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Tectonics and Structural Geology

Tectonics and Structural Geology

Geothermal Energy and Structural Geology?

Geothermal Energy and Structural Geology?

Fieldwork is a necessity to expand the brain, to kick-start 3D thinking. Field studies with a specific application in mind have – until now – usually been geared towards hydrocarbon reservoirs. However, with the increasing use of the subsurface, for example for CO2 storage and geothermal energy, alternative field studies gain importance. Here, we will focus on geothermal energy, which is in many countries a prime target of the energy transition. Around half of our energy consumption is simply the need for heat for houses, industry and other applications. Of course, some tectonic settings lend themselves easier to obtaining heat than others. In settings with volcanoes (rift and subduction zones), countries such as Iceland and Italy are frontrunners in developing high temperature (>100⁰C) geothermal energy. No matter which geographic location and which heat source is targeted, all geothermal energy reservoirs consist of the same components. This blog is about the holy trinity of a usable geothermal system: the heat source, the fluid, and the fluid pathways. With this blog, we will take you along on a yearly field trip to discover how outcrops of intrusions can be used to teach BSc, MSc and PhD students about geothermal energy. We go to the Larderello area and Elba. Tuscany, due to the extensional setting, is characterized by a high heat flux, as much as 150 mW/mm2 (compared to 40 mW/mm2 in most regions). This high heat flux means that it is a prime area for geothermal energy. This trip is organized by Prof. David Bruhn (TU Delft, the Netherlands, and GFZ, Germany), together with the invaluable aid of Prof. Domenico Liotta (University of Bari Aldo Moro, Italy).

Left: Fumaroles and geothermal power plants (with crosscutting pipelines) dominate the view in the Larderello area. Credit: Anne Pluymakers.

Discussions on top of fumaroles, where steam escapes through fracture networks. Credit: Anne Pluymakers.

 

To better understand the technological background, we first explore what a currently active geothermal system looks like. For this, we go to the Larderello area, a beautiful region near the Tuscan coast, on western edge of the Apennines. Here, geothermal energy is visible both through the local presence of fumaroles, as well as by the omnipresent pipelines and cooling towers. The pipelines are sometimes colored green to be less conspicuous. One could find them ugly, but given that they provide 3% of Italy’s energy plus employment throughout the region, they serve a great purpose! In this area abundant heat is present, which is tapped by in total 300 extraction wells and 34 power plants (run by Enel Greenpower). There are another 100 re-injector wells, to ensure the extracted steam and water are re-injected into the aquifers at depth. An example: the geothermal powerplant we visited is fed by 20 boreholes, spread over 7 km2.

An active drill site in the Larderello area. Credit: Anne Pluymakers

Tourist well: 200°C steam, at 2 bars, and 10 tonnes of steam per hour. Credit: Anne Pluymakers

The two main geothermal reservoirs in this region are (1) a shallow reservoir, with 200°C and 2 bar pressure, and (2) a deep (2 km) reservoir, with the same temperature but then at high pressure, 7 bar (and higher). The heat is carried by steam. The pressures aren’t very high, but the flow rates are enormous. This is demonstrated by the ‘tourist well’, that is now no longer in use. It contains 200°C steam, at just 2 bars – an average biketire carries between 2 and 5 bars. However, this tourist well produces 10 tonnes of steam per hour – so standing on the 85 decibel line it clearly shows the power of the earth! In this region, the power plants function on higher pressure, which renders this specific well not economically viable. Economic interest is an interplay between offer and demand, and as such is location-dependent. In other words, also here it would be technically possible to convert the heat from the tourist well into energy. To go from steam to electricity a turbine and generator are necessary. However, the steam is contaminated with fluids and other gases, which need to be separated before the steam can power the turbine and generator. Precipitation of salts in the water can lead to one of the main production issues associated with geothermal wells: scaling.

 

Scaling, or the build-up of precipitate inside pipelines, can be a big issue in geothermal power plants. Credit: Anne Pluymakers.

Scale model of the first “hot pond under cover” for boric acid extraction, where the boilers were powered by steam (display in the Geothermic Museum of Larderello, photos by Anne Pluymakers)

 

Fun fact: the first use of geothermal waters in the region was to produce boron salts. Production of boric acid and its sodium salt, borax, started as early as 1818, though mass production initiated in 1827. Francesco Larderel built a covered hot pond to gather natural steam, needed to feed the boric water evaporation boilers. By the start of the 20th century, the Larderello region produced electricity for local use.

 

Geological map of Elba island (from Rocchi, S., Westerman, D. S., Dini, A., & Farina, F. (2010). Intrusive sheets and sheeted intrusions at Elba Island, Italy. Geosphere, 6(3), 225–236. https://doi.org/10.1130/GES00551.1)

 

After this demonstration of a working geothermal system at the earth’s surface, the excursion moves on to the geological analogue. Analogue field studies are a common way of imagining what the subsurface could look like in the targeted region. Therefore, we move to the island of Elba, just off the coast of Tuscany. The equivalent heat source is the Monte Capanne pluton, on the west of the island. The first evidence of geothermal activity constitutes of precursor dike and sill intrusion into an ophiolite sequence. Large scale melting happened in the Miocene, at a depth of 6 to 7 km. To generate these volumes, water had to be present – this geological evidence then represents the ‘mother’ of the heat flux, and where the first geothermal fluids on Elba have come from. In the outcrops of the Monte Capanne pluton xenolites and large feldspar crystals are visible. The feldspar crystals indicate large convection patterns in what must have been viscous, cooling magma. After cooling, this pluton was fractured, and meter-sized mafic intrusions with fine-grained crystals share orientations with these fractures. Analysis of fluid inclusions in the outcrop indicate that the fluids are all relatively local. That is not surprising, since upon cooling, felsic magmas release large amounts of water, and also the surrounding meta-sediments (i.e. former ocean floor) were full of water.

Where there is a magma, there are interesting percolating fluids, with all kinds of rare and precious minerals. As can already be seen in the geological map, the evidence of fluids is obvious throughout the island, in the form of several mines, especially on the east side. In the east, the people have mined for iron for decades, specifically for hematite, limonite, pyrite, magnetite and ilvaite. Of these, ilvaite is a mineral that is quite specific to Elba. On the west, closer to the pluton, we can find tourmaline, beryllium, orthoclase and quartz.

Mine on Elba island. The elongated shape follows the fault path. Credit: Anne Pluymakers

Many of these mines follow distinct pathways: those of fractures and faults, demonstrating also in this fossil system the importance of fractures and faults as fluid pathways (i.e. compare the photo of the small scale fracture network on Elba to the photo of the fracture networks associated with the fumaroles). Elba hosts a famous shear zone: the Zuccale fault. This outcrop contains many textbook structures, from brittle to ductile deformation.

The less weathered mineral veins provide clear evidence of past fluid pathways. A key message for geothermal energy recovery that can be obtained from outcrops like these, is that permeability is directional and local. The fracture zone itself is asymmetric along the shear zone. The shear zone is impermeable across fault, but not necessarily as impermeable along the fault. As an engineer responsible to determine the best location to drill a well (approximately one million euro per km!), where would you go?

 

Brittle (left) and ductile (right) deformation at the Zuccale fault outcrop. Credit: Anne Pluymakers.

 

An earlier version of this blog appeared on the Focus on vips blog. This is a slightly modified version.

Trieste, where the word Karst originates

Trieste, where the word Karst originates

The city of Trieste lies in north-eastern Italy along the border with Slovenia. It is positioned at  the corner point between the Romance, Germanic and Slavic worlds and serves as an important seaport in the region. It is fascinating for both its history and geology. My relationship with Italy’s town of Science, as Trieste is often referred to, started about a year ago. I got the opportunity to start a PhD focused on imaging evaporates, at the University of Trieste and the Istituto Nazionale di Oceanografia e di Geofisica Sperimentale (OGS). Walking around the city, two elements caught my attention. First, I was impressed by the topography of the area (Figure 1), the city itself is bordered by beautiful abrupt cliffs and steep Carsic hills plunging into the Adriatic Sea.  Second, I found myself surprised I was in Italy, as the Austro-Viennese architecture dominates the streets. While I was marvelling at the beauty and size of the buildings, which nowadays testify of a thriving and multicultural past, I have started to wonder why I had never heard about this city before. The history of Trieste is out of topic of this article, but I invite you to check it out, as there are many lessons that can be learned regarding Identity politics. So, why should Trieste appear in the ‘Geology in the city’ series? First let’s zoom out and review the regional geological context.

