NCERT 7th Class (CBSE) Social Science: The Interior of the Earth

Question: What do you know about the structure of the earth?

Answer: On the basis of varying density and other physical and chemical properties of rocks, the earth can be divided into three concentric layers-crust, mantle and core.

Crust:

1. The crust is the solid outermost layer. It is also called lithosphere.
2. It is a relatively thin layer. The upper part of the crust is called continental crust (about 35 cm thick). The lower part is called oceanic crust (about 5 km thick).
3. The continental crust is made up of silica (si) and aluminia (al), and so it is called sial. The oceanic crust is made up of silica (si) and magnesium (ma) and therefore also called Sima.
4. The outer crust has soil. It provides with most of the minerals.

Mantle:

1. Just beneath the crust is a 2,900 km thick layer called mantle.
2. It lies between the crust and the core.
3. Based on the chemical composition, it is divided into the upper mantle and lower mantle.

core:

1. The core lies below the mantle and around the center of the earth. It is also called barysphere.
2. The average radius of the core is about 3,500 km.
3. The core is divided into two – the upper/outer core and the inner core.

The temperature in the core is hotter than that on the sun’s surface. The intense heat from the inner core causes material in the outer core and inner mantle to move around. The dominant minerals here are Nickel and iron. Therefore, it is also called nife (“ni” slands for Nickel and “fe” for iron).

Question: What are the main characteristics of igneous rocks?

Answer: The main characteristics of igneous rocks are:

1. These rocks are hard and compact.
2. These rocks do not occur in distinct beds or stratas.
3. These rocks do not have remains of animals or plants.
4. These rocks do not allow water to percolate through them.
5. Other types of rocks are formed from igneous rocks.
6. Valuable minerals such as iron, gold, silver and aluminum are found in these rocks.

Question: What are the main characteristics of sedimentary rocks?

Answer: The main characteristics of sedimentary rocks are:

1. These rocks are softer than igneous rocks.
2. These rocks have horizontal layers. Arranged one alone the other.
3. These rocks are mostly formed under water have marks of waver and mud-cracks.
4. These rocks also contain remains of animal and plants, called fossils, between the layers.
5. The important examples are sandstone, limestone and coal.

Question: What are the main characteristics of metamorphic rocks?

Answer: The main characteristics are as follows:

1. These rocks after changes remains in their original position.
2. They are very hard and have a high specific gravity.
3. These rocks do not have empty spaces in them.

Question: Explain the working of the rock cycle.

Answer: The rock cycle is as follows:

1. The hot and molten magma forms igneous rocks on cooling.
2. These igneous rocks are exposed to different agents of weathering and erosion on the earth’s surface.
3. The broken rock fragments, when transported and deposited in basins, form sedimentary rocks.
4. The leftover igneous rocks and the newly formed sedimentary rocks are subject to intense heat and pressure. They change into metamorphic rocks in due course of time.
5. The sedimentary and metamorphic rocks are buried again and melt to form magma.
6. This magma again cools down and forms igneous rocks.

Question: Why it is said that the rock cycle has neither a beginning an end.

Answer: The change of one type of rock into another type under certain conditions in a cyclic manner is known as rock cycle. So it has neither a beginning nor an end.

Question: What are minerals? What is the difference between Metallic and Non-metallic minerals?

Answer: Minerals are naturally occurring substances that are usually solid and inorganic. They have definite chemical composition and a crystalline structure. There are two types of minerals: Metallic and Non-metallic minerals.