Peninsular Gneiss
Encyclopedia
Peninsular Gneiss is a term coined to highlight the older gneissic complex of the abundant rock type found all over the Indian Peninsula. This term was first fashioned by W.F.Smeeth of the Mysore Geological Department in 1916 based on the first scientific study of this rare rock exposure. The Geological Survey of India
identified one of the best exposures of this rock mass, protruding high above the ground as a hillock (pictured), dated 2.5 to 3.4 billion years, in the Bangalore
city in India
at the famous Lalbagh gardens and declared it as a National Geological Monument to propagate the knowledge of the rock formation among the public, since Lalbagh is frequented by visitors from all over the world.
It is also called the Lalbagh rock.
As general information, gneiss
is a coarse-grained high grade metamorphic rock
formed at high pressures and temperatures in which light and dark mineral constituents are segregated into visible bands.
The Archean
gneiss
es and schists, which are the oldest rocks of the Indian Shield
, constitute a considerable area of Peninsular India. The Dharwar (Super Group) and the Peninsular Gneissic Complex are the classified groups of the Precambrian
rocks of India. The extent of the Archean system is depicted as the dominant system of South India
in the pictured Geological Map of India.
In most stratigraphic schemes, the peninsular gneisses are shown as the younger Archaean strata situated above the Sargur
Group. Granitization of the older sedimentary–volcogenic sequence are reported as sources of the peninsular gneisses made up of polyphase migmatite
s, gneisses and granites ranging in composition from granodiorite to tonalite.
The rocks of the Dharwar Group, which are mainly sedimentary in origin, occur in narrow elongated synclines resting on the gneisses found in Bellary district, Mysore and the Aravallis of Rajputana
.
The supracrustal rocks of the Dharwar Group of the southern Indian Peninsula, as depicted in the geological map of South India
, have the Peninsular Gneiss as the basement rock formation, also stated to be the remobilized basement. Migmatization of pre-existing metasedimentary and meta–igneous rocks are considered the contributors to formation of the composite gneiss
.
The Lalbagh hill, which has been declared as a Geological Monument (plaque pictured) to represent the Peninsular Gneiss, has dark biolite gneiss
of granitic to granodioritic composition containing streaks of biolite. Remnants of older rocks are seen in the form of enclaves. The research information of the geological record of the gneisses, collated and reported in the publication “Geological Monuments of India” published by the Geological Survey of India
, is quoted below to provide an undiluted version.
and 2.5 Ga –2.6 Ga for development of the Peninsular Gneisses of Bangalore. Recent studies carried out with precision techniques indicate that the gneisses have accreted in the following major episodes.
Thus, three major episodes, namely, 3.4 Ga., 3.3–3.2 Ga., and 3.0–2.9 Ga
., are accreted to form the Peninsular Gneiss of the region, dated 2500 to 3400 million years.
The Sargur
schist
belts within the Peninsular Gneiss could be the oldest suture zones in the Indian subcontinent
.
Lalbagh established by Hyder Ali
based on Mughal Gardens
at Sira
is not only a famous botanical garden
but has also a historical link. The creation of the Bangalore city in the 16th century is attributed to Kempegowda, the then feudal leader under the feudatory of the Vijayanagara empire
, who established four cardinal towers setting limits to the growth of the city. One of the towers at the southern end of the city is the Lalbagh tower erected over the Lalbagh hill which is made up of Peninsular Gneiss, now identified as a National Geological Monument. The city has outgrown the limits set by Kempegowda many times and is now part of the downtown area.
The topographic setting of the Lalbagh rock, from a layman’s interest, is that in the western and northern directions it slopes steeply while it merges gently with soil in the east and south. View from the North is impressive since the rock has retained its original form. The west or north–west view is panoramic and aesthetic, with grayish–white clean plates like appearance (see picture). The rock shines white after the rains and attracts attention of the visitors who excitedly climb the rock mass to the tower at the top to get a panoramic view of the gardens and the city.
) within the Lalbagh gardens is easily approachable by road (Metro Map of the city in the Info box gives exact location). Bangalore is well connected by road, rail and air with the rest of the country and is also well known internationally.
