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Important Question And Answer: Civil - Engineering Geology - Petrology

Civil - Engineering Geology - Petrology


1. Define Igneous Rocks?


All rocks that have formed from an original hot, molten material through the process of cooling and crystallization may be defined as Igneous Rocks.


2. Explain about Hypabyssal Rocks?


These Igneous Rocks are formed at Intermediate depths, generally up to 2 Km, below the surface of earth and exhibit mixed characteristics of volcanic and plutonic rocks. Porphyries of various compositions are example of Hypabyssal Rocks.


3. Define Texture of Igneous Rocks?


The term texture has been defined as the mutual relationship of different mineralogical constituents in a rock. It is determine by size, shape and arrangement of these constituents within the body of rock.


4. What are the factors Explaining Texture?


a) Degree of Crystallization Holocrystalline, Holohyaline.

b) Granularity


Coarse grained, Medium grained, Fine grained. c) Fabric


Panidiomorphi, Allotrimorphic, Hypidiomorphic.


5. Define Equigranular and Inequigranular Texture?


All those textures in which majority of constituent crystals of rock are broadly equal in size are described as equigranular textures. All those textures in which majority of constituent minerals show marked difference in their relative grain size are grouped as inequigranular textures.



6. Define Structure of Igneous Rocks?


Those feature of Igneous Rocks that are developed on a large scale in the body of an extraction or instruction giving rise 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 become apparent only when this section of such rocks is examined under microscope. In latter case they are termed microstructure.


7. What are the numbers of factor depending on Igneous Rocks?


a) The structural deposition of the host rock (also called country rock).

b) The viscosity of the magma or lava.

c)  The composition of the magma or lava.

The environment in which injection of magma or eruption of lava place.

8. Classification of Rocks?








9.. Write about igneous Rocks?




  Rocks that have formed from an originally hot molten material through the process


of cooling and crystallisation may be defined as Igneous rocks.


Important Conditions For The Original Material

         very   high


temperature and


  a molten state

10.Explain about Magma?


The  hot  molten  material  occurring  naturally  below  the  surface  of  the


Earth  is called magma.

v  It is called lava when erupted through volcanoes.


v  Igneous rocks are formed both from magma and lava.

v  It maybe mentioned here that magma is actually a hypothetical melt.


v   Lava is a thoroughly studied material that  has poured out occasionally


from volcanoes in many regions of the world again and again.


v  Magma or lava from which igneous rocks are formed may not be entirely


a pure melt: it may have a crystalline or solid fraction and also a gaseous

fraction thoroughly mixed with




v   The solid and gaseous fractions, however, form only a small part of the


magma  or  lava,  which  are  predominantly  made  up  of  liquid  material

igneous rock.


11.What is mean by hypabyssal rocks?


     These igneous rocks are formed at intermediate depths, generally up to 2 kms below the surface of the earth and exhibit mixed characteristics of volcanic and plutonic rocks.


     Porphyries of various compositions are examples of hypabyssal rocks.


12.What is mean by Batholiths?


     These are huge bodies of igneous masses that show both concordant and discordant relations with the country rock.

       Their dimensions vary considerably but it is generally agreed that to qualify as


a batholith the igneous mass should be greater than 100 square kilometers in area and its depth should not be


traceable. This is typical of batholiths: they show extensive downward enlargement


     In composition, batholiths may be made of any type of igneous rock.


They also exhibit many types of textures and structures. But as, a matter of observation, majority of batholiths shows predominantly granitic composition, texture and structure. 13.Write about Granites ?

     Definition Granites may be defined as plutonic light coloured igneous rocks.


       These are among the most common igneous rocks.


         Composition. Two most common and essential mineral constituents of granite are: Quartz and




        Quartz is always recognized by its glassy lustre, high hardness (H = 7),


and cleavage less transparent white appearance.


Felspars making granites may be of two varieties: the potash felspars, commonly orthoclase and the soda-bearing felspars like albite and oligoclase.

14.Write about Diorite ?




         It is an intermediate type of igneous rock of plutonic origin with silica percentage generally lying between 52-66 per cent.




      Diorites are typically rich in felspar plagioclase of sodic group (e.g. Albite).


      Besides plagioclase and alkali felspars, diorites also contain accessory minerals like hornblende, biotite and some pyroxenes.


      Quartz is not common but may be present in some varieties that are then specially named as quartz-diorites.


15.What is mean by Andesite ?



     These are volcanic rocks in which plagioclase felspars (sodic and sub-calcic varieties like albite, andesine and labradorite) are the predominant constituents making the potash felspar only a

subordinate member.

16.        What is mean by Dolerites ?




       These are igneous rocks of typically hypabyssal origin having formed as shallow sills and dykes


        They may be regarded as equivalents of gabbros of plutonic origin and basalts of volcanic origin.




