e.g ex 3 igneous rocks (2015-16 )

D.S.MODI, CIVIL DEPT., BVM ENGINEERING COLLEGE

VALLABH VIDYANAGAR

EXERCISE : 3

STUDY OF IGNEOUS ROCKS

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.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.To The GeologistsAny mass of minerals matters whether consolidated or not, hard or even soft irrespective of load bearing, which forms a part of the earth’s crust is rock. Rock may consist of only one mineral species, they are called monomineralic. But in nature rock are more usually consisting of aggregates of minerals.To The Civil EngineersThey 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.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

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.

DSModi BVM (CIVIL) ENGINEERING GEOLOGY MANUAL -IGNEOUS ROCKS A.Y 2015-16

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.Magma is actually a hypothetical melt. It has not been possible to see it at its place and occurrences.But it is assumed that it forms at a great depth [approximate 100 km below the earth surface, at asthenosphere] due to very high temperature and erupts at various region of the world again and again but remains below the surface. The asthenosphere layer exists in Mantle zone, which is plastic in nature having molten magmatic mass and also regarded as hypothetical molten chamber. It is also assumed that the plates move on this layer mainly responsible for plate tectonic activity.Lava is thoroughly studied material and that has poured out occasionally from volcanoes in many region of the world again and again. It provides ample proof of existence of the magma below the surface.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).

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

Volcanic Rock Hypabyssal Rock Plutonic Rock

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.

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

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 exampleComposition: - 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. DSModi BVM (CIVIL) ENGINEERING GEOLOGY MANUAL -IGNEOUS ROCKS A.Y 2015-16

Chemical composition Mineralogical Composition:-

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.The crystallization process from magma or lava may be slow or rapid. The magma may be rich in one constituent and poor in other constituents. The magma may be viscous quite mobile.All these factors lead to various shapes, sizes and arrangement of the resulting minerals and hence produce number of texture.Factors explaining Texture:-The three factors will primarily define the types of texture in a given igneous rocks.(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:-

Holocrystalline:-When all the constituents are distinctly crystallized.


SiO2 59.14%Al2O3 15.35%FeO 3.80%Fe2O3 3.08%CaO 5.08%MgO 3.45%Na2O3 3.84%K2O 3.13%H2O 1.15%Others 1.94%

Feldspar 59.5%Pyroxene and Amphibole

16.8%

Quartz 12.0%Biotite 3.8%Titanium 1.5%Apatite 0.6%Accessory Minerals

5.8%

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.

(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 (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

Texture of igneous rocks is primarily controlled by cooling rate of magma

• Extrusive igneous rocks cool quickly at or near Earth’s surface and are typically fine-grained (most crystals <1 mm)

• Intrusive igneous rocks cool slowly deep beneath Earth’s surface and are typically coarse-grained (most crystals >1 mm)

Types of Textures in Igneous rocks:

Phaneritic: rocks with mineral grains that is large enough to be identified by eye. Texture is typical of slowly cooled intrusive rocks. Intrusive or plutonic rocks are typically Phaneritic. This means that they are generally coarse grained and this texture is often quite uniform.


Grains can range in size but are often clearly visible to the naked eye (>2-3 mm). Common example is granite.Aphanitic: rocks with grain too small to be identified by eye. Texture is most common in rapidly solidified extruded magma and marginal facies of shallow intrusions. Extrusive or volcanic rocks are typically aphanitic or glassy. This means that they are generally fine grained in texture. Grains are typically 0.5 to 1 mm. Common example is Basalt. In Glassy texture very rapid cooling of molten rock Resulting rock is called ObsidianPorphyritic texture:-Many varieties are porphyritic. This means that the grain size is bimodal, with a fine grained matrix surrounding larger grains that are called phenocrysts. Minerals form at different temperatures as well as differing rates large crystals, called phenocrysts, are embedded in a matrix of smaller crystals, called the groundmassCommon example is Andesite.Pegmatitic texture:-A pegmatite is an extremely coarse-grained igneous rock (most crystals >5 cm) formed when magma cools very slowly at depth form in late stages of crystallization of granitic magmasPyroclastic texture: Various fragments ejected during a violent volcanic eruption textures often appear to more similar to sedimentary rocks

Mineralogical Classification:-Rock forming minerals make the bulk of igneous rocks.Two main classes of minerals in Igneous rocks:-


Felsic group of Minerals: - (Here Fel = Feldspars and Sic is for silica bearing) they are Light in colour, lower in density and include Quartz, Feldspar group and Feldspathoids group of minerals.

Mafic group of Minerals : (Here Ma= magnesium and fi= Fe bearing )They are Dark in colour, heavier in density and have Ferro magnesium minerals such as Mica, olivine, oxides and iron.

Colour Index: - For the Igneous rocks the colour index is accepted and designated based on the dark colour minerals present percentage wise in the given rock mass.

ROCK DIVISION COLOUR INDEX EXAMPLELeucocratic 1-30% GraniteMesocratic 31-60% GabbroMelonocratic 61-100% Peridotite

‘Shand’ and others developed methods of classifying igneous rocks saturated with silica:

Oversaturated(ACIDIC): Silica % is > 66% + Free Silica + Abundance of Quartz.

Saturated (BASIC): Silica % is 45-66% and free silica is upto 10%.

Intermediate: Rock term is used for silica % between 52-66%. Under Saturated (ULTRA BASIC): Silica % < 45% + No

Quartz.

IGNEOUS ROCK CLASSIFICATION


Generalised Classification

Nature Acidic intermediate Basic Ultrabasic

Coarsegrained

Granite Syenite - Diorite Gabbro Peridotite

DuniteQuartzPorphyry

Syenite porphyry -Diorite Porphyry

Dolerite -

Fine grained

Rhyolite Trachyte -

Andesite

Basalt -

Amorphous Glass

Silica (%)

> 65 55 - 65 45 - 55 < 45

BOWEN’S REACTION SERIES:-


ROCK CYCLE:

STRUCTURES IN IGNEOUS ROCKS:Those features of igneous rocks that are developed on a large scale in the body of an extrusion or intrusion.Giving rise to conspicuous shapes or forms are included under the term structures.They may be so well developed as to be recognized easily on visual inspection or they may become apparent only when thin sections of such rocks are examined under microscope. They are microstructures.

Types of StructuresStructures developed in igneous rocks can be grouped under three headings:Structures due to mobility of magma or Lava:The flow structures


The pillow structures the ropy and blocky lavaThe spherulitic structures Orbicular structures

Structures due to cooling of Magma:Jointing StructureRift and Grain StructureVesicular Structure

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e.g ex 3 igneous rocks (2015-16 )

Engineering