STUDENT ROCK SET
1.
2.
3.
4.
5.
6.
Basalt
Slate
Marble
Limestone
Granite
Sandstone
Igneous rocks
Molten rock is called magma. When it is forced to the earth's surface it is also called lava.
Molten rock that cools beneath the earth's surface (magma) cool down very slowly because the
environment is very hot. Some igneous rocks take many thousands of years to cool sufficiently before
crystals start to form, this long period of cooling allows crystals to grow quite large (1 -2 mm). Do you
think that the granite cooled quickly or slowly? How many different minerals can you see in
the granite?
Igneous rocks that cool on the earth's surface (lava) cool very quickly some rocks cool so quickly that
the gases do not have time to escape so are left as tiny bubbles. Looking at the size of mineral grains
is a good way of classifying igneous rocks. Examine the igneous & complete the following table.
Igneous Rocks: Name of rock
Basalt
Granite
Does that rock consist of: Large
Grains, Small Grains or Different
size Grains
Number of minerals
Colour of minerals
Is rock intrusive or extrusive
Did the magma cool:
Fast/Slowly/Crystals formed at
different rates
Colour: The type and content of minerals affect the colour of the igneous rock. Felsic minerals are
light in colour and therefore give the rock a light colour. Which igneous rocks contain felsic
minerals (which are the light coloured rocks)? Mafic minerals are dark in colour they contain
magnesium and iron.
Another way to classify igneous rocks is by examining the amount of quartz that they contain. Rocks
like granite contain a lot of quartz and are known as acid igneous rocks. Rocks like andesite do not
contain much quartz and are known as basic igneous rocks. Estimate the percentage of quartz in
the granite? (quartz is the white/colourless mineral).
Other methods of classifying igneous Rocks: A more detailed classification depends on the
varieties of feldspar found in the rock. Feldspar is used both because it is abundant in igneous rocks
and also because the proportions of different kinds of feldspar vary systematically from felsic to mafic
rocks. Granite is rich in potassium feldspar, while the more mafic rocks are dominated by sodium and
calcium feldspars.
Sedimentary Rocks
The effects of weathering constantly breaks down the igneous, metamorphic and sedimentary rocks
to produce a supply of rock fragments known as sediment. Sediment is then transported by rivers, ice,
wind and sea and deposited into layers.
Compacting and cementing of Sediments: As the layers of sediment become buried under new
deposits the increasing pressure of the new layers compacts the sediment. Rock grains are pushed
together and water between the grains is pushed out depositing the minerals that the water was
carrying as a thin layer on the surface of the sediment. This acts as cement and the sedimentary rock
is formed.
Classifying Sedimentary Rocks: Mineralogy and texture are also useful in subdividing the
sedimentary rocks. They are used in combination to set apart two main groups, the detrital and the
chemical. The detrital sediments are those that have been mechanically transported before becoming
sedimentary rocks and the chemical sedimentary rocks are those that have precipitated directly out
from sea water.
Limestones: If the sediment consists mainly of shell fragments or the mineral calcium carbonate ( the
mineral that shells are made from) the resulting sedimentary rock is called limestone. An easy test for
limestone is that it fizzes when dilute Hcl acid is dropped onto it. Oolitic limestones formed during the
Jurassic period. Small grains of sand were blown along a pre-historic beach gradually building up
layers of calcium carbonate, the tiny spheres bonded together to form an Oolitic Limesto ne.
Sandstone: Carried by rivers the boulders and pebbles often settle out first concentrating sediment of
the same size in one layer. The boulders form a layer that becomes a breccia, if they have travelled
far by river they become rounded and form conglomerate, the sandy material forms sandstone and
the fine clay and silt settle out to form marls and mudstones or if they form distinct layers: shale forms.
Metamorphic Rocks
Metamorphism is the process by which an existing rock is changed through the action of heat and
pressure.
Examine the marble and slate which one has undergone heat and pressure? Which one has
undergone just heat?
The change in the rock depends on the amount of heat and the amount of pressure. A good example
of how a metamorphic rock forms is the change from limestone to marble. The mineral called calcium
carbonate in the limestone recrystallises when heated to form the characteristic white marble
Sedimentary
Rock
Metamorphism
Metamorphic
Rock
Characteristics
Limestone
Heat
Marble
Interlocking crystals
Shale
Heat and Pressure
Gneiss
Parallel sheets or bands
On further heating new minerals may start to form. Look at the garnet gneiss. Can you see small red
crystals? These are garnets, garnets only form at temperatures of over 750 DEG C. If higher
temperatures are reached the whole rock melts and is then classified as an igneous rock such as
granite.
Magma rises from inside the earth to form igneous rocks, these rocks are gradually broken down to
form sediment which becomes cemented forming Sedimentary rocks. The Sedimentary rocks are
buried under other rocks and heated with pressure to become metamorphic rocks. If they are heated
to a point where they melt they become magma and rise to the surface to become igneous rocks, and
so on. The complete cycle takes many millions of years to complete.