Environmental changes, marked by shifting river courses, changing drainage patterns, tectonic disturbances, earthquakes, floods and monsoon fluctuations, had resulted in the disappearance and reconstruction of Mohenjodaro several times between 7,400 BC and 1800 BC, Dr J G Negi, emeritus professor at NGRI, told PTI here.

The geomorphological and temperature changes had led to cyclical increase or decrease of rainfall, which had a profound impact on ascent and decline of human civilisations and migrations in different parts of the world, he said.It was evident from pollen grain studies of Sehwan lake (in Rajasthan) that there were at least 10 cycles of floods preceded by earthquakes andother tectonic disturbances in the region around the Indus Valley civilisation during which Mohenjodaro was destructed and reconstructed, the geologist pointed out.

The movement of rivers away from the towns or destruction of vegetation due to monsoons might have ravaged the Harappan township around 1500 BC, Negi said.

The long-held belief that the Dravidian cultural sites of Harappa and Mohenjodaro were destroyed by Aryans from the North-West stands demolished with the increasing archaeological and climatological studies worldwide, Negi said.

The growing evidence suggests that the Indus Valley civilisation was destroyed not by invaders but by environmental changes, the most important of them being the drying up of the Saraswati river which found prominent mention in the Rig Veda, the geologist said.

The Rig Vedic description of Saraswati as a massive river and a source of livelihood matches well with the highest rainfall period around 3,800 BC but by 1,800 BC, the rainfall had almost stopped in theregion followed by a dry period which led to migration to the banks of the Ganga, he said.

Quoting from extensive studies on pollen remains of the lakes of North-West India to gauge monsoon fluctuations in the last 10,000 years, Negi said there was a massive earthquake around 2,000 BC, obstructing the course of the Indus river, thereby leading to major geographical changes.

The tectonic disturbances at that time had created Sehwan lake extending upstream to Mohenjodaro by more than 140 kilometres, he said.

There were evidences of repeated destruction and reconstruction of Mohenjodaro (at least five cycles) due to the appearance and disappearance of Sehwan lake, the geologist observed.