Dolerites are predominantly made up of calcic plagioclase (e.g. anorthite and labradorite).


17.What is mean by Basalts ?




Basalts are volcanic igneous rocks formed by rapid cooling from lava flows from volcanoes either over the surface or under water on oceanic floors. They are basic in character.


18.Define Sedimentary Rocks ?

     Sedimentary rocks are also called secondary rocks.


This group includes a wide variety of rocks formed by accumulation, compaction and consolidation of sediments.


          The  sediments  may  be  defined  as  particles  produced  from  the  decay  and


weathering of pre- existing rocks or may be derived from remains of dead sea or land animals in suitable


envir onme nts.


The accumulation and compaction of these sediments commonly takes place under water or at least in the presence of water.

19.            Write about sandstones ?


      Sandstones are mechanically formed sedimentary rocks of Arenaceous Group.


         These are mostly composed of sand grade particles that have been compacted and consolidated


together in the form of beds in basins of sedimentation.


     The component grains of sandstones generally range in size between 2mm and 1/16 mm. Silica in the form of very resistant mineral QUARTZ is the dominant mineral constituent of most sandstones.


20.Write about Shale ?


               Shale is a fine-grained sedimentary rock of argillaceous (clayey) composition.


    Shales are generally characterized with a distinct fissility (parting) parallel to the bedding planes and are made up of very fine particles of silt grade and to some extent of clay.


               Besides fissility, some shales show the laminated structure.


21.Write about limestone ?                                                         .





     These are the most common sedimentary rocks from the non-clastic-group and are composed chiefly of carbonate of calcium with subordinate proportions of carbonate of magnesium.


       They are formed both bio-chemically and mechanically.



         Pure limestone is invariably made up of mineral calcite (CaC03).

            In  terms       of  chemical  composition,  limestone' s  are  chiefly  made  up  of     CaO  and       CO2, Magnesium Oxide is a common impurity in most limestone' s; in some its percentage may


exceed 2 percent, the rock is then called magnesian limestone.


22.  Write about Dolomite ?



It is a carbonate rock of sedimentary origin and is made up chiefly more than 50 percent - of the mineral dolomite which is a double carbonate of calcium and magnesium with a formula of CaMg(C03h.

       Ferrous iron is present in small proportions in some varieties.

     Gypsum also makes appearance in some dolomites.

       But the chief associated carbonate is that of calcium, in the form of calcite.



23.What is mean by Coals ?



These may broadly be defined as metamorphosed sedimentary rocks of carbonaceous character in which the raw material has mostly been supplied by plants of various groups.


24.What is mean by Anthracite ?


        It is considered the highest-grade coal with fixed carbon ranging between 92-98 per cent.


      It has highest calorific value in coals and burns almost without any smoke, as the volatile matter is negligible.


25. Write about METAMORPHIC ROCKS ?


METAMORHISM is the term used to express the process responsible for all the changes that take place in an original rock under the influence of changes in the surrounding conditions of temperature, pressure and chemically active fluids.



Metamorphic  rocks  are  defined  as  those  rocks  which  have  formed  through  the operation of Stress Minerals

various types of metamorphic processes on the pre-existing igneous and sedimentary rocks involving changes in textures, structures and mineralogical compositions.

The direction of change depends upon the type of the original rock and the type of metamorphic process that operates on the rock.

Heat, pressure and chemically active fluids are the main agents involved in metamorphic processes.

Plastic  deformation,  recrystallisation  of  mineral  constituents  and development  of  parallel orientation are typical characters of metamorphic rocks.



      Metamorphic rocks have been variously classified on the basis of texture and structure, degree of metamorphism, mineralogical composition and mode of origin etc.


           A very general two-fold classification based on the presence or absence of layered structure or


(a) Foliated Rocks


Non-Foliated Rocks


28.  Write about Foliated Rocks ?


      All metamorphic rocks showing development of conspicuous parallelism in their mineralogical and structural constitution falling under the general term foliation are grouped together as foliated rocks.


     The parallelism indicating features include slaty cleavage, schistosity and gneissose structures


        Typical rocks included in this group are slates, phyllites, schists and gneisses of great variety.


29.Write about  Non-Foliated Rocks ?


      Included in this group are all those metamorphic rocks characterised with total or nearly total absence of foliation or parallelism of mineralogicaJ constituents.

                 Typical  examples  of  non-foliated  rocks  are  quartzites,  hornfels,  marbles,


amphibolites and soapstone etc.


30. What is mean by ROCKS SLATE ?




     Slate is an extremely fine-grained metamorphic rock characterized by a slate cleavage by virtue of which it can be readily split into thin sheets having parallel smooth surfaces.


      The slaty cleavage is due to parallel arrangement of platy and flaky minerals of


the slate under the dominant stresses operating during the process of metamorphism.




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