Rocks

               PTR SCIENCE

                     ★ Rock

The outermost part of lithosphere is called crust. This is the most significant part of the earth because it is occupied by humans. The materials of The crust is made up of rocks. The rocks are of different types. They are hard like granite, soft like clay and loose like gravel. Rocks have a great variety of colour, weight and hardness. 

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Rocks are composed by minerals. They are aggregates of physical mixture of one or more minerals. Minerals on the other hand are made up of two or more elements in a definite ratio. They have a definite chemical composition. Crust is made up of more than 2000 minerals, but out of these, 6 are the most abundant and contribute the maximum of this uppermost part of the earth. This are feldspar, quartz, pyroxenes, amphiboles, mica and olivine. 

Granite is a rock and its constituent minerals bound together are quartz, feldspar and Mica Which make it a hard rock. Change in the ratio of this minerals give rise to Granite of different colours and hardness. The minerals containing metals are called metallic minerals. Hematite, a major iron ore is a metallic mineral. Ores are metallic minerals which can be profitably mined. Rocks are of immense economic importance to us. 

★ Classification of rocks 

Numerous system are in use for field and petrography classification of rocks. Many classification require detailed petrography laboratory tests and thin sections, while others require limited petrography examination and field tests. The Bureau of Reclamation has adopted a classification system which is modified from R.B. Travis. Well not best entirely on field tests or field identification of minerals, many of the classification categories are sufficiently that field identification is possible. even when differences in the mineral constituent cannot be determined in precisely in the field, difference is usually are not significant enough to affect the engineering properties of the rock if classified somewhat incorrectly by lithologic name. Detailed mineralogical identification and petrography classification can be performed on hand samples or core samples submitted to a petrography laboratory. 

★ Type of the rocks

Rocks defer in their properties size of particles and mode of formation. On the basis of mode of formation rocks may be grouped into three types. 

(a) Igneous rock 

(b) Sedimentary rocks 

(c) Metamorphic rock 

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Igneous Rocks 

The word igneous is derived from the Latin word 'ignis' meaning fire. igneous rocks are formed by the cooling of highly heated molten material, known as magma. The word Magma is derived from a Greek word which means 'dough'. it requires a greater quantity of heat to melt The rocks under overlying pressure then at the surface. We do not know the exact depths at which Magma forms but probably it is formed at different depths not exceeding 40km. Molten rocks produce an increase in volume which is responsible for causing fracture or cracks In the crust. The overlying pressure gets and along these openings, thus forcing out the magma through them. Otherwise it can't escape due to great overlying pressure. 

Sedimentary rocks 

The word 'sedimentary' is derived from the Latin word sedimentum, which means settling. Rocks (igneous,sedimentary and Metamorphic) of the earth 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. The 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 - sandstone, conglomerate, limestone, shale, loess etc. are examples. (ii) Originally formed - geyserite, chalk, limestone, coal etc. are some examples. (iii) Chemically formed - chert, limestone, halite, potash etc. are some examples. 

Metamorphic rocks 

Most rocks in mountainous show regions an evidence of change. All this in course of time become metamorphic or changed form of rocks. Metamorphic rocks are formed under the influence of heat or pressure on sedimentary or igneous rocks. Tremendous pressure and high temperature change the colour, hardness, structure and composition of all types of pre-existing rocks. The process which bring about the change is known as metamorphism and the unlimite products, formed due to operation of such processes are definite as the metamorphic rocks. 

★ Structure, Texture and colour of rocks 

Rocks structure 

The structure of rocks includes holes, cavities, joints, bedding planes, fractures, cleavage, schistosity, lenses, and similar features. Rock structure is an important factor affecting the amount and direction of groundwater flow, as well as actual sliding or sleeping of any embankment under investigation. 

The term "structure" are applied to the engineering geology of a site, refers to all of your geologic structures either at the side or in a location that could affect the site. This features include faults, folds, joints, rock cleavage, and discontinuities and unconformities. Structure has an important influence on the geologic conditions of a site and the ultimate stability and safety of an engineered structure. Problems of leakage, sliding of embankments, uplift pressure in foundation, and differential settlement are often traced back to inadequate delineation and consideration of the geologic structure at the site.

Rocks texture 

Texture is defined as the geometrical aspects of the component particles of a rock, including size, shape, and spatial arrangement. Texture is also applied to unconsolidated materials as an alternate description of particle gradation. Texture is important for field identification purposes and for predicting behaviour of of Rock under load. Although specific geologic terms such as "phaneritic" and "aphanitic" imply specific descriptions of igneous rock, simpler terms such as "coarse-grained" and "fine-grained" are more useful. Descriptions of mineral constituent, degree and type of cementation, condition of weathering, fracture system, and other properties influence engineering properties. This descriptors offer mode engineering value then merely the type of rock. Standard symbols are available in the Federal geographic Data committee Digital cartographic standard for Geologic Map Symbolization (FGDC 2006) 

Rocks colour 

Rock colour is an attribute visible preception that can be described by colour names (ASTM 1986). In wide use are the munsell soil colour chart and rock colour chart ( Munsell 2009a and 2009b). For rapid field logging purposes, e.g., reconnaissance investigation, table 4-7 can be used as an alternative. Colour is difficult to describe because a perceived colour greatly depends not only on the spectral power distribution of the colour stimulus, but also on the size, shape, structure and develop of the stimulus area. For example, a given colour will appear differently when seen next to other colours; grey appears bluish when sign next to orange or brown earth colours. Perceived colour also depends on the observer's experience with similar observation; so a colour may often be named differently by different persons. 

★ Uses of the Rocks 

(1) Pyrolusite : used f roofing and cramics; helps plants to grow; a family of rocks. 

(2) Talc : used in fireworks, matches, and stink bombs; found naturally in eggs; yellow colour. 

(3) Malachite : contents of manganese that is used in railroad tracks and purple glass; named for fire. 

(4) Dolomite : ore of aluminum;the aluminum is used in cans and airplanes; form in round shapes called concrations.

(5) Feldspar : ore of zinc ;the zinc is used in pennies; looks like other rocks.

(6) Coal : used in pencils and to unstick locks; related to diamonds.

(7) Marble : used to make statues and in grand buildings; India's taj mahal is made from it.

(8) Sulfur : used as a food preservative and to melt  ice on roads; it's salt.

(9) Bauxite : used in talcum powder to keep babies dry and for fingerprint dusting; very soft 

(10) Sphalerite : used in paint, jewellery, and fancy boxes; ore of copper; green