The earths surface is constantly being eroded. This means that rocks are broken up into smaller pieces byweathering agents such as wind, water, and ice. These small pieces of rock turn intopebbles, gravel, sand, and clay. They tumble down rivers and streams. These pieces settle in a new placeand begin to pile up and the sediments form flat layers. Over a long period of time, the pieces becomepressed together and form solid rock called sedimentary rock. Most sedimentary rocks form under water.Most of the earth has been covered by water some time in the past. 70% of the earth is covered by waternow. So sedimentary rocks are common all over the world. Sedimentary rocks are often rich in fossils.Sediments can harden into sedimentary rock in two ways.pressure-As layer after layer of sediments are deposited, the lower layers are pressed together tightlyunder the weight of the layers above.cementing-Some sediments are glued together by minerals dissolved in water. Some examples of sedimentary rocks are sandstone, limestone, conglomerate, and shale. Sandstone isformed from grains of sand pressed tightly together. Sandstones are very common rocks. They are formedfrom the sand on beaches, in riverbeds, and sand dunes. Sandstones are usually made of the mineralquartz. Limestone is formed from tiny pieces of shells of dead sea animals that have been cementedtogether. Conglomerate contains sand and rounded pebbles that have also been cemented together. Shale isformed from mud or clay that has been pressed together. Shale forms in quiet waters such as swamps andbogs. Sedimentary rocks are easy to identify because you can actually see the layers.
Below is a summary of the major characteristics of sedimentary rocks. Classified by texture and composition Often contains fossils May react with acid Often has layers, flat or curved Usually composed of pieces cemented or pressed together Has great colour variety Particle size may be the same or vary Usually has pores between pieces May have cross-bedding, mud cracks, worm burrows, raindrop impressions Heres a checklist of some common sedimentary rocks and their characteristics. Look for these samecharacteristics in the rocks you find, and youll be well on your way to becoming a rockhound.Some Common Sedimentary RocksName Colour CompositionSandstone Red or Grey Sand grains cemented togetherLimestone White to Grey Calcite and sometimes fossilsShale Dark Grey Compacted mudConglomerate Different Colours Rounded cobbles and pebbles cemented together
FormationA river carries, or transports, pieces of broken rock as it flows along.When the river reaches a lake or the sea, its load of transported rockssettles to the bottom. We say that the rocks are deposited. Thedeposited rocks build up in layers, called sediments. This process iscalled sedimentation.The weight of the sediments on top squashes the sediments at thebottom. This is called compaction. The water is squeezed out frombetween the pieces of rock and crystals of different salts form.The crystals form a sort of glue that sticks or cements the pieces of rocktogether. This process is called cementation.These processes eventually make a type of rock called sedimentary rock.It may take millions of years for sedimentary rocks to form.These are the different processes in order:Sedimentation → compaction → cementation
The oldest type of all rocks is the igneous rock (IG nee us). The word "igneous" comes from a Greek wordfor fire. Deep inside the earth, the temperature is very high and the minerals there are in liquid form calledmagma. As the magma pushes towards the earths surface, it starts to cool and turns into solid igneousrock. All igneous rocks do not cool the same way. That is why they do not look all the same. Some coolslowly, deep under the earths surface. These are called intrusive igneous rocks. The slow cooling formedrocks with large crystals. Granite is an example of a rock that cooled slowly and has large crystals.Other rocks formed when the magma erupted from a volcano or reached the earths surface through longcracks. Magma is called lava when it reaches the earths surface. Lava cools quickly and forms rocks withsmall crystals. They are called extrusive igneous rocks. Basalt is an example of this type of rock. Obsidianis an example of another extrusive igneous rock that cooled so fast that it has no crystals and looks likeshiny, black glass.
Below is a summary of the major characteristics of igneous rocks. Classified by texture and composition , Normally contains no fossils , Rarely reacts with acid , Usuallyhas no layering , Usually made of two or more minerals, May be light or dark coloured , Usually made ofmineral crystals of different sizes , Sometimes has openings or glass fibres , May be fine-grained or glassy(extrusive) , Heres a checklist of some common igneous rocks and their characteristics. Look for these samecharacteristics in the rocks you find, and youll be well on your way to becoming a rockhound.Some Common Igneous RocksName Colour TextureGranite Pink/Grey IntrusiveGabbro Dark Grey to Black IntrusiveRhyolite Light Pink or Grey ExtrusiveBasalt Dark Grey to Black ExtrusiveObsidian Usually Dark ExtrusiveScoria Dark Extrusive
Heat and pressure can change many things. They can even change rocks. The name for rocks that has beenchanged is metamorphic (met uh MOR fik) rocks. Metamorphic comes from Greek words meaning "change"and "form". Metamorphic rocks form deep in the earth where high temperature, great pressure, and chemicalreactions cause one type of rock to change into another type of rock. Metamorphic rocks begin to form at12-16 kilometers beneath the earths surface. They begin changing at temperatures of 100 degrees Celsiusto 800 degrees Celsius. If you squeeze and heat a rock for a few million years, it can turn into a new kindof rock. Where does the heat come from? The heat comes from magma. Where does the pressure come from? Thepressure comes from layers of rock piled on top of layers and layers of rock. The layers on the bottom getsqueezed. The thicker the layers, the more pressure there is.
