Igneous rock, Engineering Geology, Semester IV GTU
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Igneous rock, Engineering Geology, Semester IV GTU






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Igneous rock, Engineering Geology, Semester IV GTU Igneous rock, Engineering Geology, Semester IV GTU Presentation Transcript

  • ROCKSPETROLOGY Is a branch of geology, which deals with study of rocks (Petro=rock, Logos=study)
  • Importance of Petrology in civil Engg. • It provides an opportunity to interpret the physical properties of individual rocks, likewise: texture, structure, mineral composition, chemical composition etc. • This helps in knowing the strength, durability, colour, appearance, workability etc. • These properties are very important for CE to know because different rocks are suitable for different purposes and no rock is ideal or best suited for all purpose.
  • ROCKS IGNEOUS -most abundant -primary rocks -source is magma or lava SEDIMENTARY -thin veener above the Sial and Sima in Oceanic and Continental Crusts -secondary rocks METAMORPHIC -proportion is similar to that of Igneous rocks -change of forms of Ig. And Sed. Due to Temprature, Pressure and Chemical Fluids
  • IGNEOUS ROCKS All rocks that have formed from original hot, molten material through the process of cooling and crystallization may be defined as Igneous rock. Magma and Lava
  • BROAD CLASSIFICATION OF IGNEOUS ROCKS Volcanic rocks (Extrusive rocks) -Lava or Magma flows Intermediate rocks (Hypabyssal rocks) Plutonic rocks (Intrusive rocks) Name comes from Greek god of the underworld - Pluto
  • Igneous rocks divided in three sub category Volcanic rocks Cooling and crystallization of Lava.. Cools down very fast so the grain size of the crystal is very fine. e.g. Deccan trap Ex : Rhyolite, Dacite, Trachytes and Basalts
  • Plutonic rocks This Igneous rocks formed at considerable depth (7-10 km below earth surface) Slow rate of cooling so Coarse grained Exposed to surface due to removal overlying strata due to erosion Ex. Granites, Syenites, Gabbros Hypabyssal Rocks Formed at intermediate depth, 2 kms. Below the surface of earth. Mixed characteristics of Volcanic and Plutonic, Ex. Porphyries.
  • COMPOSITION Chemical Composition SiO2 – 59.14 %, Al2O3 – 15.35 % , FeO - 3.8 %, Fe2O3 - -3.08%, CaO - 5.08% MgO - 3.49%, Na2O3 – 3.84 % , K2O 3.13% , H2O 1.15 % and others Mineralogical Composition Felspar – 59.5 %, Pyroxine and Amphiboles – 16.8 % , Quartz – 12%, Biotite - 3.8 %, Titanium 1.5%, Apatite 0.60 % and others..
  • TEXTURE OF IGNIOUS ROCKS Mutual relationship of diff. mineralogical constituents (determined by size, shape and arrangement) 3 Factors which defines Textures.. Degree of Crystallisation Holocrystalline , distinctly crystallized.( Also termed as phaneric) Merocrystalline , intermediate type Holohyline , fine size, glassy (Also termed as aphinitic) Microcrystalline Granularity (average dimension of diff compo.) Course ( > 5mm) Medium (5mm to 1mm) Fine (< 1mm)
  • Fabric Relative grain size of diff. minerals, and degree of perfection in the form of the crystals Equigranular or Inequigranular Outline of individual crystals Perfect : euhedral Semi-Perfect : subhedral Irregular : anhedral If crystals of any one form or mixture of shape Panidiomorphic : majority euhedral Allotrimorphic : majority anhedral Hypidiomorphic : all shapes
  • Types of Textures Total five Textures.. Equigranular Texture Shown by Granite and Felcites so often name as •Granitic texture (all course grained or all medium grained) •Felsitic Structure , microgranular • Orthophyric ( Between Granitic and Felsitic) Inequigranular Texture Porphyritic (presence of large size crystals (phenocrysts) embeded in fine grained ground masses or matrix), Caused due to (i) Diff. in molecular concentration (ii) change in physio chemical conditions and (iii) relative insolubility
  • Types of Textures Poiklitic (presence of fine grained crystals within large xtals) Directive Texture Indicates the result of flow of magma during formation. Parallelism of crystals in direction of magma. Ex.Trachytic and Trachytoid textures Intergrowth Textures During formation, two/more minerals crystallize simultaneously. Ex. Graphic and Granophyric Intergranular Textures Polygonal or trigonal spaces formed at early stage filled up by crystallization
  • Structure of Igneous Rock Features developed large scale in the body of an intrusion or extrusion Types (A) Structure due to Mobility of Magma or Lava (highly viscous, viscous and mobile, highly mobile) (i) The flow structures : Development of parallel layers or bands in the body of Igneous mass due to flow of magma (ii) The Pillow structure Development of bulbous, overlapping, pillow like structures in the body of Igneous rock.