 

Figure 1. The carbonate cliffs of the Strada Napoleonica at sunset, an idyllic spot for climbing and wandering. The subvertical strata represent the southwestern flank of the Trieste – Komen anticlinorium developed above the NW-SE Karst Thrust. The strata visible here are the latest limestone units, Paleocene to Eocene in age. The lower part of the slope covered by vegetation is made of Eocene Flysch. Credit: Simon Blondel.

 

A natural fortress

Trieste is built along the north-eastern Adriatic coast, where it lies on top of Eocene flysch, which consist of silty marls interbedded with sandstones. They don’t outcrop much, but you can see evidence of these all around the city. The sandstone interbeds were used as building blocks, regionally called “masegni”, to build the city. For example, the Molo Audace (an iconic pier at the harbour) and the pavement of the city’s main square: the breath-taking Piazza dell’Unità d’Italia, are made of these “masegni” (Figure 2).

 

Figure 2. View of Città nuova from Molo Audace, with Piazza dell’Unità d’Italia at the centre. The pier is built above the wreck of ship that sank in the port of Trieste in 1740. It is made of “masegni” stones extracted from the sandstone units of the Eocene Flysch. Credit: Simon Blondel.

 

As you go inland, the topography abruptly steepens toward the Karst plateau. It rises 400 m above the Gulf of Trieste, overhanging the city and confining it with cliffs and walls, to the delight of many climbers. When I say Karst I mean the geographical name given to this area and not the geological term, but I’ll come back to that a bit later. After this exhausting ascent, especially if you decide to do it by bike (my first excursion with a single-geared Dutch bike was quite gruelling), you can relax and wander around the flat “Altopiano Triestino”. It consists of limestone and dolomitic limestone formations, Valanginian to early Eocene in age. They are assimilated to a succession of carbonate platforms, whose nomenclature was recently unified within the framework of the HYDROKARST European project in 2015 (http://book.hydrokarst-project.eu/files/assets/basic-html/index.html#1). They were brought to the surface with the collision between the Eurasian and African plates, and nowadays form a wide NW-SE fold – the Trieste – Komen anticlinorium- assimilated to the external Dinaric Imbricated Belt (Figure 3).

 

Figure 3. Simplified geological map of the Gulf of Trieste. The A-B line represents the position of the cross-section in Figure 4. The green part corresponds to the seismic data, the purple part corresponds to the schematic cross-section. Trieste lies above Eocene flysch, at the front of the NW-SE Karst Thrust that delimit the Karst plateau. Offshore, the Karst’s carbonate platform is repeatedly thrusted and folded until the margin of the platform. Courtesy of Martina Busetti.

 

The anticlinorium overthrusts the Eocene flysch that deposited at its foreland (the Gulf of Trieste), along the NW – SE Karst and Palmanova thrusts (Figure 4). The Karst’s carbonates were also widely used in construction and there are many quarries in the surroundings. One of the most famous is near Villaggio del Pescatore, where a perfectly preserved skeleton of Tethyshadros insularis (Antonio to his friends) was discovered in 1994 (Figure 5).

 

Figure 4. Seismic (data acquired in the gulf) and geological schematic cross-section (of the Karst Plateau) perpendicular to the Karst Thrust, as shown on Figure 3. The Carbonate platform is folded and thrusted along the Karst thrust, overlapping the younger Eocene Flysch along the coast. Offshore the Flysch are covered by Pliocene and Pleistocene sediments, and onlap the folded Carbonate platform. Courtesy of Martina Busetti.

 

Figure 5. The reconstructed skeleton of Antonio in a former limestone quarry located between Duino and Villaggio del Pescatore). The original fossil was moved to the Museum of Natural History in Trieste. Credit: www.dinosauroantonio.it

From Kras to Karst

As stated previously, the word Karst is both a geographical and geological term in the region. At the beginning, this was just the name given to the hinterland of the Gulf of Trieste. It is supposed to be of pre-Indoeuropean origin from the root kar-, meaning rock, to describe these unusual and barren lands that were considered hostile for travellers. This term is still preserved in the Italian Carso, Slovenian Kras and German Karst. On the surface, the Karst plateau is strewed by numerous dolines and caves, which are the most dominant features that characterize it. There are many other geomorphological features that can be observed, such as the campi solcati (limestone pavement) or the kamenitze (natural ponds) in Val Rosandra (Figure 6). These natural ponds result from millions of years of erosion and dissolution by meteoritic waters of pre-existing carbonates.

 

Figure 6. From left to right, waterfall emerging from the Karst in Val Rosandra, The Val Rosandra valley with numerous Kamenitze and typical Carsic vegetation, the Škocjan caves where the Reka river plunges beneath the surface for a 40 km journey underground. Credit: Simon Blondel.

 

The use of the word Karst, as a general term to describe areas exhibiting various geomorphological features linked to erosion and dissolution, became widely accepted by the scientific community during the 19th century. Back then, Trieste was one of the most dynamic and multicultural city of the world under the Austro-Hungarian empire. Most of the population was Slovenian-speaking in the hinterland, therefore the region was referred as Kras by the locals. However, the geoscientists who started to investigate the origin of its features in more details were German-speaking, hence explaining why the German version has been retained for the geological term. What triggered these studies was principally the need for water.  As the population grew, so did the need for freshwater, which led the city to launch investigations of the nearby Karst. Unfortunately for them, the water was too deep (at about 300 m below the surface, close to the sea level) to be easily pumped for the city. But in their quest for groundwater, the scientists discovered a dense and sinuous network of cavities and tunnels dug by the groundwater, close to the current sea-level. They discovered that a clear majority of the Karst’s underground water is captured by the Reka river that plunges underground through the ponor (natural sinkhole where surface water enters) of Škocjan, a UNESCO Natural Heritage site in Slovenia (Figure 7). It reappears out from the underworld 40 km away from Škocjan, at the Timavo springs, where it nourishes the 2 km long river that outflows into the Gulf of Trieste.

The Karst Plateau hosts numerous caves and I will only mention one to conclude: the Grotta Gigante. It was discovered in 1840 and currently is both a tourist attraction and an important location for scientific studies. It hosts a seismological station, a horizontal pendulum and a clinometer to calculate the movements of the earth’s (Figure 7), as well as a measuring station for water percolation and concretion, a monitoring system for Radon gas, and finally a probe sensor measuring the temperature and the electric conductivity of groundwater. Some of these are operated by the Istituto Nazionale di Oceanografia e di Geofisica Sperimentale and the Department of Mathematics and Geosciences of the University of Trieste.

 

Figure 7. The Reka river flowing underground in the Škocjan caves (left) and the Grotta Gigante where we can notably see the two giant tubes protecting the horizontal pendulum used to calculate the movements of the Earth’s crust. Credit: Simon Blondel.

 

After reading this introduction to Trieste and its geology, it is time to come to visit us. The city is currently preparing to host the EuroScience Open Forum – ESOF 2020, so you’ll hear the name again. In the meantime, stay tuned to our ETN SALTGIANT’s research, a project funded by the European Union’s Horizon 2020 research and innovation grant, and follow us on LinkedIn and Twitter.

I would like to thank Martina Busetti and Michela Dal Cin (OGS), who reviewed and contributed to the writing of this article.

Features from the field: Ripple Marks

Features from the field: Ripple Marks

Earlier this year, Ian Kane, geologist at the University of Manchester, captured the iconic snapshot shown above. The picture reveals ripples, developed due to waves and currents in the sand of White Strand (near Killard, county Clare, Ireland) right next to Carboniferous sandstone that contains ‘petrified’ ripple marks!

The image is powerful, because it shows the basic principle of geological actualism, which can be summarized in the famous quote by Charles Lyell:

‘the present is the key to past, the past the key to the future’

The physical processes that are active in the world today occurred in the past and will continue to occur in the future. And ripples can tell us a lot about the past of our planet!

Sedimentologists study and analyze bedforms, like ripples, in present-day shorelines, river systems, deserts, and in deep marine environments like submarine fans to understand past environments. It is pretty obvious, when walking on a strand, to tell ripples were shaped by waves along the coast or by blowing wind on sand dunes, but would you be able to tell how they developed in rocks, without actually seeing the ambient where they formed?

 

Ripple marks in the Moenkopi Fm., Capitol Reef National Park (Utah). Photo credits © Daniel Mayer/Wikimedia.commons

 

The first and most obvious information we can get from the presence of ripples in sedimentary rocks is that a current must have been present- either a water current or a blowing wind. Their crests are always oriented perpendicular to the current that formed them, telling us what the direction of currents in past environments was.

Their shape, size and symmetry depend on the type of sedimentary process that is associated with their formation. There are two types of ripples: asymmetric and symmetric.

