Saturday, March 18, 2017

Comments On The 1.6 Billion Year Old Red Algae From Central India

The Proterozoic Vindhyan sedimentary basin in Central India contains sediments ranging in age from 1.7 billion years to about 600 million years ago. Bengtson and colleagues report three dimensional preservation of cellular structures which they interpret as multicellular red algae. These fossils have been found in the Tirohan Dolomite dated to about 1. 6 billion years. Before this discovery, the earliest fossils of multicellular eukaryotes was the rhodophyte Bangiomorpha, dated to about 1.2 billion years.

The Tirohan Dolomite is exposed in the Chitrakoot region of Madhya Pradesh. The fossils occur in patches of carbonate sediment which was replaced by the calcium phosphate mineral apatite just after their deposition in a shallow marine setting. Phosphotization is often a very delicate process enablng the preservation of fragile cell structures.

Here is a picture of the cellular structures of red algae imaged by SEM (scanning electron microscope)



Source: Bengtson et.al. 2017

And another rendering of the three dimensional structure of the red algae imaged using Synchrotron-Radiation X-ray Tomographic Microscopy (SRXTM). The green objects inside the cell are interpreted to be organelles, components of eukaryotic cells which aid in different physiological functions. Prokaryotes (Bacteria) lack such organelles.


Source: Bengtson et.al. 2017

I don't want to dwell on this study too much. The paper is open access for those who want to explore further.

There are two side stories that I want to comment upon.

First. The Tirohan Dolomite and its fossil assemblage has a controversial past.

They were discovered about twenty years ago by Dr Rafat Azmi, a paleontologist working with the Wadia Institute of Himalayan Geology. He reported from the Rohtasgarh area in 1998 a rich trove of filamentous and spherical forms, and odd shaped mineral fragments. He interpreted the mineral fragments as "small shelly fossils" representing fragments of animal shells and the spherical forms as possible animal embryos. Later in 2006 he reported tubular forms which he interpreted as Cambrian animal taxa. The problem was that animals are thought to have evolved by the latest Neoproterozoic- early Cambrian (600 mya -540 mya), while the understanding then was that the Tirohan Dolomite is likely 1 billion to 1.5 billion years old. Azmi's interpretation carried two enormous implications; either a) the Tirohan Dolomite was much younger in age. This would have required a major revision of the ages of Vindhyan sediments or b) that the rocks were old (~1.5 billion years), but that animals evolved much earlier than the current fossil record indicated.

These very significant implications caught the attention of geologists and media alike. The Geological Society of India sent a team to investigate Dr. Azmi's claims. They reported that they were unable to find the fossils Dr. Azmi had claimed to have found.

 Memories of an earlier scandal in Indian palaeontology were still fresh. In the late 1980's Vishwajit Gupta of Punjab University was found guilty of fraud and plagiarism. He had been misreporting fossil discoveries from the Himalayas by using museum specimens from all over the world. He had  constructed an entirely fake narrative of Himalayan fossils and stratigraphy. Scientific journals were forced to retract his papers. The Paleontological Society of India produced a book authored by S. K Shah titled "The Himalayan Fossil Fraud".  Punjab University, disgracefully, allowed Dr. Gupta to remain in service till he retired in 2004.

Under this shadow, Azmi's fossils came under similar suspicion. Fortunately, Bengtson and colleagues in a study some years later confirmed that these fossils do exist in the Tirohan Dolomite. However, they sampled the Tirohan Dolomite at Chitrakoot and not its stratigraphic equivalent (Rohtas limestone) at Rohtasgarh where Dr. Azmi's initial claims came from. They established using absolute radiometric dating that the Tirohan Dolomite is 1.6 billion years old. And they showed that the forms, similar to those Dr. Azmi found, are not multicellular animals. The spherical forms were all likely gas bubbles. Some of the larger tubular forms were revealed in the present study as red algae. Animal evolution didn't take place that early after all. The claim of the "small shelly fossils" has not been resolved fully. Bengtson and colleagues work doesn't address them. Some other researchers though have interpreted them as non-biogenic mineral growths. The stratigraphy and broader fossil content of the Rohtas limestone from where Azmi collected his fossils firmly indicates that it is not Cambrian but Proterozoic in age. .  In this present paper, these scientists have named one of the red algal forms Rafatazmia chitrakootensis in honor of Dr. Razat Azmi.

