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In this article we will discuss about:- 1. Introduction to Rocks 2. Geological Work of Atmosphere (Rock Weathering) 3. Structural Features 4. Unconformity 5. Joint Structures.
Introduction to Rocks:
Rocks exposed at the surface of the Earth are subject to direct or indirect attacks of a number of natural agencies such as atmospheric gases, heat, moisture, surface and subsurface water, wind, sea-water and ice. These agencies are ceaselessly operating, at places individually and at places in close cooperation with one another on the surface rocks, season after season and year after year.
They are thus responsible for modifying the physical features existing on the surface. For instance, we take streams. They carry away every year millions of tons of sediments to seas. Wherefrom do they (the streams) get the enormous load? Surely, from the slopes of their valleys and from the rocks at the base and sides of their channels.
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In other words, every year millions of tons of material making the mountains, plateaus, plains and valleys are washed away. As a result, the action of rivers flowing through them is modifying the existing river valleys and other features of drainage basins every year.
Similarly, huge moving bodies of ice called glaciers, covering the high mountains—the Himalayas, the Alps, the Andes and the polar-regions, scour the rocks at their bases and sides. These also succeed in scouring millions of tons of rocks and carry the same as glacial debris that they scatter on the down slope regions.
This is geological work of glaciers in nutshell. Air in the form of WIND often takes shape of storms carrying hundred and thousands of tons of loose dry material (the dust)-made up of clay, silt and sand, from one place to another. Very vast and extensive sand made regions, the deserts, are very important example of work of wind.
The work of natural agencies may be either destructive or constructive in nature in relation to the existing landform of an area at any given point of time. Let us take the example of rivers to explain this aspect. The rivers carve out valleys by eroding rocks, bit by bit, from the mountains; these valleys are constantly enlarged and deepened by them in association with other natural agencies.
This is an example of destructive work of rivers. The rivers deposit their load at appropriate places on the land, along sea-shore and even at the sea bottom, building up such features as alluvial plains, flood plains and deltas etc. This is a constructive aspect of the rivers.
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The magnitude of the geological work of these agencies, when considered on collective, cumulative and constantly occurring basis is of astounding order. Almost all the physical features existing on the Earth at present—the mountains, plateaus, plains, valleys and basins have their typical shapes due to the modifying or creative work of these agencies through geological ages. And the process is ever continuing.
An attempt has been made in the following pages to give an outline of the principles, the processes and methods by which the natural agencies operate on the land and along seashore and oceans.
Geological Work of Atmosphere (Rock Weathering):
Atmosphere, which is a gaseous envelope surrounding the planet Earth is in constant contact with the rocks of the crust wherever these are exposed on the surface. This contact is of a dynamic interactive nature in which the original structure and even composition of the rocks are changed to a great extent.
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The ruling trend in all such actions 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 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:
1. Mechanical (physical) weathering and
2. Chemical weathering.
Structural Features of a Rock:
Those structural features that are developed in the body of a rock during its formation stage are termed as Primary Structures. Stratification and lamination are the most common primary structures of sedimentary rocks; crystalline structure is typical of igneous rocks and foliation is a typical primary structure of metamorphic rocks.
All the modifications of the original (primary) structures and development of new forms, shapes and rearrangement of the component grains, crystals or mineral constituents that are induced in the rocks after their formation are grouped as Secondary Structures. Folding, faulting and jointing are some very commonly known secondary structures developed in all types of rocks.
The forces most commonly responsible for the development of secondary structures are tectonic in nature, i.e. they arise from within the earth due to certain complex type of disturbances yet incompletely understood. In some cases non-tectonic causes, that is, those operating from outside the body of the earth may also cause some changes in the body of rocks.
Unconformity in Rocks:
An unconformity is defined as a surface of erosion or non-deposition occurring within a sequence of rocks. It indicates a gap or an interval of time or a hiatus in the geological history of the area during which the normal process of deposition was interrupted. It is a structural feature in the sense that rock formations lying above and below it generally represent different conditions under which they have been formed.