Geological Survey of India
Geological Survey of India , established in 1851 is a government organization in India which is an attached office to the Ministry of Mines of Union Government of India for conducting geological surveys and studies. It is one of the oldest of such organizations in the world and the second oldest...
identified one of the best exposures of this rock mass, protruding high above the ground as a hillock (pictured), dated 2.5 to 3.4 billion years, in the Bangalore
Bangalore
Bengaluru , formerly called Bengaluru is the capital of the Indian state of Karnataka. Bangalore is nicknamed the Garden City and was once called a pensioner's paradise. Located on the Deccan Plateau in the south-eastern part of Karnataka, Bangalore is India's third most populous city and...
city in India
India
India , officially the Republic of India , is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country with over 1.2 billion people, and the most populous democracy in the world...
at the famous Lalbagh gardens and declared it as a National Geological Monument to propagate the knowledge of the rock formation among the public, since Lalbagh is frequented by visitors from all over the world.
It is also called the Lalbagh rock.
As general information, gneiss
Gneiss
Gneiss is a common and widely distributed type of rock formed by high-grade regional metamorphic processes from pre-existing formations that were originally either igneous or sedimentary rocks.-Etymology:...
is a coarse-grained high grade metamorphic rock
Metamorphic rock
Metamorphic rock is the transformation of an existing rock type, the protolith, in a process called metamorphism, which means "change in form". The protolith is subjected to heat and pressure causing profound physical and/or chemical change...
formed at high pressures and temperatures in which light and dark mineral constituents are segregated into visible bands.
Geology
Archean
The Archean , also spelled Archeozoic or Archæozoic) is a geologic eon before the Paleoproterozoic Era of the Proterozoic Eon, before 2.5 Ga ago. Instead of being based on stratigraphy, this date is defined chronometrically...
gneiss
Gneiss
Gneiss is a common and widely distributed type of rock formed by high-grade regional metamorphic processes from pre-existing formations that were originally either igneous or sedimentary rocks.-Etymology:...
es and schists, which are the oldest rocks of the Indian Shield
Indian Shield
The Indian Shield is part of the Indian Craton and occupies two-thirds of the southern Indian peninsula.The shield has remained relatively stable since its formation, over 3500 million years ago. It is delimited on the west, south and east by the present day coastline and Phanerozoic sediment...
, constitute a considerable area of Peninsular India. The Dharwar (Super Group) and the Peninsular Gneissic Complex are the classified groups of the Precambrian
Precambrian
The Precambrian is the name which describes the large span of time in Earth's history before the current Phanerozoic Eon, and is a Supereon divided into several eons of the geologic time scale...
rocks of India. The extent of the Archean system is depicted as the dominant system of South India
South India
South India is the area encompassing India's states of Andhra Pradesh, Karnataka, Kerala and Tamil Nadu as well as the union territories of Lakshadweep and Pondicherry, occupying 19.31% of India's area...
in the pictured Geological Map of India.
In most stratigraphic schemes, the peninsular gneisses are shown as the younger Archaean strata situated above the Sargur
Sargur
Sargur , a small town located about 80 km from the town of Chamarajanagar in the H D Kote Taluk of Mysore District of Karnataka, India. H D Kote is about 12 km north of Sargur. It is 55 km from the city of Mysore. Bangalore International Airport is 200 km away...
Group. Granitization of the older sedimentary–volcogenic sequence are reported as sources of the peninsular gneisses made up of polyphase migmatite
Migmatite
Migmatite is a rock at the frontier between igneous and metamorphic rocks. They can also be known as diatexite.Migmatites form under extreme temperature conditions during prograde metamorphism, where partial melting occurs in pre-existing rocks. Migmatites are not crystallized from a totally...
s, gneisses and granites ranging in composition from granodiorite to tonalite.
The rocks of the Dharwar Group, which are mainly sedimentary in origin, occur in narrow elongated synclines resting on the gneisses found in Bellary district, Mysore and the Aravallis of Rajputana
Rajputana
Rājputāna was the pre-1949 name of the present-day Indian state of Rājasthān, the largest state of the Republic of India in terms of area. George Thomas was the first in 1800 A.D., to term this region as Rajputana...
