Unit I
Definition: - Geology is the scientific study of the earth as a planet. Its scope includes study origin, age and structure of the earth as well as as evolution, modification of various surface and subsurface features.
Sub Division of the geology
a) Physical Geology:-It deals with the origin, development and ultimate fate of various surface features of the Earth and also with its internal structure. The role played by internal agents (volcanism and earthquakes) and external agents (wind, water and ice) on the physical features of the earth makes major areas of study in physical geology.
Similarly, the disposition of rock bodies, water bodies and huge moving deposits of ice on the surface and their structures also form important subjects of physical geology
b) Geomorphology:-This branch, although a part of physical geology, deals specifically with the study of surface features of the earth, primarily of the land surface. Detailed investigation regarding development and disposition of mountains, plains, plateaus, valleys and basins and various other landforms associated with them fall in the domain of geomorphology. The structure and evolution of these landforms through space and time are advanced fields of study within geomorphology
c) Mineralogy:- Minerals are the basic building units of which the solid crust of the earth is made up. Mineralogy is that branch of geology, which deals with formation, occurrence, aggregation properties, and properties use of minerals Mineralogy is sometimes itself divided into specific sub-branches such as crystallography, optical mineralogy and descriptive mineralogy and so on. Crystallography is well-established branch of mineralogy that deals exclusively with internal structure and external manifestations of minerals occurring in crystallized form in the natural process or made from synthetic processes
d) Petrology: - Minerals occurring in natural aggregated form are called rock.These rocks form the building block that makes the crust of the earth. The rocks are themselves made up or minerals already defined as building units. Formation of various types of rocks, their mode of occurrence, composition textures and structures geological and geographical distribution on the earth are all studied under petrology. It is one of the most important subdivisions of geology and is further subdivided into three distinct branches: Igneous petrology, Sedimentary petrology and metamorphic petrology. Petrography another distinct branch of geology deals specifically with nature and distribution of rock on the Earth and geological explanations governing such a distribution
e) Historical Geology: -It deals with the past history of the Earth as deciphered from the study of rocks and featuresassociated with them. Rocks may be treated as pages of the Earth's history. They contain within them enough evidence indicative of nature and time of their formation, composition, constitution, magnetism, structural disposition and in many cases, fossil (remains of ancient life), all of which when interpreted scientifically reveal a lot about the events that have passed since their formation. Thus fairly accurate estimates can be made from the above evidence about the climates, biological and environmental conditions prevailing just before, during and after the formation of these rocks in and around the areas of their occurrence. Paleo-geography, paleontology and stratigraphy are three distinct subdivisions of Historical Geology
f) Economic Geology:- This branch deals with the study of those minerals and rocks and other materials (fuels etc.) occurring on and in the earth that can be exploited for the benefit of man. These include a wide variety of ores of all the metals and non-metals, building stones, salt deposits, fuels (coal, petroleum natural gas and atomic minerals) and industrial minerals for refractories, abrasives and insulations and for manufacture of chemicals. Mode of occurrence of these materials, principles involved in their formation and accumulation, their properties, structural and other controls that help in their extraction at economical costs are important fields of study under Economic Geology.
Scope of Engineering Geology
Engineering geology may be defined as that branch of applied sciences which deals with the application of geology for a safe, stable and economic design and construction of a civil engineering project. It is now recognized as a well-established interdisciplinary subject. In qualifying career as a civil engineer, grasp of fundamentals of engineering geology is almost universally considered as essential as that of soil mechanics, strength of materials or theory of structures
Since the fifties of twentieth century, this branch has undergone very sound and rapid developments so that at present application of geological knowledge in planning designing and construction of big civil engineering projects is considered not only desirable but also absolutely essential
The basic objects of a course in Engineering Geology are two fold:
(a) It enables a civil engineer to understand engineering implications of certain conditions related to the area of construction, which are essentially geological in nature
(b) It enables a geologist to understand the nature of geological information that is absolutely essential for a safe design and construction of a civil engineering project.
It is obvious, therefore, that a civil engineer is neither expected nor required to undertake himself geological investigations of the area before designing and
implementing the constructionplans of a major civil engineering project. He must be, however, capable to understand and critically discuss a geological report of the area prepared by an experienced geologist and derive maximum useful information pertaining to the project in question
The scope of engineering geology is best studied with reference to major activities of the profession of a civil engineer which are: Construction, Water Resource Development, Town and Regional Planning
Earth-Surface Features And Internal Structure
Earth has an average radius of 6371 km However even with the most scientific Instruments we have hardly penetrated kilometres below the surface of earth.
