Saturday, August 31, 2019

Hot Water Springs Of Konkan- Geological Significance

During my recent trip to Konkan, just north of the Tural area, I came across a sign for a hot water spring.

This is located in the small settlement of Aaravali. The area around the spring has been converted into a tourist spot where locals and tourists come to enjoy a warm bath.

One interesting feature of Konkan coastal belt is the presence of hot water springs arranged in a fairly narrow linear belt from north of Mumbai to Ratnagiri area in the south. They occur somewhat midway between the Western Ghat Escarpment and the coast. Few names from north to south- Vajreshwari, Akoli kund, Ganeshpuri, Pali, Dasgaon, Unhere, Tural, Aaravali, Rajapur. Satellite image shows the area between Dabhol and Ratnagiri. Hot springs are located within the oval. The dark brown undulating line is the trace of the Western Ghat Escarpment.

What is so special about this area? See the map. Black lines are fracture zones, trending N-S, NW-SE and NE-SW. Hot water springs are located in the vicinity of these fractures roughly within the oval. The depicted area is again between Dabhol and Ratnagiri, but this relationship between fracture systems and location of hot springs applies elsewhere along the entire Konkan coastal region.

Source: Neotectonism in the Indian Subcontinent: Landscape Evolution- K.S Valdiya and Jaishri Sanwal (modified).

Water temperatures are between 50 deg C to 60 deg C. Interestingly, analysis shows significant levels of radon gas at measured sites near Tural. Presence of radon gas hints at the reason why there are hot springs here. It points to deep circulation of water.

The crust in this region is made up of a foundation of older Precambrian age granitic rocks overlain by several hundred meters of younger basalt of Late Cretaceous to Paleocene age (67-65 million years old). Radon forms by radioactive decay of uranium. The Deccan Basalts contain only tiny amounts of uranium. Granites on the other hand are enriched in uranium. Radon emission here imply that these fractures cut through the basalt pile and penetrate the 2 billion years and older granitic rocks underlying these basalts. 

Such fracture systems have provided a passageway for groundwater to percolate to great depths. Cool water comes in contact with hot rocks deep below and gets heated. This warmer buoyant water then rises to the surface, forming a hydrothermal circulation system. The cross section shows fracture/fault systems of the coastal region cutting across basalts and penetrating the underlying granitic basement. I have added a few additional fractures to the figure.

What makes the rocks hot? What is the source of heat?  Deccan volcanism ended 60 million yrs ago. It is unlikely that there is any magma underneath to provide heat. Rocks get hotter at depths due to the natural geothermal gradient. Some geologists think that many of these fractures are actually faults along which there is intermittent movement of the crust. This faulting may be causing friction between crustal blocks, generating additional heat in these zones. These fractures and faults are a legacy of the breakup of the India with Madagascar and later Seychelles during and post Deccan volcanism  68 -60 million years ago. This rifting of the Indian crust resulting in oriented fracture systems.

The schematic shows the evolution of the Western Ghat escarpment and the coastal region. Earlier, perhaps soon after Deccan volcanism ended, the escarpment was a west facing cliff formed when faulting caused the western block to subside . Subsequent erosion has resulted in this cliff retreating eastwards, creating a coastal plain. Orange lines mark the highly fractured Indian crust.

Source:  Western Ghat: The Great Escarpment of India- V.S. Kale 2010. (modified)

Next time you visit Konkan and take a dip in the invigorating warm waters, remember that grand geological forces of continental separation are responsible for the high heat flow and the ground water circulation systems that arise consequently.

Wednesday, August 21, 2019

Does Volcanism Cause Global Warming Or Cooling?

On million year time scales, does volcanism cause global warming or cooling?

The answer is both, depending upon the longevity of the volcanism and it effluents. Prolonged volcanic emissions over tens of thousands to millions of years of greenhouse gas carbon dioxide will warm the earth's surface. But magmatism and volcanism creates continental crust. During volcanic episodes and after the magmatic system dies, this new crust consumes carbon dioxide in chemical weathering reactions. This draw down of atmospheric carbon dioxide can result in global cooling, as is inferred to have resulted in the Cenozoic beginning around 30 million years ago, after collision of the India -Eurasian plates. Volcanism and tectonic activity can both warm and cool the earth's surface as magmatic arc systems grow and die.

