1) The Crust
The crust is the outermost layer of the Earth. It is solid, thin and the lightest part of the Earth. Its thickness varies from 5 km under oceans to about 60 km under continents. The crust has two layers.

1. a) Continental Crust (Sial):
   The upper part of the crust is called continental crust. It mainly consists of sedimentary and granitic rocks and forms the continents. It is rich in silica and aluminium, hence called Sial. Its average thickness is about 30 km.
2. b) Oceanic Crust (Sima):
   The lower part of the crust is called oceanic crust. Its thickness is about 5 km. It is rich in silica and magnesium, therefore known as Sima. The mean density of this layer is about 2.95 gm/cm³.
   The Conrad discontinuity separates Sial and Sima, and the Mohorovicic (Moho) discontinuity separates the crust and mantle.

2) The Mantle
The mantle lies below the crust and above the core. It extends from about 60 km to 2,900 km depth. It is composed mainly of basic silicates rich in iron and magnesium. The density of the mantle ranges from 3.3 to 5.7 gm/cm³.
The mantle is divided into two parts.

1. a) Upper Mantle (Asthenosphere):
   This is the upper layer of the mantle and is partially molten. Most earthquakes and volcanic activities originate in this layer.
2. b) Lower Mantle (Mesosphere):
   This is the lower part of the mantle and is in a solid state.
   The Repetti discontinuity lies between the asthenosphere and mesosphere, and the Gutenberg (Oldham) discontinuity separates the mantle and the core.

3) The Core
The core is the innermost layer of the Earth. It extends from 2,900 km to about 6,371 km. It is composed mainly of nickel and iron, hence called Nife. It is the densest layer of the Earth.

1. a) Outer Core:
   The outer core consists of molten materials and extends from 2,900 km to 4,980 km.
2. b) Inner Core:
   The inner core lies between 4,980 km and 6,371 km. Due to extremely high temperature and pressure, the materials remain in a solid state, hence it is called the Solid Core.
   The Lehmann / Bullen (Transition) discontinuity separates the outer core and inner core.

Question 2
What is an Earthquake? Describe the causes and effects of earthquakes.

Answer:
An earthquake is a sudden vibration or oscillation of the Earth’s crust caused by the release of energy in the form of seismic waves.
The place inside the Earth where the earthquake originates is called the Focus, and the point on the Earth’s surface directly above the focus, which receives the maximum intensity of seismic waves, is called the Epicentre.

Causes of Earthquakes:
On the basis of their origin, earthquakes can be classified into the following types:

1. Tectonic Earthquakes:
   These are caused due to folding, faulting, and displacement of rock blocks in the Earth’s crust because of tectonic forces. They are the most frequent, intense, and destructive earthquakes.
2. Volcanic Earthquakes:
   These earthquakes are associated with volcanic activities and occur due to the movement of magma beneath the Earth’s surface. They are generally low in intensity and cause comparatively less damage.
3. Man-made Earthquakes:
   These are caused by human activities such as underground nuclear explosions, mining, drilling, and the construction of large dams.
   Examples: Koyna Dam (India – 1967), Hoover Dam (USA – 1935), Mangla Dam (Pakistan).

Effects of Earthquakes:

• Earthquakes cause severe destruction in the epicentral region.
• Cracks and fissures develop on the Earth’s surface.
• Earthquake vibrations may trigger landslides in mountainous regions, blocking rivers.
• Vertical and horizontal displacement of land may occur, forming lakes or depressions.
• Dams may develop deep cracks, as seen in the Koyna Dam earthquake.
• River beds may rise or change course, for example, the Brahmaputra River during the Assam earthquake.
• Large-scale destruction of houses, roads, railway lines, bridges, factories, dams, and airports, leading to loss of life and property.
• Underground water systems are disturbed, affecting wells and springs. 

Question 3.
How does a Volcano erupt? Explain the types of Volcanoes.

Answer:
A volcano is an opening in the Earth’s crust through which heated materials such as magma, lava, gases, hot water, ash, and rock fragments are ejected from the interior of the Earth.