Therefore the study of internal structure is Outer mostly based on the Indirect geophysical methods where seismic waves are the want important source Their study divides into three well defined shells or zones on the basis of two discontinuities obtained on two seismograms. The properties of these waves are dealt with later
The Crust :-It is upper most layer of the earth. The first discontinuity called Mohorovicic discontinuity marks the lower boundary of crust
It is further divided into 2 types:
1. Continental Crust: The thickness of continental crust varies upto 80 km at different locations ex
Thickness below the Himalayas is 75 km, and under the Hindukush Mountains is 60 km .Average depth for the continental crust is 30-40 km for regions other than
mountainous regions. Continental crust is further divided into three layers on the basis of density and other properties. These are referred as A.B.C layer
A Layer: It is about 2-10 km thick, relatively of low density. It is mostly made up of sedimentary Rocks.
B Layers: It is relatively dense and made up of igneous and metamorphic rock.
C Layer: It is lower most layer of continental crust with a higher density than A and B layer. It is also refered as Basaltic layer of the crust. It is made up of basic minerals such as magnesium and silicates and hence is also called SIMA (si for silica and MA for magnesium).
2. Oceanic Crust: It is the extension of C layer of the continental crust which makes the top most layer of the ocean bed. A and B layers are practically absent here. It has on average density of about 3.00g/cc.
2 The Mantle it is the second layer of the earth This layot starting from lower part of crust continous upto a depth of 2900 km. It is further divided into upper mantle and lower mantle. SeismiMogram data indicate that upper mantle in plastic state rather than solid. It is also called asthenosphere (greek work asthenes without strength). It is on this layer only that the tectonic plates move slowly also provides the much of the volcanic activity of the earth.
Lower Mantle Layer is Solid
The entire mantle layer is also refered as SIMA layer as it is made up to silica and magnesium
3 The Core: It is the innermost concentric layer (starting from the second discontinuity also called mantle core discontinuity) extending upto the centre of earth. It is further divided into two distinct zone
Outer Core: It is a region from a depth of 2900 km to 4580 km below the surface of earth. It is a liquid layer because of the high temperature at this depth.
Inner Core: This region extends from 4580 km to the centre of earth. It is a solid layer because the immense pressure acting along with high temperature.
The entire core is also refered as NIFE as it is mostly made up of Nickel and Iron
The crust of the earth is mostly made up of silica and aluminium and it is also called SIAL.As discussed earlier the molten upper mantle allows the tectonic plates to drift slowly. These plates when each other often give rise to seismic activities (earthquakes).
Weathering of Rocks
General
Atmosphere, which is a gaseous state envelope surrounding the planets earth is in constant contact with the rocks of the crust wherever these are exposed on the surface this contact is of the dynamic interactive nature in which the original structure and even composition of the rocks are changed to great extend.The ruling trend in all such is establishing a physical and chemical equilibrium between the rocks and the atmosphere The atmospheric processes in themselves change from season to season and year after year: the rocks of the surface are, therefore always undergoing a slow , imperceptible change that ultimately results in their complete alteration and even destruction from a given place.
Weathering as already defined, is a natural process of in-situ mechanical disintegration and/or chemical decomposition of the rocks of the crust of the Earth by certain physical and chemical agencies of the atmosphere. The most important aspect of this process is that the weathered product remains lying over and above or near to the parent rock unless it is removed from there by some other agency of the nature.There are several methods by which rocks undergo weathering. These may be classified and discussed under two main classes: mechanical (physical) weathering and chemical weathering.
Mechanical (Physical) Weathering
It is a natural process of in-situ disintegration of rocks into smaller fragments and particles through essentially physical processes without a change in their composition.
A single rock block on a hill slope or a plain, for instance, may be disintegrated gradually into numerous small irregular fragments through frost action that in turn may break up naturally into fragments and particles of still smaller dimensions. These loose fragments and particles may rest temporarily on the surface if it is a plain. On slopes, however, the end product fragments and particles may roll down under the influence of gravity and get accumulated at the base as heap of unsorted debris All these fragments and particles, however, have the same chemical composition as the parent rock.
Mechanical weathering is one of the very common geological processes of slow natural rock disintegration in all parts of the world. Temperature variations and organic activity are two important factors that bring about this change under specific conditions.
Temperature variations are held responsible for extensive mechanical weathering of rocks lost on the surface. These manifests in two different ways: frost action in cold humid and thermal effects (insulation in hot arid regions. An outline of these processes is as follows:
(A Frost Action
As is known, water on freezing undergoes an increase in its volume by about ten percent. This expansion is accompanied by exertion of pressure at the rate of 140 kg/cm2 on the walls of the vessel containing the freezing water. In areas of intensive cold and humid climates, temperatures often fall below the freezing point of water repeatedly during winter .In such freezing of water in pots and water pipes and taps and in cavities and cracks in concreted roads their bursting and disintegration in many cases is a matter of common obseravation .This process of freezing of water when happening within the pores, cracks ,fracture cavities of rocks affects them considerably The original openings are widened at the first stage of attack and thereby accommodate more and more water come and freeze in subsequent cycle .A freezing cycle often followed by a thawing cycle that means melting of ice formed within the cavities.
