Monday, July 25, 2022

Field Photos: Italy Swiss Alps

A friend recently went for a trek to the Italian and Swiss Alps and sent me these stunning photos.

All Alps pics by Dr. Sushma Date.

A view along the Santa Magdalena or the Alp Suisse trail.

Imposing Pinnacles along the Tre Cime di Lavaredo hike in the Italian Alps.


 A close up of limestones and dolomites in the Italian Alps.


 A panoramic view of the distinctive landscape along the trail.


There is so much to see here in terms of geomorphology and how glacial erosion throughout the Quaternary Period has carved out the terrain. But my friend was also walking past rock outcrops that stand witness to one of the most enduring debates in sedimentary geology: the origin of that distinctive layering in these sediments.

The section of the Alps my friend was trekking in is made  up of Middle to Late Triassic age (225 -200 million years ago) limestones and dolomites. They formed in the warm tropical waters of the western Tethys Ocean. A closer examination of the layering reveals that the sediments were deposited in two broad subenvironments of a shallow sea, the intertidal zone and the subtidal zone. Intertidal and subtidal sediments alternate to form a depositional pulse or a cycle. Such couplets are stacked to form the thousands of feet of strata observed in this part of the Alps.

What could be causing the alternation of the intertidal and subtidal environments? Thick intervals of these Triassic deposits are made up tidal mud flats overlain by restricted lagoon sediments, or tidal mud flat overlain by open circulation subtidal environments, or lagoon deposits overlain by mud flats. When beds are traced laterally, these same environments grade into each other. Such inter-fingering arrangements suggest that environment adjacent to each other migrate, resulting in a vertical succession of alternating sediment types.  

Geologists recognize that such changes can be '√°utocyclic', driven by mechanisms internal to the sedimentary basin. A site of biological productivity and sediment production may choke itself by overproducing sediment. The loci of sediment production may shift to a more favorable site. Episodic storms keep redistributing sediment and reorganizing current directions . Such feedbacks result in similar environments appearing and disappearing from any one location, resulting in a cyclic sedimentary record. 

There are also successions of strata in the Triassic Alps which show a very different arrangement of sediment types. In this variation of cyclicity, intertidal mud flats may be overlain by relatively deeper water subtidal sediments which in turn are overlain by a red soil layer. The formation of soil on top of subtidal sediments deposited in water depths of up to 10 meters or so indicates a substantial drop in sea level. The top of the exposed subtidal layer was then chemically weathered to form a soil. 

Autocylic shifts in environments are gentle nudges which push one environment over another. They can't generate such a big drop in sea level. There must be drivers external to this environment that may cause sea level to rise and fall at regular intervals. These external agencies or  'allocyclic' mechanisms have been invoked to explain parts of these Triassic sequences. 

What could be controlling the regular rise and fall in sea level? Long term (over millions of years) tectonic subsidence of the basin floor certainly would have created the accommodation space for the accumulation of sediment. However, geologists look toward a different mechanism to explain the repeated deepening and shallowing events observed in these Triassic strata. 

Climate change can cause regular shifts in sea level. During the past 2.6 million years of the Quaternary ice age, sea levels have fallen by as much as 100 meters during phases of continental glacier growth, and risen during inter-glacial times when ice sheets melt. These changes have taken place at intervals of 400,000 years in the early part of the Quaternary, changing to beats of 100,000 years over the past million years. Sea level changes due to growth and decay of continental glaciers are termed glacio-eustacy. Unlike autocycles which can have variable time spans, there is a fixed periodicity to these climate driven allocycles. 

We now know that these climate cycles are controlled by periodic changes in the earth's orbital parameters which cause cyclic variation in the amount of incoming solar radiation. Such Milankovic glacio-eustatic cycles, named after the Serbian mathematician who worked out the details of earth's orbital behavior, have been recognized during other times of widespread glaciation such as the Permian. 

Milankovic worked out that there are three types of orbital movements that affect how much solar radiation reaches the top of earth's atmosphere. The shape of the earth's orbit or eccentricity cyclically varies with a period of 100,000 years and with a longer period of 400,000 years. Obliquity, or the tilt of the earth's axis with respect to its orbital plane, changes every 40,000 years. The third type are Precession cycles of 26,000 years. This is the wobble or the direction the earth's axis points to.

The Triassic though was a very hot world! The earth's land masses, amalgamated in the supercontinent Pangaea, were situated across the equator. There were no continental glaciers to wax and wane and drive sea level change. Glacio-eustacy is not a workable explanation for these cyclic Alpine sedimentary sequences.

Of late many geologists have started pointing to groundwater storage in continental aquifers as a means of causing periodic sea level change. It does sound like a fantastical idea! Such groundwater mediated sea level changes go by the name of aquifer eustacy. Milankovic climate cycles may not trigger glaciation during hot earth periods. But they can modulate long lasting humid and arid phases, each lasting tens of thousands of years. Sea levels are lowered during hot humid phases as oceans lose water by evaporation while continental aquifers get recharged. During arid phases, water is lost from aquifers by evapo-transpiration and discharge, resulting in a rise in sea level. 

An inverse phase relationship between groundwater level and sea level is thus an expectation of aquifer eustacy.

There is enough water in continental aquifers to modulate sea level change of several meters. Here is an impressive statistic. There is approximately 25 million cubic kilometer of pore space in the upper 1 km of continents above sea level.  If this is completely filled with water, the amount will equal the volume of water in continental ice caps. Even a small fraction of these pore spaces actually getting filled with water or emptying of it can change sea levels by several meters. 