  • Block Lava: is less mobile i.e. more viscous and has a Rough and irregular surface. When cools down it result into angular, chunky texture. This causes the lava to thicken and to break apart as the flow continues to move down slope.
  • Structure of Igneous Rock (iii) The Ropy or Blocky lava Developed due to different mobility Highly viscous, broken surface : Blocky Mobile lava, smoothly wrinkled surface : Ropy (iv) The Spherulitic Structure Thin minerals fibers of various size arranged in perfect or semi-perfect radial manner. (v) The Orbicular Structure. Rare type. Rock mass appeared as if composed of ball-like segregation. Each ball composed of concentric shell of different minerals
  • Ropy Lava: is more mobile i.e. less viscous, it has a wrinkled but smooth and shiny surface on cooling. .
  • Structure of Igneous Rock (B) Structure due to cooling of Magma (i) The Joining Structures Cooling of magmas often accompanied by development of cracks/ joints Sheet structure : horizontal sets, closely spaced Mural jointing: three sets of joints , cubical blocks Columnar structure : surface gets divided in polygonal cracks (ii) Rift and Grain Rock can be split with ease in two separate direction (generally at right angle) (iii) Vesicular Structure Formation of cavities in the process of cooling and crystallizations ( Due to lava rich in gases)
  • Structure of Igneous Rock (iv) Miarolitic Structure Small and distinct cavities formed during crystallization and subsequently gets filled with volatile component then enlarge and facilitate formation of minerals in them. (C) Miscellaneous structure (i)The Reaction Structure Rocks surrounded on border by some incompletely altered minerals forms this corona structure. (ii) The Xenolithic structure. Incorporation of foreign materials (from host rock) in to magma during process of crystallization.
  • Forms of Igneous Rocks Igneous rock formed from cooling and crystallization of magma, cooling acquire variety of shapes which depends upon no. of factors structural disposition of host rocks (Country rocks) Viscosity of Magma or lava Composition of Magma or lava The environment in which injection of magma takes place. Division in two broad class : Concordant and Discordant Concordant bodies : Intrusion in which magma, injected and cooled along the structural plane of host rocks. Most important concordant structure of the host rock: Sills : injected along or between the bedding plane of sedimentary sequences, thickness is much . Length may vary from few centimeters to hundreds of meters.
  • Subdivided in to (1) Simple sills : Single intrusion of magma (2)Multiple sills : Two or more injections (of same kind) (3) Composite sills : Two or more injections (of different kind) (4) Differentiated sills: Exceptionally large, sheet like injection of magma in which segregation of minerals into separate layers or zones (5) Interformational sills : The sheets of magma injected along or in between the planes of unconformity in sequence. Most common rocks composing sills are like Syenites and Gabbros
  • Phacoliths : Occupy position in the trough and crests of crystal bends called folds.(doubly convex, passes into crest or trough passively) Lopoliths: associated with structural basins, that is sedimentary beds inclined towards a common centre. Many kms. Long and thousands of meter thick. (formation of basin and injection of magma is simultaneous). Like sills : simple, multiple, complex or differentiated in character.
  • Laccoliths : Invaded strata have been arched up or deformed in to dome. The Igneous mass has flat or concave base. Discordant bodies. Intrusive bodies that have been injected in to strata without influence of structural deposition and thus traverse across or oblique to the bedding planes
  • Important types Dykes : Columnar bodies of igneous rocks that cut across the bedding plane or unconformities. Great variation in their thickness, length, texture and composition. Dykes occur in groups or sets (Dyke-set, Dyke- swarm) Like sills classified as: Simple, Multiple, Composite and differentiated Cone sheets and Ring dykes. Volcanic Necks : Vent of quiet volcanoes have sealed with intrusions. Batholiths : Huge bodies of igneous intrusion that may show both concordant and discordant relations with the invaded rock.(> 100 km2 area, depth not traceable.)
  • CLASSIFICATION OF IGNEOUS ROCK The Chemical composition of Rock The mineralogical composition of the rock The Textural variations
  • CHEMICAL CLASSIFICATIONS : •Assumption that chemical composition of ultimate rocks indicates magmetic relationship •Most noteworthy by Petrologist : Cross, Iddings, Pirson and Washington. : C.I.P.W. •Standard set minerals called Norms selected. And Chem. Compo. of given rock is defined in terms of this normative minerals. Salient features of this classification (A)Norm : Set of standard minerals of calculated chemical composition. The norm minerals are divided in two groups : Salic and Femic minerals •The Salic minerals are : Quartz, Felspar, Orthoclase, Albite and Anorthite, Lecuite, Nephelite, Corundum, Zircon and Halite •The Femic minerals are : Acmite, Diopside, Hypersthene, Olivine, Magnatite, Chromite, Hematite, Pyrite, Rutile etc.