 

Asymmetric ripples exposed in the intertidal zone near Lawrencetown (Nova Scotia). They were formed by a current (likely the tide) that was flowing from left to right. Photo credits © Michael C. Rygel/Wikimedia.commons

 

Asymmetric ripples show a gently-dipping side (stoss side) and a short inclined side (lee side). The sediment is dragged and eroded from the stoss side until it reaches the crest and deposits on the lee side, which is downstream with respect to the current. The continuous removal of sediment from the stoss side and the re-deposition on the lee side causes the ripple crest to migrate in the same direction of the current. Recognizing asymmetric ripples tells us immediately where the flow was directed. We can, for example, reconstruct the direction of a river, or a marine current, or the dominant wind in sandstone that deposited millions of years ago.

 

Symmetric ripple marks formed by waves in Permian rocks from Nomgon, Mongolia. Photo credits © Matt Affolter/Wikimedia.commons

 

Not all bedforms are the result of a single and dominant current. Symmetric ripples are formed by bidirectional currents: currents that move in one direction and then in the opposite one. Does it ring a bell? Waves!

Waves cause ripples to be symmetric because both sides of the ripple become alternatively sites of erosion and deposition while water moves back and forth. Recognizing wave ripples can tell us whether an ancient sandstone deposited on a shoreline rather than on a river bank or a dune field.

Finally, ripples are very useful in structural geology because, as they mark the surface of deposition, they are useful indicators of the stratigraphic top in a sedimentary sequence, for example when we have to deal with overturned beds.

Beyond Tectonics: Can only tectonically active planets sustain life?

Beyond Tectonics: Can only tectonically active planets sustain life?

This edition of “Beyond Tectonics” is brought to you by David Waltham. David is a professor of Geophysics at Royal Holloway who studies Geology, Astronomy and Astrobiology. His current research focus is on whether the Earth is “special” because it is habitable, or if the Earth is one of a vast amount of life-bearing planets.

“Is Earth Special? Do we live on a typical rocky world or on one of the oddest planets in the Universe? Are planets capable of harbouring life common across the Cosmos or are our nearest habitable neighbours so far away that we’ll never find them?  We don’t know!”
What does a planet need to be habitable?

This huge question can be broken down into smaller, more specific chunks. Do habitable planets have to have large moons and, if so, how common are they? Do habitable planets have to have magnetic fields and, if so, how common are they? Do habitable planets have to have just the right amount of water and, if so, how common is that?

 

A portion of the Hubble deep field image containing around 3,000 galaxies. image credit: Wikipedia

 

You might think that we’d know the answer to at least some of these more focused queries but we don’t. You can see the review of the current lack of knowledge on these, and other puzzles, here (Waltham 2019). In this edition of Beyond Tectonics, we will be focusing on the question of if habitable planets must have plate tectonics and, if so, how common is that?

 

Plate Tectonics and Habitability

The most obvious way in which plate tectonics might contribute to habitability is its influence on climate or, more specifically, climate stability. Earth’s climate is surprisingly stable. In these times of concern over climate change this probably sounds a little heretical but, when looked at over billions of years, the thing that is most striking, is how little our climate has changed. We’ve had four billion years of “good weather”, in which temperatures never dropped so low that our world froze completely (although, it did get close on a few occasions) or so high that a runaway greenhouse effect set in leading to planet-wide desiccation.

The fossil record shows that biodiversity is massively affected by even quite small temperature changes (e.g. a mere 3% temperature rise probably caused the end Permian mass-extinction (Penn et al., 2018) and so climate stability is an important, and often overlooked, component of habitability. However looking at Earth’s history, and future, the climate should be unstable. Over the 4 billion years life has existed on Earth, the Sun’s luminosity has increased by 30%. Over the same time period, the composition of the atmosphere has altered radically, as has the amount of heat our planet reflects back to space from ice, clouds and the changing continents. Temperatures should have fluctuated by hundreds of degrees, but they did not, which is probably due to plate tectonics.

The big picture here is that Earth has benefited from a coincidence. As the Sun has steadily warmed over the aeons this has been compensated by a drop in greenhouse gas concentrations at just the right rate to keep Earth habitable. Part of the explanation for this is that outgassing of carbon dioxide from the mantle has been dropping as our planet has cooled. Earth’s cooling has only been at the required fast rate because of plate tectonics. Subduction of cold lithosphere into the mantle is a much more efficient cooling mechanism than conduction of heat through a stagnant crust (as happens on every other rocky body in the Solar System) and so our planet cools much more quickly than it otherwise would.

 

The Silicate weathering cycle

Regardless of the change in outgassing rate over time, without the Silicate weathering cycle, CO2 outgassing would cause runaway warming eventually making Earth Venus-like. Carbon dioxide concentration—and hence temperature—of the atmosphere is kept low because of the silicate weathering cycle. Carbon dioxide dissolved in rainfall is slightly acidic and dissolves silicate rocks at the surface to produce, among other things, bicarbonate ions in the water draining to the sea (Raymo and Ruddiman 1992).  These, in turn, precipitate in the oceans to give carbonate sediments and, hence, atmospheric carbon is turned into solid rock thereby removing it from the atmosphere. This process only occurs because plate tectonics continuously replenishes the supply of un-weathered silicate (via mountain building/orogeny, e.g. the Himalaya) and because plate tectonics maintains Earth’s dichotomy of continents and oceans.

These carbonate sediments which build up on the ocean floor are eventually subducted with large volumes of hydrated ocean plate (water ingrained in the chemical structure of the rock). Earth’s interior contains more water than there is on the surface but, without subduction of these water-rich sediments and rocks, all of it would have found its way to the surface by now. If this were the case, then at the very least our planet would be a water-world without dry land. Although, it is more likely that without the silicate weathering cycle and with enhanced atmospheric water vapour levels, our planet would have undergone a Venus-like process of wet, runaway greenhouse warming swiftly followed by desiccation as hydrogen from dissociated water in the upper atmosphere was swiftly lost to space (Kasting 1988).

 

The Himalaya photographed from the International Space Station. Image credit: NASA

 

How important is plate tectonics for planetary habitability?

Plate tectonics should be seen as a prerequisite of planetary habitability. The arguments for plate tectonics being central to habitability are stronger than for almost any other property of our planet. They’re certainly far stronger than those arguments supporting more frequently met suggestions of the centrality of our large moon or of our strong magnetic field (Waltham 2019).

Plate tectonics is therefore important to planetary habitability but is it rare? Our planet is not special in this regard if plate tectonics is common on Earth-sized rocky worlds. Unfortunately, at this time it is not known for certain whether plate tectonics is common or rare. Sophisticated mathematic models of planet-scale convection have not given us a clear answer either. Some of these models suggest that plate tectonics will be common for planets of Earth-size or above but other models show the opposite. Many models, but not all, also suggest that the presence of water in the mantle is key. Other factors that may be important are a low surface temperature (so that the crust is rigid) or the presence of a solid-inner core (heat of fusion provides a significant proportion of Earth’s heat budget).

A different approach to determining plate-tectonic frequency would be to try and detect it on planets orbiting other stars. That’s not as outlandish as it sounds. We already have the technology to analyse the atmospheres of exoplanets and, as indicated earlier, plate tectonics massively influences atmospheric composition. Perhaps there’s a plate tectonic “signature” we can look for in these exo-atmospheres. Exo-tectonics could be a real subject, taught in undergraduate lectures, within a few decades.

References

Kasting, J.F., 1988. Runaway and moist greenhouse atmospheres and the evolution of Earth and Venus. Icarus, 74, 3, 472 – 494. Available at: https://doi.org/10.1016/0019-1035(88)90116-9

Penn, J.L., Deutsch, C., Payne, J.L., Sperling, E.A., 2018. Temperature-dependent hypoxia explains biogeography and severity of end-Permian marine mass extinction. Science, 362. Available at: doi: 10.1126/science.aat1327

Raymo, M.E., Ruddiman, W.F., 1992. Tectonic forcing of late Cenozoic climate. Nature, 359, 6391, 117 – 122. Available at: https://doi.org/10.1038/359117a0DO

Waltham, D., 2019. Is Earth Special? Earth-Science reviews, 192, 445 – 470. Available at: https://doi.org/10.1016/j.earscirev.2019.02.008

Written by David Waltham

Edited by Hannah Davies

Beyond Tectonics: How the tectonic events of 1783 were perceived by the population of Europe

Beyond Tectonics: How the tectonic events of 1783 were perceived by the population of Europe

This edition of “Beyond Tectonics” is brought to you by Katrin Kleemann. Katrin is a doctoral candidate at the Rachel Carson Center/LMU Munich in Germany, she studies environmental history and geology. Her doctoral project investigates the Icelandic Laki fissure eruption of 1783 and its impacts on the northern hemisphere.

