The Magnificent “Welded Tuff” Geoheritage Site in the Jodhpur Fort area

jodhpur Welded Tuff is located in the JodhpurFort hill area, occurs within the terrace-like weathered Malani volcanic. The welded tuff is a form of emanations, that spurted out from volcanic vents and were carried away by air to settle down. Tuff is composed of glass, quartz, and feldspar. On cooling, they develop joints that give rise to columns and terraces.

The welded tuff outcrop is about 100 m on the right side of the road before the Mehrangarh Fort while proceeding from the jodhpur town. The Malani rhyolites comprise pink, maroon, brown, purple, and grey rhyolite separated by tuff, welded tuff, and pyroclastic rocks.

Stratigraphy

The columnar joints developed are rectangular to hexagonal, attaining a length of 30m or more at places. It is overlain by deep purple-colored porphyritic rhyolite. The pyroclastics are intermixed with lava representing the final phase of emanations.

 

The late Proterozoic Malani volcanic constitute the largest suite of orogenic acid volcanic in India. The volcanism took place around 780 Ma ago.

On the basis of field observation, three stages of igneous activity have been recognized-

Volcanics of the first stage is mostly basalt with occasional andesite or trachybasalts. These are subsequently covered by the voluminous rhyolite, basalt, dacite, and trachyte flows. The third stage ceased with the outburst of ash-flow deposits.

The dominant felsic volcanic are rhyolites and spread over an area of about 31, 000 sq.km The other rock types associated with rhyolite are trachytes, dacites, pitchstone, welded tuff, volcanic breccia, and volcanic conglomerates. Malani volcanism was essentially under terrestrial conditions, although deposition by aqueous conditions is also indicated. The volcanic eruptions have been through fissures, shield volcanoes, and central cones. The site has been identified by the Geological Survey of India as a national geological monument.

What is Welded tuff

Welded tuff is a pyroclastic rock, (volcanic materials)  that was sufficiently hot at the time of eruption to weld together. During welding, the glass shards and pumice fragments stick together, deform and compact.

Pyroclastic rocks are made up of a large range of particle sizes; from the largest rocky pieces to very fine ashes and tuffs. Ash is considered to be pyroclastic because it is a fine dust made up of volcanic rock.

Purple-colored rhyolite is common in welded tuff formations. This covering over the columns gives the rocks a uniform and colorful deep purple appearance. Upon cooling rhyolite develops fractures or cooling joints which result in near-vertical columns and horizontal terraces. The columns that result are rectangular to hexagonal and are getting up to lengths of 30m or more at places.

Also Read- The Dhala impact structure, Bundelkhand craton, Central India, the largest & oldest crater in Asia

Mehrangarh Fort, Jodhpur

Mehrangarh Fort nestled atop a hill, at a height of 410 feet. Fort boasts of a history of more than 550 years and is surrounded by imposing walls. It is one of the largest forts in the country and is among the most formidable ones in Rajasthan.

Mehrangarh Fort owes its origin to Rao Jodha, the ruler of Mandore and the founder of the city of Jodhpur. When Rao Jodha decided to shift his capital to a safer and elevated location in Jodhpur, he laid the foundation of this massive fort in 1459 on a hill the Mountain of Birds.

Jodhpur is a city in Rajasthan state, India, and well connected by State Highways. The Northern Railway Main Line and Northern Railway Phalodi Branch pass through Jodhpur Town. It is also approachable by air.