Geology of Rajasthan :

Rajasthan is an important segment of the Indian shield, containing varied geological and tectonic features. Its Precambrian geology is typified by multistage reworking of the Archaean basement (BGC), a feature that has created problems concerning basement-cover relations and has led to repeated development of Mesoproterozoic and Neoproterozoic fold-thrust belts of the Aravalli and the Delhi Supergroups, having austal rifting and probable suturing history.

Aravali  Mountain System :

The Aravalli mountain system runs across the state from the north of Delhi in the north-east to the Gulf of Cambay in the south-west. The great synclinorium in the central part of the Aravalli range is occupied by rocks composed of Aravalli and Delhi rocks. The Archaen consists of the Bhilwara Super Group.

The Aravallis Super Group is a vast formation composed of quartzites, shale, conglomerates, composite gneiss and slate. These vast mountains came into existence at the close of the Archaen era when the sediments which were deposited in the seas of that age, underwent an upheaval by organic activities.

The Delhi Super Group overlies the Aravallis. Delhi Super Group is divided into lower Ralio group which is rich in crystalline limestone, grit, schistose rocks and quartzite; the middle Alwar group consists of quartzite, grits and schistose rocks and the Upper Ajabgarh group.

Vindhayas :

The lower Vindhyans in the east and Marwar in the west are the lithological formations consisting of a thick series of sedimentary rocks composed of sandstone, limestone and shale. The deposition of these rocks in western Rajasthan are mostly of an acidic nature include a thick pile of lava as its formation was preceded by igneous activity. The granite bosses and sills in Jalore, Siwana, Mokalsar and Jodhpur areas are the plutonic equivalent of these lava deposits. Rocks of the above mentioned igneous activity are designated as Erinpura Granite and Malani Igneous suit.

Jurassic Era Formations :

The Jurassic era formations are distinctly noticeable in a vast area around Jaisalmer and some of the fossils of this age are also found in the rocks here. The outcrops of these rocks are, partly, covered by wind-blown desert sands. The Bap (Jodhpur district) and Pokran (Jaisalmer district) beds composed of boulders of Malani rhyolites showing effects of glaciations during the Upper Carboniferous age have now been exploited for ground water.

Volcanic Activity :

Violent volcanic activity in the form of fissure eruptions marked the close of Mesozoic era in the lower Cretaceous age. The rocks formed then are known as Deccan Traps, found in the southern and south-eastern Rajasthan. The main characteristic of this was a stupendous outburst of covered volcanic energy, resulting into the eruption of thick streams of lava over the vast areas. Notable formations of Upper Cretaceous to Lower Eocene age are present in form of the Deccan trap extending over a vast area in Jhalawar and in the eastern parts of Chittorgarh and Banswara districts. This was the time when a large area of peninsular India was also covered with fissure eruptions of black lava.

Marine transgression seems to have inundated a large part of western Rajasthan during Eocene times and left deposition of thick beds of fossilliferous limestone.

To the north of Jaisalmer, the Jurassics are overlapped by nummulitic limestone. Over a large area of western and eastern Rajasthan the remains of Pleistocene sandy alluvium, blown sand, kankar (calcium nodules), and carbonate beds and evaporate deposits of recent and sub-recent age are found.

Several mega lineaments also traverse in the state. The Great Boundary Fault, through which the Chambal River has carved its course, passes through southeastern part of the state. This fault is visible in Begun (Chittorgarh district) and northern parts of Kota. It reappears again in Sawai Madhopur and Dholpur districts.

                        The Quaternary and recent geology is characterised by the frequent climatic fluctuations, disorganisation of the drainage systems (lost Saraswati river) and development of the Thar desert. All these and other features make the geology of Rajasthan interesting on many counts.