11 Q Science Assessment
Sedimentary rocks are formed from layers of deposits laid down in lakes or seas. Over millions of years the layers of sediment build up resulting in the weight pressing downwards, creating high pressure, which squeezes the water out. As the water is squeezed out, salts form between the particles of sediment due to crystallization and this cements the particles together. Sedimentary rocks can either rise to the surface again to be discovered, or they can descend into the heat and pressure below.
Sandstone is a type of sedimentary rock and is formed from sand. Sandstone is fine particles of sand cemented firmly together due to heat and pressure, which is why it looks very much like sand. However, there is red sandstone as well as yellow. Sandstone is commonly used for buildings.
Limestone, which can also be called chalk, is formed from seashells. It mostly contains calcium carbonate as seashells are made from calcium carbonate. It is usually white or grey in colour. The original shells are mainly crushed down how some fossilised shells can still be found in limestone.
Mudstone, which is also known as shale, is formed from mud. Mud is basically finer particles of sand cemented together due to heat and pressure. Mudstone is often grey in colour and unlike other rocks; it tends to split onto its original layers very easily.
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Coal is a fossil fuel formed from the remains of plants and vegetation. Most of our coal was formed about 300 million years ago when the earth was covered by steamy swamps. As plants and trees died, their remains sank to the bottom of the swampy areas building up in layers. Due to the heat and pressure, the particles are compressed together is coal is formed.
The chemical composition of limestone is CaCO3, which makes it a compound of calcium and carbon dioxide, (calcium carbonate).
As limestone is mainly calcium carbonate CaCO3, a simple test to see whether a given rock is limestone would be to add hydrochloric acid to it. It the substances is limestone, carbon dioxide should be given off. When hydrochloric acid is added to limestone, it the solution should fizz giving off a colourless gas that turns limewater milky, which is carbon dioxide CO2.
Calcium carbonate + Hydrochloric acid = Calcium chloride + water + carbon dioxide
CaCO3 + 2HCl = CaCl2 + H2O + Co2
When limestone is heated at a high temperature, carbon dioxide and calcium oxide, which is also known as quicklime is produced.
Calcium carbonate calcium oxide + carbon dioxide
CaCO3 CaO + CO2
Calcium oxide, or quicklime reacts violently with water to produce calcium hydroxide, which is also known as slaked lime. Slaked lime is a white powder and a very strong alkali, and because it is an alkali it can be used in agriculture to reduce the acidity soil, which is too acidic to healthy growth of crops. The advantage that slaked lime has over quicklime is that it is much faster at reducing acidity of the fields.
Calcium oxide (quicklime) + water = calcium hydroxide
CaO + H2O Ca (OH) 2
Limestone can be used for making building, however acid rain can corrode the building made from limestone. Limestone can be heated with clay to make cement. When limestone is heated with sand and sodium carbonate, it makes glass. Any rocks, which are not eroded away, are eventually returned to the mantle when plates descend in tectonic activity.
Only sedimentary rocks contain fossils, as metamorphic and igneous rock would have gone through higher levels of heat and pressure, where the fossils would not be able to survive. Whereas sedimentary rocks have only been gentle crushed for a few million years, which is why all sedimentary rocks are likely to contain fossils.
Fossils are formed from the remains of plants and animals get trapped. Other minerals replace most of these trapped remains but this leaves the structure and trace of the original plant or animal. When the sedimentary layers are undisturbed, the lower the layer, the older is it, so the using this, the geological sequence of formation can be worked out.
Fossils can be used to date the age of the rock form the species and the sort of environment present at the time of the follies formation, for example the climate and nature of the land. However, using fossils to date the age of rocks is not absolutely accurate, which is why radioisotope studies are used for more accurate results. Fossils can also be used as evidence for the evolution of species because the development of individual species and their structure can be followed. When the sedimentary rock that the fossils lie in erodes, the fossils start to appear. Usually, the original harder parts of the organism tend to be better preserved, such as the shells and bones, and these have to be extracted carefully form the materials around them.
You would expect to find fossils in igneous rocks because they are formed from molten mixed up magma. Fossils would not survive in such conditions. Fossils are rare to find in metamorphic rocks, however traces can sometimes be preserved in rocks like slate. Fossils are likely to be unclear or destroyed completely by the heat and pressure involved in the formation of metamorphic rocks.
There are several features in sedimentary rocks, which help scientists to work out what has happened to the rocks over the years and the origin of the rocks. Such features include:
- The order of the layers, as the deeper the layer is, the older the rock is.
- Discontinuous deposition, where layers of different rocks are laid down at different times.
- A younger layer of rock could cut across an older one.
- Ripple marks in the rock can show that it has been formed from the seabed due to waves or currents.
- When rock formations are tilted, it causes large-scale movements and the angles can be followed over distances to show the relationship between distant rock formations.
- The folding of rocks shows the compression of layers due to plate movement.
- When fractures or fault signs are present in rocks, it shows evidence of earthquakes in the past.