“A Violent Revolution of Planet Earth” – The Calabrian Seismic Sequence of 1783
The 1783 “Year of Awe”

In 1783, a sequence of several strong earthquakes devastated Calabria, later that year the Icelandic Laki fissure eruption blanketed parts of the northern hemisphere with a strange, dry sulfuric fog. Contemporaries coined 1783 to be an annus mirabilis, a year of awe, that saw many unusual phenomena: A temporary “burning island” appeared off the coast of Iceland; earthquakes rocked parts of Western Europe and even the Middle East; there were erroneous reports of at least three volcanic eruptions in Germany (this news was later retracted); and in September, news of a volcanic eruption at or near Mount Hekla, Iceland’s most infamous volcano, reached the European mainland. All of these events inspired the notion that somewhere “a violent revolution of planet Earth” was underway (Münchner Zeitung 1783). The most popular theory at the time was that the earthquakes in Calabria had caused the sulfuric-smelling dry fog of 1783.

The volcanic eruptions in Iceland, and earthquakes in Italy are both caused by their respective geology. Iceland sits on the Mid-Atlantic Ridge, a divergent boundary between the Eurasian and the North American plate, and on top of the Iceland mantle plume, making it very volcanically active. The Italian peninsula is dominated by a subduction zone – between the Eurasian and African plates – which reaches from Sicily to Northern Italy and causes many earthquakes.

How do we know the strength of these historic earthquakes, when they occurred before seismometers had been invented? Geologists estimate the magnitude of historical earthquakes from written reports that describe the damage caused. The Mercalli-Cancani-Sieberg scale (MCS), named after Giuseppe Mercalli, Adolfo Cancani, and August Heinrich Sieberg, is one scale to infer the magnitude of an earthquake from historical descriptions: the MCS scale reaches from level I (earthquake “not felt”) to level XII (“extreme”, causing total damage, waves seen on ground surfaces, objects being thrown upward into the air).

 

“The most terrible and destructive of any earthquake”

With the presence of the Calabrian Arc – characterized by normal faulting and uplift – and the volcanoes of Etna and Stromboli nearby, Southern Italy and Sicily experience regular earthquakes and volcanic eruptions. However, the earthquakes of early 1783 did not follow the normal pattern of one strong quake and weaker fore- and/or aftershocks. Instead, there was a seismic sequence of five strong earthquakes. A seismic sequence is an unusual event, in which one earthquake increases the stress on other parts of the fault system, which triggers subsequent earthquakes. This process is called Coulomb stress transfer.

 “The Earthquakes in Italy were, perhaps, the most terrible and destructive of any that have happened since the Creation of the World. Four hundred towns, and about four or five times as many villages, were destroyed in this dreadful calamity. The number of lives lost, are estimated at between forty and fifty thousand.” An Account of the Earthquakes in Calabria, Sicily, and other parts of Italy, in 1783. Communicated to the Royal Society [of London], by Sir William Hamilton, the British Ambassador to the Kingdom of the Two Sicilies in Naples, May 23, 1873.”

Today we know, these were not, in fact, the most destructive earthquakes of all time, although in 1783 these unusual earthquakes, and the hundreds of aftershocks that occurred throughout the year certainly seemed that way.

 

A contemporary print of the first of the 1783 Calabrian earthquakes. The earthquake caused severe damage and destruction in Reggio Calabria in the foreground, with the Strait of Messina in the background. Image source: Wikipedia.

 

The earthquakes in Calabria and their consequences

The print above illustrates the first of the Calabria earthquakes, which occurred on February 5, 1783, at noon, near Oppido Mamertina in Calabria, which was a XI on the MCS scale (Richter magnitude of 7.0). It gives us an idea of the extreme destructive force of this earthquake. The residents of Messina and Calabria were knocked off their feet by the shaking as they tried to flee, avoiding cracks in the ground, and falling trees and rubble (Jacques et al., 2001). Contemporary reports show the initial earthquake destroyed almost all of the nearby buildings, and the initial and subsequent earthquakes caused total casualties in the tens of thousands.

 

Map of the intensity of the first large earthquake on 5 February 1783, which reached a XI of the Mercalli scale, here symbolized in dark purple. A star denotes the point of the epicenter of the earthquake (credit CFTI).

 

The second strong earthquake, which also reached VIII-IX on the MCS scale (“severe” to “violent”), struck only half a day later, at 0:20 am. The first earthquake had scared many people in the region, and they did not want to spend the night in their houses. In Scilla (in NW Calabria), many people decided to camp on the beach overnight. This proved to be a fatal error: The second earthquake triggered a rockslide, which created a tsunami that killed 1500 people on the beach (Bozzano et al., 2011; Mazzanti et al., 2011).

 

The extraordinary weather phenomena of 1783

During the summer of 1783, another highly unusual phenomenon was present in Europe: A sulfuric-smelling dry fog hung in the air for weeks on end. Many naturalists and amateur weather observers around Europe noticed this phenomenon and speculated as to its cause. At the time, it was believed that sulfuric fogs were a precursor to strong earthquakes, a dry fog was observed in the days before the 1755 Lisbon earthquake – most likely produced by an eruption of the Icelandic volcano Katla. A similar fog was also reported in Calabria on February 4, 1783 (Kiessling, 1888; von Hoff, 1840).

We now know that the Icelandic Laki Fissure eruption, of 1783, released large amounts of gases and ash, which were carried towards continental Europe via the jet stream. However, news of this took almost three months to reach Europe, by which time the dry fog had vanished again, making it difficult to explain the phenomenon at the time.

 

The Southwestern part of the Laki Fissure in Iceland today (credit Katrin Kleemann).

 

How to explain earthquakes without the theory of Plate Tectonics

The sheer number of unusual subsurface phenomena observed during this time seemed overwhelming. Many theories were developed to explain the “year of awe,” one suggested the Calabria earthquakes had created a crack in the Earth, which was releasing the sulfuric fog observed over Europe. For a very long time, it remained only one theory among many.

In the late eighteenth century, it was believed that all volcanoes, most often coined “fire (spitting) mountains,” were connected via fire channels inside the Earth. Earthquakes and volcanic eruptions were believed to be caused by chemical reactions—between gas or metals and water for instance—in subterranean passages and caverns (Reinhardt et al., 1983; Oldroyd et al., 2007). Today, we have the theory of plate tectonics and mantle plumes, however, even today, the geology of the Calabrian Arc seems very complex and is far from fully understood.

 

“Subterraneus Pyrophylaciorum“: Fire canals connecting all volcanoes on the planet, depicted in Athanasius Kircher’s Mundus Subterraneus, 1668 (credit Wikimedia commons).

 

Written by Katrin Kleemann
Edited by Hannah Davies

 

References/extra reading

  • Bozzano, F., Lenti, L., Martino, S., Montagna, A. and Paciello, A., 2011, Earthquake triggering of landslides in highly jointed rock masses: Reconstruction of the 1783 Scilla rock avalanche (Italy). Geomorphology, 129 (3-4), 294 – 308.
  • Graziani, L., Maramai, A. and Tinti, S., 2006, A revision of the 1783-1784 Calabrian (southern Italy) tsunamis. Natural Hazards and Earth System Sciences, 6 (6), 1053 – 1060.
  • Hamilton, W., 1783, An account of the late earthquakes in Calabria, Sicily, &c. Colchester: J. Fenno.
  • Jacques, E., C. Monaco, P. Tapponnier, et al., 2001, Faulting and earthquake triggering during the 1783 Calabria seismic sequence. Geophysical Journal International, 147, 499 – 516.
  • Kiessling, K. J., 1888, Untersuchung über Dämmerungserscheinungen zur Erklärung der nach dem Krakatau-Ausbruch beobachteten atmosphärisch-optischen Störungen. Hamburg: L. Voss, 26.
  • Kleemann, K, 2019. Living in the Time of a Subsurface Revolution: The 1783 Calabrian Earthquake Sequence. Environment & Society Portal, Arcadia (Summer 2019), no. 30. Rachel Carson Center for Environment and Society. http://www.environmentandsociety.org/node/8767.
  • Kleemann, K., 2019, Telling stories of a changed climate: The Laki Fissure eruption and the interdisciplinarity of climate history. Edited by K. Kleemann and J. Oomen, RCC Perspectives: Transformations in Environment and Society no. 4, 33-42, doi.org/10.5282/rcc/8823. http://www.environmentandsociety.org/sites/default/files/03_kleemann.pdf
  • Kozák, J., and Cermák, V., 2010, The illustrated history of natural disasters. Dordrecht: Springer Netherlands.
  • Mazzanti, P. and Bozzano, F., 2011, Revisiting the February 6th 1783 Scilla (Calabria, Italy) landslide and tsunami by numerical simulation. Marine Geophysical Research, 32 (1-2), 273 – 286.
  • Oldroyd, D., Amador, F., Kozáko, J, Carneiro, A, and Pinto, M., 2007, The study of earthquakes in the hundred years following the Lisbon earthquake of 1755. Earth Sciences History 26 (2), 321 – 370.
  • Placanica, A., 1985, Il filosofo e la catastrophe: Un terremoto del Settecento. Turin: Einaudi.
  • Reinhardt, O., and Oldroyd, D. R., 1983, Kant’s theory of earthquakes and volcanic action. Annals of Science, 40 (3), 247 – 272.
  • Von Hoff, K. E. A., 1840, Chronik der Erdbeben und Vulcan-Ausbrüche: mit vorausgehender Abhandlung über die Natur dieser Erscheinungen 1 Vom Jahre 3460 vor, bis 1759 unserer Zeitrechnung. Volume 1. Gotha: Perthres, 108.

