Petrology and igneous rocks 06/28/2022


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Petrology and igneous rocks


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• Petrology: (Petra: Rocks and Logy: Logos: the science)

• The branch of Geology dealing with various aspects of rocks such as formation of rocks, chemical and mineralogical classification- occurrence of rocks, their properties, textures, structures, etc.

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• Rocks:-• They are natural solid, massive and aggregate

of minerals forming the crust of the earth. They are regarded as building blocks of the Earth crust.

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• To The Civil Engineers• They regard rocks as something hard,

consolidated and load bearing material which where necessary has to be removed by blasting. This concept also accords with the popular idea and the meaning of the world.

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• There are three major /principal types of rocks on Earth’s crust. It is based on way of formation:-

• (1)Igneous Rocks • (2)Sedimentary Rocks • (3)Metamorphic Rocks

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• Igneous Rocks:-• All rocks that have formed from an originally

hot molten material through the process of cooling and crystallization may be defined as igneous rocks. The source material for the formation of Igneous rocks is Lava and Magma.

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• Magma Rocks:-• The hot molten material

occurring naturally below the surface of the earth is called magma.

• The hot molten material when erupted through volcanoes as an eruptive mass is lava. Igneous rocks are formed from both lava and magma.

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• Magma or lava from which igneous rocks are formed may not be entirely a pure melt; it may have a crystalline or solid fraction thoroughly mixed with it. It consists of three components:

• A liquid portion, called melt that is composed of mobile ions

• Solids, if any, are silicate minerals that have already crystallized from the melt

• Volatiles, which are gases dissolved in the melt, including water vapor (H2O), carbon dioxide (CO2), and sulfur dioxide (SO2).

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• The magma can exist as a melt as long as physical and chemical environment surrounding it remains unchanged. But as and when there is a change in one or more of these conditions cooling and crystallization of magma may start and end up with the formation of an igneous rocks.

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• Igneous rocks are divides into three sub groups based on their occurrence:-

• Volcanic Rock • Hypabyssal Rock • Plutonic Rock

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Volcanic Rock:-

• These are the igneous rocks formed on the surface of the earth by cooling and crystallization of lava erupted from volcanoes.

• Lava cools down very fast compare fine grained. The grain sizes formed in these rocks are fine grained.

• The cooling may take place on the surface or even under water of sea and oceans. Ex: - Deccan traps of India – More than 4 lakh Sq. Km spreaded in Peninsular India., the Columbia plateau and Prana plateau of Brazil. Trap of Kashmir Himalayas.

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Hypabyssal Rock:-

• They are at intermediate depth at about 2 km. They exhibits mixed characters of volcanic and plutonic rocks. Ex: - Porphyries.

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Plutonic Rock:-

• These are formed are considerable depth of 7 to 10 km below the surface of the Earth. As these molten materials do not come immediately with atmospheric contact, the rate of cooling is very slow. Because of slow rate of cooling the rocks resulting from magma are coarse grained.

• These rocks are exposed on the surface of the earth as a consequence and erosion of the overlying strata. Ex: - Granites, Gabbros are few example

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• Composition: - Igneous rocks show great variation in chemical composition.

• Clark and Washington – Shown that on an average the following elements (expressed in % of oxide) are present in the igneous rocks.

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• Texture of Igneous Rocks:-• The term texture is defined as the mutual

relationship of different mineralogical constituent in a rock.

• It is determined by the size, shape and arrangement of these constituents within the body of the rock.

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• Factors explaining Texture:-• The three factors will primarily define the

types of texture in a given igneous rocks.• (A) Degree of Crystallization:- (B) Based on Granularity:- (C) Based on Fabric:-

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• (A) Degree of Crystallization:- In an igneous rock all the constituent minerals may be present in distinctly crystallized form and easily recognized by unaided eye, or they may be poorly crystallized even glassy or non crystallized. The resulting textures are:-

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• Holocrystalline:-When all the constituents are distinctly crystallized.

• Holohyalline: - When all the constituents are very fine in size and glassy or non crystalline.

• Merocrystalline: - It is intermediate type. Some are crystallized and some are glassy.

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• (B) Based on Granularity:- Average dimension of different constituents in the given rock mass are Coarse, Medium, Fine.

• Coarse size: > 5 mm Medium size: 1-5 mm Fine size: < 1 mm

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• (C) Based on Fabric:-This is a composite term expressing the relative grain size of different mineral constituent in a rock as well as the degree of perfection in the form of the crystals of the intermediate minerals.

• Equigranular: All the minerals are equal in size throughout the rock mass

• In equigranular. All the minerals are not equal in size throughout the rock mass

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Thank youMade by:Jariwala Harekrishna H.