The second comment I have is on multicellularity. These red algae are the oldest multicelluar eukaryotes found anywhere. Plants, Fungi, Protists (amoebas) are eukaryotes.  They share a common eukaryote ancestor which was unicellular. That means there was just one origin of the eukaryotic cell type. However, multicellularity has evolved many times independently in different branches of the eukaryote family.

Multicellularity comes in different flavors. In simple forms of multicellularity, organisms are made up of sheets and aggregates of cells sticking to one another. There is differentiation of somatic and reproductive cells. Communication between cells is limited. One important aspect is that all the cells are in direct contact with the environment, since in these organisms, nutrient transfer takes place by diffusion from the environment to the cell. More complex types of multicellularity require the evolution of not just cell to cell adhesion, but elaborate cell to cell communication systems and a division of labor i.e. cells specialized for different functions. Also, these organisms have a three dimensional arrangement of cells wherein only few cell types are in direct contact with the environment. Diffusion is not efficient enough to supply internal cells with all the necessary life support. Molecular conduits and tissues that facilitate bulk transport and circulation of nutrients need to evolve to build this type of multicellularity.

The figure below shows the many origins of the complex type of multicellularity (in red) in different eukaryotes branches.


Source: Andrew H Knoll 2011

Based on cell type, life is divided into two domains. The Prokaryotes (Bacteria and Archaea) have smaller simpler cells. Eukaryotes are generally larger and are made up of more complex cells. This cell type evolved by a symbiotic merger between two types of prokaryote cells. Prokaryote fossils have been found in rocks older than 3 billion years. The eukaryote fossil record begins in rocks younger than 2 billion years. The timing of the origin of eukaryotes is unclear. Estimates range from  2.5 billion to 1.5 billion years ago. These red algae fossils show that eukaryotes had already diverged into different branches by 1.6 billion years ago, which means that the unicellular ancestor of eukaryotes evolved before that. It also means that red algae took the road to multicellularity much earlier than animals.

Does complexity evolve necessarily whenever genetic potential is available or does it depend on ecologic opportunity? If the cellular machinery and the underlying genetic regulatory systems required for multicellularity evolved in the ancestors of red algae by 1.6 billion years ago, why did multicellular animals not evolve earlier as well? It could well be that there were ecologic conditions limiting the evolution of physiologically demanding creatures like animals. The end of Neoproterozoic ice-ages by about 650 million years ago and the break up of supercontinent Rodinia impacted sea water chemistry. Sea water oxygen increased to threshold levels permitting a more active life style. Increased weathering of continents brought into the oceans metals like zinc which are crucial for physiological functions. Creation of larger continental shelves and shallow water zones due to continental breakup provided varied ecologic spaces for diversification. Animal evolution was triggered in this ecological context.

Wednesday, March 8, 2017

Papers: Tectonics And Physical Volcanology Of Deccan Traps

There are plenty of research papers on the geochemistry of the Deccan Basalts. But nature lovers and trekkers like me come face to face not with chemistry but with the physical forms of lava and the structural elements of the volcanic pile.

I found this list of papers most useful. They have helped me sort out my confusions regarding lava morphology and taught me something about the structural fabric of the western margin of the Deccan Volcanic Province.

1) Near N–S paleo‑extension in the western Deccan region, India: Does it link strike‑slip tectonics with India–Seychelles rifting? - Achyuta Ayan Misra Gourab Bhattacharya, Soumyajit Mukherjee, Narayan Bose

This is a structural analysis of the fracture systems that cut across the western margin of the Deccan province.  The area of study is the coastal plains, about 100 km north and south of Mumbai. The Indian western margin is a rifted margin i.e. it formed by the breakup of India with Madagascar (88 million years ago) and then Seychelles (64 million years ago). This type of margin is formed by tensional forces splitting apart continents and so you would expect normal faults, wherein blocks of crust have moved down along inclined fault planes.  Except here, the researchers find evidence of strike slip movement along sub-vertical fault planes. This means crustal blocks slid past each other. This implies oblique rifting with components of both extension and transverse movement between India and Seychelles. There are some really revealing field photos of this transverse (strike slip) movements.