Features from the field: Foliation

Features from the field: Foliation

Have you ever walked on a mountain trail, passing past outcrops of rocks and noticed that many rocks appear to be split along a well-defined orientation? If you have, you might have seen one of the most important structures in metamorphic rocks – called foliation.

The term ‘foliation’ derives from the Latin folium, meaning ‘leaf’. A rock with a foliation looks like a pile of ‘leaf-sheets’ that appear piled up one upon the other. Any set of planes that is pervasively repeated in a rock volume defines a foliation, irrespective of its origin, thickness or composition. In the example shown on top of the page, the rock easily splits along foliation surfaces that dip to the left. In this case, the foliation is defined by the preferred orientation of tiny platy minerals – called phyllosilicates – that cause the rock to break apart easily along a specific direction.

Triassic metabreccia from Punta Bianca (La Spezia, Italy). Note the foliation defined by deformed clasts of white marble. The image is 90 cm in width. Photo © Samuele Papeschi

In the example on the left, a deformed breccia, the foliation is defined by clasts that were flattened by tectonic forces and that are now all oriented parallel to each other.

There are many processes that can lead to the development of a foliation in rocks (I will save them for another post). What is important is that a foliation is a pure and simple geometric term, which means that you can use it to describe any pervasive set of planes in a rock, even if you don’t know their origin or composition.

Given this definition, foliations are not restricted just to metamorphic rocks. Bedding planes in sedimentary rocks define a foliation, and so do flow structures in volcanic rocks or compositional bands (schlieren) in intrusive rocks. This kind of foliation is called primary foliation – formed during deposition or igneous crystallization of rocks – to distinguish them from secondary (or tectonic) foliation that develops when rocks are deformed.

 

There is a foliation inside this granitic dyke from the Calamita Peninsula (Elba Island, Italy). In this case it is defined by black bands rich in tourmaline and changes in grain size. Photo © Samuele Papeschi

 

Tectonic foliations are widespread in metamorphic rocks. They form as a result of different processes, which require two basic ingredients:

  • First, you need deformation processes. Luckily, we are on a trembling planet and at depth rocks are squeezed by tectonic forces.
  • Second, you need heat and pressure. You can squeeze rocks as much as you want, but if temperature is not high enough, they will break rather than develop a foliation. Some rocks can deform at relatively low temperature (claystones for instance) while others -for example a granite, require several hundreds of degrees to start deforming.

If all the conditions above are satisfied, congratulations! Your rock will develop a foliation. During deformation, old grains rotate and parallelize with each other. Eventually, new minerals will grow already oriented parallel to the foliation. This is because the foliation plane lies perpendicular to the direction of maximum tectonic force (called stress in geology) and mineral grains find less resistance to their growth along the foliation plane. Difficult? Here is a sketched example, modified after Raymond (1995).

 

The progressive development of a foliation overprints primary structures leading to the development of a new, metamorphic structure. Redrawn after Raymond (1995).

 

The more deformation goes on, the more primary structures are obliterated. A foliation progressively overprints primary structures, becoming more and more penetrative as deformation continues. If rocks face only limited deformation or are deformed at relative low temperatures, they can preserve original structures, but if the intensity of deformation or temperature (or both) are high, primary structures will likely be destroyed.

We have barely started to scratch the surface of this interesting topic. In the next chapters of the ‘Features from the Field’ series I will look in greater detail at the complicated world of foliation. Stay tuned!

 

References and further reading

Dieterich, J.H., 1969. Origin of cleavage in folded rocks. American Journal of Science 267, 155-165.
Fossen, H., 2016. Structural Geology. Cambridge University Press.
Ramsay, J.G., and Huber, M.I., 1983. The techniques of Modern Structural Geology. Vol. 1: Strain Analysis. Academic Press, London.
Raymond, L. A., 1995. Petrology: the study of igneous, sedimentary, metamorphic rocks. Dubuque, IA : Wm. C. Brown (editors)

Minds over Methods: Dating deformation with U-Pb carbonate geochronology

Minds over Methods: Dating deformation with U-Pb carbonate geochronology

Credit: Nick Roberts

For this edition of Minds over Methods, we have invited Nick Roberts, a research scientist at the British Geological Survey, working within the Geochronology and Tracers Facility (GTF) running a LA-ICP-MS laboratory. Nick has a background in ‘hard-rock’ geology, incorporating geochemistry, geochronology, and magmatic and metamorphic petrology across a wide range of tectonic settings, and is now involved in projects covering a breadth of earth system science, from Archean ore deposits to early hominid evolution. Nick is experienced in developing LA-ICP-MS U-Pb methods. He was involved in the first characterisation of a natural carbonate for use as a reference material, and in demonstrating the applicability of LA-ICP-MS U-Pb carbonate geochronology to a number of key applications, such as dating brittle deformation, ocean crust alteration, and paleohydrology.

 

Faults and fractures are ubiquitous in the Earth’s upper crust. As well as providing deformation histories of basins and orogens, they are critical for understanding the formation, migration and storage of natural resources. Determining the absolute timing of fault slip and fracture opening has lacked readily available techniques. Most existing methods require specific fault gouge mineralogy that is not always present, e.g. K-Ar illite dating. Other methods require a specific composition of fault-hosted mineralisation, e.g. U-Th/He dating of hematite, Th-Pb dating of hydrothermal monazite, U-Pb/U-Th dating of opal, and U-Pb/U-Th dating of carbonate. The latter is the most widely applicable, since carbonate minerals (e.g. calcite, dolomite) are common to many fault and fracture systems throughout a wide range of geological settings. The ability to date carbonate mineralisation with the popular method of U-Pb Laser Ablation Inductively Coupled Mass Spectrometry (LA-ICP-MS), is opening up new doors in tectonics and structural geology.

 

The method

The key to the LA-ICP-MS method is its high spatial resolution (~20-200 microns); the method involves sampling of material with a laser and measuring the composition of that material with an ICP-MS. Uranium concentrations in carbonate are low when compared to most other U-Pb chronometers, typically 10 ppb to 10 ppm, which is one or two orders less than a typical zircon. Uranium concentration, particularly in vein-filling calcite, can also be highly variable within in a single sample, spanning orders of magnitude over length-scales of 10s of microns or less. A major benefit of LA-ICP-MS dating, is that this variation in composition can be targeted and sampled with the laser, which can lead to precise age estimates through measurement of wide-ranging U/Pb ratios (see example in Figure 1). High uranium zones can also be rather elusive and searching for a needle in a haystack is often an appropriate analogy. Another benefit of LA-ICP-MS is that many samples can be screened in a single session, to select those of the most favourable composition for dating, and to find within those samples the regions with highest U. The method can be utilised on polished thick sections or blocks, or chips or grains mounted in epoxy-resin blocks. This in situ technique makes it highly versatile and allows for complementary analysis that are crucial for providing context to the dates, such as cathodoluminescence, optical microscopy, trace element mapping, and in-situ Sr isotope analyses.

