The geology of India is diverse.
Different regions of India contain rocks belonging
to different geologic periods, dating as far
back as the Eoarchean Era.
Some of the rocks are very deformed and altered.
Other deposits include recently deposited
alluvium that has yet to undergo diagenesis.
Mineral deposits of great variety are found
in the Indian subcontinent in huge quantity.
Even India's fossil record is impressive in
which stromatolites, invertebrates, vertebrates
and plant fossils are included.
India's geographical land area can be classified
into the Deccan Traps, Gondwana and Vindhyan.
The Deccan Traps covers almost all of Maharashtra,
a part of Gujarat, Karnataka, Madhya Pradesh
and Andhra Pradesh marginally.
During its journey northward after breaking
off from the rest of Gondwana, the Indian
Plate passed over a geologic hotspot, the
Réunion hotspot, which caused extensive melting
underneath the Indian Craton.
The melting broke through the surface of the
craton in a massive flood basalt event, creating
the Deccan Traps.
It is also thought that the Reunion hotspot
caused the separation of Madagascar and India.
The Gondwana and Vindhyan include within its
fold parts of Madhya Pradesh, Chhattisgarh,
Odisha, Bihar, Jharkhand, West Bengal, Andhra
Pradesh, Maharashtra, Jammu and Kashmir, Punjab,
Himachal Pradesh, Rajasthan and Uttarakhand.
The Gondwana sediments forms a unique sequence
of fluviatile rocks deposited in Permo-Carboniferous
time.
The Damodar and Sone river valleys and Rajmahal
hills in the eastern India contain a record
of the Gondwana rocks.
== Plate tectonics ==
The Indian Craton was once part of the supercontinent
of Pangaea.
At that time, what is now India's southwest
coast was attached to Madagascar and southern
Africa, and what is now its east coast was
attached to Australia.
During the Jurassic Period about 160 Ma (ICS
2004), rifting caused Pangaea to break apart
into two supercontinents, namely Gondwana
(to the south) and Laurasia (to the north).
The Indian Craton remained attached to Gondwana,
until the supercontinent began to rift apart
about in the early Cretaceous, about 125 million
years ago (ICS 2004).
The Indian Plate then drifted northward toward
the Eurasian Plate, at a pace that is the
fastest known movement of any plate.
It is generally believed that the Indian Plate
separated from Madagascar about 90 Million
years ago (ICS 2004), however some biogeographical
and geological evidence suggests that the
connection between Madagascar and Africa was
retained at the time when the Indian Plate
collided with the Eurasian Plate about 50
Million years ago (ICS 2004).
This orogeny, which is continuing today, is
related to closure of the Tethys Ocean.
The closure of this ocean which created the
Alps in Europe, and the Caucasus range in
western Asia, created the Himalaya Mountains
and the Tibetan Plateau in South Asia.
The current orogenic event is causing parts
of the Asian continent to deform westward
and eastward on either side of the orogen.
Concurrently with this collision, the Indian
Plate sutured on to the adjacent Australian
Plate, forming a new larger plate, the Indo-Australian
Plate.
Tectonic evolution
The earliest phase of tectonic evolution was
marked by the cooling and solidification of
the upper crust of the earth's surface in
the Archaean Era (prior to 2.5 billion years)
which is represented by the exposure of gneisses
and granites especially on the Peninsula.
These form the core of the Indian Craton.
The Aravalli Range is the remnant of an early
Proterozoic orogen called the Aravali-Delhi
Orogen that joined the two older segments
that make up the Indian Craton.
It extends approximately 500 kilometres (311
mi) from its northern end to isolated hills
and rocky ridges into Haryana, ending near
Delhi.
Minor igneous intrusions, deformation (folding
and faulting) and subsequent metamorphism
of the Aravalli Mountains represent the main
phase of orogenesis.
The erosion of the mountains, and further
deformation of the sediments of the Dharwarian
group (Bijawars) marks the second phase.
The volcanic activities and intrusions, associated
with this second phase are recorded in the
composition of these sediments.
Early to Late Proterozoic(2.5 to 0.54 billion
years) calcareous and arenaceous deposits,
which correspond to humid and semi-arid climatic
regimes, were deposited the Cuddapah and Vindhyan
basins.
These basins which border or lie within the
existing crystalline basement, were uplifted
during the Cambrian (500 Ma (ICS 2004)).
