Handout 5.9

Term II Class 8
The City School

Handout #: 5.9
Topic: The Rock Cycle
By reading this handout, learners will be able to:
• Recognize that sedimentary rocks can be formed by pressure from layers of sediment e.g; Sandstone
and chalk
• Describe characteristics of sedimentary rocks; e.g. Non-inter-locking, porous, contain fossils
• Describe some observable differences between limestones and relate the composition of limestone to
the process of formation
• Generalise that rocks are mixtures and vary in composition
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. The changes in
mineral composition of rocks is due to changes in the temperature and pressure conditions of
metamorphism.
What is porosity?
Porosity is the percentage of void space in a rock.
Types of Rocks
There are three types of rocks:
• Igneous Rocks
• Sedimentary Rocks
• Metamorphic Rocks
Sedimentary rocks
Sedimentary rocks are formed from the broken remains of other rocks that become joined
together.
Term II Class 8
A river carries, or transports, pieces of broken rock as it flows along. When the river reaches a
lake or the sea, its load of transported rocks settles to the bottom. We say that the rocks
are deposited. The deposited rocks build up in layers, called sediments. This process is
called sedimentation.
The weight of the sediments on top squashes the sediments at the bottom. This is
called compaction. The water is squeezed out from between the pieces of rock and crystals of
different salts form. The crystals stick the pieces of rock together. This process is
called cementation.
These processes eventually make a type of rock called sedimentary rock. It may take millions of
years for sedimentary rocks to form.
Here are the different processes in order:
What are sedimentary rocks like?
Sedimentary rocks contain rounded grains in layers. The oldest layers are at the bottom and the
youngest layers are at the top. Sedimentary rocks may contain fossils of animals and plants
trapped in the sediments as the rock was formed. Sedimentary rocks are formed from
sediments that have settled at the bottom of a lake, sea or ocean, and have been compressed
over millions of years. The sediment comes from eroded rocks carried there by rivers or ice, and
from the skeletons of sea creatures.
Examples of sedimentary rocks include sandstone, limestone, chalk and clay.

Term II Class 8
Properties of a sedimentary rocks

Sedimentary rocks are largely found on the Earth’s surface. They cover 75% area of the Earth.
These rocks are generally not crystalline in nature. They are soft and have many layers as they
are formed due to the deposition of sediments.
Interesting Facts about Sedimentary Rocks

Sedimentary rocks are formed by sediment that is deposited over time.
• Flint is a hard, sedimentary form of the mineral quartz.

• Sedimentary rocks cover most of the Earth’s rocky surface, but they only make up a very small
percentage of the planet’s crust.
• They often contain fossils of plants and animals millions of years old.
• Limestone is often made from the fossilized remains of ocean life that died millions of years
ago.
• These rocks form layers called strata which can often be seen in exposed cliffs.
Limestone
Sedimentary rock composed mainly of calcium carbonate (CaCO3). Most limestones have a
granular texture. Their constituent grains range in size from 0.001 mm (0.00004 inch) to visible
particles. In many cases, the grains are microscopic fragments of fossil animal shells.
Different types of limestones:
Shelly Limestone
This rock is made up of well cemented bioclasts. The bioclasts (mainly shell fragments) are
poorly sorted, ranging from <1mm up to a few 10s of mm.
Fine-grained cement fizzes with acid, indicating that it is carbonate. It has weathered to a pale
yellow-grey.
Term II Class 8
Chalk
It is composed mainly of fine-grained calcium carbonate formed from the remains of certain
types of micro-organisms. Like any other high-purity limestone, chalk is used for making lime
and cement and also used as a fertilizer.
Oolitic limestone
It is composed mainly of sand sized pellets of calcium carbonate which are formed in warm
shallow water.

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Term II Class 8

The City School

By reading this handout, learners will be able to:

What are Rocks?

Porosity is the percentage of void space in a rock.

Here are the different processes in order:

What are sedimentary rocks like?

Examples of sedimentary rocks include sandstone, limestone, chalk and clay.

Properties of a sedimentary rocks

Interesting Facts about Sedimentary Rocks

• Flint is a hard, sedimentary form of the mineral quartz.

Different types of limestones:

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