Mumbai’s birth was an outcome of an explosive central type eruption with lava and debris thrown upwards with full force, unlike the quiescent eruption responsible for the formation of the Deccan trap from layers of volcanic lava (that eventually create basalt rocks).
The tell tale signs of Mumbai’s birth are found in the diversity of rocks post the massive explosions. The hills of Kanheri and Jogeshwari are made of Agglomerate rocks made of angular lava fragments of different sizes and shapes. The mines of Kurla and Malad are rich in Rhyolite rocks, tailor made for construction work. A variant of this rock, Trachyte, is found in Kandivali mines. The Sanjay Gandhi National Park in Borivali has ample stocks of Pitchstone, a dull black glassy volcanic rock of higher water content which is remarkably erosion-resistant. Mahakali Caves in the Andheri suburb of Mumbai is made of Tuff, a rock comprised of volcanic ash. It was also found in large volumes while a tunnel was being dug for the Bandra sewage plant.
Popular belief is that the volcanic explosion split Mumbai into seven islands – Bombay (comprised of Walkeshwar, Girgaon, Fort, Dongri), Mazagaon, Shiv-Matunga-Parel, Worli, Mahim, Little Colaba (Old Woman's Island, originally Al Oman named after the Omanese Arabs who inhabited the region), and Colaba.
However, Tim Riding, a postdoctoral fellow at the University of Edinburgh and a well known Indian Ocean colonialism researcher, contends that Bombay was not seen as seven islands until the nineteenth century. The archipelago (group of islands with the sea around them) was reckoned as one island by the English, and four by the Portuguese. On the other hand, there are references that suggest that Greek geographer Ptolemi referred to the seven islands as ‘Heptanesia’.
Whatever the name and number, life in the Mumbai Islands was exceedingly difficult in the seventeenth century as they were an filthy expanse of muddy swamp and foul stench, and hence a staple breeding ground for mosquitoes, flies, and other insects that caused epidemic after epidemic.