Eventually, repetition of the freezing and thawing cycles over many years leads to gradual disintegration of the rocks because of internal stresses exerted in the process
The frost formed fragments are angular, sub angular and irregular in outline and remain spread over the parent rock having flat surface or flat slopes. If original surface forms slope, as in commonly the case in the hilly and mountainous regions, these frost
fragments get heaved up from the crevices and cavities and then roll down the slope under the influence gravity. Finally, the fragments accumulate at the base as heaps commonly called as Scree deposits. In some cases, especially when the slopes are stabilized and the pull of gravity is weaker the fragment remain unevenly strewn over the surface of the slopes. Such slopes covered by frost formed by scree are the often refered to as talus slopes.
Exudation is process similar to frost action but in this case disintegration takes place due to formation of crystals of sodium chloride, etc. within cavities of rocks thereby causing their disintegration. This process is seen in good measure in porous rocks near seashore
b) Thermal Effects (Insolation)
In arid, desert and semi-arid regions where summer and winter temperatures differ considerably, rocks undergo physical disintegration by another phenomenon related to temperature. As we know, rocks, like many other solids, expand on heating and contract on cooling. They (rocks) are,of course classed as bad conductors of heat but even then prolonged exposure to direct heating by the Sun induce appreciable volumetric changes in them. In arid and semiarid regions, thedifference between and night temperatures and also between average temperature in summer and winter is quite considerable. In some deserts, for instance Kara Qum, rocks are exposed to a high temperature 70-80°C in summer and are then cool down -10 °C in winter. Such repeated variations in one experienced by a body of rock gradually break it into smaller pieces, especially in the top layers, by development of tensile stress developing from a team expansion and contraction
Exfoliation. In a thick rock body or where the rock is layered, these are the upper layers get affected most due to the temperature Variation. As a result, the upper layers may virtually peal off from the underlying rock mass. In many cases such a change is also accompanied by chemical weathering, especially margins and boundaries of the separated layer developing curved surfaces
This phenomenon pealing off of curved shells from rocks under the influence of thermal effects in association with chemical weathering is often termed as exfoliation.
It is a large-scale phenomenon resembling in some details with spheroidal weathering that results from predominantly chemical weathering on smaller rock blocks( and is described separately)
c) Unloading
This is another process of mechanical Weathering where large scale development of fracturing in confined rock masses is attributed to removal of the overlying rock cover due to prolonged erosional work of other agencies. These rock masses remain confined from sides but due to relief of pressure from above, they expand upwards consequently joints develop in them parallel to the uncovered surface dividing them into sheets. This rupturing or jointing in itself is a mechanical breakdown of rocks and makes them available For further weathering or decay along the joint planes It is believed by many that unloading of deeply buried pluton is often the cause of development of concentric
joints in them. Further mechanical weathering along these joints leads to pealing off of slabs and converting the pluton into an exfoliation dome
Chemical Weathering
It is a process of alteration of rocks of the crust by chemical decomposition brought about by atmospheric gases and moisture. The chemical change in the nature of the rock takes place in the presence of moisture containing many active gases from the atmosphere such as carbon dioxide, nitrogen, hydrogen and oxygen. As we know, rocks are made up of minerals, all of which are not in chemical equilibrium with the atmosphere around them Chemical weathering is essentially a process of chemical reactions between the surfaces of rocks and the atmospheric gases in the direction of establishing a chemical equilibrium. The end product of chemical weathering has a different chemical composition and poorer physical constitution as compared to the parent rock.
Chemical weathering cats up the rocks in a number of ways depending upon their mineralogical composition and the nature of chemical environment surrounding them.
Following are some of the main processes of chemical weathering : solution, hydration and hydrolysis, oxidation and reduction, carbonation, base-exchange and formation of colloids.
(a) Solution
Some rocks contain one or more minerals that are soluble in water to some extent. Rock salt,gypsum and calcite are few common examples. It is also well known that though pure water is not a good solvent of minerals in most cases, but when it (the water) is carbonated, its solvent action for many common minerals is enhanced. Thus limestone is not easily soluble in pure water but carbonated water dissolves the rock effectively.
Limestone gets pitted and porous due to chemical weathering (b) Hydration and Hydrolysis
These two processes indicate the direct attack of atmospheric moisture on the individual minerals of a rock that ultimately affect its structural make up. It is believed that though the Interior Work of many minerals is in electric equilibrium, the surfaces of many crystals are not; they may have partially unsatisfied valences. When the polarised water molecules come in contact with such crystals, it may cause any one of the following two reactions
First. The ions tend to holds the polarized side of the water molecule and form hydrate.
