MINERALS, ROCKS, Metallic Minerals, Non Metallic Minerals, Igneous rocks, Sedimentary rocks, Metamorphic rocks, Geography notes UPSC, Geography optional Subject notes UPSC, Geography notes IAS, Geography optional Subject notes IAS
MINERALS & ROCKS
A mineral is a naturally occurring substance that is solid and stable at room temperature, representable by a chemical formula, usually abiogenic, and has an ordered atomic structure
Mineral are divided as follows:
- Metallic Minerals These minerals contain metal content and can be sub-divided into three types:
(i) Precious metals: gold, silver, platinum etc.
(ii) Ferrous metals: iron and other metals often mixed with iron to form various kinds of steel.
(iii) Non-ferrous metals: include metals like copper, lead, zinc, tin, aluminium etc.
- Non-Metallic Minerals These minerals do not contain metal content. Sulphur, phosphates and nitrates are examples of non-metallic minerals. Cement is a mixture of non-metallic minerals.
Some Major Minerals and Their Characteristics
- Feldspar Silicon and oxygen are common elements in all types of feldspar and sodium, potassium, calcium, aluminium etc. are found in specific feldspar variety. Half of the earth’s crust is composed of feldspar. It has cream to salmon pink colour. It is used in ceramics industries.
- Quartz It is one of the most important components of sand and granite. It consists of silica. It is a hard mineral virtually insoluble in water. It is white or colourless and used in radio and radar. It is one of the most important components of granite.
Pyroxene consists of calcium, aluminum, magnesium, iron and silica. Pyroxene forms 10 per cent of the earth’s crust. It is commonly found in meteorites. It is in green or black colour.
Aluminium, calcium, silica, iron, magnesium are the major elements of amphiboles. They form 7 per cent of the earth’s crust. It is in green or black colour and is used in asbestos industry. Hornblende is another form of amphiboles.
It comprises of potassium, aluminium, magnesium, iron, silica etc. It forms 4% of the earth’s crust. Commonly found in igneous and metamorphic rocks, it is used in electrical instruments.
Magnesium, iron and silica are major elements of olivine. It is used in jewellery. It is usually a greenish crystal, often found in basaltic rocks. Other minerals like chlorite, calcite, magnetite, haematite, bauxite and barite are also present in some quantities in the rocks.
The earth’s crust is composed of rocks. A rock is an aggregate of one or more minerals. Rock may be hard or soft and in varied colours. For example, granite is hard, sandstone is soft. Gabbro is black and quartzite can be milky white. Rocks do not have definite composition of mineral constituents. Feldspar and quartz are the most common minerals found in rocks.
The crustal rocks are classified on the basis of mode of formation, physical and chemical properties, location etc. On the basis of mode of formation the rocks are divided into three categories
(i) igneous rocks (ii) sedimentary rocks (iii) metamorphic rocks.
- Igneous rocks
As igneous rocks form out of magma and lava from the interior of the earth, they are known as primary rocks. The igneous rocks are formed when magma cools and solidifies. When magma in its upward movement cools and turns into solid form it is called igneous rock. The process of cooling and solidification can happen in the earth’s crust or on the surface of the earth.
Igneous rocks are characterized on the basis of texture. Texture depends upon size and arrangement of grains or other physical conditions of the materials. If molten material is cooled slowly at great depths, mineral grains may be very large. Sudden cooling (at the surface) results in small and smooth grains. Intermediate conditions of cooling would result in intermediate sizes of grains making up igneous rocks. Granite, gabbro, pegmatite, basalt, volcanic breccia and tuff are some of the examples of igneous rocks.
Igneous rocks are roughly hard rocks and water percolates with great difficulty. They do not have strata and are less affected by chemical weathering. They don’t contain fossils. The number of joints increases upwards. They are mostly associated with volcanic activity.
They are classified on several grounds as mentioned below:
(a) On the basis of silica content:
(i) Acidic igneous rocks have more silica e.g. Granites
(ii) Basic igneous rocks have less silica e.g. Gabbro.
(b) On the basis of chemistry and mineralogical composition:
(i) Felsic igneous rock (feldspar is dominant)
(ii) Mafic igneous rock (magnesium and ferrous are dominant)
(iii) Ultra mafic igneous rock (Peridotite and dunite are dominant).
(c) On the mode of occurrence:
(i) Intrusive igneous rocks— they are cooled and solidified below the surface of the earth. They are further divided into plutonic and hypabyssal igneous rocks. Plutonic rocks cool deep beneath the earth e.g. Granite. Hypabyssal rocks cool just beneath the earth surface e.g. Batholith, laccolith, phacolith, lapolith, sills, dykes, etc.
(ii) Extrusive igneous rocks are formed due to cooling and solidification of hot and molten lava at the earth’s surface e.g. Basalt, Gabbro, obsidian.