Deep inside the Earth, intense heat melts rocks to form magma. Due to high pressure and temperature, this magma rises through cracks and weak zones in the crust. When it reaches the surface, it erupts violently through an opening called a vent.
The funnel-shaped hollow at the top of the volcanic cone is called a crater. A large basin-shaped crater with steep sides is known as a caldera.
Types of Volcanoes:
On the basis of periodicity or frequency of eruption, volcanoes are classified into three types:

1. Active Volcanoes:
   Volcanoes which erupt frequently and continuously eject lava, ash, and gases are called active volcanoes.
   Examples: Mount Etna (Sicily), Stromboli (Italy), Mauna Loa (Hawaii).
2. Dormant Volcanoes:
   Volcanoes which remain inactive for a long period but may erupt suddenly and violently in the future are called dormant volcanoes. They often cause great destruction to life and property.
   Example: Mount Vesuvius.
3. Extinct Volcanoes:
   Volcanoes which have not erupted for a very long time and show no signs of future eruption are called extinct or dead volcanoes.
   Examples: Narcondam, Krakatoa.

Question 4.
What is a rock? Describe the different types of rocks.

Answer:
A rock refers to the hard and resistant materials of the Earth’s crust. Scientifically, the term rock includes both hard and soft materials such as granite, chalk, clay, etc. Thus, a rock is any natural solid material, hard or soft, that forms the Earth’s crust.
Rocks do not have the same chemical composition, whereas minerals have definite chemical composition and physical properties. The Earth’s crust is composed of various types of rocks.

Types of Rocks:
On the basis of origin or mode of formation, rocks are classified into three major groups:

• Igneous Rocks:

The term igneous is derived from the Latin word “Ignis”, meaning fire. Igneous rocks are formed by the cooling and solidification of molten magma. They are called primary rocks because they were the first rocks to be formed and are the parent rocks from which other rocks are derived.

Igneous rocks are classified into two types:

• Intrusive Igneous Rocks:

When magma cools and solidifies inside the Earth’s crust, it forms intrusive igneous rocks. These rocks cool slowly and are coarse-grained.
Examples: Granite, Dolerite.

Intrusive rocks are of two types:

1. Plutonic rocks: Formed at great depth inside the Earth due to slow cooling of magma.
2. Hypabyssal rocks: Formed in cracks, fissures, and crevices at shallow depths.

• Extrusive Igneous Rocks:

When magma reaches the Earth’s surface as lava and cools rapidly, it forms extrusive igneous rocks.
Example: Basalt.

• Sedimentary Rocks:

Sedimentary rocks are formed by the deposition, accumulation, compaction, and cementation of sediments derived from older rocks, plants, and animal remains by rivers, wind, glaciers, and sea waves. As they are formed in layers, they are also called stratified rocks.

Sedimentary rocks are classified into three types:

• Mechanically Formed Rocks:

These rocks are formed from rock fragments transported and deposited by rivers, glaciers, winds, or waves and cemented together.
They are classified into:

1. Rudaceous rocks
2. Arenaceous rocks
3. Argillaceous rocks

• Chemically Formed Rocks:

These rocks are formed by the evaporation and deposition of minerals dissolved in water.
Examples: Rock salt, Gypsum.

• Organically Formed Rocks:

These rocks are formed from the remains of plants and animals.

They are of two types:

1. Calcareous rocks: Contain calcium carbonate or lime.
   Examples: Limestone, Chalk.
2. Carbonaceous rocks: Formed from plant remains under heat and pressure.
   Example:
3. Metamorphic Rocks:

Rocks which undergo change in form or composition without disintegration due to heat and pressure are called metamorphic rocks. These rocks are formed from igneous or sedimentary rocks.

The intense heat and pressure inside the Earth alter the structure and composition of rocks partially or completely to form new rocks.
Examples: Marble, Quartzite, Diamond, Ruby, Emerald.

Question 5.
What is a mineral? Explain the types of minerals.

Answer:
A mineral is a natural, inorganic substance found in the Earth’s crust, having a definite chemical composition and distinct physical properties. The scientific study of minerals is known as Mineralogy. Minerals are very important natural resources and are widely used by human beings for various economic and industrial activities.

Types of Minerals:

Minerals are classified into different types based on their nature, composition, and use. The important types are:

1. Metallic Minerals:

Metallic minerals contain metals and are sources of metal extraction. These minerals have metallic lustre and are good conductors of heat and electricity.
They are of two types:

• Ferrous minerals: Minerals which contain iron.
  Example: Iron ore.
• Non-ferrous minerals: Minerals which do not contain iron.
  Examples: Copper, Bauxite, Zinc, Lead.

2. Non-Metallic Minerals:

These minerals do not contain metals and do not yield metal on processing. They are mainly used in industries and for domestic purposes.
Examples: Mica, Asbestos, Sulphur, Phosphate, Limestone.

3. Precious Minerals:

These minerals are rare and valuable and are mainly used for making ornaments and decorative items.
Examples: Gold, Silver, Platinum, Diamond.