This process of addition the water molecule is termed as hydration. Examples are provided by hydration of iron oxides and calcium sulphate crystals. In some mineral with ferrous ion , the Fe++ ion holds the water molecule and form water -iron complex or a hydroxide. Similarly, CaSO4 anhydrite get slowly converted to gypsum by hydration
CaSO4 + 2H2O− − −→ CaSO42H2O
Second, Ion may be changed whereby some ions from water may enter into the crystal lattice of the mineral. This process of exchange of ions is called hydrolysis. It is very process of weathering or silicate minerals (which are quite abundant in rocks) and is best with reference to weathering of mineral Orthoclase, feldspar.
K+ ALSi3O8+H+− − −→ HAlSi3 O8+ K+
(c) Oxidation and Reduction
Iron is a content of many minerals and rocks. The iron bearing minerals and hence are especially chemical weathering through the process of oxidation and reduction Oxidation. Feru iron (Fe++) of the minerals is oxidized to ferric ion (Fe+++) on exposure to air rich in moisture. Ferric iron is not stable and is further oxidized to a stable ferric hydroxide
(i) 4Fe+ 3O2− − −→ 2Fe2O3
(ii) Fe2O3+ H2O− − −→ Fe2O3.H2O
Similarly, Pyrite (FeS), a natural and common iron mineral present in many rocks in amounts (eg in limestone), may undergo oxidation and hydration in a sequence forming sulphuric acid in the process that may further corrode the carbonate rock (limestone) 2FeS2+7O2+ 2H2O− − −→ FeSO4 +2H2SO4
Reduction. In specific types of environment, such as where soil is rich in decaying vegetation (swarmps), minerals and rocks containing in oxide may undergo a reduction of the oxides to iron. In this case the decaying vegetation supplies the carbon content causing reduction. The effects of oxidation (and to some extent of reduction) weathering are easily observed from the colour changes produced in Iron bearing rocks.
The rocks in which the iron has been oxidized to ferric state show a marked brown colour, especially in oxides, hydroxides and hydrate .But where the oxidation has reached only the ferrous state, the typical colours developed in the rocks are various shades of green, blue and grey
(d) Carbonation
It is the process of weathering of rocks under the combined action of atmospheric carbon dioxide and moisture, which on combination form a mildly reacting carbonic acid. The acid formed exerts an especially corrosive action over a number of silicate bearing rocks. The silicates of potassium, sodium and calcium are particularly vulnerable to decay under conditions of carbonation.
A typical example is that feldspar orthoclase, a very common and important constituent of many igneous, sedimentary and metamorphic rocks, which decomposes according to following reaction
2KalSi3O8+CO2+ 2H2O− − −→ Al2Si2O5 (OH)+K2CO3 +4SiO2
The end products in the above reaction are a clay mineral, a soluble bicarbonate and silica. Further, in the above equation, Na or Ca may be present instead of K if the mineral in question is another type of feldspar. The main end product, Kaolinite, is formed in all such cases. Only the soluble carbonate differs in accordance with the metallic ions of the feldspar type. This chemical change in the rock produces definite alteration in the physical constitution of the rock: a soft (H =1) clay mineral is formed in place of a hard mineral (felspar, H=6), thereby affecting the strength of the rock very significantly. Carbonates are removed in solution and silica forms colloids; this may result in partial or total conversion of a strong igneous rock like granite into a mass of soft clay like product in the zone of weathering. Many igneous rocks like granite, granodiorite, syenite, basalts and porphyries suffer this type of weathering on
a massive scale, as felspars are their chief constituent minerals
(e) Colloid Formation
The processes of hydration, hydrolysis, oxidation and reduction operating on the rocks and minerals under different atmospheric conditions may not always end in the formation of stable end products. Often they result in splitting of particles into smaller particles - the colloids characterized by atoms with only partially satisfied electrical charges. Formation of colloidal particles is especially common in the weathering of clay minerals, silica and iron oxides. The colloids of these minerals are, however,soon precipitated as their charges are satisfied and they form stable products
Spheroidal Weathering
It is a complex type of weathering observed in jointed rocks and characterized with the breaking of original rock mass into spheroidal blocks. Both mechanical and chemical weathering are believed to actively cooperate in causing spheroidal weathering. The original solid rock mass is split into small blocks by development of parallel joints due to thermal effects (insolation). Simultaneously the chemical weathering processes corrode the borders and surfaces of the blocks causing their Spheroidal Weathering shapes roughly into spheroidal contours.
