The Indian rocks on the basis of their evolution, mode of deposition and physical characteristics are classified into various categories. The broader category is called ‘group’ their sub-division as ‘system’ and further sub-division as ‘series’.
The Peninsular India is a geological museum of various groups of rocks belonging to different periods of earth’s geological history. It begins with the Achaean group of oldest rocks and terminates with the deposition of recent sediments of Aryan group (recent delta deposits, recent raised beaches, coral coasts, etc.). This process of rock formation is still continuing in the peripheral regions of the Peninsula. Shows major rock-groups and rock-systems in India.
Achaean Group
Achaean group of rocks consists of two systems-(a) Achaean granites and gneisses, and (b) Dharwarian sedimentary. The geological history of India begins with this group of most ancient rocks. The term ‘archaic’ literally means the most ancient or belonging to the earth’s earliest geological period (R.C. Mehdiratta, 1962, p. 48). It refers to all the rock systems lying beneath the first and the oldest Eparchaean Unconformity. These rocks from foundations of ancient plateaus and cores of mountains and are found exposed in the most ancient segments of the earth like Peninsular India, Siberia, Canada, S. Africa and Brazil etc. That is why these are called as the ‘Basement complex’, the ‘Fundamental Complex’ or the ‘Achaean Complex.’
ADVERTISEMENTS:
Achaean rocks have been formed in more than one ways: (i) as first formed crust of the earth due to cooling and solidification of the molten material, (ii) as first-formed sediments accumulated in the primitive oceans and later on subjected to an extreme degree of thermal and regional metamor- phism, (iii) as intruded magma and the extruded lava masses poured out of the earth’s interior and were subsequently metamorphosed into gneisses and schist, etc. (R.C. Mehdiratta, 1962, p. 48).
Lithology
The composition of the Achaean rocks is very complex. These rocks are so much metamorphosed that these have almost lost their original characteristics. They are all azoic, thoroughly crystalline, extremely contorted, faulted and foliated and Tourmaline, magnetite, ilmenite etc. It is mainly found in West Bengal, Jharkhand (Chotanagpur, Ranchi and Hazaribagh districts), Orissa and Karnataka. It also occurs in the Sun Valley and Assam.
The Bundelkhand Gneiss-it is coarse-grained gneiss which looks like a granite. It differs from the Bengal gneiss due to the absence of accessory minerals and rocks such as dolomites, marbles and quartzites. It is found in the Bundelkhand region, and states like Maharashtra, Rajasthan, Andhra Pradesh and Tamil Nadu.
ADVERTISEMENTS:
Charnockite.-the name charnockite is given after Job Charnock, the founder of Calcutta, whose tombstone is made of this rock. It is also called the Nilgiri gneiss. It is a bluish-grey to dark-coloured rock, medium to coarse-grained in texture composed of hy persthene, enstatite, blue coloured quartz, microline, plagioelase, hornblende, augite, biotite with some accessories like zircon, magnetite, ilmenite, graphite and garnet. It varies in composition from acid plutonic to basic plutonic or even ultra-basic. It is widely found in Tamil Nadu (South Arcot, Palni hills, Nilgiri, Shevaroys), Andhra Pradesh (Nellore),
Karnataka, Kerala (Coorg, Malabar), Orissa (Balasore), Bihar, Jharkhand, Madhya Pradesh and Rajasthan.
The Himalayan Archaeans-the central ranges of the Himalayas are mainly composed of granites, gneisses, schists, phyllites, etc. Although most of these crystalline rocks belong to the Archaean system, it is very likely that some of these are of much younger age later intruded into the older rocks. According to recent findings ‘ortho gneiss and granite no doubt build the substratum of these mountains, but the peaks rising above them often art of stratified, even of fossiliferous, sediments’ (D.N. Wadia, 1975, p. 86).
Life in the Achaean Rocks
ADVERTISEMENTS:
The Achaean rocks are all azoic or unfossiliferous. It is possible that these were formed at a time when no life existed on the earth or sale would have disappeared due to intense metamorphism. But the presence of calcareous rocks such as marble and carbonaceous material such as graphite has been considered by some scholars to mean that some sort of organic matter did exist during the Achaean period.
Economics
The Achaean rocks are the repositories of the mineral wealth of India. The metalliferous ore deposits, non-metallic minerals of economic utility, rare minerals and gem stones and building materials are all found in these rocks. These rocks are very rich in iron-ores, copper ores, manganese ores, mica etc.