.
The supracrustal rocks of the Dharwar Group of the southern Indian Peninsula, as depicted in the geological map of South India
South India
South India is the area encompassing India's states of Andhra Pradesh, Karnataka, Kerala and Tamil Nadu as well as the union territories of Lakshadweep and Pondicherry, occupying 19.31% of India's area...
, have the Peninsular Gneiss as the basement rock formation, also stated to be the remobilized basement. Migmatization of pre-existing metasedimentary and meta–igneous rocks are considered the contributors to formation of the composite gneiss
Gneiss
Gneiss is a common and widely distributed type of rock formed by high-grade regional metamorphic processes from pre-existing formations that were originally either igneous or sedimentary rocks.-Etymology:...
.
Geology of the Lalbagh hill monument
Gneiss
Gneiss is a common and widely distributed type of rock formed by high-grade regional metamorphic processes from pre-existing formations that were originally either igneous or sedimentary rocks.-Etymology:...
of granitic to granodioritic composition containing streaks of biolite. Remnants of older rocks are seen in the form of enclaves. The research information of the geological record of the gneisses, collated and reported in the publication “Geological Monuments of India” published by the Geological Survey of India
Geological Survey of India
Geological Survey of India , established in 1851 is a government organization in India which is an attached office to the Ministry of Mines of Union Government of India for conducting geological surveys and studies. It is one of the oldest of such organizations in the world and the second oldest...
, is quoted below to provide an undiluted version.
.
A common enclave is a dark grey to black rock called amphiboliteAmphiboliteAmphibolite is the name given to a rock consisting mainly of hornblende amphibole, the use of the term being restricted, however, to metamorphic rocks. The modern terminology for a holocrystalline plutonic igneous rocks composed primarily of hornblende amphibole is a hornblendite, which are...
containing the minerals plagioclasePlagioclasePlagioclase is an important series of tectosilicate minerals within the feldspar family. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a solid solution series, more properly known as the plagioclase feldspar series...
feldsparFeldsparFeldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....
and hornblendeHornblendeHornblende is a complex inosilicate series of minerals .It is not a recognized mineral in its own right, but the name is used as a general or field term, to refer to a dark amphibole....
. Due to interactions with graniteGraniteGranite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite usually has a medium- to coarse-grained texture. Occasionally some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic...
fluids, these enclaves have developed a border rich in biotiteBiotiteBiotite is a common phyllosilicate mineral within the mica group, with the approximate chemical formula . More generally, it refers to the dark mica series, primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more aluminous endmembers...
micaMicaThe mica group of sheet silicate minerals includes several closely related materials having highly perfect basal cleavage. All are monoclinic, with a tendency towards pseudohexagonal crystals, and are similar in chemical composition...
. Later magmatic activity represented by grey poryphiritic graniteGraniteGranite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite usually has a medium- to coarse-grained texture. Occasionally some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic...
followed by pink massive granite is also seen at several places. The gneisses and granites have been profusely invaded by still younger veins of coarse grained pegmatitePegmatiteA pegmatite is a very crystalline, intrusive igneous rock composed of interlocking crystals usually larger than 2.5 cm in size; such rocks are referred to as pegmatitic....
which have cut up the gneisses into several irregular and lenticular shapes.
The earliest rock was a dark coloured amphibolite which was converted into grey biotite gneiss during migmatization. The grey biotite gneiss during migmatization was first intruded by grey porphyritic granite and later by pink granites. Pegamatities of several generations have traversed all these rocks.