Origin:
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An unconformity is developed due to a change in the process of deposition of sedimentary rocks. If the process proceeds uninterruptedly for considerable time, layer after layer will be deposited one above another. This will normally result in a sequence of rocks in which the oldest beds are those that occupy a position at the base of the sequence and every upper layer will be younger in geological age in ascending order, the youngest being at the top. Such a sequence, uninterrupted in its deposition is known as conformable.
Suppose that in another case, the process of deposition is interrupted at a certain stage and the rocks already deposited are uplifted and exposed to the conditions of erosion whereby a few of the top layers are eroded. This is followed again by a phase of subsidence and deposition during which still new beds are deposited over the eroded surface. The sequence of rocks so formed is not conformable.
It has a break or hiatus in it in the form of a surface, which demarcates a gap in succession indicating layers that were eroded from in between before the deposition of new layers above the surface. The lower layers might have even suffered tectonic deformations as evidenced by their folding and faulting. The surface indicating a hiatus in a normal order of succession is the surface of unconformity.
Types:
(1) Angular Unconformity:
It is characterized by different inclinations and structural features above and below the surface of unconformity. The sequence below the unconformity may be steeply inclined, folded and faulted. This represents the older formations.
The sequence above the surface of unconformity represents the younger formations and may be either horizontal or gently inclined. Since the angular deposition is different in the two sequences at the particular surface, the latter is commonly referred as angular unconformity.
(2) Disconformity:
It is that type of unconformity in which the beds lying below and above the surface of erosion (or non-deposition) are almost parallel. In other words, there is no angular variation in the deposition of the rocks of the entire sequence. In these cases, there is generally no folding or faulting or tilting of the strata.
Such an unconformity becomes evident only after thorough investigations involving drilling through the strata, recovery of cores and other representative samples from the deeper layers from which the evidence of erosion or non-deposition can be deciphered and comparative study of the strata exposed in the nearby areas.
(3) Nonconformity:
It is the term used for unconformity in a sequence of rocks composed of plutonic igneous rocks (like granites) as older or underlying rocks and sedimentary or volcanic rocks as the overlying younger or newer rocks. The non-conformity in such cases is the surface of contact between the rocks having different mode of formation.
(4) Local Unconformity:
When an unconformity is traceable only in a small area or in a few rock formations of a given area, it is termed as local unconformity. Evidently, it is due to changes in the conditions of sedimentation in a very limited area of a major basin.
(5) Regional Unconformity:
When an unconformity is traceable over a large area, extending for hundreds of kilometers, it is conveniently called a regional unconformity. It is generally of angular unconformity type and is of great significance in historical geology as it establishes the genetic relationship of rocks of a very wide area.
Three regional unconformities are recognized in Indian geology:
(a) The Eparchean Unconformity
(b) The Post-Vindhyan Unconformity and
(c) The Palaeozoic Unconformity
Of these, the first separates Archean Group and the Purana in the Peninsular India. The post – Vindhyan unconformity is traceable in both Peninsular and extra peninsular India. The Palaeozoic unconformity separates the rocks of Dravidian group from that of Aryan group and is traceable in extra Peninsular India.
Some other less common types of unconformities are:
(a) Blended Unconformity:
It is a type of unconformity in which there is a thick layer of residual soil present between the two layers which mark the sharp unconformable contact between the sequences.
(b) Breakup Unconformity:
This type is generally observed in basins created by the faulting. These basins often get filled up by salt evaporites rising as pillars and domes in the basins, which are then covered up by layers of younger sediments. The break in the sequence is often referred as breakup unconformity.
(c) Buttress Unconformity:
It is similar to breakup unconformity in that it is related to fault generated basins and scarps. But in this case the material that fills up the basins fast is the elastic material derived from surrounding rocks thereby creating an unconformable contact. The Interarc basins are the ideal places for such unconformities.
Detection of Unconformity:
Presence of unconformity in a sequence of rocks may be established by one or more of the following features that may be observed during fieldwork.
