52
3.
ROCKS
Rocks, like minerals, have properties that allow us to identify them, for example,
composition
and
texture
.
❚
The
composition
of a rock refers to the minerals that make up the rock.
Some rocks only have one mineral. For example, limestone is composed only
of calcite. These rocks are called
simple
or
homogeneous rocks
. Others are
made up of a variety of minerals. For example, granite is made up of quartz,
feldspar and mica. These rocks are called
complex
or
heterogeneous
rocks
.
There are exceptions such as petroleum and coal, which are not of mineral
origin as they come from animal remains and so do not contain minerals.
❚
Texture
refers to the size and arrangement of the minerals in the rock. It is not
always possible to observe the texture of a rock without using a magnifying
glass or even a microscope.
These instruments allow us to identify the minerals that make up rocks. For
example, without a magnifying glass, granite appears to have different colours:
greys, whites and blacks. With a magnifying glass we can see that it is made up
of crystals of quartz (greys), feldspar (whites) and mica (blacks).
There are many ways of classifying rocks, but geologists prefer to classify
them according to their origin. Rocks can be
metamorphic
,
igneous
or
sedimentary
.
Rocks
are natural aggregates made up of one or various different minerals.
Conglomerate
Types of rocks
Formed by the
transformation of other
rocks subjected to high
pressure conditions
and/or temperatures,
without reaching a
melting state.
Sediments are
fragments of other
rocks, minerals and
organic remains.
Sedimentary rocks
form when sediments
consolidate.
These originate when
magma from the
Earth’s interior cools
and solidifies.
Sedimentary
Metamorphic
Igneous
Composition and texture of granite
quartz
mica
feldspar
Understand
17.
Explain the difference
between homogeneous
and heterogeneous rocks.
Whyarecoal andpetroleum
exceptions?
18.
L
isten and identify the
rocks:
igneous
,
sedimentary
or
metamorphic
.
Analyse
19.
Study the photo below.
What type of rock is a
conglomerate?
Is it homogeneous or
heterogeneous? Explain
your answer.