  • Nomenclature: In the C.I.P.W. Classification, Igneous rocks are divided into five classes, named according to SALIC FEMIC ratio Prefixes “PER” and “DO” (High and moderate respectively.) No prefix when two groups are equally represented Salic femic ratio Nomenclature for class  >7.0 Persalic 7.00 – 1.66 Dosalic 1.66 – 0.60 Salfemic 0.60 – 0.14 Dofemic  < 0.14 Perfemic
  • (C) Calculations : Most important part of classification and involve frequent use of tables for molecular weight, % weights of oxide and molecular values etc. Steps for result •Rock chemical analyzed, data converted to oxides of respective elements (with help of table) •% weight of each oxide / molecular weight = Molecular number •The molecular numbers and various constituents oxides are arranged in an orderly manner •From molecular numbers, different normative minerals are calculated according to sets of rules and with help of tables •The % weights of different minerals are calculated from the data obtain. From this the ratio between salic and femic minerals can be easily computed and classified.
  • MINEROLOGICAL CLASSIFICATION o Expression of chemical composition of parent magma. o Possible to recognize most common rock forming minerals megaocopic or microscopic examination. o Felsic Group: Light in colour, lower in density, includes quartz, feldspar and felspathoids o Mafic Group Dark in colour, heavier, Ferro- magnesian group of elements. Represented in rock by amphiboles, pyroxenes, micas, olivine, oxides of iron etc. oThen Rock division and color index.
  • TEXTURAL CLASSIFICATION Indicative of condition in which rock have formed.  Phanerites: Course grained, can be identified megascopically.  Aphanites: Fine grain constituents, Microscopic examination.  Glasses : Uncrystallized. Or glassy in nature. Zero grain size Also Plutonic, Volcanic and Hypabyssal
  • Composition Phaneritic Aphanitic Color index (% dark minerals) Felsic Granite Syenite Monzonite Rhyolite Trachyte Latite 10 15 20 Intermediate Granodiorite Diorite Dacite Andesite 20 25 Mafic Gabbro Basalt 50 Ultramafic Peridotite 95
  • Chief Mineral Constituents Acidic Rocks Intermediate Rocks Basic Rocks Ultra Basic Rocks Quartz & Felspar Felspar, both varieties Plagioclase felspar & Iron & magnesium minerals Iron & Magnesium minerals Plutonic Rocks (Massive & coarse grained) GRANITE SYENITE GABBRO PERIDOTITE Hypabyssal Rocks (fine gr.) PORPHYRITIC ROCKS DOLERITE Volcanic Rocks (glassy,cellular) RHYOLITE ANDESITE BASALT Silica % > 65 % 50 TO 65 % < 50 % Colour Light coloured rocks Dark coloured rocks Av. Sp. Gravity 2.6 2.8 3.1 3.6
  • To Remember : Composition and Classification of Igneous Rocks • Chemistry: e.g. % SiO2 • Mineralogy: e.g. – Felsic (Feldspar and Silica) – Intermediate – Mafic (Magnesium and Ferric) – Ultramafic
  • Why last figure is so important • The color and mineral distribution indicate an increasing density and melting temperature. Darker igneous rocks generally weigh more and are formed at higher temperatures and pressures. This reflects the density- stratification of the whole Earth!
  • Felsic Igneous Rocks: -Igneous rocks rich in minerals high in silica. They include: Granite Rhyolite
  • Intermediate Igneous Rocks: -Igneous rocks in between in composition between felsic and mafic igneous rocks. They include: Granodiorite Dacite Diorite Andesite
  • Mafic Igneous rocks -very low silica content, and consist primarily of mafic minerals. The most common ultramafic rock is: Peridotite
  • LIGHT COLORED Felsic INTERMEDIATE COLORED DARK COLORED Mafic VERY DARK COLORED Ultramafic COARSE- GRAINED (You can see different minerals) GRANITE: Can see crystals. Usually gray or pink. Can see quartz - gray, glassy grains. Can see feldspar - pink, buff, or white. DIORITE: Can see crysta ls with somewhat more light colored feldspar grains than dark colored minerals. A mix of light and dark but with no quartz. Salt & pepper appearance. GABBRO: Can see crystals — lots of flat shiny cleavage surfaces. Usually black to greenish black. FINE- GRAINED (You can NOT see crystals, for the most part) RHYOLITE: Usually gray, pink, pastel. Might see small clear, rectangular crystals. Sometimes banded. ANDESITE: Light to dark gray. Normally has small black crystals BASALT: Usually black or rust red. May have some or lots of gas bubble holes, some holes may be filled. May see small green grains. GLASSY OBSIDIAN: Black, red, green, GLASS PERLITE: Usually pearly gray. May contain Apache Tears. PUMICE: LOTS of gas bubble holes, very lightweight, will float on water. Abrasive. PORPHYRITIC (2 grain sizes) ALL CRYSTALLINE IGNEOUS VARIETIES may exhibit porphyritic texture FRAGMENTAL TUFF: Compacted volcanic fragments generally less than 4mm diameter (ash) VOLCANIC BRECCIA: Mixed tuff and angular large (>32mm diameter) fragments AGGLOMERATE: Mixed ash and rounded/sub-rounded large (>32mm diameter) fragments PERIDOTITE: Composed of 90- 100% olivine PYROXENITE: Composed of pre- dominately pyroxene AMPHIBOLITE: Composed of pre- dominately amphiboles (such as hornblende) CLUES TO IGNEOUS ROCKS COLOR TEXTURE
  • Granite is a coarse-grained, light colored, intrusive igneous rock that contains mainly quartz and feldspar minerals.