Some examples of how metamorphic rocks were changed: sandstone becomes quartzite, shale becomesslate and limestone becomes marble.Below is a summary of the major characteristics of metamorphic rocks.Classified by texture and composition ,rarely has fossils, may react with acid , may have alternate bandsof light and dark minerals , may be composed of only one mineral, ex. marble & quartzite , may havelayers of visible crystals , usually made of mineral crystals of different sizes, rarely has pores or openings, may have bent or curved foliation Heres a checklist of some common metamorphic rocks and their characteristics. Look for these samecharacteristics in the rocks you find, and youll be well on your way to becoming a rockhound.Some Common Metamorphic RocksName Colour TextureGneiss Pink/Grey FoliatedMarble Light Coloured UnfoliatedQuartzite Light Coloured UnfoliatedSlate Dark Grey to Black Foliated
Physical weathering is caused by physical changes such as changes in temperature,freezing and thawing, and the effects of wind, rain and waves. Temperature changeswhen a rock gets hot it expands a little, and when a rock gets cold it contracts a little. If arock is heated and cooled many times, cracks form and pieces of rock fall away. This typeof physical weathering happens a lot in deserts, because it is very hot during the day butvery cold at night. Wind, rain and waves wind, rain and waves can all cause weathering.The wind can blow tiny grains of sand against a rock. These wear the rock away andweather it. Rain and waves can also wear away rock over long periods of time. Freeze-thaw Water expands slightly when it freezes into ice. This is why water pipes sometimesburst in the winter. You might have seen a demonstration of this sort of thing at school -a jar filled to the brim with water eventually shatters after it is put into a freezer. Theformation of ice can also break rocks. If water gets into a crack in a rock and then freezes,it expands and pushes the crack further apart. When the ice melts later, water can getfurther into the crack. When the rock freezes again, it expands and makes the crack evenbigger. This process of freezing and thawing can continue until the crack becomes so bigthat a piece of rock falls off.
The weathering of rocks by chemicals is called chemical weathering. Rainwater is naturally slightly acidicbecause carbon dioxide from the air dissolves in it. Minerals in rocks may react with the rainwater,causing the rock to be weathered. Some types of rock are easily weathered by chemicals. For example,limestone and chalk are made of a mineral called calcium carbonate. When acidic rainwater falls onlimestone or chalk, a chemical reaction happens. New soluble substances are formed in the reaction. Theseare washed away and the rock is weathered.Chemical weathering can hollow out caves form and make cliffs fall away.Some types of rock are not easily weathered by chemicals. For example, granite and gabbro are hard rocksthat are weathered only slowly. Still some of their minerals do react with the acids in rainwater to formnew, weaker substances that crumble and fall away.Acid rainWhen fossil fuels such as coal, oil and natural gas are burned, carbon dioxide and sulphur dioxide escapeinto the air. These dissolve in the water in the clouds and make the rainwater more acidic than normal.When this happens, we call the rain acid rain.Acid rain makes chemical weathering happen more quickly. Buildings and statues made from rock aredamaged as a result. This is worse when the rock is limestone rather than granite. Acid rain also kills treesand fish.
Plants can cause biological weathering. Animals and plants can wearaway rocks. This is called biological weathering. For example,burrowing animals such as rabbits can burrow into a crack in a rock,making it bigger and splitting the rock. You may have seen weedsgrowing through cracks in the pavement. If you have gone for a walk inthe countryside, you may even have seen bushes or trees growing fromcracks in rocks or disused buildings. This is because plant roots can growin cracks. As they grow bigger, the roots push open the cracks and makethem wider and deeper. Eventually pieces of rock may fall away. Peoplecan even cause biological weathering just by walking. Over time, pathsin the countryside become damaged because of all the boots and shoeswearing them away.
Sedimentation creates layers or rock particlesCompaction and cementation presses the layers and sticks the particles together. This createssedimentary rock.Rocks underground that get heated and put under pressure are changed into metamorphic rock.Rocks underground that get heated so much they melt turn into magma. Magma is liquid rock.Magma also comes from deeper inside the Earth, from an region called the mantle.Pressure can force magma out of the ground. This creates a volcano. When the magma cools it turnsinto solid rock.Magma that cools underground forms solid rock called intrusive igneous rock.Areas of rock can move slowly upwards, pushed up by pressure of the rocks forming underneath.This is called uplift.Weathering breaks down rocks on the surface of the Earth. There are three types of weathering -physical, chemical and biological.Wind and water move the broken rock particles away. This is called erosion.Rivers and streams transport rock particles to other places.Rock particles are deposited in lakes and seas, where they build up to form layers. This starts theprocess of sedimentation which will create sedimentary rock.