RocksRocks 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 rocksIgneous 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 rocksMetamorphic rocks are rocks that have changed over time, such as limestone which is made into marble because of the heat generated by magma.
Sedimentary rocksSedimentary rocks are rocks that are made from the slow hardening of sediments into layers, or strata. There are many kinds of sedimentary 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 chalkLimestone 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.