  • Granite : Composition : •Quartz (glassy lusture, H=7, white transparent appearance •Felspar - 1, Potash felspar (orthoclase), 2 Soda- bearing (abite, oligoclase) •Mica (muscovite, biotite) and accessory minerals. Texture : course-medium grained, holocrystalline, equigranular. Massive structure Types : depend upon minerals & textures, eg. Muscovite-granite, biotite granite, hornblend, augite etc. Use : ??
  • Granite - intrusive quartz feldspar biotite mica
  • Granite - intrusive Pegmatite Graphic Granite Porphyritic mica feldspar
  • Diorite is a coarse-grained, intrusive igneous rock that contains a mixture of feldspar, pyroxene, hornblende and sometimes quartz.
  • DIORITE Intermediate Igneous rock of plutonic origin with 52-66 silica% Composition : Rich in felspar plagioclase of sodic group, also contain hornblende, biotite and some pyroxene Texture: Resemblance to granite, course-medium grained. But absence of Quartz Occurrence : In form of small dykes, sills, stocks. feldspar biotite Generally it has a salt and pepper appearance (about ½ black and ½ white).
  • Andesite is a fine-grained, extrusive igneous rock composed mainly of plagioclase with other minerals such as hornblende, pyroxene and biotite.
  • Andesite - extrusive ANDESITE is the fine- grained equivalent of DIORITE. It tends to be a darker gray than rhyolite and is often porphyritic, with visible hornblende. hornblende phenocrysts
  • Gabbro is a coarse-grained, dark colored, intrusive igneous rock that contains feldspar, augite and sometimes olivine..
  • Gabbro - intrusive GABBRO is a coarse- grained rock that is high is iron & magnesium- bearing minerals (pyroxenes, amphiboles, plagioclase feldspar, olivene). The rocks will be dark in color, somewhat heavier than granitic rocks and devoid of quartz. Black minerals are primarily amphibole (like hornblende) and plagioclase feldspar.
  • Basalt is a fine-grained, dark-colored extrusive igneous rock composed mainly of plagioclase and pyroxene.
  • Basalt - extrusive BASALT occurs as thin to massive lava. flows, sometimes accumulating to thicknesses of thousands of feet and covering thousands of square miles. The volcanoes that produce basaltic lavas are relatively quiet, Basalt is dark, fine-grained and often vesicular (having gas pockets). The pockets may be filled with secondary minerals, e.g. quartz, zeolite minerals, calcite, opal, etc. and then it is called amygdaloidal (a- mig-duh-loy-dal) basalt.
  • Basalt - extrusive Gases released near the surface of a lava flow create bubbles or vesicles that are “frozen” in stone. Vesicular (Scoria) Amygdaloidal (*)
  • Obsidian is a dark-colored volcanic glass that forms from the very rapid cooling of molten rock material. It cools so rapidly that crystals do not form.
  • Pegmatite is a light-colored, extremely coarse-grained intrusive igneous rock. It forms near the margins of a magma chamber during the final phases of magma chamber crystallization. It often contains rare minerals that are not found in other parts of the magma chamber.
  • Peridotite is a coarse-grained intrusive igneous rock that is composed almost entirely of olivine. It may contain small amounts of amphibole, feldspar, quartz or pyroxene.
  • Pumice is a light-colored vesicular igneous rock. It forms through very rapid solidification of a melt. The vesicular texture is a result of gas trapped in the melt at the time of solidification..
  • Rhyolite is a light-colored, fine-grained, extrusive igneous rock that typically contains quartz and feldspar minerals.
  • Volcanic Breccia - extrusive VOLCANIC BRECCIA is pyroclastic (fire-formed fragments) and forms in explosive eruptions. It is a mix of large angular fragments and small ash.
  • The end !
  • The following are Spare pictures…
  • intrusive extrusive Into the Bedrock On top (Exit) (crystals) (no crystals)