CHAPTER 1 THE SCOPE OF GEOLOGY 1. The study of earth materials goes back to the first use of metals, but the interpretative study of the minerals and an understanding of the true origin of rocks dates effectively from the beginning of the nineteenth century. The study of geology has subsequently become enlarged by the growth of numerous subdisciplines, some like geophysics being now as well known as geology itself. The term earth science (or geoscience) has been coined to embrace once again the whole field. 2. The various sub-disciplines which are defined below are concerned with: a. Basic description of earth minerals. b. The application of other sciences to earth materials. c. Methods of obtaining information for historical synthesis of earth evolution. d. Practical applications to human demands and problems. 3. Definitions: (1) Mineralogy is essentially descriptive of the naturally occurring com pounds and chemical elements that in various combinations constitute the rocks of the earth; the investigation and classification of these minerals is mainly chemical and crystallographic. (2) Petrology is first descriptive (petrography) of rocks that are naturally occurring associations of minerals. Secondly from the nature and distribution of rocks, the sequence of their mode of formation is inferred (pedogenesis). (3) Geochemistry is the application of chemical methods and theory to explaining the pattern of distribution of minerals and rocks, and it makes a major contribution to the prediction of occurrence. (4) Geomorphology is descriptive of the present exposed surfaces of the rocks of the crust of the earth, and seeks to interpret these surfaces in terms of natural processes (chiefly erosion) which lead or have led to their formation. The term 'Physical Geology' has also been used in this sense. (5) Pedology (Soil Science) is concerned with those parts of the present earth surface which have become weathered or otherwise modified 1 Downloaded by [ Griffith University] on [25/10/17]. Copyright © ICE Publishing, all rights reserved. Chapter 1 in situ by solar energy and by the effects of organisms to form a soil which is of primary importance to man in agriculture. (6) Sedimentology comprises the description and interpretation of the products of erosion (deposits) and of large assemblages of organisms which result in accumulations of their mineral skeletons. The study is extended to include processes of sedimentation, the interpretation of past deposits of this kind and of the natural changes they have sub sequently undergone (diagenesis). (7) Stratigraphy (Historical Geology) includes first the description of rocks locally in their observed sequence of formation. Secondly a com posite reference scale of layered rocks is selected to represent a con tinuous period of time. Thirdly the synthetic activity is the time-correlation of any other set of rocks with the reference scale of rocks by comparison of evolutionary (or occasionally unique) events inferred from observations on the two sets of rocks. (8) Paleobiology (Palaeontology) is concerned with recording details of all traces of past plant and animal life. Usually an attempt is made to interpret the mode of life of the organisms (mostly extinct); this is however only a step towards establishing the evolutionary history, for its use in stratigraphy, of the group of organisms concerned. P a l e o biology is currently the principal evolutionary basis of stratigraphic correlation. (9) Tectonic geology is the study of the gross arrangement of major rock bodies in the crust of the earth, and the elucidation of the origin and development of the vertical and horizontal movements that have led to this arrangement. (10) Structural geology is the study of the effects of forces causing flow folding and fracture structures to develop in previously consolidated rocks. Interpretation from the observed fractures and folds and their patterns, of the forces and of their directions and constraints, is the main basis of tectonic reconstructions. (11) Metamorphic geology is the study of the effect of natural heat and/or pressure on the mineral content of previously formed rocks in terms of the chemical and physical stability of individual minerals under these conditions, but short of complete melting. (12) Vulcanology is the study of high-temperature melted rock phenomena resulting from internal earth processes, either confined beneath the observable crustal surface, or sometimes unconfined at the surface in the form of volcanoes. (13) Geophysics is the study of all the gross physical properties of the earth and its parts, and is particularly associated with the detection of the nature and shape of unseen subsurface rock bodies by measurement of such properties and property contrasts. Small scale applied geo physics is now a major aid in geological reconnaissance. 