The sediments are generally undeformed and
have in many places preserved their original
horizontal stratification.
The Vindhyans are believed to have been deposited
between ~1700 and 650 Ma (ICS 2004).Early
Paleozoic rocks are found in the Himalayas
and consist of southerly derived sediments
eroded from the crystalline craton and deposited
on the Indian platform.
In the Late Paleozoic, Permo-Carboniferous
glaciations left extensive glacio-fluvial
deposits across central India, in new basins
created by sag/normal faulting.
These tillites and glacially derived sediments
are designated the Gondwanas series.
The sediments are overlain by rocks resulting
from a Permian marine transgression (270 Ma
(ICS 2004)).
The late Paleozoic coincided with the deformation
and drift of the Gondwana supercontinent.
To this drift, the uplift of the Vindhyan
sediments and the deposition of northern peripheral
sediments in the Himalayan Sea, can be attributed.
During the Jurassic, as Pangea began to rift
apart, large grabens formed in central India
filling with Upper Jurassic and Lower Cretaceous
sandstones and conglomerates.
By the Late Cretaceous India had separated
from Australia and Africa and was moving northward
towards Asia.
At this time, prior to the Deccan eruptions,
uplift in southern India resulted in sedimentation
in the adjacent nascent Indian Ocean.
Exposures of these rocks occur along the south
Indian coast at Pondicherry and in Tamil Nadu.
At the close of the Mesozoic one of the greatest
volcanic eruptions in earth's history occurred,
the Deccan lava flows.
Covering more than 500,000 square kilometres
(193,051 sq mi) area, these mark the final
break from Gondwana.
In the early Tertiary, the first phase of
the Himalayan orogeny, the Karakoram phase
occurred.
The Himalayan orogeny has continued to the
present day.
== Major rock groups ==
=== 
Precambrian super-eon ===
A considerable area of peninsular India, the
Indian Shield, consists of Archean gneisses
and schists which are the oldest rocks found
in India.
The Precambrian rocks of India have been classified
into two systems, namely the Dharwar system
and the Archaean system.
The rocks of the Dharwar system are mainly
sedimentary in origin, and occur in narrow
elongated synclines resting on the gneisses
found in Bellary district, Mysore and the
Aravalis of Rajputana.
These rocks are enriched in manganese and
iron ore which represents a significant resource
of these metals.
They are also extensively mineralised with
gold most notably the Kolar gold mines located
in Kolar.
In the north and west of India, the Vaikrita
system, which occurs in Hundes, Kumaon and
Spiti areas, the Dailing series in Sikkim
and the Shillong series in Assam are believed
to be of the same age as the Dharwar system.
The metamorphic basement consists of gneisses
which are further classified into the Bengal
gneiss, the Bundelkhand gneiss and the Nilgiri
gneiss.
The Nilgiri system comprises charnockites
ranging from granites to gabbros.
=== Phanerozoic ===
==== Palaeozoic ====
===== Lower Paleozoic =====
Rocks of the earliest part of the Cambrian
Period are found in the Salt range in Punjab
and the Spiti are in central Himalayas and
consist of a thick sequence of fossiliferous
sediments.
In the Salt range, the stratigraphy starts
with the Salt Pseudomorph zone, which has
a thickness of 450 feet (137 m) and consists
of dolomites and sandstones.
It is overlain by magnesian sandstones with
a thickness of 250 feet (76 m), similar to
the underlying dolomites.
These sandstones have very few fossils.
Overlying the sandstones is the Neobolus Shale,
which is composed of dark shales with a thickness
of 100 feet (30 m).
Finally there is a zone consisting of red
or purple sandstones having a thickness of
250 feet (76 m) to 400 feet (122 m) called
the Purple Sandstone.
These are unfossiliferous and show sun-cracks
and worm burrows which are typical of subaerial
weathering.
The deposits in Spiti are known as the Haimanta
system and they consist of slates, micaceous
quartzite and dolomitic limestones.
The Ordovician rocks comprise flaggy shales,
limestones, red quartzites, quartzites, sandstones
and conglomerates.
Siliceous limestones belonging to the Silurian
overlie the Ordovician rocks.
These limestones are in turn overlain by white
quartzite and this is known as Muth quartzite.
Silurian rocks which contain typical Silurian
fauna are also found in the Vihi district
of Kashmir.