(1) Angular Relations:
When the layers of a sequence on either side of a contact are not parallel, an unconformity is indicated. When pronounced tilting and/or folding or faulting is easily observed in the rocks on one side of the contact, the presence of the angular unconformity is confirmed.
(2) Basal Conglomerate:
New deposition in basins is generally indicated by the presence of a layer of conglomerates made up of rounded, semi-rounded pebbles and gravels. These are indicative of shallow water conditions. As such, when during the field study of a sequence of rocks, a layer of conglomerates of any thickness is observed, it indicates an unconformity at the base of the layer of overlying rock.
(3) Residual Soil:
Presence of a layer of residual soil (which is always a product of weathering) within a sequence of rocks is also indicative of erosion having taken place during the deposition period of that particular sequence. So position of soil layer within a sequence of rocks is a likely location of the unconformity.
(4) Other Evidence:
The contrasting behaviour of rocks on either side of a particular contact surface with respect to induration, type and degree of metamorphism, intensity of folding and the fossil content are some other features that help in confirming the contact surface as unconformity.
Engineering Considerations:
Unconformities of all types indicate a break or discontinuity in the sequence of the rocks. As such, their presence, if overlooked, can be a source of considerable error in qualitative judgement about the site for a particular project.
The behaviour of rocks above and below the unconformity will necessarily show a variation in their mechanical properties and hence affect the stability of the project. Unconformity marks a weak contact, which will allow percolation of water and may behave as bedding plane or fault plane towards forces imposed from above.
Joint Structures in Rocks:
Almost all classes of rocks—igneous, sedimentary and metamorphic, invariably show joint structures to lesser or greater extent. We may find quite a large proportion of outcrop of any of these rocks practically free from joints at some places, but at other places the same type of rock may be heavily jointed, showing cracks of a greater variety. Hence it is not only the genesis of the rocks which is responsible for these structures but also the forces to which these rocks have been subjected to after their formation.
Joints are of great practical importance for all those dealing with the rocks as sites, materials of construction, in prospecting for minerals, groundwater and oil and gas reservoirs. Hence elementary knowledge about their geometry, classification and style of occurrence in different rocks is important.
Joints are defined as divisional planes or fractures along which there has been no relative displacement. These fractures divide the rocks into parts or blocks and unlike the faults, the parts have not suffered any movement along the fracture plane. There may be or may not be an opening up of blocks perpendicular to the joint planes.
Nature:
Joints may be open or closed in nature.
Open joints are those in which the blocks have been separated or opened up for small distances in a direction at right angles to the fracture surface. [Fig. 7.26(a) JT-1] These may be gradually enlarged by weathering processes and develop into fissure in the rocks.
In closed joints, there is no such separation. Even then, these joints may be capable of allowing fluids (gases and water) to pass through the rock [Fig. 7.26(b) JT-3].
Similarly, the joints may be smooth or rough on the surface and the surface may be straight or curved in outline [Fig.7.26(b) JT-3, JT-4].
The joints may be small in their extension, confined to only a part of a layer or mass of rock, or they may be quite prominent and extending for considerable depth and thickness. The former are called discontinuous joints whereas the extensive joints are referred as continuous joints. The more prominent continuous joints are often called the master joints. [Fig. 7.26(c) JT-6], Almost all the joints are discontinuous in the strict sense because these disappear with depth in the crust of the earth.
In many cases, open joints get filled up by solutions of secondary materials which crystallise or precipitate there forming thin or thick streaks or bands of the filling material. These are simply called veins when thin or in strands and fissure veins when their thickness is greater than 20 cm.
Attitude:
Joints are fracture planes or surfaces and their occurrence often takes place in such a way that their position in space or attitude (dip and strike) may be described conveniently either independently or with respect to the attitude of the rocks in which they occur. In other words, joints have dip and strike, the former being their inclination with the horizontal and the latter being the direction of intersection of a joint plane with a horizontal plane [Fig. 7.27(a)].
Grouping:
Joints generally occur in groups of two or more joint planes.
A Joint set is a group of two or more joint surfaces trending in the same direction with almost the same dip.
A joint system is a group of two or more joint sets.