Volcanism also ejects sulfur particles into the atmosphere. These particles block and reflect sunlight away and this albedo effect may result in cooling of the earth's surface. Volcanic ash falling on both land and sea may act as a fertilizer, enhancing organic productivity and further drawing down and sequestering carbon dioxide through increased organic carbon burial. A recent paper published in Geology by Gerilyn Soreghan and colleagues (open access) points to a temporal coincidence between explosive eruptions and glacial conditions during the Late Paleozoic. The researchers suggest that the prolonged icehouse conditions from around 360 million years ago to 260 million years ago resulted from explosive volcanism and effects of sulfate aerosols.

This paper has prompted a thoughtful commentary (open access) by Rice University geologists Cin-Ty Lee and Sylvia Dee on the broader controls of volcanism and crustal weathering on global climate. On the particular question of whether the Late Paleozoic ice age was a result of sulfate ejections, they differ somewhat from the authors of the study. Cin-Ty Lee and Sylvia Dee point out that the residence time of sulfur particles in the atmosphere is just a few years. To maintain a global icehouse for a 100 million year period would require large explosive eruptions every few years over tens of millions of years.

They point to an example of another period of enhanced magmatic activity in the Cretaceous Period. Field evidence from the continental interior of the U.S. shows just about 200 eruptions over a 10 million year period. Only a few of these were large enough to have ejected significant amounts of aerosols into the stratosphere. That is not to say that sulfate aerosol albedo cannot cool the planet. But it may happen over shorter 1000-10,000 year time scales. In case of the Late Paleozoic icehouse, they suggest that the pattern of cooling may hint at the causative factor. Numerous short-lived cooling events would be suggestive of explosive volcanism as the cause.

On longer time scales carbon dioxide will play a larger role in modulating climate. Explosive eruptions and resulting sulfate particle emissions are only a small component of magmatic flux. On the other hand, CO2 degassing is taking place even without eruptive activity. Long lived magmatic activity will result in a warming trend due to an increase in atmospheric CO2.  Post magmatism, a drop in atmospheric CO2 levels and cooling due to silicate weathering also takes place on longer million year time scales.

An extract from Cin-Ty Lee and Sylvia Dee commentary:

More broadly, the nature by which volatiles are exchanged between planetary interiors and their surfaces is rich with complexity. The magnitude and style of magmatism not only controls volatile degassing but also erosion, weathering, radiative balance, and biological productivity. How magmatic processes change through time and with geodynamic states is an area ripe for interdisciplinary research and new discoveries. Soreghan et al.’s work is an example of how investigating these processes from deep time to the present, as well as on Earth and other planets, will force us to rethink how planetary systems operate.

The geologic record shows that enhanced phases of volcanic activity sustained over thousands of years can cause the earth's climate to tilt towards a long lasting greenhouse or an icehouse. People who claim that the warming of the earth's surface over the past few decades is due to natural causes like volcanic eruptions and not fossil fuel burning must understand the time scales and amounts involved. Even big volcanic eruptions that occur every few years emit only a few million tons of CO2. Awkwardly, for anthropogenic warming deniers, these eruptive events may result in a short term cooling phase due to the effects of sulfate aerosols. A recent survey puts the total global emissions due to volcanic eruptions and non-eruptive degassing of magma to be about 0.3 billion tons per year. In contrast, human activity is putting 30-40 billion tons of CO2 in the atmosphere every year.

Explosive volcanism as a key driver of the late Paleozoic ice age.
Does volcanism cause warming or Cooling?

Saturday, August 17, 2019

Darwin: Victorian Menage A Trois

In 1864, a few years after the publication of the Origin of Species, Darwin, fighting a bout of illness took up some botanical work on Lythrum, a genus of flowering plants known as loosestrifes. He had been breeding them on and off for many years. His interest was in their sexuality as they gave him a deeper understanding on the evolution of sterility and reproductive isolation. His theory of population divergence and the origin of new species depended upon the evolution of traits that prevented individuals and populations from mating with each other.