2) Geology of the Elephanta Island fault zone, western Indian rifted margin, and its significance for understanding the Panvel flexure- Hrishikesh Samant, Ashwin Pundalik, Joseph D’souza, Hetu Sheth, Keegan Carmo, LoboKyle D’souza, Vanit Patel

Wait a minute. There are normal faults with downthrown blocks in this region too. And from the famous Elephanta Island. The fault planes dip eastwards producing easterly downthrows. That means the easterly crustal block has moved down. Again, some good field photos of fault planes and slickensides ( fault surfaces which get a polished striated appearance due to the frictional movement of rocks). These faults with easterly downthrows are found all along the west coast.  There is one near the proposed site of the nuclear power plant at Jaitapur in southern Maharashtra, which shows signs of intermittent movement over the past fifty thousand years. So, there is a very practical reason for understanding these faults.

3) Deccan Plateau Uplift: insights from parts of Western Uplands, Maharashtra, India- Vivek. S Kale, Gauri Dole, Devdutta Upasani and Shilpa Patil Pillai

This is a study of part of the Deccan plateau. I visited this region a few weeks back.  Very useful information of the various fracture systems that cut across the stacks of lava and their significance in terms of recent (Quaternary) crustal movements and controls on the drainage systems. Well thought out block diagrams illustrate the authors ideas very clearly.

4) Pahoehoe–a'a transitions in the lava flow fields of the western Deccan Traps, India-implications for emplacement dynamics, flood basalt architecture and volcanic stratigraphy-  Raymond A. Duraiswami, Purva Gadpallu, Tahira N. Shaikh, Neha Cardin

Good explanations of the morphology of basalt lava flows.  I really liked the sketches showing the internal structure of lava flows and the emplacement of pahoehoe lava fields with its transformation into transitional and a'a type lavas. Very useful guide for my next outing into the Deccan basalts!

Saturday, February 25, 2017

Field Photos: Western Uplands And Giant Plagioclase Basalts

Last Sunday I visited Chavand fort near the town of Junnar, about 110 km north of Pune. This is a rugged terrain marked by several NW-SE oriented ridges separated by broad U shaped valleys. In the map below the black cross marks the location of Chavand. WGE refers to the Western Ghat Escarpment and KCB refers to the Konkan Coastal Belt. Trekkers familiar with this region will recognize the hill ranges, especially the Bhimashankar range and the Harishchandragad range. And near the town of Junnar is Shivneri fort, birthplace of the Maratha king Shivaji.


Source: Kale et. al, 2016

When Deccan volcanism ended, this region would have been a vast flat- to- gently undulating lava surface. At that time, some 60 million years ago, you could have walked from where Bhimashankar temple now stands to the present location of Harishchandragad along broadly the same elevation without the need to climb down several hundred feet or so into a valley and then climb up again. Over time however, south easterly flowing rivers and tributaries have gouged out grooves within this large plateau, dissecting it into a valley and ridge terrain.

The geomorphology of this region therefore reflects the creation of relief due to removal of material by erosion. This contrasts with other areas like the famous Basin and Range Province in western United States, where parallel faults have moved blocks of crust hundreds of feet to form a system of flat bottomed valleys (grabens) and flat topped ridges (horsts).

The Western Uplands end abruptly along the Western Ghat Escarpment, a sinuous west facing cliff overlooking the Konkan coastal plain. The escarpment is the edge of the Deccan Plateau.

There is another factor that has shaped this landscape. Take a look below at a satellite imagery of this area. The arrows mark fracture systems that have broken this plateau.


Erosion along these zones of weakened rock results in slabs of basalts peeling on rock faces. Over time, the result is a landscape that fragments into mesas, buttes and pinnacles. Chavand is one such mesa. Notice its straight edged polygonal shape suggestive of erosion along fracture planes.

Many of these fracture systems originated in the tensional forces that the western margin of India experienced during rifting and associated Deccan volcanism. After its separation from Madagascar around 88 million years ago, the Indian continent's rifted and the fractured western margin migrated over the Reunion hotspot, an unusually hot area of the mantle. The result, beginning around 68 million years ago was Deccan volcanism. Some of these linear structures, common in the area around Sangamner, are dikes. They are the pipes which brought up magma from deep subsurface chambers to the surface. Continued rifting of the western margin resulted ultimately in crustal blocks subsiding along a series of N-S oriented parallel faults. The Western Ghat Escarpment likely originated as a west facing fault scarp, but it would have been located as much as a 100 km to the west of its present location. Erosion over million of  years has resulted in an eastward retreat of this feature to where it stands now.

Picture below shows how hill ranges have been broken by a fracture system, resulting in isolated pinnacles.