Geological age
Initial studies on the Gneiss samples of the Lalbagh hill and other locations in Bangalore, carried out in the early 1970s, have attributed two major events of 2.9 –3.0 GaGya
In astronomy, geology, and paleontology, Gya or Ga is a year multiplied by one of the SI prefix multipliers. It is often used as a unit of time to denote length of time before the present...
and 2.5 Ga –2.6 Ga for development of the Peninsular Gneisses of Bangalore. Recent studies carried out with precision techniques indicate that the gneisses have accreted in the following major episodes.
| Geological/Radiometric ages | Study Region/Belt |
|---|---|
| 3.4 Ga. | The oldest Peninsular Gneiss found in areas in the Hosur Hosur Hosur is a town and a municipality in Krishnagiri district in the Indian state of Tamil Nadu.The name of the town is from Kannada language which means 'New settlement/town' . It is a taluk of Krishnagiri district. It is located about 40 kilometres south-east of Bangalore and 38 kilometres from... – Gorur – Holenarsipur – Hunsur Hunsur Hunsur is a taluk in Mysore district in the Indian state of Karnataka.Its main claim to fame was that former Chief minister of Karnataka state Sri D. Devaraj Urs hailed from this constituency of Karnataka Legislative Assembly and famous film director Hunsur Krishnamurthy hailed from here. It... belt |
| 3.3–3.2 Ga. | The second generation Peninsular Gneiss found in the Bangalore– Chickmagalur –Holenarsipur region |
| 3.0–2.9 Ga | a) Grandiorite facies Facies In geology, facies are a body of rock with specified characteristics. Ideally, a facies is a distinctive rock unit that forms under certain conditions of sedimentation, reflecting a particular process or environment.... in the Bangalore Gneisses established by the Single Zircon Zircon Zircon is a mineral belonging to the group of nesosilicates. Its chemical name is zirconium silicate and its corresponding chemical formula is ZrSiO4. A common empirical formula showing some of the range of substitution in zircon is 1–x4x–y... Kober Kober Kober is a surname shared by the following people:*Adolf Kober , German rabbi and medievalist*Alice Kober , U.S... evaporation Evaporation Evaporation is a type of vaporization of a liquid that occurs only on the surface of a liquid. The other type of vaporization is boiling, which, instead, occurs on the entire mass of the liquid.... 207Pb/206Pb data. b) In the Dharwar Craton Craton A craton is an old and stable part of the continental lithosphere. Having often survived cycles of merging and rifting of continents, cratons are generally found in the interiors of tectonic plates. They are characteristically composed of ancient crystalline basement rock, which may be covered by... studies done on the trondhjemites of Holenarsipur, major crust forming event are attributed through the Rb–Sr whole rock isochron Isochron In the mathematical theory of dynamical systems, an isochron is a set of initial conditions for the system that all lead to the same long-term behaviour.-An introductory example:... ages. c) In Karighatta Karighatta Karighatta is a hill situated a few kilometres outside the 'island' town of Srirangapatna. It is situated off the Bangalore-Mysore Highway just before Srirangapatna. The name Karighatta translates to "Black Hill" in Kannada. The hill has a Hindu temple devoted to a form of Hindu god Vishnu, called... – Kunigal Kunigal Kunigal is a town in Tumkur district in the Indian state of Karnataka. It is the headquarters of Kunigal taluk. Kunigal is famous for stud farm owned by Vijay Mallya.- Geography :Kunigal is located at... areas also, a similar age is indicated. In this case, data of U–Pb SHRIMP of Zircons from amphibolite Amphibolite Amphibolite is the name given to a rock consisting mainly of hornblende amphibole, the use of the term being restricted, however, to metamorphic rocks. The modern terminology for a holocrystalline plutonic igneous rocks composed primarily of hornblende amphibole is a hornblendite, which are... facies gneisses from Kobaladurga and Kober Kober Kober is a surname shared by the following people:*Adolf Kober , German rabbi and medievalist*Alice Kober , U.S... single zircon evaporation ages of grey gneisses have been considered |
Thus, three major episodes, namely, 3.4 Ga., 3.3–3.2 Ga., and 3.0–2.9 Ga
Gya
In astronomy, geology, and paleontology, Gya or Ga is a year multiplied by one of the SI prefix multipliers. It is often used as a unit of time to denote length of time before the present...
., are accreted to form the Peninsular Gneiss of the region, dated 2500 to 3400 million years.
