Rocks are the main part of earth. There is a great role of rocks in the formation of earth


The oldest rocks 
 The oldest known rocks on Earth are 3900 million years old - They are the Acasta gneiss rocks from Canada. 
There are three main kinds of rocks 
(1) Igneous rock 
(2) sedimentary rock 
(3) metamorphic rock. 

Igneous rocks 

 Igneous means is‘fiery’. These rocks are made when hot molten magma is produced .Volcanic rocks, such as basalt, are igneous rocks that form from lava that has erupted from volcanoes. 

Metamorphic rocks 

 Metamorphic rocks are rocks that have changed over time, such as limestone which is made into marble because of the heat generated by magma. 

Sedimentary rocks 

 Sedimentary rocks are rocks that are made from the slow hardening of sediments into layers, or strata. There are many kinds of sedimentary rocks. 
Sandstone rocks 
 Sandstone, are made from sand and silt.Other rocks are broken down into these materials by weathering and erosion.Most sediments form on the sea-bed. Sand is washed down onto the sea-bed by rivers. 

Limestone and chalk 

 Limestone and chalk are sedimentary rocks made mainly from the remains of sea creatures.Streams and rainwater absorb carbon dioxide gas from soil and air. It turns them into weak carbonic acid. Carbonic acid corrodes (wears away by dissolving) limestone in a process called carbonation. When limestone rock is close to the surface, carbonation can create spectacular scenery. Corroded limestone scenery is often called karst, because the best example of it is the Karst Plateau near Dalmatia, in Bosnia. On the surface, carbonation eats away along cracks to create pavements, with slabs called clints. 
The slabs are separated by deeply etched grooves called grykes. Limestone rock does not soak up water like a sponge. It has massive cracks called joints, and streams and rainwater trickle deep into the rock through these cracks.Streams drop down into limestone through swallow-holes, like bathwater down a plughole. Carbonation eats out such holes to form giant shafts called potholes.


 Some potholes are eaten out to create great funnel-shaped hollows called dolines, up to 100 m across. Where water streams out along horizontal cracks at the base of potholes, the rock may be etched out into caverns. Caverns may be eaten out so much that the roof collapses to form a gorge or a large hole called a polje. 

 Recycling of rocks

 Rocks are continually recycled. Whether they  from volcanoes or sediments, all rocks are broken down into sand by weathering and erosion. 

Rock cycle

 The sand is deposited on sea-beds and river-beds where it hardens to form new rock. This process is the rock cycle. 

Strata of rocks 

 Rocks usually form in flat layers called strata. Tectonic plates can collide with such force that they crumple up these strata. Sometimes the folds are just tiny wrinkles a few centimetres long. Sometimes they are gigantic, with hundreds of kilometres between crests (the highest points on a fold). 

The shape of folds 

The shape of a fold depends on the force that is squeezing it and on the resistance of the rock. 

The slope of a fold 

The slope of a fold is called the dip. The direction of the dip is the direction in which it is sloping. The strike of the fold is at right angles to the dip. It is the horizontal alignment of the fold.


 Some folds turn right over on themselves to form upturned folds called nappes .As nappes fold on top of other nappes, the crumpled strata may pile up into mountains. Syncline A downfold is called a syncline; an upfolded arch of strata is called an anticline. The axial plane of a fold divides the fold into halves.

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