The Earth’s crust is made up of rocks and minerals. Minerals combine in different proportions to form rocks, while rocks form the base of soil, natural vegetation and land use.
This chapter explains the major elements of the Earth’s crust, important rock-forming minerals, types of rocks and the rock cycle.
Table of Contents
Major Elements Of The Earth’s Crust
About 98% of the Earth’s crust is made up of eight major elements:
| S. No. | Element | By Weight |
|---|---|---|
| 1 | Oxygen | 46.60% |
| 2 | Silicon | 27.72% |
| 3 | Aluminium | 8.13% |
| 4 | Iron | 5.00% |
| 5 | Calcium | 3.63% |
| 6 | Sodium | 2.83% |
| 7 | Potassium | 2.59% |
| 8 | Magnesium | 2.09% |
| 9 | Others | 1.41% |
The remaining part of the crust contains elements such as:
- Titanium
- Hydrogen
- Phosphorus
- Manganese
- Carbon
- Sulphur
- Nickel
These elements combine in different proportions to form minerals.
Minerals
A mineral is a naturally occurring, inorganic, solid substance with a definite chemical composition, orderly internal arrangement of atoms and specific physical properties.
Minerals are the basic building blocks of rocks.
Some Major Rock-Forming Minerals
Most rock-forming minerals originate from magma inside the Earth. When magma cools and solidifies, minerals crystallise and form rocks.
Feldspar
Feldspar forms about half of the Earth’s crust.
It is generally light cream to salmon pink in colour.
Its main constituents are:
- Silicon
- Oxygen
- Aluminium
- Sodium
- Potassium
- Calcium
Feldspar is widely used in ceramics and glass-making.
Quartz
Quartz consists of silicon and oxygen.
Its chemical formula is SiO₂.
It is hard, colourless or white and lacks cleavage.
It breaks like glass and is commonly found in sand and granite.
It is used in electronic and optical instruments.
Bauxite
Bauxite is a hydrous oxide of aluminium.
It is the chief ore of aluminium.
It is generally non-crystalline and occurs in small pellets.
Cinnabar
Cinnabar is mercury sulphide.
It is the principal ore of mercury.
It is usually reddish-brown in colour.
Dolomite
Dolomite is a double carbonate of calcium and magnesium.
It is commonly white in colour.
It is used in cement and iron-steel industries.
Gypsum
Gypsum is hydrous calcium sulphate.
It is used in cement, fertiliser and chemical industries.
Haematite
Haematite is a red ore of iron.
Magnetite
Magnetite is a black iron oxide.
It is an important ore of iron.
Amphibole
Amphiboles form about 7% of the Earth’s crust.
They contain:
- Silica
- Aluminium
- Calcium
- Iron
- Magnesium
They are usually green or black in colour.
Hornblende is a common variety of amphibole.
Mica
Mica contains:
- Potassium
- Aluminium
- Magnesium
- Iron
- Silica
It forms about 4% of the Earth’s crust.
It is commonly found in igneous and metamorphic rocks.
It is used in electrical and electronic equipment.
Olivine
Olivine mainly contains:
- Magnesium
- Iron
- Silica
It is greenish in colour and is commonly found in basaltic rocks.
Pyroxene
Pyroxenes contain:
- Calcium
- Magnesium
- Iron
- Silica
They are green or black in colour.
They are commonly found in igneous rocks and meteorites.
Other minerals such as chlorite, calcite and barite are also found in rocks.
Rocks
The crust of the Earth is composed of rocks.
A rock is an aggregate of one or more minerals.
Rocks form the basis of soil and influence:
- Natural vegetation
- Land use
- Soil type
- Human activities
Feldspar and quartz are the most common minerals found in rocks.
The scientific study of rocks is known as Petrology.
Types Of Rocks
Based on their mode of formation, rocks are divided into three major types:
- Igneous rocks
- Sedimentary rocks
- Metamorphic rocks
Igneous Rocks
Igneous rocks are formed when magma or lava cools and solidifies.
