I recently read Peter Brannen’s excellent book “The Ends of the World: Volcanic Apocalypses, Lethal Oceans And Our Quest To Understand Earth's Past Mass Extinctions”. He has packed quite a few details on the geologic triggers and ecologic upheavals the earth has witnessed from time to time, resulting in the episodic reorganization of the earth’s biosphere.
In the chapter on the Late Cretaceous mass extinction he writes, referring to the Deccan Basalts, .. “today in Western India, 11,500 -foot-tall bar-coded mountains, like the jagged banded peaks of Mahabaleshwar have been carved from this surfeit of molten rock”. He presents a lively discussion on what impact such a prolonged phase of volcanism could have had on environmental health and biodiversity.
The view below of a section of the Deccan Basalts at Warandha Ghat, SW of Pune city, illustrates nicely the 'bar coded mountains" that Brannen mentions.
Repeated effusion of lava which spread rapidly away from eruptive vents has produced the distinct layered architecture. Differential weathering of softer and hard layers produces intermittent rubbly vegetated slopes alternating with scarps resulting in a "bar coded" edifice.
But what about the 11,500 foot tall reference? "Tall" will be read by most as elevation. That is a curious number since the "jagged banded peaks" of the popular hill station of Mahabaleshwar are around 4700 feet high. The highest region in the Deccan Volcanic Province is Kalsubai near Nasik, standing at 5400 feet.
Why such a large discrepancy between Brannen and the measured elevation in the Deccan Volcanic Province? I suspect what Brannen is referring to is a composite stratigraphic thickness of the lava in Western Maharashtra. Refer to the table below. It presents two different approaches to organizing the lava pile into discrete units.
Source: Vivek S Kale and coworkers 2017- Geological Society of London, Special Publications.
"Lithostratigraphy" relies on systematic physical differences in lava packages to classify them into formal units. The "Chemostratigraphy" classification uses chemical differences in successive phases of eruption to subdivided the lava pile.
Add up the thickness of the individual units and you end up with a 3400 meters or 11,150 feet thick section of lava. Branner may have used a slightly different estimate of lava thickness to arrive at a 11,500 foot thickness for the Deccan Basalts.
If, as seen in the section at Warandha Ghat, the lava is nearly horizontally disposed, why then is the stack of lava "only" 4700 feet at Mahabaleshwar and caps out at 5400 feet at Kalsubai?
The answer is in the way the different subunits of the lava are exposed all across the volcanic province. The map below shows sections of the Deccan Traps at different locations.
Source: L Vanderklyusen and coworkers 2011- Journal of Petrology.
In this map, the chemostratigraphic subdivisions are shown. What is important is that all the subunits never stack up in any one place. The reason is in the regional structure of the volcanic pile. It is in the form of a gently waveform with younger packages of lava offlapping towards the south.
Source: M. Widdowson and K.G. Cox 1996: Earth and Planetary Science Letters.
This north to south cross section of the Deccan Traps along the Western
region shows how different subgroups are exposed along the profile. You
can see how at any one place the lava thickness and altitude is between
1000 to 1500 meters ASL.
It is important to mention that this is a regional view of the lava section across a distance of around 650 km depicted with a 40x vertical exaggeration. The inclination of the lava layers is very subtle and has been deduced from careful measurements of the altitude of the subunit boundaries at different locations along the profile.
The section below shows a close up of the lava stratigraphy along two north to south profiles in mid Western Maharashtra in the well known Harishchandragarh - Bhimashankar area. Notice again how only a few of the units stack in any one location.
Source: Vivek S Kale and coworkers 2017- Geological Society of London, Special Publications.
Why did such an arrangement of the lava units emerge? Is the waveform a regional volcanic dome with lava radiating from one central eruptive center? Or is it due to post-eruption arching of the crust? Have younger lava sections been removed by erosion from the northerly locales, or has there been a southerly migration of eruptive centers over time, resulting in the offlapping arrangement? Anne E. Jay and Mike Widdowson in a study published in the Geological Society of London estimate that as much as 1.5 km of lava section has been removed by erosion from the Nasik area. That region would have stood much taller tens of millions of years ago. We will leave these questions lingering for another time.
What I do wish is that Mahabaleshwar really stood 11,500 feet tall. I could have ice skated on Lake Venna.