Pillow Basalt - A feature seen today forming only in oceans along active rift margins or submarine volcanoes. But the Pillow basalt lava structures of Karnataka are far away from any active submarine volcanoes or mid-oceanic ridges (see the white down arrow showing the location of Pillow basalt far away from any active submarine volcano vents or oceanic ridges).
|Position of Mardahalli Pillow basalt w.r.t to the active Andaman Sea Back-Arc Basin (BAB) in the east and mid-oceanic Carlsberg ridge (diverging spreading center) in the south west. (Image courtesy: Google Earth)|
They were formed ~2.8 billion years ago, and provides an important clue to the evolution of precambrian peninsular India. I visited the site during my masters study in 2009, and this small mound hosts an important clue to submarine volcanic activity of Peninsular India during the Precambrian times.
You might be wondering how does it look like? They look like buns or pillows - a feature formed when hot molten basaltic magma slowly erupts under water and solidifies so rapidly to form roughly spherical or rounded pillow-shape. Such pillow basalts can be seen forming even today along the mid-oceanic ridges or where sub-marine volcano erupts in the ocean. The lava gets chilled so fast that part of the flow separates into discrete rounded bodies a few feet or less in size. The pillow basalts when erupted under water forms a chilled rind on all sides of the extrusion.
|Features of pillow structure - rind, vesicle zone and core seen on pillow lava cliff section at Mardihalli hill (marker pen for scale). (Photo by: R. Thomas)|
The hot rind is solid but elastic (like a balloon), and continuous injection of lava into it leads to bulbous, spherical, or tubular lobes that eventually solidify to form pillow-shaped masses of basalt. Have a look at the video of pillow lava formation.
The pillow lavas structures of Mardihalli hill in Karnataka is one of the featured national geological monument on the Geological Survey of India website. It is located near Maradihalli, a small village situated in Chitradurga District, Karnataka. But not many know about it or is not part of the travel itineraries of many tourist. It is nestled within Chitradurga schist belt of Dharwar Group and is one of the best preserved of its kind in the world. However, there is another similar pillow lava monument located in the Iron ore belt of Nomira, Keonjhar District, Odisha. To get there is very easy, it can be approached by a metalled road via Ayamangala which is about 180 km from Bangalore. Maradihalli is 16 km southeast of Chitradurga town and 4 km north of Ayamangala village, along the NH-4 (Bangalore –Pune).
|Pillow lava location just south of Maradihalli village. (Image courtesy: Google Earth)|
The visit to the site was part of geological mapping study exercise together with Geological survey of India. What I remember so fondly is standing beside this cliff with my chisel end hammer, and being carried away in the geological past, imagining the under sea eruption. But what has made this moment come alive once again is the paper by Duraiswami et. al. published last year in the Journal of Volcanology and Geothermal Research on the 'Emplacement of pillow lavas from the ~2.8 Ga Chitradurga Greenstone Belt, South India: A physical volcanological, morphometric and geochemical perspective'.
Stacked pillow lava structure at Mardihalli hill- a national Geological Monument. (Hammer for scale) (Photo by: R. Thomas)
How were they placed there? The research article attributes its placement to marginal shallow marine inter/ back arc basin. The study also shows repeated budding of larger pillows, producing a series of interconnected pillow units indicating fluid lava that was emplaced on steeply dipping flanks.
The study was also carried on another hill along the road northwest of Mardihalli near Kunchiganalu marked as Chitradurga hill in the literature, which shows individual pillowed flows alternating with massive submarine sheet flows that are devoid of any vesicles (I did not visit this one). Both Mardihalli and Chitradurga hills are part of the north-south trending hills of the Chitradurga greenstone belt that mimics a back arc submarine basin. The main outcomes of the study of the pillow lavas shows - they were emplaced at low to moderate effusion rates (≤ 5 m3/s), and erupted as tholeiites (not as andesites as previously interpreted). The study shows Back Arc-Basin affinities based on the geochemistry, and envisages that the Chitradurga basin witnessed distinct episodes of submarine tholeiite eruptions that produced pillowed lavas, that later variably interacted with sea water to produce spilitized geochemistries. The field and stratigraphic relationships of the volcanics and associated clastic sediments suggest that the pillow lavas were emplaced in a shallow marine marginal inter/back arc basin. - excerpts from the paper. For further read on this interesting article (only abstract available) - Duraiswami, R.A., et al. (2013)