The domain of geology abounding its widened scope is further divisible into a seemingly great number of subdivisions termed as the branches for the sake and convenience of systematic study. The main and allied branches of geology may be named as follows:
Main (Principal) Branches
(1) General geology
(2) Physical geology (Geomorphology)
(3) Geotectonics
(4) Mineralogy (Crystallography inclusive) (13)
(5) Petrology (Igneous, Sedimentary and (14) Metamorphic)
(6) Structural geology
(7) Historical geology or Stratigraphy
(8) Palaeontology
(9) Economic geology
Allied Branches
(10) Engineering geology (11) Marine geology Geophysics Geochemistry Geo-hydrology / Hydrogeology (15) Environmental geology
The cardinal themes of the main branches are briefed as follows:
General Geology:
It is the formal branch of geology that deals with the broad features / aspects of the earth in particular and the other members of solar family with the Sun as the kingpin and sole controller.
It also deals with certain principal aspects of. The cosmos – the ordered universe. The features of the earth include its origin, age, constitution, internal structure and the depth zones of the marine (oceanic) realms.
It may be mentioned here for clarification that the contents of ‘general geology’ discussed in this chapter are exclusively and expressedly according to the syllabus prescribed for +2 first year (Xlth Class) course in geology under unit-I in part, although the contents dealt with here are included as a forepart of physical geology in many standard text and reference books on geology. Thus, without any bias to the standard text and reading books on geology, the descriptions in this chapter exclusively denote the course-frame prescribed for the +2 First yc’ar syllabuses in geology.
Physical Geology:
It serves as a tool to understand the physical process which moulds the earth surface. The terms, synonymous with this branch, are ‘geomorphology’ and ‘dynamic geology’. This branch deals with (a) the geometry (b) origin and developmental history of landform features of mountains, plateaux, valleys, rivers, lakes, glaciers, deserts, oceans and ground water (c) geological work of the exogenetic (external) geological agents mentioned above, in constantly moulding the features of the earth’s surface and (d) aspects of natural geologic phenomena such as denudation, weathering, erosion, mass wasting, landsides, soil creep, avalanches and soil erosion.
Geotectonics:
This branch deals with the major and very large sized structures of the earth’s lithosphere (the crust and its lower part) and their changes produced by crustal deformation brought about by the interplay of the earth’s endogenetic (internal) forces. It describes and explains the geometry and mode of formation of the mega (very large scale) crustal features and their causative processes.
These features are lofty fold mountains, Block Mountains, rift valleys, mid-oceanic ridges, geosynclines, and island arcs etc. It also deals with the global concepts of isostasy, eustasy (sea level changes), continental drift, convection cells, seismicity, ocean-floor-spreading and the plate-tectonics. In a simple sense, this sub-discipline deals with the movements of various crustal parts and the formation of resultant large scale crustal and infracrustal features.
Mineralogy:
It. is a formal branch that deals with the atomic structure, physico-chemical and optical properties of the minerals present in the earth’s lithosphere and the crust in particular. Crystallography is an important sub-branch which describes the internal atomic structure in a three dimensional perspective and the external geometric forms of the crystalline minerals. The mode of occurrence, genesis and uses of minerals are also studied in this branch.
Petrology:
It is the branch that studies the mode of occurrence, textures, structures, mineralogical and chemical compositions, classification and genesis of the rocks of igneous, sedimentary and metamorphic classes, present in the crust and below it. Each major rock class is further divisible into sub-branches such as plutonic, hypabyssal and volcanic types in case of igneous rocks; residual, mechanical, chemical and organic types in case of sedimentary rocks and into contact (thermal), dynamo-thermal, plutonic and cataclastic types of metamorphic rocks. All the three major classes of igneous, sedimentary and metamorphic rocks are, thus, further divisible into relatively smaller sub-types.