Lythrum was known for its triple sexuality. There were three kinds of flowers. The female stigma could be tall, medium or short-styled. Each was accompanied by two sets of male stamens.  If the female was tall, the males were short or medium sized.

Darwin realized through his breeding experiments that greater the difference in height between these sexual organs, the greater the frequency of sterility. Mating between the unequal sized sexes in the same plant produced sterile seeds.  This was evolution preventing inbreeding and favoring cross pollination.

Adrian Desmond and James Moore in their biography, Darwin: The Life of a Tormented Evolutionist, write about Darwin's mischievous side:

" Talk about illegitimacy might have shocked the ladies guilds, at least coming from Erasmus Darwin's grandson. (Grandfather's own bastardizing experiments were still supplying tittle-tattle. At this moment Darwin suspected the widow of a botanical friend, Francis Boott, to be an illegitimate granddaughter of old Erasmus). But Darwin was stolid, methodical, reducing the love of the plants to cold, clinical calculation. Sterile seeds counts somehow fitted an unromantic, data-crunching age. Not for him Erasmus's flowery personifications, as styles and stamens bent to embrace in a kiss:

Two knights before thy fragrant altar bend,
Adored Mellissa! and two squires attend.

Still, he could tease. Solicited by a Mrs Becker for something edifying for her ladies' literary society, he posted 'On the Sexual Relations of the Three Forms of Lythrum salicaria'. Goodness knows how many red faces left after hearing that 'nature has ordained a most complex marriage-arrangement, namely a triple union between three hermaphrodites,- each hermaphrodite being in its female organ quite distinct from the other two hermaphrodites and partially distinct in its male organs, and each furnished with two sets of males' ".

There is no doubt that Darwin was an outstanding thinker. But he also was a hands on guy. His thinking was not vacuous or overly speculative.  He was a tinkerer and putterer.  He produced data. By cross breeding plants. By dissecting barnacles. By co-opting pigeon breeders and noting down the variation in the size and shapes of different breeds. Documenting tiny differences between individuals within groups was crucial to his case that complex structures could evolve by incremental changes over generations.

From life's little details he built the grandest theory of all.

Friday, August 2, 2019

Secondary Mineralization In Deccan Basalts: Timing And Precipitation Environments

When and how did these minerals form?

These are the world famous zeolites and other secondary minerals (green apophyllite) that fill cavities and cracks in Deccan Basalt lava flows.  Zeolites and other secondary minerals like apophyllite are calcium, sodium, potassium bearing alumino-silicates with varying amounts of water and other volatile elements like fluorine (apophyllite). They are prized by mineral collectors and by petrologists who study them to understand the geologic conditions that affected the lava after their eruption. This has broader implications for understanding the initiation and evolution of fluid circulation systems during the burial and exhumation of the lava pile.

A recent study has taken a step towards understanding the timing and precipitation conditions of these secondary minerals in basalts.

Exceptional Multi Stage Mineralization of Secondary Minerals in Cavities of Flood Basalts from the Deccan Volcanic Province, India - Berthold Ottens, Jens Götze, Ralf Schuster, Kurt Krenn, Christoph Hauzenberger, Benkó Zsolt and Torsten Vennemann. 

The most exciting part of this study is the publication of  absolute ages of mineralization of apophyllite using Rubidium-Strontium (Rb-Sr) and Potassium-Argon (K-Ar) radiometric methods. As far as I know , these are the first ever published absolute dates of secondary mineralization in Deccan lavas.

The scientists studied lava flows from the famous Savda quarries near the town of Jalgaon and also from quarries near the town of Nasik, both in the state of Maharashtra.The photograph below shows a portion of the lava sequence that was studied in one Savda quarry. The occurrence of secondary minerals in lava cavities is shown in the lava profile to the right. All the following images in this post are from the paper linked to above.

This study concentrated on examining the mineral sequences found in the large cavities in the central portions of a lava flow. Smaller amygdules (fully filled cavities) and vesicles (partially filled small cavities) occurring at the bottom and top of lava flows won't contain the entire sequence of minerals.

