Inside Our Earth: CBSE Class 7 Social Science Geography Chapter 2 Notes

The Layers of the Earth: A Journey to the Centre

Our Earth is structured like an onion, with several concentric layers. Understanding these layers is key to understanding many geographical phenomena. The three main layers are the Crust, the Mantle, and the Core.

1. The Crust: This is the outermost, solid layer where we live. It's the thinnest of all the layers. There are two types of crust:

• Continental Crust: This makes up the landmasses. It is thicker (about 35 km) and is mainly composed of silica and alumina, hence it's called sial (si-al).
• Oceanic Crust: This is found beneath the oceans. It is thinner (about 5 km) and consists of silica and magnesium, so it's called sima (si-ma).

2. The Mantle: Just beneath the crust lies the mantle, which extends up to a depth of 2900 km. It is a thick layer of semi-molten rock called magma. It's much hotter and denser than the crust.

3. The Core: This is the innermost layer with a radius of about 3500 km. It is characterized by very high temperature and pressure. The core is further divided into:

• Outer Core: This is in a liquid state.
• Inner Core: This is a solid ball. The central core is primarily made of nickel and iron, and is called nife (ni-fe).

Key Terms for 'Inside Our Earth'

Crust

The thinnest, outermost solid layer of the Earth, on which we live.

Mantle

The layer of the Earth between the crust and the core, composed of hot, dense, semi-molten rock.

Core

The innermost layer of the Earth, made mainly of nickel and iron (nife), with a liquid outer part and a solid inner part.

Rock

Any natural mass of mineral matter that makes up the Earth's crust.

Mineral

A naturally occurring substance with a definite chemical composition and physical properties.

Magma

Extremely hot, molten rock found beneath the Earth's surface, within the mantle or crust.

Lava

Magma that has erupted onto the Earth's surface through a volcano or fissure.

Sediments

Small particles of rocks, minerals, or organic matter that are moved and deposited by wind, water, or ice.

Fossils

The preserved remains or traces of ancient organisms (plants and animals) trapped in layers of sedimentary rock.

Rock Cycle

The continuous process by which rocks are created, changed from one form to another, destroyed, and then formed again.

Comparison of Rock Types

The Rock Cycle Explained

1. —
2. —
3. —
4. —
5. —

Must Remember Facts

• The radius of the Earth is approximately 6371 km.
• The Crust makes up only 1% of the Earth's volume, the Mantle 84%, and the Core 15%.
• Continental crust is made of Sial (Silica + Alumina). Oceanic crust is made of Sima (Silica + Magnesium).
• The core's main constituents are Nickel and Iron (Nife).
• Igneous rocks are formed from molten magma and are called primary rocks.
• Intrusive igneous rocks (like Granite) cool slowly inside the Earth, forming large crystals.
• Extrusive igneous rocks (like Basalt) cool quickly on the surface, forming small crystals.
• Sedimentary rocks are the only type of rocks that contain fossils.
• Limestone changing to Marble and Clay changing to Slate are examples of metamorphism.
• Rocks are made of different minerals, which have definite chemical compositions and are used as fuels, in industries, and as gems.

Examples of Rocks and Their Uses

• {"title":"Granite (Igneous)","bodyMarkdown":"A hard rock used in construction, for making kitchen countertops, and paving stones."}
• {"title":"Sandstone (Sedimentary)","bodyMarkdown":"Made from sand grains. Used to build historical monuments like the Red Fort in Delhi."}
• {"title":"Marble (Metamorphic)","bodyMarkdown":"Formed from limestone. A beautiful stone used for flooring, statues, and decorative items. The Taj Mahal is made of white marble."}
• {"title":"Coal & Petroleum (Minerals)","bodyMarkdown":"These are fossil fuels. Coal is used for generating electricity and in industries. Petroleum provides us with petrol, diesel, and other products."}

Exam Revision Tip

For your exams, you must practice drawing and labelling the diagram of the Earth's interior and the rock cycle. These are very frequently asked questions. Also, be very clear about the difference between magma (inside the Earth) and lava (on the surface). Creating a comparison table for the three rock types, like the one in these notes, is an excellent way to remember their key features.

Practice Questions with Solutions

• What are the three main layers of the Earth? The three main layers are the Crust, Mantle, and Core.
• What is a rock? A rock is any natural mass of mineral matter that makes up the earth's crust.
• Name the three types of rocks. The three types of rocks are Igneous Rocks, Sedimentary Rocks, and Metamorphic Rocks.
• What are fossils? Fossils are the remains of dead plants and animals that get trapped in the layers of sedimentary rocks.

Frequently Asked Questions

Frequently Asked Questions