Rock is an aggregate of minerals that forms an appreciable part of the lithosphere. On the basis of origin , and mode of formation.
Rocks may be classified into : (i) igneous (ii) sedimentary and (ii) metamorphic.
The rocks are formed mainly by minerals that are feldspars, micas, amphiboles, pyroxenes, olivine, quartz, calcite, dolomite, clays, and gypsum etc.
Igeneous Rocks : Rocks formed by the cooling and solidification of molten silicate minerals ( magma ). Igneous rocks include volcanic and plutonic rocks.
Igneous rocks are classidied on the basis of texture and composition. The major kinds of igneous rocks are basalt, gabbro, andesite, diorite, rhyolite, and granite. Igneous rocks may also be classified under intrusive and extrusive rocks:
Intrusive Igneous rocks :
these are igneous rocks that, penetrated into or between other rocks in their fluid state and solidified. It can later be exposed at the Earth’s surface after erosion of the overlying rock.
Extrusive Igneous Rocks :
Rocks formed by solidification of magma above the Earth’s surface are known as extrusive rocks.
Characteristics of Igneous Rocks are as under :
>These rocks are solidified from molten magma.
> They usually do not occur in distict beds of strata.
>Igneous rocks are generally not fossiliferous.
>They are generally hard and water percolatesin them with great difficulty along the joints.
> Igneous rocks are granular and crystalline.The size of Crystals, however, differs from one rock to another.
> These rocks are generally weathered by mechanical(physical) weathering.
>Igneous rocks are mostly associated with the volcanic activity.
>Most igneous rocks consist of silicate minerals.
>The valuable minerals like iron-ore, copper, gold, silver, zinc, lead, mica, etc. are generally found in the igneous rocks.
Rocks formed by the accumulation and consolidtion of sedimentnt are known as sedimentary rocks. Sedimentary rocks may be as following types:
Agrilacous( Aqueous Rocks) Sedimentary Rocks –
A group of detrital sedimentary rocks deposited in water. They are commonly, clay, shale, mudstones, siltstone, and marl.
Three typs of Agrillacous rock are Marine , Lacustrine , Riverine.
Marine Agrillaceous sedimentary rocks
are formed due to deposition and consolidation of sediments in the oceans and seas mainly in their littoral zones.
Lacustrine Agrillaceous sedimentary rocks
are formed due to deposition and consolidation of sediments in Lake Environment. Generally, the sediments are deposited at the floor of the lakes.
Riverine Agrillaceous sedimentary rocks
are those which are formed due to deposition of sediments in the riverine environment. The sediments may be deposited in the beds of the rivers and in the flood plains.
Aeolian sedimentary rocks–
Aeolian processes create a number of distinct features, through both erosion and deposition of sediment.
Sediments deposited after transported by wind are known as aeolian deposits. Loess is one of the most important aeolian deposits. The loess deposits are found away from the source region and outside the desert. The dust particles of loess are so small that they hold together even when dry and once deposited cannot be easily lifted by the wind.
Glacial sedimentary rocks
The sediments deposited by glaciers are called glacial sedimentary rocks. These deposits are also known as moraines.
Characteristics of sedimentary rocks are as under
>The material of sedimentary rocks is derived from the pre-existing rock of any of the three rock classes as well as from newly formed organic matter. However, igneous rock is the most important original source of the inorganic minerl matter that forms sedimentary rock.
>The sedimentary rocks contain layers or strata. They are, therefore, often called stratified rocks.
>The layers of the sedimentary rocks are sometimes horizontal, and sometimes tilted due to lateral compressive and tensile forces.
>Sedimentary rocks are deposited in seas and oceans in a sequential order.
These rocks are formed from pre-existing rocks within the Earth’s crust due to changes in temperature, pressure and chemical action of fluid. The main examples of metamorphic rocks are slate, marble, quartzite, schist, and gneiss.
Metamorphic rocks : The origin of common metamorphic rocks is complex. In some cases, such as quartzite, marble, and metaconglomerate, the nature of the original rocks is easily determined. In other cases, such as schist and gneiss, it is difficult and sometimes impossible to determine the type of source rock.
Characteristics of metamorphic rocks are as under
>Metamorphic rocks are formed due to change in texture and mineral composition of the pre-existing rocks.
>Some rocks after metamorphism become more hard than their original form. For example, marble from limestone, quartzite from sandstone, and diamond from carbon.
> The fossils of the original sedimentary rocks are destroyed and, therfore, they do not contain fossils.
> The coarse-grained metamorphic rocks are imperfectly foliated, e.g. gneisses, from granites, while the fine-grained metamorphic rocks are perfectly foliated, e.g. schist from shale.
>Many of the metamorphic rocks are impervious ( marble and slate ), and some of them are pervious, e.g. gneiss.
> Many of the metamorphic rocks may slipt along the bedding planes, e.g., micaschist.
>Most gneissic metamorphic rocks comprise bands of granular quartz and feldspar.