- Sedimentary rocks
The word ‘sedimentary’ is derived from the Latin word sedimentum, which means settling. Rocks (igneous, sedimentary and metamorphic) of the earth’s surface are exposed to denudational agents, and are broken up into various sizes of fragments. Such fragments are transported by different exogenous agencies and deposited. These deposits through compaction turn into rocks. This process is called lithification. In many sedimentary rocks, the layers of deposits retain their characteristics even after lithification. Hence, we see a number of layers of varying thickness in sedimentary rocks like sandstone, shale etc.
Depending upon the mode of formation, sedimentary rocks are classified into three major groups: (i) mechanically formed – e.g. sandstone, conglomerate, shale, loess etc. (ii) organically formed – e.g. chalk, limestone, coal etc. (iii) chemically formed – e.g. chert, halite, potash etc.
These rocks are formed due to aggregation and compaction of sediments. These rocks contain fossils of plants and animals. They cover 75 percent of surface area of the globe. However they form only 5 percent of the volume of earth’s crust. They contain several layers or strata but these are seldom crystalline rocks. They are seldom found in original and horizontal manner. They may be well consolidated, poorly consolidated and even unconsolidated. They are characterized by different sizes of joints. Most sedimentary rocks are porous and permeable.
The formation of sedimentary rocks takes place in three stages:
- Transportation: after weathering and erosion the fragments of parental rocks are transported by the agents of erosion like stream, wind, air, etc.
- Deposition: transported materials are deposited in sea, lakes, etc. The particles are deposited in parallel layers and their process of layer formation is called “stratification”.
- Consolidation: when the number of layer is large, the weight of upper layer begins to affect the lower layers and the further compression solidifies the sediments into rocks.
They are classified under different schemes:
- On the basis of nature of sediments:
(a) Mechanically formed or clastic rocks e.g. Sandstones. Conglomerates, clay rock, shale, loess.
(b) Chemically formed sedimentary rocks e.g. gypsum, salt rock.
(c) Organically formed sedimentary rocks e.g. Limestone, dolomites, coal, peats, etc.
- On the basis of transporting agents:
(i) Argillaceous or aqueous rocks:
(a) Marine rocks,
(b) Lacustrine rocks,
(c) Riverine rocks
(ii) Aeolian rocks e.g. Loess.
(iii) Glacial sedimentary rocks e.g. Till, moraine.
- Metamorphic rocks
The word metamorphic means ‘change of form’. These rocks form under the action of pressure, volume and temperature (PVT) changes. Metamorphism occurs when rocks are forced down to earth’s interior by tectonic processes or when molten magma rising through the crust comes in contact with the crustal rocks or the underlying rocks are subjected to great amounts of pressure by overlying rocks. Metamorphism is a process by which already consolidated rocks undergo recrystallization and reorganization of materials within original rocks.
Mechanical disruption and reorganization of the original minerals within rocks due to breaking and crushing without any appreciable chemical changes is called dynamic metamorphism. The materials of rocks chemically alter and recrystallize due to thermal metamorphism. There are two types of thermal metamorphism – contact metamorphism and regional metamorphism.
In contact metamorphism the rocks come in contact with hot intruding magma and lava and the rock materials recrystallize under high temperatures. Quite often new materials form out of magma or lava are added to the rocks.
In regional metamorphism, rocks undergo recrystallization due to deformation caused by tectonic shearing together with high temperature or pressure or both. In the process of metamorphism in some rocks grains or minerals get arranged in layers or lines. Such an arrangement of minerals or grains in metamorphic rocks is called foliation or lineation.
Sometimes minerals or materials of different groups are arranged into alternating thin to thick layers appearing in light and dark shades. Such a structure in metamorphic rocks is called banding and rocks displaying banding are called banded rocks. Types of metamorphic rocks depend upon original rocks that were subjected to metamorphism.
Metamorphic rocks undergo complete alteration in the appearance of pre-existing rocks due to change in mineral composition and texture through temperature and pressure changes. Gneiss, granite, slate, schist, marble, quartzite etc. are some examples of metamorphic rocks. They are classified as mentioned below:
- Contact or thermal metamorphism: here metamorphism occurs when the mineral composition of the surrounding rocks is changed due to intense heat e.g. Limestone is changed to marble.
- Regional or dynamic metamorphism: here pressure plays an important role so that rocks are altered in their forms in an extensive area.
Rocks do not remain in their original form for long but may undergo transformation. Rock cycle is a continuous process through which old rocks are transformed into new ones. Igneous rocks are primary rocks and other rocks (sedimentary and metamorphic) form from these primary rocks. Igneous rocks can be changed into metamorphic rocks. The fragments derived out of igneous and metamorphic rocks transform into sedimentary rocks. Sedimentary rocks themselves can turn into fragments and the fragments can be a source for formation of other sedimentary rocks. The crustal rocks (igneous, metamorphic and sedimentary) once formed may be carried down into the mantle (interior of the earth) through subduction process (parts or whole of crustal plates going down under another plate in zones of plate convergence). The same can melt down due to increase in temperature in the interior and turn into molten magma, the original source for igneous rocks.