Tuesday, May 30, 2023

Homo Sapiens Origins

Did our species evolve from an isolated population of ancient humans in one part of Africa in a relatively quick branching event? Or, was our evolution a continent wide process with many groups of ancient people from different parts of Africa exchanging genes over long periods of time? The time period of interest based on genetics and fossils is from roughly 700,000 years ago to about 50,000 years ago.

There is a wealth of recent reading material on the topic of Homo sapiens origins. I am sharing two articles which I found very useful in understanding how paleo-anthropologists are using fossil and genetic data to come to a better understanding of our origins.

1) The Origin and Evolution of Homo sapiens -  Chris Stringer

2) Pan‐Africanism vs. single‐origin of Homo sapiens -  Putting the debate in the light of evolutionary biology- Andra Meneganzin, Telmo Pievani, Giorgio Manzi. 

I liked the second one better. I felt it outlined the differing views clearly, pointing out the problems genetics and evolutionary theory present for some of the more extreme scenarios out there. It assesses the African Middle Pleistocene fossil record (0.7 -0.1 million years ago) very well, while attempting to reconcile it with expected evolutionary patterns. 

There are articles that concentrate on the new genetic data of human populations. But being a geologist, my worldview and indeed my understanding of the subject matter biases me towards fossils. 

So have at it. Both are open access.

Tuesday, May 23, 2023

Panchachuli Glacier Area Landscapes

After a gap of four years, I visited Darma Valley again earlier in the month. This well known trekking destination is situated in the Kumaon Himalaya of Uttarakhand. With me were a bunch of nature lovers eager to understand the geology of the Himalaya. This was a geology tour organized by Deep Dive India. I accompanied the group as the geology expert.

We spent most of our time in the high grade metamorphic terrain of the Greater Himalaya and the low grade metamorphic Tethyan realm. The Panchachuli Glacier and its deposits provided for discussions on climate change and  a more recent geologic past. Reaching these remote locations meant a day long drive through the winding roads of the Lesser Himalaya. We made quite a few stops to observe different types of strata. The highlight was a road cut that exposed a synclinal fold, a beautiful example of the nature of deformation pervading the Lesser Himalaya.

I'm sharing a few images of the landscapes around the Panchachuli Glacier, which is the main attraction of this region. Although, as our group discovered, there is more exciting and varied geology to observe here.

Dantu, our base village was cold when we arrived in the evening. A light snowfall made for a memorable experience. Permanent link for this video- Dantu Snowfall

A dusting of white lingers on the forest tops after the night's snowfall. A view from Dantu.

A stand of Bhojpatra (Himalaya Birch) lights up the High Himalaya slopes. 

The weather had cleared up by the next day and we were treated to majestic views of the Greater Himalaya. 

A burbling icy stream made for a soothing break on the high walk towards Panchachuli base camp. In case you are unable to view the video in the post, click on this link - Icy Stream Panchachuli.

The Post Master's home in village Naagling. For decades this beautiful home doubled up as a guest house for weary trekkers. The family no longer lives here. I hope there is a way to maintain this heritage structure. 

Explaining Miocene granite intrusions and Himalaya mountain building. Can anyone ask for a better classroom? 

Join me next time!

I'll also be writing about some conglomerate and sandstone boulders that I keep observing in these high Kumaon valleys. The outcrops are not accessible to me, but I have some ideas on what these rocks signify.

Monday, May 1, 2023

Pavement Geology: Ophicarbonate

You must have come across polished slabs of this dark green rock with white veins, used on stairways and as wall panels. It is an ophicarbonate.

The term encapsulates a range of geological processes, often separated by tens of millions of years. Oceanic lithosphere that is being generated by magmatism at mid ocean ridges, where plates diverge, is made up of igneous rocks arranged in a sequence. The top layer is basalt lava erupting and forming oceanic crust. Much lower down are ultramafic rocks like peridotites, composed mainly of the mineral olivine. In this lithospheric rock sequence, the boundary between the mantle and the crust is taken to be the transition from denser ultramafic rocks like peridotite to less dense feldspar containing rocks like a gabbro. Peridotites are rocks from the earth's mantle.   

As this newly formed lithosphere (tectonic plate) moves away from the active ridge, faulting can exhume these peridotites to shallower levels. The lithosphere is still warm and magmatic fluids and heated sea water hydrate and alter this peridotite to the mineral serpentine. Calcite can also form during these reactions, if these fluids are alkaline and bicarbonate rich. Such processes of serpentization have been observed, for example, at the Lost City hydrothermal vents in the Atlantic Ocean, where highly alkaline fluids are altering peridotite and precipitating large quantities of carbonate on the sea floor.

Being a part of a drifting tectonic plate, such serpentinized and carbonated peridotites eventually arrive at a subduction zone, where oceanic lithosphere is sliding underneath another plate. Slices of oceanic lithosphere including this altered peridotite gets scraped off along thrust faults and emplaced in a growing mountain chain. These fragments of oceanic lithosphere preserved on land along zones of plate convergence are called ophiolites. And the serpentine and calcite bearing altered peridotite is called an ophicarbonate.  

Alteration of the peridotite to serpentine and calcite can also occur during subduction by reaction with hydrothermal fluids expelled during metamorphism of buried sediments. During this alteration process the peridotite is crushed and acquires a broken fragmented appearance with veins of calcite surrounding blocks of serpentine. 

This example shows very clearly the brecciated nature of an ophicarbonate. You can observe fragments of green serpentine floating in and surrounded by large veins of calcite. 

Ophiolites and pockets of ophicarbonate are found all across the northern margin of the Indian tectonic plate from Ladakh and tracing the convergence zone southeastwards and south to the Naga Hills and the Andaman Island chain. 

Such deep sea processes are of interest to geologists who study the long term cycling of chemical elements on Earth. Alteration of  peridotites via carbonation reactions traps dissolved inorganic carbon in carbonate minerals like calcite and dolomite. The vast bulk of such altered rocks sink into the mantle along subduction zones, sequestering that carbon in the earth's interior for tens to hundreds of millions of years. Eventually that carbon may return to the surface as a gas via volcanic eruptions or geometrically bound as diamonds!

The next time you are climbing a stairway paved by this rock you can ponder on its fantastical journey from deep ocean to mountain front. And don't forget, you are stepping on a piece of the earth's mantle!