The city of Pune is built on rocks formed during one of the greatest volcanic eruptions in earth history. These are the Deccan Basalts, made up of an iron and magnesium rich lava. They started erupting around 68 million years ago. The last of the volcanism ended around 60 million years ago. The pace of eruptions was not constant during this time span. Geologists have determined that around 70%-80% of the lava volume erupted around 66 million years ago in a relatively short time space of 200,000 years. Hot ground water began circulating in these lavas. From these mineral rich waters were precipitated large pretty faceted minerals. These are the well known zeolites and large quartz and calcite crystals which are treasured by mineral collectors.
The earth also underwent a major biological transformation at this time. Around 70% of species, including the dinosaurs, were wiped out in a mass extinction. The main culprit of this great dying is a meteorite which crashed into what is now the Yucatan Peninsula in Mexico. But there is a lively debate on what role these volcanic eruptions could have played in this extinction event.
The basalts cover most of the state of Maharashtra and parts of Madhya Pradesh and Gujarat. They also extend about a 100 km underneath the Arabian Sea. The western part of the volcanic region subsided along faults by 64 million years ago, putting a large region under the sea and bringing the Mumbai region to sea level. After volcanism, prolonged weathering and river erosion over the past tens of millions of years has cut into this volcanic pile and carved out quite a spectacular landscape of the Sahyadri mountains.
The most eye-catching feature is the Western Ghat Escarpment, a line of steep slopes and cliffs, separating an elevated plateau to the east from the coastal plains to the west. The famous Tiger’s Leap at Lonavala, and Konkan Kada north of Pune, are striking examples of these cliffs. Millions of years ago this escarpment was located about a 100 km to the west of its present location. It has retreated eastwards since due to the forces of erosion. Deep valleys, mesas, and pinnacles, make for a rugged terrain that trekkers and outdoor enthusiasts take full advantage of, especially during the monsoon and winter months.
Go out and explore this great natural wonder of India
Suvrat Kher is a sedimentary geologist. He did his PhD from Florida State University in the United States, specialising in limestones and sedimentary basin evolution. He also has work experience in Geographic Information Systems and Remote Sensing. He resides in Pune, Maharashtra, and divides his time between consulting in geology and coaching football and rugby. Over the past 10 years he has pursued geology outreach via his science blog Rapid Uplift. His Twitter handle is @rapiduplift . He also takes geology excursions to the Western Ghats and the Himalayas.