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.
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).
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 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 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.
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.