3
The Geosphere
112
3.
Rocks
Start this section by showing the different types of rocks. This
is a good opportunity to review the differences between rocks
and minerals just to make sure all the students understood these
concepts. Students can look for the variations between the
different types of rocks. To help them you could use the following
chart by copying it on the board and filling it in with the whole
class:
Rock
Density Visible
sediments
Visible
minerals
Aligned
minerals
Ask students about the concepts of composition and texture of rocks.
You can add two more columns to the chart (one for composition
and one for texture) and students should copy it in their notebook
and complete it as they work through the unit.
The practical activity about
The texture of rocks
on page 64 will help
students to clearly understand these concepts.
There are many ways of classifying rocks, but geologists prefer to
classify them according to their origin. Rocks can be metamorphic,
igneous or sedimentary.
Now students could de questions 17 to 20.
Tips: Before doing question 17, check that students understand
the meaning of homogeneous and heterogeneous in other
contexts. For question 18, play the audio at least twice. The
second time, stop after each sentence and elicit the answer. For
question 19, ask students to do this in pairs.
Curricular adaptation:
4. ROCKS
Section adapted according to the curriculum.
3.1.
Igneous or magmatic rocks
Igneous or magmatic rocks are the ones that originated from the
solid magma, in the interior or the exterior of the crust. At this
point, you can highlight the difference between magma and lava.
Magma is formed in the Earth’s interior while the lava is in the
Earth’s exterior.
Now students can do question 20. Ask students to do this in-
dividually, and then write various answers on the board ( some
incorrect), then ask the students in groups to choose the correct
answers.
Curricular adaptation:
5. TYPES OF ROCKS
Section adapted according to the curriculum.
Answer key
Understand
17.
Explain the difference between homogeneous and
heterogeneous rocks. Why are coal and petroleum
exceptions?
Homogeneous rocks only have one mineral, for example,
limestone that is composed only of calcite. On the other
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3. Thegeosphere
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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 rocksare called
simple
or
homogeneous rocks
.Othersare
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
.
3.1.
Igneous rocks
In some areas of the upper mantle, minerals melt due to high temperatures.
When magma rises to the surface it can remain in the crust or it can exit to the
exterior. Inbothcases,away from the sourceofheat,magmacoolsuntil itbecomes
solid and forms
igneous rocks
or
magmatic rocks
.
These rocks are made up of one or several minerals compressed together, so they
are extremely compact. The resistance of igneous rocks to pressure makes them a
perfect material to construct buildings.
Igneous rocks are classified as volcanic or plutonic depending on where they were
formed.
❚
Volcanic igneous rocks
or
extrusive
rocks are formed when magma rises and
leaves the Earth’s crust as lava. Lava cools quickly and forms volcanic rocks.
❚
Plutonic igneous rocks
or
intrusive rocks
are formed when rising magma
cools slowly inside the Earth’s crust. The mineral crystals are easily visible as they
had more time to form.
Rocks
arenaturalaggregatesmadeupofoneorvariousdifferentminerals.
Magma
is a substance made up of melted materials mixed with gases and
water, formed in the Earth’s interior.
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
Obsidian
is shiny black
and originates in the
continental crust. It has a
glassy texture. The crystals
have not had time to
form.
Pumice
or
pumice stone
is a continental rock. It is
easily recognised because
of the many small holes
in the surface. These were
formed by gases that
were in the rock before it
solidified.
Basalt
originates in
submarine volcanoes.
It contains very small
minerals due to its rapid
cooling. It is the most
abundant rock found in
the Canary Islands and
the ocean floor.
Granite
is the most
abundant plutonic rock on
the continental crust.
Syenite
is a continental
plutonic rock.
Gabbro
is a plutonic rock
found in the ocean’s crust.
Composition and textureofgranite
quartz
mica
feldspar
Understand
17.
Explain the difference
between homogeneous
and heterogeneous rocks.
Whyarecoalandpetroleum
exceptions?
18.
L
isten and identify the
rocks:
igneous
,
sedimentary
or
metamorphic
.
Analyse
19.
Study the photo. What
type of rock is a
conglomerate?
Is it homogeneous or
heterogeneous? Explain
your answer.
Understand
20.
Explain why crystals in
the minerals that make
up plutonic rocks are
visible,butarenotvisible
inthemineralsofvolcanic
rocks.