They are also called primary rocks because other rocks are derived from them.
Features Of Igneous Rocks
- Formed from cooling and solidification of molten material.
- Harder than sedimentary rocks.
- Non-stratified.
- Fossils are absent.
- Do not get easily eroded.
- Non-porous.
Minerals Found In Igneous Rocks
Igneous rocks commonly contain metallic minerals such as:
- Iron
- Copper
- Manganese
- Silver
- Gold
Examples Of Igneous Rocks
- Granite
- Gabbro
- Pegmatite
- Basalt
- Volcanic breccia
- Tuff
Sedimentary Rocks
Sedimentary rocks are formed by the deposition and lithification of fragments produced by exogenous processes.
The word sedimentary is derived from the Latin word sedimentum, meaning settling.
Rocks exposed on the Earth’s surface are broken into fragments by denudational processes. These fragments are transported by water, wind or ice and deposited.
Over time, the deposited material undergoes compaction and cementation.
This process is called lithification.
Features Of Sedimentary Rocks
- Formed from eroded materials deposited in layers.
- Least hard among the three types of rocks.
- Stratified.
- Fossils may be present.
- Erode faster than igneous and metamorphic rocks.
- Porous.
Minerals Found In Sedimentary Rocks
Sedimentary rocks commonly contain fossil fuels such as:
- Coal
- Petroleum
- Natural gas
Types Of Sedimentary Rocks
Sedimentary rocks are classified into three types.
Mechanically Formed Or Clastic Rocks
These are formed from rock fragments.
Examples:
- Sandstone
- Conglomerate
- Shale
Organically Formed Or Biogenic Rocks
These are formed from organic remains.
Examples:
- Chalk
- Coal
- Some types of limestone
Chemically Formed Rocks
These are formed by chemical processes.
Examples:
- Chert
- Halite
- Gypsum
Examples Of Sedimentary Rocks
- Sandstone
- Conglomerate
- Limestone
- Slate
- Loess
- Geyserite
- Chalk
- Coal
- Chert
Metamorphic Rocks
Metamorphic rocks are formed from pre-existing igneous or sedimentary rocks through the process of metamorphism.
Metamorphism involves changes in:
- Pressure
- Temperature
- Chemical environment
These rocks form when rocks are subjected to intense heat and pressure due to tectonic activity or when magma intrudes into surrounding rocks.
Features Of Metamorphic Rocks
- Formed by metamorphism of igneous and sedimentary rocks.
- Harder than igneous and sedimentary rocks.
- Non-stratified.
- Fossils are absent.
- Erode slower than igneous and sedimentary rocks.
- Non-porous.
Minerals Found In Metamorphic Rocks
Metamorphic rocks commonly contain non-metallic minerals such as:
- Mica
- Precious stones
Types Of Metamorphism
Contact Metamorphism
Contact metamorphism occurs when rocks come into contact with hot magma.
It causes recrystallisation due to high temperature.
Regional Metamorphism
Regional metamorphism occurs over large areas due to tectonic compression, high pressure and temperature.
Foliation And Banding
During metamorphism, minerals may arrange in layers.
This layered structure is called foliation.
When light and dark mineral bands appear alternately, the structure is called banding.
Such rocks are called banded rocks.
Classification Of Metamorphic Rocks
Metamorphic rocks are classified into:
- Foliated rocks
- Non-foliated rocks
Examples Of Metamorphic Rocks
- Granite
- Gneissoid
- Syenite
- Slate
- Schist
- Marble
- Quartzite
Igneous, Sedimentary And Metamorphic Rocks: Comparison
| Point Of Difference | Igneous Rocks | Sedimentary Rocks | Metamorphic Rocks |
|---|---|---|---|
| Origin | Formed by cooling and solidification of molten material | Formed by hardening of eroded material in layers | Formed by metamorphism of igneous and sedimentary rocks |
| Hardness | Harder than sedimentary rocks | Least hard | Harder than igneous and sedimentary rocks |
| Strata | Non-stratified | Stratified | Non-stratified |
| Fossils | Absent | Present | Absent |
| Resistance To Erosion | Does not get easily eroded | Erodes faster | Erodes slower |
| Porosity | Non-porous | Porous | Non-porous |
| Minerals | Iron, copper, manganese, silver, gold | Coal, petroleum, natural gas | Mica and precious stones |
| Examples | Granite, Gabbro, Pegmatite, Basalt, Volcanic breccia, Tuff | Sandstone, Conglomerate, Limestone, Slate, Loess, Geyserite, Chalk, Coal, Chert | Granite, Gneissoid, Syenite, Slate, Schist, Marble, Quartzite |
Rock Cycle
Rocks do not remain in their original form permanently.