The investigation revealed three broad stages of mineralization. Stage 1 sequence precipitated first. It consists of an iron-magnesium and potassium bearing clay layer (containing celadonite and smectites) coating the walls of cavities and microbial films and filaments.  Stage 2 sequence consists of calcite (calcite 1), fine grained zeolites (zeolite 1), and plagioclase, followed by a layer of chalcedony and quartz.  This is followed by a second generation of larger calcite (calcite 2) and zeolite (zeolite 2) crystals. The common zeolites, both in zeolite 1 and zeolite 2 stages are heulandite, and stilbite with additional mordenite observed in zeolite 1 assemblage. Stage 3 sequence is made up of a third generation of calcite (calcite 3) , along with apophyllite and rare powellite (calcium molybdate). 

This multi-generation mineralization sequence could only be ascertained by carefully noting down the mineral sequences appearing in hundreds of different large cavities. Such a broad examination is necessary since the entire sequence may not have crystallized in any one cavity. The two pictures below demonstrate this problem.

On the left is a cavity which shows two generations of calcite (Stage 2) grown on a clay-chalcedony substrate (Stage 1). On the right is apophyllite (Stage 3) directly overlying Stage 1 mineral encrusted bio-filaments. In both these cavities, the Stage 2 zeolites have not precipitated. In some other cavity one might encounter a sequence of Stage 2 large zeolites overlain by Stage 3 calcite, but not the earlier calcite and zeolites, nor the Stage 3 apophyllite. Reconstruction of the entire sequence thus requires an examination of a large number of cavities in order to work out the true order of mineral succession.

This reconstructed mineral paragenetic sequence is presented in the graphic below.

The researchers used fluid inclusion studies and carbon and oxygen isotope analysis to narrow down the temperature conditions during precipitation (fluid inclusion) and the source of mineralizing fluids (isotope analysis).

Fluid inclusions are tiny volumes of fluid that get trapped during crystal growth. The fluid may be a liquid or a vapor or both. For ascertaining temperature of entrapment, a bi-phase inclusion is selected. The sample is heated until it reaches a temperature where the inclusions change from a heterogeneous (bi-phase) state to a homogeneous (one phase) state. This homogenization temperature (range) is taken to be the temperature of fluid entrapment i.e. initiation of crystal growth.

Fluid inclusion analysis of calcite 1, calcite 2 and quartz indicated temperatures between 94 deg C to 173 deg C. Inclusions in stage 3 calcite and apophyllites indicated higher temperatures between about 140 deg C to 244 deg C.

The carbon isotopes of calcites (presented as the ratio of C13 to C12) showed depleted values due to enrichment of the lighter C12 isotope. This was taken to indicate a substantial biogenic contribution for the carbon (the lighter isotope is preferentially taken up by organisms during photosynthesis). The oxygen isotope values for calcite 1 and calcite 2 were enriched in the heavier isotope O18. Values of delO18 were + 14 - +15 for calcite 1 and +19 - +27 for calcite 2. DelO18 is a measure of the ratio of the two isotopes of oxygen O18 and O16. Such enriched values indicate magmatic source fluids, although the slightly lighter values of calcite 1 suggests mixing with meteoric water, which is enriched in the lighter isotope O16 as compared to magma and sea water (meteoric water is derived from rainfall and ends up percolating through rock as groundwater). Isotope analysis of calcite 3 is not presented in this study.

Rb-Sr and K-Ar geochronology of apophyllites shows that precipitation of apophyllite took place repeatedly as discrete events spread over a large time span. The Nasik sample yielded ages of 58 mya (million years ago) and 21 mya. Apophyllites from Savda quarry near Jalgaon yielded ages of 45 mya and 27 mya.

The observed mineral sequence along with data from fluid inclusions, isotopes and geochronology have enabled the researchers to propose the following sequence of events depicted in the schematic below.

After the eruption of a lava flow, interaction of the hot lava surface with meteoric water resulted in filling up of cracks and cavities with water, accompanied by the alteration of mineral olivine, plagioclase and volcanic glass.  Fe, Mg, Si, Al released from the rock was recombined to form clay mineral and iron hydroxide coatings on the walls of cavities and on microbial films and filaments.

