 2 × 6400 = 12800 km.

(2) Choose the incorrect statement.

• a) The edges of a plateau are generally steeply sloping.
• b) The peaks of mountains become narrow.
• ✗ c) A wide and shallow valley is called gorge.
• d) Not all plains are fertile.

Answer: c — a gorge is a narrow and deep valley, not a wide and shallow one.

(3) Choose the correct option of the pair.

• A) 1-a, 2-c, 3-e, 4-b, 5-f
• ✓ B) 1-d, 2-f, 3-e, 4-a, 5-c
• C) 1-f, 2-d, 3-b, 4-c, 5-a
• D) 1-b, 2-d, 3-a, 4-f, 5-c

Answer: B) 1-d, 2-f, 3-e, 4-a, 5-c

The movement of the liquid metals creates the magnetic field of the Earth. This magnetic field protects the biosphere from harmful solar winds.

Because of the tremendous pressure, the inner core remains in a solid state, even though its temperature is about 6000° C.

The friction between the moving lithospheric plates causes earthquakes.

1. Mountain range: Continuously high, steeply sloping mountains standing in a series are called a mountain range. The mountains follow one line or direction. Example: the Sahyadri, the Satpuda.
2. Mountain chain: A series of mountains spread in any directions is called a mountain chain. It is made of several ranges together. Example: the Himalaya, the Andes, the Rocky.

1. Valley: The low-lying, elongated landform between mountains, hills and hillocks is a valley. Most valleys are broad, with gentle slopes, and are shaped like the letter ‘V’ or ‘U’. Streams flow through them. Example: the Narmada river valley, the Spiti valley.
2. Gorge: A narrow and deep valley with very steep, almost vertical walls is called a gorge. Example: the Kundalika valley (Raigad).

1. Position: The continental crust is the upper layer, forming the landmass. The oceanic crust is the lower layer, lying below the oceans.
2. Thickness: The continental crust is thick — on average 30 to 70 km. The oceanic crust is comparatively thin — on average 3 to 5 km.
3. Composition: The continental crust is made of lighter elements, mainly Silica and Aluminium, so it is called SiAl. The oceanic crust is made mainly of Silica and Magnesium, so it is called SiMa.
4. Density: The continental crust is lighter (about 2.7 gm/cm³). The oceanic crust is denser (about 3.0 gm/cm³).
5. Main rocks: Granite in the continental crust; basalt and gabbro in the oceanic crust.

The continental crust is mainly composed of the lighter elements Silica (Si) and Aluminium (Al). The name SiAl is formed from the first letters of these two elements (Si + Al). Hence the continental crust is called SiAl.

Scientists have inferred that the core is made mainly of heavy elements such as Nickel (Ni) and Ferrous, i.e. iron (Fe). The name NiFe is formed from the symbols of these two elements (Ni + Fe). Hence the core is called NiFe.

At the time of its origin, the Earth was a mixture of extremely hot dust and gases, which later turned into a high-temperature liquid. While the Earth was liquid, its light and heavy elements separated according to their weight. The lighter elements, such as Silica and Aluminium, settled in the upper parts. The heavier elements, such as Nickel and Ferrous, sank and settled in the deep middle part of the Earth. That is why the core contains the maximum quantities of Ferrous and Nickel.

• Heat from the Earth’s interior: The temperature increases towards the centre. Hot material in the mantle rises and cooler material sinks, forming slow circular currents that drag the plates along.
• Rising magma: Molten material rising through cracks in the ocean floor pushes the plates apart.
• Gravity: Where a heavier plate meets another plate, its edge sinks into the mantle and pulls the rest of the plate with it.

1. Location: The outer core lies between the mantle and the inner core. The inner core is the innermost layer, at the centre of the Earth.
2. State: The outer core is in a liquid state. The inner core is in a solid state because of the extreme pressure at the centre.
3. Temperature: The outer core is about 3000° C. The inner core is about 6000° C.
4. Density: The outer core has a density of about 10 gm/cm³. The inner core is denser, about 13 gm/cm³.
5. Main element: The table in the chapter lists Nickel (Ni) for the outer core and Ferrous (Fe) for the inner core; together the core is called NiFe.
6. Role: The movement of liquid metals in the outer core creates the Earth’s magnetic field.

The temperature of the outer mantle is similar to the melting point of the rocks it is made of. As a result, the rocks there are in semi-molten condition, neither fully solid nor fully liquid. Therefore the outer mantle is in a semi-fluid state. The study of earthquake waves also shows that this layer could be flowing.

The radius of the Earth is about 6400 km, so it has not been possible to reach its centre directly with any tool. Geologists have studied the interior indirectly:

• By studying how earthquake waves travel through the Earth.
• From the increasing temperature observed from the surface towards the centre, for example in deep mines.
• From inferences about the density of the elements found at different depths.
• By examining the materials that come out during volcanic eruptions.
• From knowledge of the Earth’s magnetism and gravity.

• Plains: Plains are flat and many are made of fertile river soil. Farming, transport and building are easy there, so plains are densely populated. Example: the North Indian Plains.
• Mountains: Steep slopes, cold climate and thin soil make life and farming difficult, so mountains are thinly populated. But they give us forests, rivers that start there, hill stations for tourism, and act as natural barriers.
• Plateaus: Plateaus are often rich in minerals and useful for mining. Their flat tops are used for farming and grazing, and their steep edges were used to build forts. Example: the Deccan Plateau.
• Valleys: Valleys have rivers and fertile land, so villages and towns grow there. They also provide routes through mountains.
• Hills and hillocks: They are used for grazing, forests, places of worship and forts. Example: the Parvati hillock (Pune).

Landforms therefore decide where people live, what occupations they follow, how they travel and even how places are defended.