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Class Rock Pack
Soils: sandy soil, Clay Soil.
Soils are formed from the weathering of rocks.
Weathering can be classified into two main processes. The first is the disintegration of rock by
mechanical or physical processes. The second is the decomposition of rock by chemical
processes.
Physical Weathering
A physical breaking up of rocks into small fragments or into their constituent mineral grains. There
are many different types of physical weathering, some of the more common ones are:
Changes in temperature
The minerals contained in a rock may expand by different amounts. For example the expansion
of quartz is twice that of feldspar. When rocks are heated to high temperatures during the day
and then rapidly cool at night (conditions commonly found in deserts) the effect of all of the
minerals expanding at different rates may cause the rock to disintegrate.
Task: Look at the granite. How many different types of mineral can you see?
Freeze thaw action
Frost action is a very powerful weathering force in temperate climates (such as in the UK). When
water freezes it expands by around 1 a percent, this creates a pressure of around 1 ton per
square inch (enough to force most rocks apart).
Chemical Weathering
Rain water is naturally acidic either because of absorbing carbon dioxide while in the atmosphere
or absorbing various organic acids when on the ground. The slight acidity of the rain water can
slowly dissolve away certain minerals. One mineral that is easily weathered by acidic rain water is
Feldspar.
Task: Look at the sample of granite. What do you think would happen to the granite if the crystals
of feldspar were weathered away?
Feldspar is a key mineral in a great many igneous, metamorphic, and sedimentary rocks.
Feldspar is an important mineral in the rock granite, in the UK we tend to think of granite as being
one of the hardest of rocks. However in tropical countries granite boulders can easily be kicked
into a heap of mineral grains. The rock falls apart because the original interlocking crystal network
of quartz and feldspar no longer holds together when the feldspar weathers to loose clay.
Rocks
Rocks can be divided into three groups:
Igneous: Rocks with crystals that have formed by molten magma cooling above or below the
earth's surface.
Sedimentary: Broken fragments of older rocks that have been cemented together.
Metamorphic: Rocks that have been altered by the effects of heat and pressure.
Sedimentary Rocks: sandstone, limestone, chalk.
The effect 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 a 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 precipitate 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 a limestone. An
easy test for limestone is that it fizzes when dilute HCl acid is dropped onto it.
Oolitic limestone is made up of tiny spheres or Ooliths. Ooliths are formed when tiny grains of
sand are blown across a desert shoreline rich in calcium carbonates. The sand of grain gradually
builds up a number of layers of calcium carbonate (a bit like a snow ball gathering snow as it rolls
across the ground) the ooliths become cemented together to form an Oolitic Limestone.
Chalk
Chalk is a very fine-grained pure-white limestone. Chalk is made from the shells of a number of
very tiny, microscopic sea creatures, mainly: foramifera, coccoliths and sponges.
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.
Igneous rocks: gabbro, granite, basalt
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) cools down very slowly because the
environment is very hot. Some igneous rocks take many thousands of years to cool sufficiently for
crystals to form. The faster the rock cooled the smaller the crystals.
Looking at the size of mineral grains is a good way of classifying igneous rocks. Examine the
igneous rocks in the pack and complete the following table.
Igneous Rocks: Name of rock
Gabbro
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, MAFIC rocks contain more minerals made
from magnesium and iron (darker/heavier minerals) and therefore give the rock a dark colour.
Quartz
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 gabbro
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).
Gabbro
Gabbro tends to display a mottling due to the scattering of pale feldspars through the dark green
iron and magnesium minerals. Because gabbro contains a large quantity of iron and magnesium
it is much denser than the other igneous rocks in fact much of the earth's interior consists of
gabbro.
Metamorphic Rocks: slate, garnet-gneiss, marble.
Metamorphism is the process by which an existing rock is changed through the action of heat and
pressure.
Examine the sample of slate. The slate has undergone the effects of heat and pressure.
Examine the sample of marble. The marble has undergone the effects of heat without pressure.
When a limestone is heated the calcium carbonate re-crystallizes to give the marble a white
sugary texture.
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 shale to slate to schist. The flaky
clay minerals that make up the soft shale are squeezed by the intense earth movements and line
up in one direction to make the characteristic thin sheets of slate.
Previous
rock
Metamorphism
Metamorphic
Rock
Characteristics
Limestone
Heat
Marble
Interlocking crystals
Shale
Heat and Pressure
Slate
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.
The Rock Cycle
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.