Types of Rocks

You might have seen different types of rocks in your surroundings. Have you ever

wondered how they are formed? In this article, we will learn about the different

types of rocks and how they are formed.

What Are Rocks?

Rocks are mineral aggregates with a combination of properties of all the mineral

traces. Any unique combination of chemical composition, mineralogy, grain size,

texture, or other distinguishing characteristics can describe rock types. Additionally,

different classification systems exist for each major type of rock. There are different

types of rocks existing in nature.

Rocks which are found in nature rarely show such simple characteristics and

usually exhibit some variation in the set of properties as the measurement scale

changes.

Types of Rocks

There are three types of rocks:

●​ Igneous Rocks
●​ Sedimentary Rocks
●​ Metamorphic Rocks

Igneous Rock

Igneous rock is one of the three main rock types. Igneous rock is formed through

the cooling and solidification of magma or lava. Igneous rock may form with or

without crystallisation, either below the surface as intrusive (plutonic) rocks or on

the surface as extrusive (volcanic) rocks.

This magma can be derived from partial melts of existing rocks in either a planet’s

mantle or crust. Typically, the melting is caused by one or more of three processes:

an increase in temperature, a decrease in pressure, or a change in composition.

Types of Igneous Rock

Following are the two types of igneous rock:

1.​ Intrusive igneous rock: These rocks crystallise below the earth’s surface resulting in
large crystals as the cooling takes place slowly. Diorite, granite, and pegmatite are
examples of intrusive igneous rocks.
2.​ Extrusive igneous rock: These rocks erupt onto the surface resulting in small crystals as
the cooling takes place quickly. The cooling rate for a few rocks is so quick that they
form an amorphous glass. Basalt, tuff, and pumice are examples of extrusive igneous
rock.

Igneous Rock Examples

Basalt Diorite
 Granite Mica and quartz

Sedimentary Rock

The sedimentary rocks are formed by the deposition and subsequent cementation

of that material within bodies of water and at the surface of the earth. The process

that causes various organic materials and minerals to settle in a place is termed

sedimentation.

The particles that form a sedimentary rock by accumulating are called sediment.

Before being deposited, the sediment was formed by weathering and erosion from

the source area and then transported to the place of deposition by water, wind, ice,

mass movement or glaciers, which are called agents of denudation. Sedimentation

may also occur as minerals precipitate from water solution or shells of aquatic

creatures settle out of suspension.

Types of Sedimentary Rock

Following are the three types of sedimentary rock:

1.​ Clastic sedimentary rocks: These rocks are formed from the mechanical
weathering debris. Sandstone, and siltstone are examples of clastic
sedimentary rocks.
2.​ Chemical sedimentary rocks: These rocks are formed from the dissolved
materials that precipitate from the solution. Iron ore and limestones are
examples of chemical sedimentary rocks.
3.​ Organic sedimentary rocks: These rocks are formed from the accumulation
of plant and animal debris. Coal, and some dolomites are examples of
organic sedimentary rocks.

Sedimentary Rock Examples

Halite Limestone

Sandstone Siltstone

Metamorphic Rocks

The metamorphic rocks make up a large part of the Earth’s crust and are classified

by texture and by chemical and mineral assemblage. They may be formed simply

by being deep beneath the Earth’s surface, subjected to high temperatures and the

great pressure of the rock layers above it.

Metamorphic rocks arise from the transformation of existing rock types, in a

process called metamorphism, which means “change in form”. The original rock is

subjected to heat with temperatures greater than 150 to 200°C and pressure

around 1500 bars, causing profound physical and/or chemical change.

Types of Metamorphic Rock

Following are the two types of metamorphic rock:

1.​ Foliated metamorphic rocks: These rocks are produced by exposure to heat and
pressure which makes them appear layered. Phyllite and gneiss are examples of
foliated metamorphic rocks.
2.​ Non-foliated metamorphic rocks: These rocks don’t have layers. Marble and quartzite
are examples of non-foliated metamorphic rocks.

Metamorphic Rock Examples

Marble Quartzite

Slate Phyllite
Types of rocks in Singapore

The main types of rocks in Singapore are igneous, sedimentary, and

metamorphic.