As the lava layer got buried under younger lavas, the composition and temperature of the fluids evolved resulting in the precipitation of calcite 1, fine grained zeolites 1 and chalcedonay and quartz. The oxygen isotope values of calcite 1 indicates a mix of magmatic residual fluids and meteoric water. Continued burial resulted in a diminishing contribution from meteoric water. Zeolite 2 and calcite 2 phases precipitated from magmatic residual fluids as indicated by the oxygen isotope values which are enriched in the heavier isotope. Maximum burial temperatures at this stage have been estimated to be about 150 deg C.

This interpretation of Stage 1 and Stage 2 mineral assemblages having formed immediately following volcanism and during burial is in line with previous thinking regarding secondary mineralization in Deccan basalts. Data about the timing of Stage 3 however has thrown up a surprise.

Geochronology indicates that precipitation of stage 3 minerals like calcite 3 and apophyllite took place much later. Deccan volcanism ended by 64 mya across most of the province. Mineralization ages of 47 mya and 27 mya indicate that by this time considerable erosion of the lava pile would have resulted in exhumation of deeper layers and much lower burial temperatures. The fluid inclusions in calcite 3 and apophyllite indicate crystallization at temperatures between 144 deg C and 244 deg C. Some earlier work by Shrikantappa and Mookherjee on fluid inclusions in apophyllite from Savda indicate even higher temperatures reaching 280 deg C. Such boiling conditions at a shallow burial level implies the formation of a hydrothermal system. The presence of powellite, a calcium molybdate, indicates oxidizing fluids. Such a system must have formed repeatedly at widely separated time intervals. The study does not put forth an explanation of the geological events that could have triggered the formation of these high temperature fluid circulation systems.

In summary, Stage 1 and Stage 2 involves a locally formed circulation system. Elements were scavenged from adjacent regions of the lava flow and incorporated into growing secondary minerals. Stage 3 involves a larger circulation system. The concentration of vanadium in basalts is low, in the range of few tens of ppm (parts per million). In Apophyllite, the concentration of vanadium is on the order of 3000 ppm.  This suggests that fluids attained this element concentration by circulating and reacting with a large volume of basalts over a widespread area.

The researchers have stressed that their proposed explanation applies to the specific mineral sequence observed at Savda quarries near Jalgaon and the lava flows near Nasik and is not to be taken as a general explanation for secondary mineralization covering the entire Deccan basalts. That would require much more extensive sampling from different regions and stratigraphic levels. The observations from other studies and the mineral sequence I have personally observed in the Pune area though does suggest that Stage 1 and Stage 2 sequences at least are common everywhere, although the specific combinations of zeolites may vary. Early near surface reaction of hot lava with groundwater and then progressive burial with mineralizing fluids getting contributions from both meteoric water and magmatic residual fluids would have been a common trajectory of fluid rock interaction across the volcanic province.

Such a timing of mineralization, contemporaneous with volcanism and continuing after burial, has been noted from lava provinces in Iceland and Iran too. The Stage 3 event though is much younger and would depend on later geologic triggers that may differ from place to place. In eastern Iceland for example, a late stage of mineralization has been linked to heat provided by the intrusions of dikes ( sheet like bodies of magma injected along fractures). The geochronology of apophyllites in the Deccan Volcanic Province needs to be validated by more such work.

Some questions do remain.

First, nearly 20 to 40 million years after Deccan volcanism ended, what could be the source of heat for the initiation of the late stage fluid circulation systems that precipitated calcite 3 and apophyllite?

And secondly, the lack of carbon and oxygen isotope analysis of calcite 3 prevents us from identifying the source of precipitating fluids. That is a lacunae that future studies must aim at filling.


Srikantappa, C.; Mookherjee, A. Water, Aqueous, H2O-CO2 and Gaseous Inclusions in Cavity Minerals in the Basaltic Lava flows around Pune, India: Evidence for Boiling. In Proceedings of the Second Meeting of the Asian Current Research on Fluid Inclusions (ACROFI–2), Kharagpur, West Bengal, India, 12–14 November 2008; p. 176.