 

Figure 1. Example of vein-filling calcite with LA-ICP-MS elemental map of uranium, and corresponding U-Pb concordia (Tera-Wasserburg) plot from LA-ICP-MS U-Pb spot analyses across the same region. Credit: Nick Roberts

 

Dating deformation

Carbonate, primarily calcite, occurs as vein-filling mineralisation in fractures and along fault planes (see Figure 2 for examples). Such occurrences of carbonate have precipitated from a fluid, and thus, provide a record of past fluid-flow. Carbonate vein-fill can also be used to provide absolute timing constraints on brittle deformation. Carbonate that infills a fracture provides a minimum age for the timing of fracture opening. Many fractures and fault planes exhibit multiple opening and/or slip events that lead to multiple episodes of carbonate precipitation. From these, carbonate dates can be used to bracket the timing of fault slip, and potentially provide constraints on the longevity and periodicity of fault slip events.

Figure 2. Examples of calcite vein-fill in various faults and fracture types. Credit: Nick Roberts and Jonny Imber

 

Good dates need good petrography

The key to dating fault slip and fracture opening is detailed petrography. Vein mineralisation takes on a wide variety of growth textures, and these can used to infer the history of mineral precipitation as well as fault kinematics. For example, veins may exhibit syntaxial (growth from the margin towards the centre), antitaxial (growth from the centre towards the margin) or with no preferred growth direction. Calcite morphologies are wide-ranging, from flattened, to blocky, to elongate, to fibrous (see example in Figure 3). Calcite can also exhibit deformation twinning which can be used to infer the strain regime. These textures and morphologies, along with features such as inclusion trails, shear bands and healed fractures, allow us to interpret the combined history of fracture opening, displacement and fluid precipitation.

Figure 3. Example of a complex calcite fracture-fill from a mudstone-hosted normal fault, showing cathodoluminescence and reflected light imaging, schematic textural interpretation, LA-ICP-MS trace element mapping, and a simple model of vein evolution. Credit: Nick Roberts

 

Carbonate – a treasure trove of geochemical proxies

One of the key benefits to dating carbonate mineralisation, is that its elemental and isotopic composition provides a wide-ranging archive of fluid chemistry and temperature. Traditional methods include stable carbon and oxygen isotopes and radiogenic Sr isotopes; these provide information on the source of fluids and allow the characterisation of subsurface fluid pathways. Trace elements provide further information on fluid source and rock-fluid interaction and are particularly useful for characterising proxies such as the reducing potential of precipitating fluids. Clumped isotopes are a novel method that has been explored over the last decade and provide a proxy for the fluid temperature during carbonate precipitation. In combination with U-Pb dating, clumped isotopes can characterise ancient hydrothermal fluid-flow in the upper crust (MacDonald et al., 2019). A benefit of LA-ICP-MS geochronology, is that various in-situ methods can be combined, allowing fluid composition and dating to be analysed from the same domain. For example, carbon and oxygen isotopes can be measured in situ using an ion microprobe, and Sr isotopes and trace elements can be measured using LA-ICP-MS.

 

Application to tectonics

To date, LA-ICP-MS carbonate geochronology has been applied successfully to a range of tectonic settings to constrain the timing of brittle deformation. These include the far-field effects of the Pyrenean orogeny in southern England (Parrish et al., 2018) and Alpine orogeny in Sweden (Goodfellow et al., 2018), rift-related faulting during opening of the North Atlantic in the Faroe Islands (Roberts & Walker, 2016), graben formation in the Alpine orogen (Ring & Gerdes, 2016), the timing of the Dead Sea and North Anatolian transform fault zones (Nuriel et al., 2017 and 2019, respectively), nappe stacking in the Arabian Peninsula (Hansman et al., 2018), foreland deformation of Sevier-Laramide orogenesis (Beaudoin et al., 2018) and brittle deformation within an accretionary wedge (Smeraglia et al., 2019).

 

Edited by Derya Gürer

References

Beaudoin, N., Lacombe, O., Roberts, N.M. and Koehn, D., 2018. U-Pb dating of calcite veins reveals complex stress evolution and thrust sequence in the Bighorn Basin, Wyoming, USA. Geology46(11), pp.1015-1018.

Goodfellow, B.W., Viola, G., Bingen, B., Nuriel, P. and Kylander‐Clark, A.R., 2017. Palaeocene faulting in SE Sweden from U–Pb dating of slickenfibre calcite. Terra Nova29(5), pp.321-328.

Hansman, R.J., Albert, R., Gerdes, A. and Ring, U., 2018. Absolute ages of multiple generations of brittle structures by U-Pb dating of calcite. Geology46(3), pp.207-210.

MacDonald, J.M., Faithfull, J.W., Roberts, N.M.W., Davies, A.J., Holdsworth, C.M., Newton, M., Williamson, S., Boyce, A. and John, C.M., 2019. Clumped-isotope palaeothermometry and LA-ICP-MS U–Pb dating of lava-pile hydrothermal calcite veins. Contributions to Mineralogy and Petrology174(7), p.63.

Nuriel, P., Craddock, J., Kylander-Clark, A.R., Uysal, I.T., Karabacak, V., Dirik, R.K., Hacker, B.R. and Weinberger, R., 2019. Reactivation history of the North Anatolian fault zone based on calcite age-strain analyses. Geology47(5), pp.465-469.

Nuriel, P., Weinberger, R., Kylander-Clark, A.R.C., Hacker, B.R. and Craddock, J.P., 2017. The onset of the Dead Sea transform based on calcite age-strain analyses. Geology45(7), pp.587-590.

Parrish, R.R., Parrish, C.M. and Lasalle, S., 2018. Vein calcite dating reveals Pyrenean orogen as cause of Paleogene deformation in southern England. Journal of the Geological Society175(3), pp.425-442.

Ring, U. and Gerdes, A., 2016. Kinematics of the Alpenrhein‐Bodensee graben system in the Central Alps: Oligocene/Miocene transtension due to formation of the Western Alps arc. Tectonics35(6), pp.1367-1391.

Roberts, N.M. and Walker, R.J., 2016. U-Pb geochronology of calcite-mineralized faults: Absolute timing of rift-related fault events on the northeast Atlantic margin. Geology44(7), pp.531-534.

Smeraglia, L., Aldega, L., Billi, A., Carminati, E., Di Fiore, F., Gerdes, A., Albert, R., Rossetti, F. and Vignaroli, G., 2019. Development of an Intrawedge Tectonic Mélange by Out‐of‐Sequence Thrusting, Buttressing, and Intraformational Rheological Contrast, Mt. Massico Ridge, Apennines, Italy. Tectonics38(4), pp.1223-1249.

Minds over Methods: Virtual Microscopy for Geosciences

Minds over Methods: Virtual Microscopy for Geosciences
The next “Minds over Methods” blogpost is a group effort of Liene Spruženiece (left) – postdoctoral researcher at RWTH Aachen and her colleagues Joyce Schmatz, Simon Virgo and Janos L. Urai.

Credit: Liene Spruženiece

The Virtual Microscope is a collaborative project between RWTH Aachen University and Fraunhofer Institute for Applied Information Technology (Schmatz et al., 2010; Virgo et al., 2016).

In the multitude of tools to analyze rocks, the optical microscope is still one of the first go-to methods for rock characterization. A look on a petrographic thin section under an optical microscope gives a quick overview of the mineralogy and fabric of a rock and helps to determine the areas of interest for more detailed analysis with other methods. It is also a crucial part for teaching geology classes.

Most geoscience institutions already have a petrographic microscope that is equipped with a camera and maybe an automated stage for capturing image mosaics. However, these images make only limited use of the vast amount of information available, such as the overview of the sample fabric at different magnifications or the change of optical mineral properties (pleochroism and extinction behavior) with the rotation of polarizers. Although there have been recent developments in the field of virtual microscopy (NASA, 2007; The Open University; 2010; Tetley and Daczko, 2014), to our knowledge, none of the existing systems has been able to fully emulate a user experience in a virtual environment that is equal to using a traditional analogue microscope.

 

Figure 1. The setup of the PetroScan microscope. Credit: Fraunhofer FIT

How does it work?

The idea behind the virtual microscope is to capture the full information of entire rock thin-sections in a digital format, including the possibility to switch between plain light and crossed polarizers, rotate the polarizers and zoom everywhere in the sample at high magnifications.

The hardware part of our system consists of a fully automated petrographic microscope, connected to a computer (Figure 1). The software contains three modules; a PetroScan launcher for setting up the image acquisition, optimizer for combining and interpolating the scanned images and a viewer program (Figure 3).