And here is a picture of Chavand.


Recently my friend Vivek Kale and colleagues complied some very interesting geomorphologic, structural and sedimentology data to suggest that these western uplands have experience some tectonic movements during Quaternary times (past 2.58 million years). They emphasize that the Deccan plateau and Western Upland should not be regarded as a monolithic stable crust block. They point to three major fracture systems (F1, F5, F7 in the map below) which have segmented this part of the western upland. The central segment, i.e. the area north of Chavand, roughly between fracture systems F1 and F5 has moved up relative to the blocks to the north and south. The presence of sediments deposited in the Pravara river system and along F1 and some streams to the south  is evidence that the these blocks subsided somewhat, resulting in stretches of streams becoming sediment traps.


Source: Adapted in Kale et. al 2016 from Dole et.al. 2000, Dole et. al. 2002, Bondre et.al. 2006

These sediments represent deposition over the past 100,000 years or so. At some localities along the Mahalungi river, they have been deformed. Soft sediment deformation structures such as slumping, load structures and sand dykes have been recorded by Dole and colleagues. Such structures are evidence of ground shaking and sediment liquefaction and remobilization during earthquakes.  Also observed at one locality is reverse faulting. The faulting has been inferred to be of Holocene age, as recent as the past 10,000 years.

These structural movements have also disrupted and modified the antecedent drainage of this region. The map above shows several easterly flowing streams (R. Mahaludi, R. Adula, R. Mula, R. Madvi, R. Pushavati) in this region abruptly turning southeast as they intersect NW-SE trending fractures. The yellow overlay on the map indicates sedimentary deposits. The major fracture systems F1, F5, F7 likely reflect faults in the Precambrian continental crust underlying the Deccan volcanics. They have been rejuvenated in Quaternary times and have cut across and caused dislocations of the volcanic pile.

There are other interesting drainage features in this region. Many streams have stretches with potholes (Nighoj on R. Kukdi), cascades (stretches of R. Pushpavati, R. Mula, R. Ghod, R. Bhama) and entrenched meandering channels (R. Pravara, R. Mula, R. Ghod, R. Vel) all suggesting episodes of increased vigor of stream down cutting. Whether this is a climatic signal (e.g. increase in rainfall will increase water flow and stream erosive power) or is tectonically triggered (e.g. slight uplift and  tilting of land will increase stream gradient, resulting in more vigorous stream flow and down cutting), as Kale and colleagues have recently argued, is an active area of research.

Finally, the giant plagioclase basalts. Plagioclase, which belongs to the feldspar family of minerals, is a major component of basalts. The entire Deccan volcanic lava sequence is subdivided into three subgroups based on geochemical differences. The giant plagioclase basalt lava flows occur predominantly in the lower part of the volcanic sequence. The table on the left (Kale et. al. 2016) shows the geochemical stratigraphy of the Deccan volcanic sequence with the location of the giant plagioclase basalts (GPB). The GPB flows cap individual formations within the Kalsubai Subgroup. This has been  interpreted by many geologists to mean that they mark the final eruptions of a magmatic cycle. Because of their distinctive appearance these GPB flows have proved to be useful as marker horizons in stratrigraphic mapping.

The plagioclase crystals are greater than 1 cm and often have grown to several centimeters long. They are surrounded by a fine grained to glassy matrix. The picture below is a close up of a giant plagioclase basalt, showing tabular plagioclase phenocrysts (arrows).


Geologists agree that these giant crystals grew slowly in magma chambers tens of kilometers deep in the subsurface. These crystals were then brought to the surface by ascending magma, which then cooled rapidly on the surface forming a fine grained to glassy matrix. This two stage crystallization history is the reason for the two distinct grain sizes in this rock.

The picture below shows Chavand fort hill face. The giant plagioclase basalt lava flow makes up the shrubby gentler slope.  These basalts are vesicular (containing pits and holes due to trapped gas bubbles in the lava) and softer and have weathered to form the gentler slopes. The upper harder and more compact basalt forms cliffs.  


There are differing views however on how long these crystals were growing in the subsurface and what that implies for the mode and duration of Deccan eruptions. I"ll leave the details for another post. Meanwhile, here is another picture of the giant plagioclase basalt showing lath shaped plagioclase grains (1) and a rosette of plagioclase phenocrysts (2).


...And a few more pictures of the terrain as seen from the top of Chavand.

