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Grouping rocks
Rock is made of grains that fit together. Each grain in the rock is made from a
mineral, which is a chemical compound. The grains in a rock can be different
sizes, colours and shapes.
Some types of rock have interlocking grains that fit tightly together, for example
granite. Rocks such as marble and slate do not let water soak through them.
They are called impermeable rocks.
Other types of rock have rounded grains, for example sandstone. These are
usually softer and more crumbly than rocks with interlocking grains. Rocks with
rounded grains are more likely to absorb water than rocks with interlocking
grains. This is because the water can get into the gaps between the grains.
Rocks that absorb water are called porous. Some rocks, such as sandstone or
chalk, let water soak through them. They are called permeable rocks.
Wherever you are standing, there are rocks underneath you. If you are standing
in your garden, walking in the park, building a sandcastle on the beach,
swimming in the sea or crossing a river, there are rocks somewhere beneath
your feet.
Rocks come in many different forms, from soft mudstone and crumbly sandstone
to hard granite. Some rocks split across in sheets - like slate. Other rocks are
rare and valuable - like gold and diamonds.
Look at all the different forms of rocks in these photographs. See the variation in
shape and colour.
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So why do we see earth and stones? Rocks break down - ice cracks them; water
erodes them. They are rubbed together and break into smaller and smaller
pieces. These small bits are mixed with the remnants of plants and animals and
make earth or soil.
Rocks can be divided into three types:
1. Igneous rocks are formed from molten material called magma that comes
from deep inside the Earth. Sometimes this material cools slowly under the
surface, but sometimes it explodes out from a volcano as liquid lava. Igneous
rocks are also called volcanic rocks. At one time the whole of the Earth’s
crust was formed from igneous rocks. It has taken millions of years for the
other types to be formed. The changing of rocks from one form into another
is called the rock cycle.
Igneous rocks include:
Basalt
Dolerite
Granite
Obsidian
Pumice
Basalt Rhyolite
2. Metamorphic rocks are formed from igneous and sedimentary rocks that
have been changed in some way by heat or pressure inside the Earth. The
elements that make up the rocks may be remixed, but no new ones are
added or old ones taken away. Marble, for example, is a ‘changed’ form of
limestone.
Metamorphic rocks include:
Gneiss
Marble
Mylonite
Schist
Slate
Marble 3. Sedimentary rocks are formed from the remains of older rocks that have
been worn away. Layer upon layer of rock pieces may be gathered together
in one place by water, wind or ice. Many were originally part of the sea bed
and also contain fossils of shells and tiny sea creatures. Over thousands of
years, the weight of the layers above presses the bottom layers together so
hard that they join together into solid rock again.
Sedimentary rocks include:
Chalk
Conglomerate
Limestone
Sandstone
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Use the websites listed below to help you complete this table.
Name of rock
Description
(use the Word
Bank to help
you)
Where did you
see it?
What is it
used for?
Why is it used
for this?
Granite
Pumice
Marble
Slate
Chalk
Limestone
Word Bank
smooth rough hard soft dull shiny light heavy porous grainy strong brittle sharp
crystalline black white grey brown red yellow orange crumbly
Further information and research
This is a useful website that shows pictures of various rocks, divided into
igneous, sedimentary, metamorphic and minerals. http://www.fossweb.com/modules36/EarthMaterials/activities/rockdatabase/metamorphic_rocks/index.html
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Learn about the rock cycle here –
http://www.oum.ox.ac.uk/thezone/rocks/cycle/index.htm
This site is full of information about rocks.
http://alex.edfac.usyd.edu.au/BLP/websites/Hibba/index.html
Minerals
Minerals are the ingredients that make up rocks. They may be composed of a
single element, such as gold or copper. These are called native elements. Other
minerals are made of compounds of elements.
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Properties of minerals
Each type of mineral has its own combination of properties that identify it. Some
of the properties are:
Colour Many minerals come in a wide variety of colours. Different
minerals can be the same colour.
Crystal form describes the geometric shape of a crystal.
Hardness is the resistance of a mineral to scratching.
Lustre is how a mineral reflects light or how it shines. Some ways
to describe lustre include glassy or vitreous, metallic, dull, and
pearly.
Magnetism is a special property of some minerals, especially magnetite.
Samples are attracted by a magnet. Lodestone, a special form of magnetite, is a
magnet itself.
Streak is the colour of a mineral in powdered form.
Transparency describes how a mineral transmits light. Some minerals are
transparent (you can see through them); others are translucent (some light
passes through a sample) or opaque (no light passes through a sample).
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