===== Upper Paleozoic =====
Devonian fossils and corals are found in grey
limestone in the central Himalayas and in
black limestone in the Chitral area.
The Carboniferous is composed of two distinct
sequences, the upper Carboniferous Po, and
the lower Carboniferous Lipak.
Fossils of brachiopods and some trilobites
are found in the calcareous and sandy rocks
of the Lipak series.
The Syringothyris limestone in Kashmir also
belongs to the Lipak.
The Po series overlies the Lipak series, and
the Fenestella shales are interbedded within
a sequence of quartzites and dark shales.
In many places Carboniferous strata are overlaid
by grey agglomeratic slates, believed to be
of volcanic origin.
Many genera of productids are found in the
limestones of the Permo-Triassic, which has
led to these rocks being referred to as "productus
limestone".
This limestone is of marine origin and is
divided into three distinct lithostratigraphic
units based on the productus chronology: the
Late Permian Chideru, which contains many
ammonites, the Late — Middle Permian Virgal,
and the Middle Permian Amb unit.
==== Mesozoic ====
In the Triassic the Ceratite beds, named after
the ammonite ceratite, consist of arenaceous
limestones, calcerous sandstones and marls.
The Jurassic consists of two distinct units.
The Kioto limestone, extends from the lower
the middle Jurassic with a thickness 2,000
feet (610 m) to 3,000 feet (914 m).
The upper Jurassic is represented by the Spiti
black shales, and stretches from the Karakoram
to Sikkim.
Cretaceous rocks cover an extensive area in
India.
In South India, the sedimentary rocks are
divided into four stages; the Niniyur, the
Ariyalur, the Trichinopoly(a district in the
Madras Presidency, covering present-day districts
of Tiruchirappalli, Karur, Ariyalur and Perambalur),
and the Utatur stages.
In the Utatur stage the rocks host phosphatic
nodules, which constitute an important source
of phosphates in the country.
In the central provinces, the well developed
beds of Lameta contain fossil records which
are helpful in estimating the age of the Deccan
Traps.
This sequence of basaltic rocks was formed
near the end of the Cretaceous period due
to volcanic activity.
These lava flows occupy an area of 200,000
square miles (520,000 km2).
These rocks are a source of high quality building
stone and also provide a very fertile clayey
loam, particularly suited to cotton cultivation.
==== Cenozoic ====
===== Tertiary period =====
In this period the Himalayan orogeny began,
and the volcanism associated with the Deccan
Traps continued.
The rocks of this era have valuable deposits
of petroleum and coal.
Sandstones of Eocene age are found in Punjab,
which grade into chalky limestones with oil
seepages.
Further north the rocks found in the Simla
area are divided into three series, the Sabathu
series consisting of grey and red shales,
the Dagshai series comprising bright red clays
and the Kasauli series comprising sandstones.
Towards the east in Assam, Nummulitic limestone
is found in the Khasi hills.
Oil is associated with these rocks of the
Oligo-Miocene age.
Along the foothills of the Himalayas the Siwalik
molasse is composed of sandstones, conglomerates
and shales with thicknesses of 16,000 feet
(4,877 m) to 20,000 feet (6,096 m) and ranging
from Eocene to Pliocene.
These rocks are famous for their rich fossil
vertebrate fauna including many fossil hominoids.
===== Quaternary period =====
The alluvium which is found in the Indo-Gangetic
plain belongs to this era.
It was eroded from the Himalayas by the rivers
and the monsoons.
These alluvial deposits consist of clay, loam,
silt etc. and are divided into the older alluvium
and the newer alluvium.
The older alluvium is called Bhangar and is
present in the ground above the flood level
of the rivers.
Khaddar or newer alluvium is confined to the
river channels and their flood plains.
This region has some of the most fertile soil
found in the country as new silt is continually
laid down by the rivers every year.
== Earthquakes ==
The Indian subcontinent has a history of devastating
earthquakes.
The major reason for the high frequency and
intensity of the earthquakes is that the Indian
plate is driving into Asia at a rate of approximately
47 mm/year.
Geographical statistics of India show that
almost 54% of the land is vulnerable to earthquakes.
A World Bank & United Nations report shows
estimates that around 200 million city dwellers
in India will be exposed to storms and earthquakes
by 2050.
National Disaster Management Authority says
that 60% of Indian landmass is prone to earthquake
and 8% susceptible to cyclone risks.
== References ==
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).