 

Figure 2. Automated sample stage allows capturing high resolution images of entire thin sections by combining thousands of images taken sequentially along a predefined grid. Credit: Jochen Hürtgen

High-resolution mosaics (Figure 2) containing up to a million images can be acquired in a few hours. With crossed polarizers, the scans are performed for several rotation angles. The movement accuracy of the sample stage is about 10 nm. This ensures precise overlap of the scanned mosaics at different rotation angles, where each corresponding pixel has the same x-y coordinates. Thus, the images of each rotation angle are precisely overlapping and allow to interpolate the change in brightness values for every individual pixel in the mosaics, producing smooth curves (Figure 3) that reflect extinction behavior in minerals (Heilbronner and Pauli, 1993).  Each attribute of these interpolated curves (phase, amplitude) of the sample can be viewed at any rotation angle.

 

Quantification of optical images

Figure 3. Screenshot of the PetroScan TileViewer window showing a thin section that is scanned under crossed polarizers. On the right side the scanned image is overlain by the phase map, where each pixel is color-coded according to the mineral extinction angles. Credit: Liene Spruženiece, Joyce Schmatz, Simon Virgo and Janos L. Urai

Virtual microscopy offers powerful means for quantitative analysis of giga pixel images to extract multi-scale information. It has a resolution of up to few micrometers for areas in sizes of 10 square centimeters. In its current form the viewer software that we developed contains a built-in toolbox for displaying and tresholding the intensity, saturation and hue values of the scanned images. This can be used for a quick estimation of sample porosity or proportions of different minerals. In addition, the scans with interpolated crossed-polarizer rotation also contain information of the mineral extinction behavior that is used to produce a “phase map” (Figure 3). The “phase map” displays the variations in the mineral optical axis orientations. Although it cannot provide absolute values for the crystallographic orientations, it allows a clear distinction between differently oriented mineral grains and allows easy visualization of qualitative lattice misorientations inside individual mineral grains. Furthermore, import and export functions are incorporated in the image viewing software. Thus more advanced image analysis can be carried out in specifically designated external softwares (e.g. Fiji/ImageJ, GiS, Matlab, Python), then inserted back in the PetroScan viewer software.

 

Applications in teaching

Figure 4. Mineralogy and fabrics of the scanned samples can be segmented and quantified using built-in toolboxes. The recognition of features is greatly improved by the possibility to zoom in the samples, switch between the plain light and crossed polarizers and rotate the polarizers. Credit: Liene Spruženiece, Joyce Schmatz, Simon Virgo and Janos L. Urai

At RWTH Aachen University, virtual petrography has been used as a teaching tool for several years. For example, it is extensively used in the microtectonics course. Each class consists of a short introduction in some of the common rock microstructures, such as veins, cataclasites, mylonites, dissolution-precipitation features and others. This is followed by an hour-long presentation by a student group, where they describe and discuss the respective microstructure in a scanned thin-section, projected to a screen with a high-resolution beamer. The students use laser pointers for characterizing the features and origins of the microstructure, switch between views in plain-polarized or crossed-polarized light, zoom in and out across the sample, adjust illumination settings, rotate polarizers and do basic image analysis (Figure 4). This has proven to be a highly engaging teaching method. The students in the audience question the presenting group asking to provide a closer look or more details on any interesting microstructural feature. Often hypotheses are formed by the listeners and immediately tested by the presenters, allowing to perform an investigation and agree on a reasonable explanation at the end of the class. Virtual petrography can be especially important in teaching institutions that do not own well-equipped microscopy labs. It only requires a computer and a projector. The thin section scans and PetroScan viewing software will be available for download online. Thus, such a method will not incur additional costs to the institutions, at the same time will provide a large collection of a high variety of geological samples.

 

The vision

We imagine a world-wide community built around the platform of virtual microscopy, where the information from different analytical methods can be exchanged between users and stored in open databases, available for teaching or research purposes. Many advantages arise from such a transformation. Work can be carried out anywhere without a need to access microscopes, several users can simultaneously view the same samples, data can be easily quantified and integrated between different methods, and thin-section libraries can be shared and exchanged by the user community.

Further plans for developing this method include collaboration with the computer vision department at RWTH Aachen University in order to create deep learning algorithms that allows quick and precise segmentation of rock microfabrics, such as mineral content and distribution, grain boundaries, grain sizes, porosity, etc. This has been made possible by recently obtained funding from the RWTH Aachen University “Exploratory Research Space – ERS”. The final data set will be public and shared with communities.

 

Edited by Derya Gürer

 

References

Heilbronner, R.P., Pauli, C. (1993). Integrated spatial and orientation analysis of quartz c-axes by computer-aided microscopy. Journal of Structural Geology, 15, 369-382.

Tetley, M.G., Daczko, N.R. (2014). Virtual Petrographic Microscope: a multi-platform education and research software tool to analyse rock thin-sections. Australian Journal of Earth Sciences 61, 631-637.

NASA (2007). Virtual Microscope. Available at: http://virtual.itg.uiuc.edu/

Schmatz J., Urai J.L., Bublat, M., Berlage, T. (2010). PetroScan – Virtual microscopy. EGU General Assembly, EGU2010-10061.

The Open University (2010). The Virtual Microscope for Earth Sciences Project. Available at: http://www.virtualmicroscope.org/.

Virgo S., Heup, T., Urai J.L., Berlage, T. (2016). Virtual Petrography (ViP) – A virtual microscope for the geosciences. EGU General Assembly, EGU2016-14669.

From Mountains to Modernists: the geological foundations and inspirations of Barcelona

From Mountains to Modernists: the geological foundations and inspirations of Barcelona

Barcelona is a vibrant city on the Mediterranean coast, nested snugly between the sea and the Collserola Ridge of the Catalan Coastal ranges. The story of Barcelona starts around 2000 years ago as an Iberian settlement, owing to its strategic location on the coastal route connecting Iberia and Europe. The combination of easily defendable ground and the fertile soils of the Besos and Llobregat deltas have helped shape its history as an important trade hub and place of varying political importance. However, the rocks on (and of) which the city is built tell a much longer and intriguing story. Today we take a close look at the rocks that built Barcelona, and the fascinating history that produced them.

Our story begins in the Paleozoicum, when the oldest rocks outcropping near Barcelona were formed. Iberia collided with the remains of Gondwana and became part of the supercontinent Pangea. During this period, the Hercynian orogeny, intense metamorphism produced  part of the rich mineral assemblage found in the Collserola Ridge today (although it should be noted that collecting rocks is forbidden, as the ridge enjoys a Natural Park status). During the Mesozoic, when Pangea broke apart and Iberia developed into a separate plate, the area that is now Catalunya was mostly submerged under a sea. Most of the sediments deposited during this stage have however been eroded away. This was due to the uplift of the Coastal Ranges during the Pyrenean orogeny in the Paleogene, as Iberia reconnected with Europe between 55 and 25 million years ago. The Catalan Coastal Range was rotated anticlockwise, making it run oblique to the N-S contraction in the Pyrenees. As a result of this uplift, the Collserola Ridge still has an elevation of up to 500 meters, providing sweeping vistas over the city and Mediterranean. During the middle Miocene (around 14 million years ago) the current Barcelona was a coastal area, where the erosional products of the Coastal Ranges were deposited under varying sea levels. Extension started in this period, and continues up to today along reactivated normal faults associated with the Valencia Trough.

This geological map shows the modern city and the distribution of bedrock types underlying it. Credit: Google Earth, Geological Map adjusted from Sergisai.

 

Montjuïc – foundations and façades

So how has this geological history affected the city of Barcelona? The most obvious influence is in the very materials used for constructing it, leaving clues to the geological history of Catalonia all over the city. Situated to the south of Barcelona’s centre is Montjuïc, a ~200 m high block of Tertiary deltaic sediments, product of erosion from the Collserola mountains. The block was tilted due to recent extension, and the surrounding materials eroded, leaving it as an isolated hill surrounded by the alluvial plain filled with modern sediments.

Left: Study of Montjuïc Quarry Artists: Enric Galwey, Barcelona, 1864-1931. Credit: Museu Nacional. Right: View of Montjuïc castle. Credit: barcelonaconnect.com

On top of the Montjuïc hill sits an imposing castle with 360views over the city, built in the seventeenth century. It was used for defensive purposes, for example as a base by Catalan forces fighting a Spanish army during the Battle of Montjuïc in 1641, but also to control the city itself, being used for bombardments and imprisonment at various occasions since the fall of Barcelona during the War of the Spanish Succession (1701-1714). Its bloody history continued into the Spanish Civil War (1936-1939). Montjuïcs most important mark on the city however has been the sandstone quarried there and used in many of Barcelona’s most iconic structures. The very short distance from the source area has caused a rich and poorly sorted mineral assemblage, with the metamorphic minerals of the Collserola Ridge somewhat preserved. The sediments are a sequence of conglomerates, sandstones, mudstones and marls due to the changing sedimentary conditions with variation of sea levels. Although the main mineral assemblage is quartz and feldspars, it includes small amounts of biotite, muscovite, zircon, chlorite and tourmaline, adding to the vibrant colours of the sediments. Montjuïc stone’s high silica content (making it exceptionally durable) and vibrant colours made it a cherished resource, therefore being quarried from Iberian times well into the 20th century. Amongst others, the Palau de la Genaralitat, City hall, Catedral de Barcelona and Barcelona University were constructed from Montjuïc stone. A notable example is the Santa Maria del Mar church, of which the construction formed the backdrop for the 2006 novel (and 2018 Netflix drama) by Idefonso Falcones, La Catedral del Mar, telling the story of a stoneworker and illuminating the hardships and triumphs of medieval life in Barcelona.

