Rocks are made up of different combination of minerals. According to their conditions of formation, rocks are of three types.
Sedimentary rocks are formed when layers of rock pieces and eroded soil get squeezed together. These rocks are generally formed under water. As the rivers rush to the sea, they pick up mud, sand and pebbles and carry them along. When the water of the rivers reaches the sea, it slows down and drops a part of its load. The heavier pebbles are dropped first. Next, it drops the sand. It may carry the fine particles of mud for miles together into the sea, before it drops them.
Waves also beat upon the shores and break up the rocks along the shores into pebbles, sand and mud. These are eventually dropped to the floor of the sea.
As the mud, sand and pebbles sink to the bottom of the sea, they form layers on the ocean floor. In course of time, their fragmented particles are cemented together to form solid rocks.
1. Grains of sand form sandstone
2. Lime deposits form limestone.
3. Organic substances form coal.
4. Mud becomes shale.
Pre-existing rocks break down due to excessive heat, frost, wind, waves, glaciers and swift streams. These particles settle down in the form of different layers under the action of water or when blown or deposited by wind.
Sandstone is usually grey or yellow. It can also be pink or red. When the grains of sandstone are not well-cemented together, it can easily be crumbled up into sand in your hand.
Conglomerate is sometimes called pudding stone because of the mixture or rock material in it. Small, rounded pebbles get cemented together to form conglomerate. It often forms along riverbeds or at beaches.
River water dissolves lime and salt in it. Normally, it does not drop its load of lime and salt when it reaches the sea. At times, however, it does lose some of its lime. This lime mixes with mud and settles down at the bottom of the sea. Later on, it hardens into limestone. Shells of countless little sea animals also help in making the layers of limestone.
Shale is made of mud. It may be red, bluish grey, greenish grey or black. Like sandstone, it can often be broken up easily. The easiest way to test shale is to moisten it with water and smell it. It smells like mud.
None of the other rocks, which look like, coal burn. Coal is usually black in colour but it can also be brown. Organic matter buried long ago under extremely high pressure and temperature turns into coal, peat and amber.
Often dead organisms are covered with sand or silt. These layers of sand or silt turns into rocks. If the body of a dead organism gets trapped between these layers of rocks, it does not decay but gets preserved. Similarly, sometimes layers of ice trap organisms and preserve them for years together. Sticky substances harden to form amber rocks. These preserved remains of plants and animals are called fossils. A fossil can be a piece of bone, a part of an organism, outline of an organism or an organism that has changed to rock. Archaeopteryx and woolly mammoth are examples of fossilized organisms.
During volcanic eruptions, molten material called magma is continuously ejected to the surface of the earth. On reaching the earth’s surface, this magma starts flowing as lava. As the lava cools down, it solidifies to form rocks. These rocks are called igneous rocks. The word ‘igneous’ comes from a Latin word ignis which means fire.
Igneous rocks are of the following types.
Pumice is formed when the lava cools very quickly. It is actually formed from the foam on the top of the lava stream. It is light in weight and porous. This is due to the empty spaces present in it, which originally were occupied by the bubbles of gas while it was hot. Usually, pumice is pale grey in colour.
It is also formed when the lava cools very fast. It is different from pumice as it does not have pores in it. It is black and very shiny, in fact, blacker and more lustrous than coal. It looks like black glass therefore it is sometimes called volcanic glass.
Basalt is formed from the lava, which cools rather slowly. It is dull black in colour. It often breaks up into six-sided columns when it cools. It has sharp edges thus it can be used to make tools.
At times the lava is ejected out of a volcano with such a force that it goes high up into the air. This lava hardens into small pieces of rock before it reaches the ground. If the bits of rock are very small they are called volcanic ash. If they are larger, they are called volcanic cinders. Sometimes, volcanic ash is cemented together to form a solid rock after it falls to the ground. The rock formed in this way is dull grey. It is called tuff.
Granite is a common igneous rock, always formed underground. The hot liquid rock (magma) of which it is made, does not rise to the surface. If granite is seen on the surface of the ground, it is because of the fact that the rocks above it have been worn away.
For granite formation, the hot liquid rock must cool very slowly. The hot rock, which forms granite, cools so slowly that the minerals in it separate themselves from one another and form crystals.
Granite is always a speckled rock because the crystals in it are never of the same colour. Quartzite and feldspar are minerals always found in granite. Mica, tourmaline and garnet are some other minerals present in granite rocks.
Most metamorphic rocks are evolved from either sedimentary of igneous rocks. As rocks are compressed more and more by other layers of rocks, the lower rocks may change due to the weight of the upper layers. Heat from the interior of the earth may sometimes help to change the rock. Water seeping down from above into the layers of the rocks that have already been formed also helps to change the rock. Some of the ways in which mountains are formed involve a great deal of sideways pushing. This sideways pushing is another cause for changes in the rocks.
Slate is formed from shale. Slate has the property of splitting into thin layers. Slate has a muddy smell when it is wet. Classrooms are often provided with slate blackboards.
Sandstone changes to quartzite. The sand grains are no longer felt in the quartzite. It is so hard that paving stones are sometimes made of it.
Limestone results in the formation of marble. Marble may be black, pink, grey or white. Sometimes it is streaked with another colour. The Taj Mahal is made of white marble. The chemical composition of marble is calcium carbonate (CaCO3).
Schist is a rock with very thin layers. Conglomerate changes to schist. Schist may also be formed from other rocks.
Granite is changed by the heat and weight of the rocks above it into a bonded rock called gneiss.