Sunday, July 23, 2023

Septarian Concretion from Khambhat

My friend Bhushan Panse, who is a geology enthusiast and an avid rock and mineral collector, handed this specimen to me over a coffee meeting. He had bought it from a mineral supplier from Khambhat, Gujarat.

I commented that it is a septarian concretion. These hard ellipsoidal or oval shaped lumps form in mud and silt layers by the precipitation of calcite  around a nucleus. Khambhat and many other parts of Gujarat are underlain by Mesozoic and Cenozoic age sedimentary rocks. The process of concretion formation would have taken place at shallow burial depths when these sediments were still porous and water saturated. Mineral deposition in pore spaces often takes place in concentric layers. The calcium carbonate comes from saturated marine pore water or is derived from shells as they start dissolving during shallow burial. Notice the rust to brown color of the concretion. It is likely due to the presence of iron oxide and hydroxides which formed in the pore spaces from the iron contained in clay minerals.

The term Septarian Concretion refers to the radiating cracks or Septaria (derived from Septum). Cracks come in a variety of shapes. There are radiating cracks as seen in this specimen. These cracks are wider near the center and taper outwards. Other concretions may show concentrically oriented cracks, or overlapping sigmoidal shapes. Cracks may intersect, pointing to multiple cracking events. They are filled with either calcite or silica. The crystals filling these cracks are sometimes broken and displaced, and cracks may contain mud and silt. These features indicate a variety of stresses at play in concretions interiors. 

There are many ideas on how these cracks form. They have been interpreted as shrinkage cracks due to desiccation and hardening of mud. Dehydration during chemical transformation of clay minerals is another explanation.  A third hypothesis links the formation of cracks to gas expansion released during putrefaction of organic material. 

Sedimentologist Brian Pratt has offered another novel explanation. He proposed that these cracks result due to shaking of sediment during synsedimentary earthquakes. Shaking during ground motion results in variable stress fields in the interior of the concretion forming a large variety of crack geometries. These concretions may be preserving signals of  seismicity affecting that sedimentary basin!

Here is his compilation of the large variation in septarian concretion cracks from various sedimentary basins across Canada.


 Source: B. Pratt: Septarian concretions: internal cracking caused by synsedimentary earthquakes

A geologist friend who worked with the Geological Survey of India suggested another intriguing explanation. Parts of the region near Khambhat experienced explosive volcanic activity towards the waning phases of Deccan Volcanism. Ash expelled from volcanoes can coat small broken lava fragments forming lumps known as  '√°ccretionary lapilli'. Aggregations of ash and pyroclastic material if larger than 64 mm are known as volcanic bombs. This concretion fits the size range of a bomb. The dark fragments in the center of the concretion do resemble a fine grained igneous rock. A closer examination under a microscope is needed for a confirmation of its origin.

It is fun to examine hand specimens that friends collect from various part of the world and try to identify the rocks and minerals. But often a clear cut answer is not possible due to the need for additional information from a higher resolution or the chemical makeup. But a guessing game over coffee is always welcome. 

Geodes, nodules, and concretions found in volcanic and sedimentary rocks are mystery objects. You never know what you will see inside when you break open one of these lumps. There may be an array of perfectly faceted purple amethyst crystals and multicolored calcite. Or a trapped fossil. Or a crack network filled with bright and shiny calcite and quartz. These crystal rich interiors give us important information on the composition of fluids which react with rock at many different times during their geologic history. This water rock interaction is of interest to mineralogists and  economic geologists who want to understand the history of fluid flow through sedimentary basins and the conditions that lead to the concentration and deposition of metals. 

Geological investigation at all scales inform us about how the earth works. One can stand and gape at great mountain ranges and wonder about the movement of tectonic plates. But you can also crack open a rather dull colored lump from a shale and marvel at its insides, all telling a story of groundwater flow and chemical reactions, and who knows, past earthquakes as well. 

Sunday, July 16, 2023

Links: Early Life, Critical Minerals, Net Zero Emissions

Some interesting readings over the past couple of weeks.

1) In a Fierce Desert, Microbe ‘Crusts’ Show How Life Tamed the Land. Zack Savitsky describes a fascinating research program underway to study microbial life in the fierce Atacama Desert in Chile. These organic communities are called 'grit crust'. They coat pebbles and large sand grains on the desert floor. A closer examination has showed that they are composed of hundreds of species of cyanobacteria, algae, fungi, along with lichen combinations. These microbes play an important role in the biogeochemical fixation and cycling of elements and over millennia break down rock and enrich the soil in nutrients. They also give scientists a glimpse on how life may have first colonized land, perhaps as early as the Archean. 

2) Digging into India’s critical minerals dash. Recently, the Indian Ministry of Mines released the Critical Minerals for India report. India has also approved commercial mining for minerals containing the following elements —lithium, beryllium, titanium, niobium, tantalum and zirconium. These elements are raw materials for magnets used in wind turbines and EV motors. M Rajshekhar writes about the geological distribution of these elements and global supply chains, pointing out the challenges India faces in procuring a reliable supply of these materials necessary for its energy transition away from fossil fuels. 

3) Explainer: Will global warming ‘stop’ as soon as net-zero emissions are reached? Even if we stop emitting CO2 today, we are locked into hundreds of years of warming because of past emissions. I too have uttered these words, but it turns out that I, and many others, have been conflating two separate scenarios: constant concentrations versus zero emissions. Zeke Hausfather clarifies what these two different situations mean and how they will differently impact CO2 levels and future warming. 

Net zero CO2 scenario will very quickly stabilize temperatures or even slightly cool the earth, but that will also depend on emissions of  other confounding variables like aerosols and other greenhouse gases. Aerosols by blocking sun's energy have a cooling effect. On the other hand, achieving net zero too far in the future might still result in warming due to the feedback effects of other greenhouse gases like methane and nitrous oxide. Good explainer!

Tuesday, July 4, 2023

Chronostratigraphy: Beats Of The Geological Time Scale

The International Commission of Stratigraphy has published an updated chronostratigraphic chart.

You can download a high resolution pdf as well as a jpg version. 

This is a good resource if you want to know the time spans for all those hard to pronounce units that geologists keep referring to in their talks and articles. 

The columns in the figure are hierarchical divisions of geologic time based on the rock record. Take for example the columns on the extreme right. Proterozoic Eon is an interval of time defined by rocks that formed between 2.5 billion years ago and ~539 million years ago. The Neoproterozoic Era is a subdivision defined by rocks formed between 1 billion years ago and ~539 million years ago. And the Ediacaran Period is a time unit represented by rocks formed between 635 million and ~539 million years ago. A significant change occurred around 539 million years ago. Shelly fossils began appearing in the rock record. This marks the end of the Proterozoic and the beginning of the Cambrian Period.  

Notice something interesting? 

Take a look at the internal divisions of the Archean and the Proterozoic Eons. The Archean is divided into fairly uniform 400-300  million years units, while most of the Proterozoic is divided into roughly 200 million years units.

In contrast, the Phanerozoic Eon subdivisions have unequal age spans. By around 550 million years ago, unicellular and multicellular creatures evolved the ability to build hard skeletons made up of calcium carbonate, calcium phosphate, and silica. This resulted in better preservation of their remains in the rock, giving geologists the ability to track evolutionary changes. The boundary between different subdivisions you see on that chart is a recognition of the appearance of a new species or species assemblage. Evolution's clock has an irregular beat, making for unequal chrono units.

More uncommonly, boundaries are defined by chemical shifts that mark a change in the earth's environment. For example, the newest subdivision of the Quaternary Period, the Meghalaya Age, is recognized by shifts in the oxygen isotope composition of stalactites from a cave in Meghalaya, signalling global drought events beginning 4.2 thousand years ago.

The Precambrian was a microbial world. Microbes too were undergoing continual evolutionary changes, but they did not build hard skeletons and their tiny soft remains don't preserve well at all. Geologists can't use microbial evolution to subdivide the Archean and the Proterozoic. There are no systematic temporal changes in rock types as well.

The boundary between the Archean and the Proterozoic does represent a significant transition in earth processes. It marks a shift from a hotter and more tectonically active Archean to a cooler earth, where large rafts of continents had formed. Many sedimentary basins that developed on this continental crust did not undergo the intense deformation and metamorphism observed in Archean sedimentary and volcanic rock associations. But, like the Archean, through most of the Proterozoic too, there is no uniquely changing fossil record to act as anchors for subdivision. 

By around 700 million years ago, rapid global changes in climate and the evolution of larger organisms has allowed for a finer resolution of the rock record. Both the Cryogenian, which preserves signals of widespread glaciations, and the Ediacaran, with a better fossil record of larger multicellular creatures, are smaller subdivisions of the Proterozoic.

Through my geology career, I have been lucky to have worked on rocks from across the time scale.  My M.Sc. thesis was on Orosirian, Statherian and Tonian age rocks from the Proterozoic. My Ph.D research was on Katian and Hirnantian age rocks from the Ordovician Period. And I have done smaller projects on the Oxfordian of Jurassic, and some imprecisely dated rocks from the Eocene Period.