2 Downloaded by [ Griffith University] on [25/10/17]. Copyright © ICE Publishing, all rights reserved. Chapter 1 ( 1 4 ) Seismology is a branch of geophysics concerned with recording of earthquakes and the interpretation of these records. Increasingly impor tant is the art of prediction of earthquakes with the eventual possibility of ameliorating their surface effects. ( 1 5 ) Hydrogeology is the study of the natural (and artificial) distribution of water in rocks, and its relationship to those rocks. In as much as the atmosphere is a continuation of the hydrosphere, and is in physical and chemical balance with it, there is a close connection with meteorology. (16) Glacial geology is the study of the direct effects of the formation and flow under gravity of large ice masses on the earth's surface. Glaciology is concerned with the physics of ice masses. ( 1 7 ) Impact geology is the study of the effects of collisions of extra terrestrial masses with the earth's surface, and has derived great impetus from study of similar features of the moon. Use of terms 'rock* and 'soil' 4 . Unfortunately engineers and geologists have developed distinct uses of these terms which cannot be reconciled and which will cause confusion in the present context unless each use is prefixed. TABLE 1. R E C O M M E N D E D U S E S O F T H E T E R M S 'SOIL' AND 'ROCK' Current geological usage Recommended geological usage Example of a typical section Recommended engineering usage Current engineering usage (RefstoCP2001: 1957 appx) W (« (c) <<0 (*) Soil (pedology definition) Soil profile Temperate weathered profile •Drift' deposits Uncon solidated deposits Glacial till (Pleistocene) Semiconsolidated rock with concretions Oxford Clay with concretions (Jurassic) Consolidated rock Coal Measure shales (Carboni ferous) (Solid) Rock Topsoil (humic) Topsoil (C170) (humic) Engineering soil Soil (C161) (with boulders) Bedrock Rock (CI 55) (Bedrock) NOTE: It is recommended that 'soil' and 'rock* should not be used in writing without qualification. 3 Downloaded by [ Griffith University] on [25/10/17]. Copyright © ICE Publishing, all rights reserved. Chapter 1 As indicated in Table 1, geologists have accepted the normal pedological definition of 'soil' meaning the whole weathered profile down to unweathered rock; they have also classified as 'rocks' all ancient deposits, whether consolidated or not (because consolidation is an accident of local crustal circumstances). Engineers on the other hand have used 'topsoil' for the upper humic part of the pedological soil profile, and 'soil' for everything underneath that is movable by normal earth moving equipment; 'rock' or 'bedrock' was sufficiently consolidated to require blasting or comparable effort to break it up. The usage of these terms is as described in the relevant paragraphs of the appendix to CP 2001: 1957: C155. Rock. In the engineering sense, hard and rigid deposits forming part of the earth's crust, such as sandstones, limestones, metamorphic formations and igneous masses, as opposed to deposits classed as soil. Geologists define rock as any naturally occurring deposits be they hard or soft, but excluding topsoil. CI 61. Soil. In the engineering sense, any naturally occurring loose or soft deposits forming part of the earth's crust, particularly where they occur close enough to the surface of the ground to be encountered in engineering works, but excluding topsoil. The term covers such deposits as gravel, sands, silts, clays and peats. It should not be confused with the agricultural or pedological soil which embraces only the topsoil and subsoil as here defined. Pedological soil may come within the meaning of the word soil in the engineering sense when it is excessively deep as in some tropical and con tinental regions, but in the British Isles the two conceptions of the word can be kept distinct. CI 65. Subsoil. The weathered portion of the earth's crust that lies between the topsoil as here defined and the unweathered material below. The term is sometimes used to refer to the soil in the engineering sense (e.g. subsoil drainage) but its use in this sense is deprecated. CI 70. Topsoil. The superficial skin of the deposits forming the earth's crust that has, by processes of weathering and the action of organic and other agencies, been transformed into material capable of supporting plant growth. The term thus embraces the upper or humus-bearing horizons of the soil of pedology. The differences between the two well established procedures are clear. In the long term it will be essential to avoid these difficulties of confusion by developing modifications of both words which add more precision to any use, and to persuade all users to subscribe to this practice, particularly in any written statement or record. This persuasion will take time and will only be accepted if it is simple; the uses shown in the centre of Table 1 are therefore suggested and will be used where appropriate in this book. 5. Engineer, geologist and prediction. Geotechnical subjects with particular relation to engineering are discussed in the later chapters of this book; in the early chapters however, an attempt is made to isolate those elements of the whole of 4 Downloaded by [ Griffith University] on [25/10/17]. Copyright © ICE Publishing, all rights reserved. Chapter 1 geology which are necessary to a general understanding of 90 per cent of the problems in this field likely to be encountered by an engineer. This applies particularly to the direct finding or tracing of either rocks or underground water. The geologist is trained to predict conditions by means of a very wide range of observations, by understanding of processes, and by integration of results. It cannot be too strongly stressed that field observations which have no obvious direct relevance to engineering, such as the collection of fossils, may be crucial to a geologist in making an effective assessment of a problem; the geologist will normally need all the assistance he can get in recording such observations. REFERENCE LIST—CHAPTER 1 CP 2001: 1957 Site Investigations. Code of Practice 2001, British Standards Institution, London (under revision). 5 Downloaded by [ Griffith University] on [25/10/17]. Copyright © ICE Publishing, all rights reserved.