The Sargur
Sargur
Sargur , a small town located about 80 km from the town of Chamarajanagar in the H D Kote Taluk of Mysore District of Karnataka, India. H D Kote is about 12 km north of Sargur. It is 55 km from the city of Mysore. Bangalore International Airport is 200 km away...
schist
Schist
The schists constitute a group of medium-grade metamorphic rocks, chiefly notable for the preponderance of lamellar minerals such as micas, chlorite, talc, hornblende, graphite, and others. Quartz often occurs in drawn-out grains to such an extent that a particular form called quartz schist is...
belts within the Peninsular Gneiss could be the oldest suture zones in the Indian subcontinent
Indian subcontinent
The Indian subcontinent, also Indian Subcontinent, Indo-Pak Subcontinent or South Asian Subcontinent is a region of the Asian continent on the Indian tectonic plate from the Hindu Kush or Hindu Koh, Himalayas and including the Kuen Lun and Karakoram ranges, forming a land mass which extends...
.
Lalbagh
Lalbagh established by Hyder Ali
Hyder Ali
Hyder Ali was the de facto ruler of the Kingdom of Mysore in southern India. Born Hyder Naik, he distinguished himself militarily, eventually drawing the attention of Mysore's rulers...
based on Mughal Gardens
Mughal Gardens
Mughal gardens are a group of gardens built by the Mughals in the Islamic style of architecture. This style was heavily influenced by the Persian gardens particularly the Charbagh structure. Significant use of rectilinear layouts are made within the walled enclosures...
at Sira
Province of Sira
The Province of Sira in southern India was a province of the Mughal empire that was established in 1687 and lasted until 1757. The province, which comprised the Carnatic region south of the Tungabhadra river, had its capital in the town of Sira...
is not only a famous botanical garden
Botanical garden
A botanical garden The terms botanic and botanical, and garden or gardens are used more-or-less interchangeably, although the word botanic is generally reserved for the earlier, more traditional gardens. is a well-tended area displaying a wide range of plants labelled with their botanical names...
but has also a historical link. The creation of the Bangalore city in the 16th century is attributed to Kempegowda, the then feudal leader under the feudatory of the Vijayanagara empire
Vijayanagara Empire
The Vijayanagara Empire , referred as the Kingdom of Bisnaga by the Portuguese, was an empire based in South Indian in the Deccan Plateau region. It was established in 1336 by Harihara I and his brother Bukka Raya I of the Yadava lineage. The empire rose to prominence as a culmination of attempts...
, who established four cardinal towers setting limits to the growth of the city. One of the towers at the southern end of the city is the Lalbagh tower erected over the Lalbagh hill which is made up of Peninsular Gneiss, now identified as a National Geological Monument. The city has outgrown the limits set by Kempegowda many times and is now part of the downtown area.
The topographic setting of the Lalbagh rock, from a layman’s interest, is that in the western and northern directions it slopes steeply while it merges gently with soil in the east and south. View from the North is impressive since the rock has retained its original form. The west or north–west view is panoramic and aesthetic, with grayish–white clean plates like appearance (see picture). The rock shines white after the rains and attracts attention of the visitors who excitedly climb the rock mass to the tower at the top to get a panoramic view of the gardens and the city.
Access
The monument, located in the southern part of Bangalore, 4 kilometres (2 mi) from the Legislature office complex of the Karnataka Government (Vidhana SoudhaVidhana Soudha
The Vidhana Soudha, located in Bengaluru , is the seat of the state legislature of Karnataka. It is an imposing building, constructed in a style sometimes described as Neo-Dravidian, and incorporates elements of Indo-Saracenic and Dravidian styles...
) within the Lalbagh gardens is easily approachable by road (Metro Map of the city in the Info box gives exact location). Bangalore is well connected by road, rail and air with the rest of the country and is also well known internationally.
External links
- Locations Map of the 26 Nartional Geological Monuments of India
- Trans. Min. Geology Institute India, 1, 47 (1906).
- Rec. Geology Survey India, 69, 109 and 458 (1935).
- Mem. Geology Survey India, 70 (1936 and 1940).
- Explanatory brochure on Geological and Mineral Map of Karnataka and Goa Aug 1981