A large water tank dug out from the hard basalt


Mesa top grasslands give way to the rugged ranges of the Western Uplands. This is a north facing view with the Harishchandra range at the far end.


A south facing view with the Bhimashankar range with its forested plateaus.


..did I mention it was a pretty steep climb?


Until next time...

Monday, February 20, 2017

Aristotle And Darwin: Why?

A thought provoking passage from The Lagoon: How Aristotle Invented Science by Armand Marie Leroi-

The history of Western thought is littered with teleologists. From fourth-century Attica to twenty-first century Kansas, the Argument from Design has never lost its appeal. Aristotle and Darwin, however, share the most unusual conviction that though the organic world is filled with design there is no designer. But if the designer is dead for whose benefit is design? It's the prosecutor's question: cui bono?

Darwin answered that individuals benefit. Biologists have batted the question about ever since. The answers they've essayed are : memes, genes, individuals, groups, species, some combination or all of the above. Aristotle, however, generally appears to agree with Darwin: organs exist for the sake of the survival and reproduction of individual animals. This is why so much of his biology seems so familiar.

Yet there is a deep difference between Aristotle's teleology and Darwin's adapatationism, one which appears when we follow the chain of explanation that any theory of organic design invites. Why does the elephant have a trunk? To snorkel. Why must it snorkel? Because it's slow and lives in swamps. Why is it slow? Because it's big. Why is it big? To defend itself. Why must it defend itself? Because it wants to survive and reproduce. Why does it want to survive and reproduce? Because..

Because natural selection has designed the elephant to reproduce itself. Darwin gave teleology a mechanistic explanation. He halted the march of whys.

Aristotle was an eternalist. In his cosmos, organic beings were produced by a union of their parents. They in turn by a union of their parents.. continued into infinite regress. Organisms wanted to survive and reproduce so that their "kind" persist for ever. This static world had always existed. He saw divinity in immortality.

In Aristotle's world organic beings don't change and transform into anything else. He never did come around proposing a theory of transmutation or change or evolution. He had some of the raw material to advance such thoughts.

His extensive dissections and comparative anatomy had given him an understanding that life is arranged in a hierarchical manner. Dogs and foxes are more closely related to each other than either is to lions and leopards. Both, canids and felines though are part of a larger mammal family. Members within this family are more similar to each other than they are to members of reptiles. Aristotle recognized that there are groups within groups. He termed the basic taxonomic units as genos. These are part of the larger magiste gene. Ikthis (fish), entoma (insects),ornis (birds), zootoka tetrapoda (live bearing tetrapods- mammals), oiotoka tetrpoda (egg laying tetrapods -reptiles, amphibians) were some of his "greatest kinds" or  magiste gene.  Yet, he didn't ponder upon the relationship between life's groupings and never realized that this pattern is a tree like structure, resulting from common ancestry and subsequent lineage branching. The figure to the left is Darwin's sketch (Notebook B 1837) of the tree of life depicting common ancestry and the branching nature of life.

Aristotle was aware that there is variation within each "kind" or genos. Organisms within a genos varied in their eidos or form. He also had a theory of inheritance. Parents passed on their eidos to progeny, often not exactly. Progeny then, occasionally, could be somewhat different from parents. Moreover, his understanding of inheritance was surprisingly modern. A child might inherit her father's nose or her mother's nose but not something in between. So, he had a particulate view of inheritance. This also meant that traits remain stable and may pass on unaltered over many generations.  Father's and mother's contributions don't blend to form some average feature, but remain discrete. By this he did not mean that actual particles were being transmitted. Rather, traits were reproduced by the movement and heat of either the semen or 'menses' (menstrual fluid). Whichever was stronger determined whether the child resembled her father or mother.

Darwin never understood inheritance very well. He was sure that variation is the  fuel of evolution. But he was troubled that blending would wipe out variation in populations. He struggled with the problem of inheritance for decades.

Aristotle's limitation was that he didn't think anomalous features or differences offered any advantages to the individual. His view was that creatures are born within the limits of their physiology and there was no room for improvement. In that, he should have listened to Socrates! In a Greek society with very particular notions of beauty, Socrates, with his snub nose and flabby lips was an anomaly. He boasted though that his lips worked better than anyone else's. The mutant feature provided an advantage. Aristotle rejected such notions.

