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

Solution: The Earth is composed of three main concentric layers. These layers, from the outermost to the innermost, are:

1. Crust: This is the outermost, thin, solid layer of the Earth. It is the part we live on and is made up of various minerals and rocks.
2. Mantle: Located beneath the crust, the mantle is a much thicker layer. It is primarily composed of semi-molten rock, known as magma, which can flow very slowly.
3. Core: This is the innermost layer of the Earth. It is the hottest part and is divided into the outer core and the inner core. The core is believed to be the 'engine' of the Earth due to its extreme temperature and pressure.

Solution: A rock is defined as any natural, solid aggregate of one or more minerals or mineraloids. It forms a significant part of the Earth's crust. Rocks can vary greatly in their composition, texture, size, shape, and colour, depending on how and where they were formed.

Solution: The three main types of rocks found on Earth are:

1. Igneous Rocks: These rocks are formed when molten rock material, called magma (beneath the surface) or lava (on the surface), cools down and solidifies.
2. Sedimentary Rocks: These rocks are formed from the accumulation, compaction, and cementation of small particles derived from pre-existing rocks, organic matter, or chemical precipitation. These particles are often transported by wind, water, or ice.
3. Metamorphic Rocks: These rocks are formed when existing igneous, sedimentary, or even other metamorphic rocks are transformed by intense heat, pressure, or chemical reactions deep within the Earth's crust, without melting completely.

Solution: Extrusive and intrusive rocks are both types of igneous rocks, distinguished by where the molten rock cools and solidifies:

Extrusive Rocks: These rocks are formed when molten rock, known as lava, erupts onto the Earth's surface during volcanic activity. As the lava is exposed to the cooler atmosphere or water, it cools down rapidly and solidifies, forming extrusive igneous rocks. Examples include basalt.

Intrusive Rocks: These rocks are formed when molten rock, called magma, cools and solidifies beneath the Earth's surface. Since the magma is insulated by the surrounding rock layers, it cools much more slowly, allowing larger crystals to form. Examples include granite.

Solution: The rock cycle is a fundamental geological process that describes the continuous transformation of rocks from one type to another over vast periods of time. It involves various processes like melting, cooling, weathering, erosion, deposition, compaction, and metamorphism. For instance:

1. Molten magma cools and solidifies to form igneous rocks.
2. Igneous rocks can be weathered and eroded into smaller pieces (sediments), which are then transported, deposited, compacted, and cemented to form sedimentary rocks.
3. Both igneous and sedimentary rocks can be subjected to intense heat and pressure deep within the Earth, causing them to transform into metamorphic rocks.
4. Metamorphic rocks, under even greater heat, can melt to form magma again, restarting the cycle. Alternatively, they can be weathered and eroded to form sediments, leading to sedimentary rocks.

This cycle illustrates that rocks are not static but are constantly changing and being recycled.