Recent short term measurements of the hydrological cycle supports the notion that groundwater storage can influence sea level. For example, very high rainfall over Australia and part of the southern Hemisphere in 2011 resulted in a drop of 7 mm in global sea level that lasted a few months. And the Gravity Recovery and Climate Experiment satellite data since 2002 indicates that increased land water storage has actually slowed down the rate of sea level rise by a small amount.

Can some of the Triassic sedimentary cycles of the Alps be attributed to aquifer eustacy? How can one track periodic groundwater change in geologic history and test whether they coincide with sea level changes? One proxy is to use lake sediments of the same age as marine sequences.  Lakes are connected to aquifers.  High lake levels are indicators of saturated aquifers. Lake levels drop as aquifers discharge. Geologists have been studying Late Triassic age lake sediments from the Newark Basin in  northeastern U.S. They have identified sedimentary cycles formed during alternating humid (high lake levels) and arid climate (low lake levels) phases. 

The broad time span of these lake sequences coincide with the time frame of some thick intervals of marine sedimentary cycles of the Alps. Whether individual lake and marine cycles are out of phase could not be worked out due to limitations in age resolution of strata. However, a Milankovic band 400,000 year periodicity has been estimated for these cycles, a finding strongly suggestive of  climate driven eustacy.  From another time period, some analysis of  Cretaceous age lake sediments of Songliao Basin of NE China indicated lake level highs coinciding with global sea level lows. This finding also hints that aquifer recharge and discharge may be primarily responsible for periodic sea level changes during a greenhouse earth when there are no continental glaciers to modulate sea levels. 

Such questions continue to be asked and the mechanisms behind generating sedimentary cycles of the Triassic has by no means been satisfactorily worked out. There are many types of cycles in the Triassic Alps, observed R.A. Fischer, whose seminal work in the 1960's opened up avenues of debate that continue unabated. Perhaps it is the spectacular setting and stark rock faces that lend themselves to bold hypothesis making, linking sedimentary rhythms to the celestial dance of our planet.  

Thursday, July 14, 2022

Field Photos: Iceland

More pictures arrived from different parts of the world. My friends visiting Iceland and the Alps sent me some stunning photos of landscapes and geology. 

Iceland. 

All pics by Biju Mohan.

Lava flows forming gentler slopes and steep rock faces. Notice the rough columnar jointing in the upper lava flow.

Where basalt plateau meets the sea. Cliffs and a wave cut platform.

Volcanic cone and crater.

A fissure or a crack through which lava would have poured out. These are present all over Iceland.  

Iceland predominantly has basalt volcanism, broadly the same rock type as the Deccan. It is one of the few locations where the Mid-Atlantic spreading center is exposed above sea level. This is a divergent plate boundary, where the European and North American tectonic plates (along with some micro-plates) are moving away from each other.

Biju asked me an interesting question; "Did the deccan area looked like present day Iceland sometime in the past? Is there evidence for numerous volcanoes in the Deccan?

Yes, a young Deccan volcanic terrain would have looked similar to Iceland in some aspects. Since in both places, the crust was pulled apart by extensional forces, long fissures or cracks formed and were the main passageways for magma to come to the surface. These fissures from where lava came out would have been visible in a young Deccan. They have eroded away now. What is left are dike swarms, essentially cracks plugged by sheets of magma. Many of these dikes represent the feeder passages from which lava ascended to the surface. So, an exhumed lower level is now visible. Volcanic cones would also have been visible. These have mostly been eroded away in the Deccan.

As such Deccan would not have seen the development of very large steep cones, since the lava type is runny, and does not pile up much to build cones. Iceland though, besides basalts,  has more of a silica rich sticky lava type, with more explosive volcanism,  and a more pronounced development of steeper volcanic cones. Remember the Eyjafjallaj√∂kull volcano that erupted in April 2010?

Fresh lava fields would have been clearly demarcated. In young volcanic terrains it is easier to pick out discrete eruptive episodes. Lava fields erupting from different vents overlap. Slightly older lava will change color due to weathering and also get colonized by plants. Fresher lava fields will be barren and likely steaming as well! In the much older Deccan , erosion has erased such differences. Exhumation doesn't always expose a pristine surface, rather a patchwork of vertical sections where one gets a two dimensional view is the common outcrop pattern, making recognition of such lava fields challenging to the untrained eye. 

Another similarity would have been the presence of active hydrothermal systems. Today, the Deccan volcanic system is extinct, but 65 million years ago, groundwater would have been heated by flowing through hot rock and proximity to magma. Fumaroles and hot springs would have been a common phenomenon. I have been collecting secondary minerals from the Deccan Traps since my college days, and I would have loved to have wandered through a young Deccan volcanic terrain, where hot mineral saturated water were depositing silica, calcite, and zeolite minerals in cracks and cavities of the basalts. 

The oldest lava flows in Iceland are mid- Miocene in age. Erosion has been sculpting landscapes for a good 15 million years. The result is some uncanny similarities with the Deccan. The 'Trap' topography, alternations between gentler and steeper slopes is also seen in Iceland. And along the Konkan coast, basalt and laterite sea cliffs look over flat wave cut platforms just like the Iceland coast. 

Sea cliff and a wave cut bench, Harnai, Konkan.

 

I'll close with this beautiful Iceland landscape. 


Coming soon.. Dolomite Alps and a geological conundrum.