They undergo transformation over geological time.
The rock cycle is a continuous process through which old rocks are converted into new types of rocks.
Igneous, sedimentary and metamorphic rocks are interconnected.
Each rock type can transform into another under suitable conditions such as:
- Heat
- Pressure
- Weathering
- Erosion
- Melting
- Solidification
Example Of Rock Cycle
An igneous rock may change into a sedimentary rock through weathering and deposition.
It may also become a metamorphic rock under heat and pressure.
Similarly, sedimentary rock may become metamorphic rock.
Any rock type may melt to form magma.
When magma cools and solidifies, igneous rock forms again.
FAQs On Rocks And Minerals
What is a mineral?
A mineral is a naturally occurring inorganic solid substance with a definite chemical composition, orderly atomic arrangement and specific physical properties.
Which element is most abundant in Earth’s crust?
Oxygen is the most abundant element in Earth’s crust.
What is a rock?
A rock is an aggregate of one or more minerals.
What is petrology?
Petrology is the scientific study of rocks.
Which are the three major types of rocks?
The three major types are igneous rocks, sedimentary rocks and metamorphic rocks.
Why are igneous rocks called primary rocks?
Igneous rocks are called primary rocks because other rocks are derived from them.
How are sedimentary rocks formed?
Sedimentary rocks form through deposition, compaction and cementation of eroded materials.
What is lithification?
Lithification is the process of compaction and cementation by which sediments become rock.
Which rocks commonly contain fossils?
Sedimentary rocks commonly contain fossils.
What is metamorphism?
Metamorphism is the change in existing rocks due to heat, pressure and chemical environment.
What is contact metamorphism?
Contact metamorphism occurs when rocks come into contact with hot magma.
What is regional metamorphism?
Regional metamorphism occurs over large areas due to tectonic compression, pressure and temperature.
What is foliation?
Foliation is the layered arrangement of minerals in metamorphic rocks.
What is the rock cycle?
The rock cycle is the continuous process through which one type of rock changes into another over geological time.
Last Moment Exam Cheat Sheet – Rocks And Minerals
- About 98% of Earth’s crust is made up of eight elements.
- Oxygen is the most abundant element in Earth’s crust.
- A mineral is a naturally occurring inorganic solid with definite chemical composition.
- Feldspar forms about half of Earth’s crust.
- Quartz is made of silicon and oxygen, or SiO₂.
- Bauxite is the chief ore of aluminium.
- Cinnabar is the principal ore of mercury.
- Haematite and magnetite are important ores of iron.
- A rock is an aggregate of one or more minerals.
- The scientific study of rocks is called Petrology.
- Igneous rocks are primary rocks formed by cooling and solidification of magma or lava.
- Sedimentary rocks are stratified, porous and may contain fossils.
- Lithification means compaction and cementation of sediments.
- Metamorphic rocks form from existing rocks due to heat, pressure and chemical changes.
- Contact metamorphism occurs due to contact with hot magma.
- Regional metamorphism occurs over large areas due to tectonic pressure and temperature.
- Foliation means layered mineral arrangement in metamorphic rocks.
- Granite, basalt and gabbro are igneous rocks.
- Sandstone, limestone, coal, chert and gypsum are sedimentary rocks.
- Marble, slate, schist and quartzite are metamorphic rocks.
- Rock cycle explains the continuous transformation of one rock type into another.