CBSE Class 7 Social Science Chapter 2 – Inside Our Earth NCERT Solutions

The three main layers that make up the Earth are:

1. Crust: This is the outermost, thin, solid layer of the Earth where we live.
2. Mantle: Located beneath the crust, this is a much thicker layer composed of semi-molten rock.
3. Core: This is the innermost layer, consisting of a solid inner core and a liquid outer core, primarily made of iron and nickel.

What is a rock?

A rock is a naturally occurring solid aggregate of one or more minerals or mineraloids. It forms an essential part of the Earth's solid outer layer, known as the crust. Rocks can vary significantly in their colour, size, and texture.

Name three types of rocks:

The three main types of rocks are:

• Igneous rocks (formed from molten rock)
• Sedimentary rocks (formed from accumulated sediments)
• Metamorphic rocks (formed from existing rocks transformed by heat and pressure)

How are extrusive and intrusive rocks formed?

Extrusive igneous rocks are formed when molten rock (lava) erupts onto the Earth's surface. This lava then cools down rapidly and solidifies, forming rocks on the crust. Examples include basalt.

Intrusive igneous rocks are formed when molten magma cools and solidifies deep beneath the Earth's surface. Because the cooling process is slower underground, these rocks often have larger crystals. Examples include granite.

What do you mean by a rock cycle?

The rock cycle is a fundamental concept in geology that describes the dynamic processes through which the three main rock types—igneous, sedimentary, and metamorphic—transform from one type into another over geological time. For instance, igneous rocks can be weathered and eroded into sediments, which then form sedimentary rocks. When subjected to intense heat and pressure, both igneous and sedimentary rocks can transform into metamorphic rocks. If metamorphic rocks melt, they form magma, which can then cool to become igneous rocks again, completing the cycle.

What are the uses of rocks?

Rocks have numerous uses in human life. Hard rocks are commonly used for constructing roads, houses, and other buildings. Stones are also used in various traditional games, such as 'seven stones' (pitthoo) and 'hopscotch' (stapu/kitkit).

What are metamorphic rocks?

Metamorphic rocks are formed when existing rocks (either igneous or sedimentary) are transformed by significant heat, pressure, or chemical reactions, usually deep within the Earth. These conditions cause the original minerals to recrystallize or change into new minerals. For example, clay can be transformed into slate, and limestone can be transformed into marble.

(i) The correct answer is (a) Igneous. Igneous rocks are formed directly from the cooling and solidification of molten magma or lava.

(ii) The correct answer is (b) Core. The core is the central and innermost layer of the Earth.

(iii) The correct answer is (b) Minerals. Gold, petroleum, and coal are all considered valuable minerals or resources extracted from the Earth.

(iv) The correct answer is (a) Sedimentary rocks. Sedimentary rocks are formed from layers of sediment, and these layers often trap and preserve the remains of ancient organisms, known as fossils.

(v) The correct answer is (a) Crust. The Earth's crust is the thinnest of all the layers, varying in thickness from about 5 km under the oceans to 70 km under the continents.

Here are the correct matches:

• (i) Core matches with (e) Innermost layer. The core is the central part of the Earth.
• (ii) Minerals match with (d) Has definite chemical composition. Minerals are naturally occurring substances with a specific chemical formula and structure.
• (iii) Rocks match with (b) Used for roads and buildings. Many types of rocks are hard and durable, making them suitable for construction purposes.
• (iv) Clay matches with (f) Changes into slate. Clay is a type of sediment that can be transformed into metamorphic rock (slate) under heat and pressure.
• (v) Sial matches with (c) Made of silicon and alumina. Sial is a term used to describe the upper layer of the Earth's crust, which is rich in silica and alumina.

(i) We cannot go to the centre of the earth because of extreme conditions. To reach the Earth's center, one would need to dig a hole approximately 6,000 kilometers deep through the ocean floor or land. The immense heat and pressure at such depths make this task practically impossible with current technology. Therefore, direct exploration of the Earth's core remains unachievable.

(ii) Sedimentary rocks are formed from sediments because of the process of lithification. Rocks on the Earth's surface are constantly broken down into smaller fragments by weathering and erosion. These fragments, called sediments, are then transported by agents like wind, water, and ice and eventually deposited in layers. Over long periods, these layers of sediments are compacted under the weight of overlying material and cemented together by minerals dissolved in water, hardening to form sedimentary rocks.

(iii) Limestone is changed into marble due to metamorphism. Limestone is a sedimentary rock. When limestone is subjected to intense heat and pressure deep within the Earth, its minerals recrystallize and rearrange. This transformation process, known as metamorphism, converts the limestone into a harder, more crystalline metamorphic rock called marble.

The uppermost layer of the Earth is called the crust.

The special feature of the crust is that it is the thinnest of all the Earth's layers. Its thickness varies, being thinner under the oceans and thicker under the continents.