Perhaps this is why he couldn't take the next step.... That advantageous variation could be selected upon by nature. Possessors of that trait would on average leave behind more progeny. A certain new form would thus become more common over generations and while some other form disappeared. Some people argue that the lack of a fossil record may have been one reason why Aristotle never appreciated that organic beings have changed over time. However, as Leroi describes, Greek travelers and physiologoi (naturalists) from his time had written about fossil sea shells found high on mountains and fish imprints on stone. Theophrastus, his student and protege, describes dug up ivory. Dwarf elephants were among the many remains from the Pleistocene megafaunal fossil beds of Samos, Kos and Tilos islands.

To someone wedded to an unchanging cosmos, this may not have made any difference, Leroi argues.

It would be more than two millennia before Darwin and Wallace put all the pieces together.

Highly Recommended.

Thursday, February 9, 2017

Why Is There A "Lost Continent" Underneath Mauritius

Yes, the term "lost continent" brings up visions of a lost world full of fantastic creatures that once existed deep in the earth's past. Or, of a civilization that once was, but was swallowed up by rising seas and which now only remains on the margins of human memory.

The "lost continent" underneath Mauritius is making news. It is more accurate to say that there is continental crust underneath the oceanic lavas of Mauritius. And that continental crust is very old. Geologists found crystals of zircon in young lava that erupted on Mauritius about 5.7 million years ago. The age of the zircon is however Archean in age, between 2.5 billion - 3 billion years. That means the zircon crystal did not form in the young lava, but belongs to the older foundation of the island. They were extracted from this Archean crust by rising molten material and brought to the surface about 5. 7 million years ago.

The crust making up the earth continents is primarily made up of granitic and andesitic rocks and sedimentary cover. This crust is light and thick (30km-40 km) and it sticks above sea level.  On the other hand, the crust making  up the ocean basins is made up of basalt and is denser and thinner (~10 km). So, what is Archean continental crust doing in the middle of the Indian ocean, surrounded by Cretaceous-Cenozoic oceanic basaltic crust?

The answer lies in the way Gondwanaland broke up, or rather the way India broke away from Madagascar about 88 million years ago. This was a continuation of the progressive breakup of Gondwanaland that began in the late Jurassic about 150 million years ago. A large rigid continent need not break into two clean pieces. Very often, the edges splinter. Several smaller fragments of continental crust are left isolated near the edges of the two continents.

The map below shows these continental splinters scattered in the Indian ocean as Madagascar and India broke apart and drifted away from each other.


Source: Lewis D. Ashwal, Michael Wiedenbeck, and Trond H. Torsvik 2017

Mauritius is part of a series of splinters that collectively are called Mauritia. These splinters were part of the Archean continental nucleus that made up Madagascar and the western Dharwar craton in south India.

Here is the interesting part that many news reports haven't touched on. Look closely at the map above. Trace the Carlsberg Ridge southwards. The Indian plate, which is drifting northwards, lies to the east of this ridge and the African/Somali plate to the west. Today, Seychelles and Mauritius is on the African/Somali plate and Chagos and the Laccadives on the Indian plate. But, when the initial separation happened about 84 million years ago, Seychelles and most of Mauritia were on the northerly drifting Indian plate. This is because 84 million years ago, the plate boundary between the Indian and African plates was formed by sea floor spreading in the Mascarene basin.

This is depicted in the  paleo-geographic reconstruction below. At 65 million years, the CIR or Central Indian Ridge is where sea floor spreading is forming the Mascarene basin. Seychelles and Mauritia lie to the east of this ridge on the Indian plate.

Source: Shankar Chatterjee et. al. 2013

Later, beginning around 62 million years ago and continuing up to about 41 million years ago, the loci of sea floor spreading jumped eastwards. The result was the formation of new plate boundaries between the Seychelles and Laxmi Ridge (62 mya) and between Mauritius and Chagos/Laccadives (42 mya).  These "ridge jumps", as they are called, formed the Carlsberg Ridge and  transferred Seychelles and Mauritius on to the African/Somali plate. Continued northward drift of India coupled with sea floor spreading and the formation of new oceanic crust along the Carlsberg Ridge has formed the broad oceanic basin of the Arabian Sea/ Indian ocean.

The process of continental breakup involves extensional forces that stretch and thin the crust. Fault movements cause a subsidence of crustal blocks. Many of the splinters at the edge of major continental margins are such thinned downfaulted blocks. They thus often get submerged under the sea.

Open Access