Name:
There are three main types of rock: Igneous
Rock, Metamorphic Rock and Sedimentary Extend your knowledge
Rock.
Igneous Rock
How sedimentary rock forms
Sedimentary rock is formed over a very long
time. It is important because of the way
it forms and the time it takes to form and
preserve fossils within it.
These are the steps in the process of the
formation of sedimentary rock:
Step 1: Soil and rock on the surface
breaks down
Igneous rocks are called ‘fire rocks’
and are formed either underground
or above the ground. They are
formed underground when the
melted rock, called magma, deep
within the earth, becomes trapped
in small pockets. As these pockets
of magma slowly cools, the magma
becomes igneous rock. Ash and
lava come out of a volcano. When
the lava cools down it sets into
solid rock. This type of rock is called
Igneous rock.
All Igneous rocks are made of
minerals. Each mineral consists of
different chemicals. When these
chemicals cool down, they form
crystals.
Metamorphic Rock
Metamorphic rocks are rocks that
This process is called erosion. Over time, big have changed into another kind
of rock. These rocks were once
rocks break down into smaller pieces and
igneous or Sedimentary rocks. How
eventually into grains. Erosion happens for
do Sedimentary and Igneous rocks
many reasons:
change? The rocks are exposed
• Plant roots grow in the cracks of a rock to lots and lots of pressure, which
and force them to break apart as they creates heat buildup, which causes
grow
them to change. If you examine
• Chemicals in the air or in the water cause Metamorphic rock samples closely,
you’ll discover how flattened some
them to break down into smaller pieces
of the grains in the rock are. Slate,
• When wind bangs on rocks and when marble and quartzite are some
water streams onto them continually, examples of Metamorphic rocks.
they are broken down piece by piece
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GRADE 5
Grade 5 Term 4 Natural Science and Technology: Planet Earth and Beyond and Systems and Control: Sedimentary Rocks: Formation of Sedimentary Rock
Formation of sedimentary rock
GRADE 5
The grains do not often stay in one place. Tiny grains of clay
are blown into the air, forming dust and are moved from the
area where they were formed to other places. When it rains,
the tiny grains are washed away from the area where they
formed and are eventually transported to rivers.
Step 3: Grains are deposited in low-lying areas
The wind can only carry the grains if it is blowing at a fast
speed. As soon as it slows down these grains drop to the
ground. This is what happens in low-lying areas. The wind
slows down and deposits the grains on the ground in those
areas.
Rivers deposit grains in a similar way. They carry the grains
from the higher areas (escarpment) at a faster speed. When
the river levels out in the low-lying areas (flood plains), the
water slows down. The grains are deposited on the floor of
the river or stream, washed onto the riverbed, or trapped by
the plants in the wetlands.
When these grains are deposited, they are called sediments.
This is how layers of sediments are formed.
Step 4: New layers of sediments are deposited on existing layers
Grains are continually being washed or blown down into these regions. As one layer
of sediment starts, another is formed on top of it. Sometimes these layers are thicker
and sometimes they are thinner.
Step 5: Layers are compacted and hardened over time
When the layers of sediment are first formed, they are loosely packed and can be moved
around by movement of the water. However, as new layers form on top of these, the
sediments are “pushed down” (compacted) and they become harder over time. This
hardening process forms rock. One can actually see the layers that have formed over time.
Helpful illustration: Think about a thick book you’ve read or have at home. If you feel
one page of the book, it feels very thin and light. If you take about 10 pages together
in your fingers, they feel heavier and thicker. Now hold the whole book in your hand
– it is heavy and thick. This is how sediments are compacted – one layer is thin and
light, but as soon as other layers start forming, they become thicker and heavier and
this is what causes sedimentary rock to form.
Activity 1: Summarise the formation of Sedimentary rock
Fill in each of the steps that explain the formation of sedimentary rock and draw a
picture to show how this happens.
© e-classroom 2014 www.e-classroom.co.za
Grade 5 Term 4 Natural Science and Technology: Planet Earth and Beyond and Systems and Control: Sedimentary Rocks: Formation of Sedimentary Rock
Step 2: Grains are moved by the wind and water
Step 4:
GRADE 5
Step 2:
Step 3:
Step 5:
Different types of sedimentary rock
Shale, Sandstone, and Limestone are three examples of Sedimentary rock. There are
many other types as well.
Shale and Sandstone
Shale is made from mud that has been compacted into sedimentary layers. The
grains of clay are too tiny to see so the rock looks solid.
Sandstone is made from sand grains that have been compacted over time. As the
sand grains are slightly bigger than the clay grains, they can be seen in the rock.
Sandstone is a very soft rock which crumbles easily.
Limestone
Limestone is different to Shale and Sandstone because it is not formed by grains
of sand or clay. Limestone is formed from the remains of dead sea animals. When
the animal dies, its body falls to the ocean floor where it decomposes. The bones or
shells are broken down and layers are formed. Over time, other sediments are layered
on top of this layer and it is compacted and hardened to form limestone.
© e-classroom 2014 www.e-classroom.co.za
Grade 5 Term 4 Natural Science and Technology: Planet Earth and Beyond and Systems and Control: Sedimentary Rocks: Formation of Sedimentary Rock
Step 1: