Darwin's Earthworms, Ocean Currents, Geology Heritage Lost

My latest set of readings.

1) Why Darwin Admired the Humble Earthworm. A delightful essay by Philip Ball on Darwin's work on earthworms. Published towards the end of his career, this book apparently sold more copies than the Origin of Species! As Philip Ball wittily observes, that tells us something about the English passion for gardening. Darwin's research on earthworms consisted of detailed observations and cleverly designed experiments, often carried out with the help of family members. 

His powers of observation and analysis remained undimmed - "Darwin reported that 80 percent of leaves he removed from worm burrows had been inserted tip first—a far from random distribution".

2) No, the Gulf Stream isn't going to shut down. The premise of the movie The Day After Tomorrow is that of a catastrophic cold snap engulfing north America and Europe, triggered by the shutting down of the Gulf Stream. This massive ocean current forms in the subtropics in the western side of the Atlantic and transports heat from the lower latitudes to northern Europe, moderating the temperatures in these northern regions. Media reports claim that recent work might be pointing to a collapse of the Gulf Stream, but as Frank Jacobs explains, people are conflating two very different current systems. 

Some studies are suggesting that the Atlantic Meridional Overturning Circulation, a much smaller and restricted circulation system, might be slowing down and might even collapse by 2050. This will result in some cooling in the Greenland and Norwegian seas, but will not affect the larger Gulf Stream. The article has a nice animation of global ocean currents which I found informative.

3) They Have Put Geology in Coffins. For long, geologists have been complaining about the utter indifference shown by successive Indian governments to our natural heritage. Here is one more example from Himachal Pradesh. Along the Kalka-Shimla highway, on the stretch between Parwanoo and Solan lay a treasure. This was a section of sedimentary rocks recording the retreat of the Tethys Sea which began after the collision between India and Asia started creating high topography. Along this stratigraphic section, marine sediments give way to freshwater deposits. The outcrop was a natural outdoor laboratory for students and researchers. Now it is gone. The National Highway Authorities of India has covered it with concrete and stone walls. Science is the big loser again. 

Arundeep Ahluwalia expresses the anguish of geologists who knew and loved this part of the Himalaya- "It forever denies coming generations any chance to study the long stretches of such highways and to the nature lovers in society the excitement of the history and grandeur of the earth".  

Read and weep. 

Field Photos: Boudinage In Sandstone

Boudinage is a structure that is developed when rock layers are being pulled and stretched. The term is derived from the French word for sausage. A rock layer is deformed by necking and is segmented into a string of sausages. The structure is best developed when there is contrast in competence or strength in a rock pile. The stronger layer is broken up in boudins, while the weaker layer 'flows' around and fills the gaps created in the neighboring boudin layer.

I wanted to showcase examples of boudins from igneous, sedimentary, and metamorphic rocks. 

The first example has been recently documented in a paper on silica rich magmatism from Sausrashtra by Anmol Naik and colleagues. The photograph shows a rhyolite with boudinage (black arrow) developed in a lava flow band. This deformation is due to layer parallel stretching during flow of a viscous lava.

 Source: Anmol Naik et.al. Geological Magazine 2023

The second instance is from Darma Valley, Kumaon,  near the village of Philum, not so far from the Panchachuli Glacier. It is a remarkable instance of boudins developing in a sandstone. Black arrows highlight elliptical and oval knobs, fragments of a once continuous sand rich layer. Notice how the surrounding thinly layered material has flown into the gaps between these boudins. The more competent sand layer broke up, while the softer clay rich material got wrinkled and warped but did not break. 

And lastly, here is an example of the more common variety of boudinage encountered in the field. This has developed in a high grade metamorphic rock. The lighter more competent layer made up of quartz and feldspar has been deformed into a series of boudins, while the mica rich grey layers are ductile. This outcrop is also from Darma Valley, Kumaon, near the village of Baaling. 

The three examples showcase deformation which produced similar structures in very different settings. Internal forces generated during flow of a viscous lava can result in folding and boudins. In sedimentary basins, similar forces may  affect a sand rich slurry moving down slope as a gravity flow or on semi consolidated sediment shaken during an earthquake. And forces acting on high grade metamorphic rocks  produce some spectacular boudins at high temperatures during episodes of mountain building.

Documenting rock deformation in the field is fun! 

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  'áccretionary 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. 

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!

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.