TS
Tectonics and Structural Geology
Hannah Davies

Guest

Hannah Davies is currently a PhD student in Lisbon University, Portugal. By combining GPlates and numerical modelling she is exploring the link between plate tectonics and tides. As ocean basins change over geological time scales due to continental drift, and the Wilson cycle, the tides in those ocean basins also changes. Hannah's work is currently focused on quantifying this change in the tides over geological time.

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.

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