Igneous rocks

●​ Granite: The most common igneous rock in Singapore, found in

Bukit Timah, Woodlands, and Pulau Ubin
●​ Gabbro: Found in Little Guilin, Bukit Gombak
●​ Norite: Found in Singapore

Sedimentary rocks

●​ Sandstone: Found in the western part of Singapore

●​ Mudstone: Found in the western part of Singapore
●​ Limestone: A common sedimentary rock in Singapore
●​ Siltstone: A common sedimentary rock in Singapore

Metamorphic rocks

●​ Quartzite: Found in the northeastern part of Singapore and on

Pulau Tekong
●​
●​ Gneiss: A metamorphic rock that forms when igneous or
sedimentary rocks are subjected to high temperatures and
pressures
●​

Location of these rocks

●​ Bukit Timah Hill, the highest point in Singapore, is made of granite

●​ Hills and valleys in the northwest of Singapore are made of
sedimentary rock
●​ The eastern region of Singapore is made of sandy and flatter land
Metamorphic rocks are the least common in Singapore because the region
doesn't usually experience the high temperatures and pressures needed
to form them.

Rock Cycle
We see rocks in our surroundings, from different shapes to sizes. Rocks never

remain the same, and they are always changing with time. The Rock cycle is a basic

concept in geology that describes the time-consuming transitions through geologic

time.

Rocks are constantly being recycled and going through several processes of

chemical and physical changes. There are three main rock types:

●​ Sedimentary
●​ Metamorphic
●​ Igneous

The diagram below shows that rocks are altered or destroyed when it is forced out

of its equilibrium conditions. This cycle of rock formation and wearing out is

constantly recycled the earth’s minerals. The rock cycle is nothing but a process by

which rocks of one type with certain characteristics change into rocks of another

kind.
Rock Cycle: Transition to Igneous

When rocks are pushed deep under the earth’s surface, they may melt into magma.

If the conditions no longer exist for the magma to stay in its liquid state, it will

solidify into an igneous rock.

Rock Cycle: Transition to Metamorphic

The rock exposed to high temperatures and pressures can be changed physically or

chemically to form a different rock, called metamorphic.

Rock Cycle: Transition to Sedimentary

Rocks exposed to the atmosphere are very unstable and subject to the processes

of weathering and erosion. This process breaks the original rock down into smaller

fragments and carries away dissolved materials. Sedimentary rocks form from

deposits that accumulate on the Earth’s surface.

Weathering

What is Weathering?

The scenes of the world are continuously mutating. Sun, rain, frost, and wind are

breaking down even the most solid of the rocks into smaller bits before they are

taken away. This process is termed as weathering. The action of the elements of

climate and weather, animals, and plants on the land surfaces to break them down

biologically, chemically, and physically is called weathering. It is the breakdown

and decay of rocks in situ. It can be a very elongated slow procedure, taking

hundreds of years. However, weathering can also be rapid, such as the damage to

pavements or roads after a cold, frosty winter.

Types of Weathering

Weathering is the initial step in soil creation. Weathering happens in four different

manners: These include chemical weathering, physical weathering, biological

weathering, and mechanical weathering. The three major ways weathering

happens include chemical, physical, and mechanical weathering which are

explained below.

How is Weathering different from Erosion?

Weathering has no moving agent of transport, whereas in Erosion the rock and

upper layer soil particles are worn away and moved elsewhere by water, wind, etc.
How is Weathering different from Erosion?

​
Physical weathering happens when rocks are fragmented into minor fragments

while ensuring no alterations in their chemical makeup. The main causes behind

physical weathering include spontaneous fluctuations in temperature like too high

or too low heat or cold. Variations in would be the causal element of freeze-thaw

action and this occurs when water gets collected in the fissures in rocks all through

the day and during the temperature drops at night that would be sufficient to

freeze water into ice. It is a known fact that almost every substance grows in size

when heated and contracts when exposed to low-temperature situations. Unlike

these cases, water converts to ices when the temperature goes below zero and

actually expands. The resultant action would result in the expansion of the fissure

and thereby the structure of it weathers.

Chemical Weathering

Chemical weathering happens when the chemicals get diluted and dissolved in

water and seep and percolate down the rock surfaces. Chemical weathering
happens in areas having high temperatures and enough supply of water alike

tropical environments that are humid. While the chemical weathering happens, the

composition of minerals alters due to the reaction of chemicals in water or air. This

also results in the rock decomposition. Some parts of the rock decomposition are

carried away pinging through rocks and soil, a procedure called leaching. The

water ultimately may carry these materials to rivers and then to the sea. This is the

source of the salinity of the oceans. The instances of chemical weathering are

oxidation, the decay of calcium carbonate, etc.

Mechanical Weathering

Rocks are also fragmented by mechanical force. This procedure is called

mechanical weathering. Because of the frequent variations in temperature, rocks

begin to contract and expand. This action results in the breakdown of rocks. It is

also caused by the roots of plants. They are growing through the cracks in the

rocks and cracks the rocks apart. Chemical and mechanical weathering work

together to break down rocks. Often, mechanical cracks and water seep into the

rock and weather it chemically.