 

Antoni Gaudí – the modern face of Barcelona

House Casa Mila (La Pedrera) in Barcelona building by the great Spain architect Antonio Gaudi. Credit: Adobe Stock.

Apart from the materials found in Barcelona’s buildings, their design has also been influenced by the impressive geology found on the Iberian peninsula. The most obvious example of this is found in the works of Antoni Gaudí, the architect who has had perhaps the biggest single-handed influence on the face of the city.

Besides from his most famous works, the (still unfinished) Sagrada Família Cathedral, Casa Battló, Casa Milà and Parc Güell, he designed about ten more iconic buildings, and hundreds of other objects. Part of his architectural genius came from the inspiration he pulled from landscapes surrounding Barcelona, leading him to become one of the founders of the school of organic architecture. The Prades Mountains in the region of Terragona bear a striking resemblance to some of the design element used in Casa Battló and La Pedrera,while the beautiful l´Argenteria in the Collegats gorge in the Pyrenees has been rumoured to have inspired the Nativity façade of the Sagrada Familia (see the photos below). The gorge was incised into Tertiary conglomerates deposited on the interface between the (then endorheic) Ebro Basin and the Pyrenees, subsequently affected by karstification and finally covered in travertine precipitated from natural spring water, resulting in this spectacular geological feature. Gaudi’s works give the city an unique and organic feel, and form part of the main tourist attractions.

 

Left: L’Argenteria, Colgats gorge. Right: Nativity Façade of the Sagrada Familia. It is easy to see how Gaudi could have drawn inspiration from the beautiful cliff for some of the elements of the façade. Credit: Chris Kuijper.

 

As with most modern cities, the geological features visible in Barcelona are only a tiny fraction of the richness of rocks and structures hidden underground and as the city rapidly expanded in the 20th century, very little of this information was properly recorded. But, a curious eye and wandering feet can discover many clues to the rich history of the region, and any travelling geologist would be more than satisfied with what Barcelona has to offer.

Beyond tectonics: The present-day tides are the biggest they have been since the formation of Pangea

Beyond tectonics: The present-day tides are the biggest they have been since the formation of Pangea
“Beyond tectonics” is a blog series which aims to highlight the connections between tectonics and other aspects of the Earth system. In this iteration of the “Beyond tectonics” series we talk about how plate tectonics have affected the tides on Earth over geological timescales. We will talk about tectonics on the Earth since the formation of Pangea to the present day, and into the future, ending with the formation of the next Supercontinent in around 200 – 250 Million years from now.

 

Tides of the Planet Earth

The Earth’s tides are predominantly caused by the gravitational pull of the Moon, and the centripetal force of the Earth due to its rotation. The force of the Moon and the spin of the Earth cause two tidal bulges to form, one that follows the Moon, and one on the opposite side of the planet. These two tidal bulges move around the Earth with a period of 12.5 hours. When the buldge moves over a coast, a high tide occurs, and when a bulge is not over a coast, a low tide occurs. This is why there are two low and high tides each day. These tides vary in strength around the world because ocean and coastal morphology plays a large role in how the tidal energy is distributed.

 

How do tectonic plates affect the tide?

The surface of the Earth is broken up into pieces like the shell of an egg. These pieces, or plates, can be divided into oceanic and continental plate. The main difference between the two types of plate is their buoyancy. Both types of plate are buoyant, however, after it is formed at a mid ocean ridge, ocean plate becomes less and less buoyant with age. Eventually, after around 30 million years, ocean plate is less buoyant than the underlying mantle meaning it could sink if it reached a subduction zone. In the present-day oceans, almost all plate older than 180 million years old has sank back into the mantle at a subduction zone. This recycling of ocean crust is what causes the oceans to change shape. The continents are pulled together by the closing oceans (sinking ocean plate), and pushed apart by the opening ones (creation of ocean plate). In the present day, the Pacific is closing and the Atlantic is opening, causing the plate drift map to look like this:

The major subduction zones on Earth. Black arrows illustrate trench migration vectors, while open arrows illustrate plate velocity (credit – Schellart et al., 2007)

As the oceans grow and shrink, their width changes. This means the tidal wave in those oceans has either too little, too much, or just the right amount of space to flow in the ocean. In the present-day the Pacific is too big, the Indian ocean too small, but the Atlantic ocean is just the right size to make the tide resonant.

 

What is resonance?

To explain resonance, imagine the tidal wave moving across the Atlantic ocean and back, like a child on a swing. If you apply the force on the swing when it is at the highest point, you are applying a force at one of the natural frequencies of the system, so the energy you input will make the swing go higher. If you push the swing before or after it reaches its peak, then you might input some energy, but it won’t be as efficient. The Atlantic ocean is just the right width to allow the wave to “swing” back and forth. The input of energy is applied at just the right point, one of the natural frequencies of the tide, to allow resonance.

It is possible for this to happen in any ocean basin. An ocean basin can house resonant tides when the width of the basin (L) is equal to a multiple of half wavelengths of the tide  (𝜆 = √gHT),  where (T) is the tidal period, (g) is gravity, and (H) is water depth. Essentially, when the ocean basin has a width that intersects with a natural frequency of the tide, it will become resonant.

 

The Super-tidal cycle

The reason why the present day tides are the biggest since the formation of Pangea is because the Atlantic is currently resonant with the tide, i.e. it is in a Super-tidal period. Looking at the energy of the tides from when Pangea existed to the present-day (Green et al., 2017), we can see that during Pangea’s life the tides were weak. That status quo continued from 180 to 1 million years ago, when the Atlantic suddenly developed large tides. The Atlantic had been growing all that time, and had finally reached a width where the tide became resonant.

M2 tidal amplitudes since the breakup of Pangea (credit – Green et al., 2017)

The large present-day Atlantic tide is unlikely to last. Green et al., (2018) predict that this super-tidal period will last around 20 million years, and Davies et al., (2019) predict another won’t occur for tens of millions of years, depending on how the future Earth develops.

Therefore, what are the implications of the present-day Atlantic having such large tides? Further testing of the implications of the Super-tidal cycle is needed before any conclusions can be made on how it may affect the Earth system. However, the larger energy input into the oceans during a Super-tidal period may enhance tidal mixing, which means the ocean will have a better distribution of nutrients and oxygen, i.e. it is less likely for it to become stratified. What we do know right now, is that Supercontinents generally have very weak tides (Green et al., 2018), and periods of continent divergence similar to the present day, have larger, or sometimes Super-tides (Balbus 2014; Davies et al., 2019).

 

References

  • Balbus, S. A. 2014, Dynamical, biological and anthropic consequences of equal lunar and solar angular radii. Proc. R. Soc. A, 470. Available at: http://doi.org/10.1098/rspa.2014.0263
  • Davies, H. S., Green, J. A. M., Duarte, J. C., 2018, Back to the future: testing different scenarios for the next supercontinent gathering. Global and planetary change, 169, 133 – 144. Available at: https://doi.org/10.1016/j.gloplacha.2018.07.015
  • Davies, H. S., Green, J. A. M., Duarte, J. C., 2019. Back to the future 2: Tidal modelling of four potential scenarios for the next Supercontinent gathering. Presented at EGU 2019. Available at: https://meetingorganizer.copernicus.org/EGU2019/EGU2019-1004-1.pdf
  • Green, J.A.M., Molloy, J.L., Davies, H.S., Duarte, J.C., 2018. Is there a tectonically driven super-tidal cycle? Geophys. Res. Lett. 45 (8). Available at: https://doi.org/10.1002/2017GL076695.
  • Schellart, W.P., Freeman, J., Stegman, D.R., Moresi, L., May, D. 2007. Evolution and diversity of subduction zones controlled by slab width. Letters to Nature, 446, 308 – 311. Available at: doi:10.1038/nature05615