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Tectonics and Structural Geology

wilson cycle

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

Lisbon at the dawn of modern geosciences

Lisbon at the dawn of modern geosciences

Here, where the land ends and the sea begins...
Luís de Camões (Portuguese poet)

Lisbon. Spilled over the silver Tagus River, it is known by its beautiful low light, incredible food and friendly people. Here, cultures met, and poets dreamed, as navigators gathered to plan their journeys to old and new worlds. Fustigated by one of the greatest disasters the world has ever witnessed, Lisbon is intertwined with the course of Earth Sciences. For some, modern seismology was born here. For others, this might even have been the place where it all begun; what we now call geology.

On the morning of All Saints day of 1755, a giant earthquake struck the city of Lisbon. With a magnitude of ~8.7, the event was so powerful that it was felt simultaneously in Germany, as well as in the islands of Cape Verde. The main shock occurred around 9.40 am, when a significant portion of the population was attending the mass in churches. Lasting several minutes, many of the roofs collapsed and thousands of candles set fires that would last for days. While people were looking for safety at open areas near the river, three giant tsunami waves were on their way. Forty minutes after the main shock, the waves rose the Tagus River and flood the city’s downtown. The death toll in Lisbon reached up to 50,000 people, about one quarter of Lisbon’s population at the time. This event is known as the Great Lisbon Earthquake of 1755.

 

Painting depicting the day of the 1755 Great Lisbon Earthquake. Credit: Wikipedia.

 

The 1755 Lisbon Earthquake was a terrific natural disaster. A few years ago, the French magazine L´Histoire, considered this earthquake as one of the 10 crucial events that changed history. At the time, Lisbon was a maritime power in a maritime epoch. This was also the age of Enlightenment, when man started to realize that many events such as earthquakes, volcanoes and storms, had natural causes, and were not sent by gods.

Convento do Carmo, destroyed during the 1755 earthquake and kept as a ruin for memory. Credit: Flickr.

Lisbon was in the spotlight of the modern world and some of the most prominent philosophers like Kant, Voltaire and Rousseau focused on the destructive event of the 1st of November, 1755. In particular, Emmanuel Kant published in 1756 (yes, 1756!) three essays about a new theory of earthquakes (see Duarte et al., 2016 and the reference list below for two of the Kant’s essays). I recommend all geoscientists to read these documents. It is incredible how Kant understands and describes how earthquakes align along linear features that are parallel to mountain chains. Does this sound familiar? Moreover, he uses the then new physics of Newton to calculate the forces that were needed to set the seafloor off Lisbon in movement in order to generate the observed tsunami. He even refers to experiments with buckets full of water to explain how the tsunami formed (analogue modelling!?). And Kant was not alone…

The minister of the King of Portugal at the time, the Marquis of Pombal, sent an enquiry to all parishes in the country with several questions. While some of the questions were intended to evaluate the extent of the damage, it is now clear that the Marquis was also trying to gain (scientific) knowledge about the event (see Duarte et al., 2016 and references therein). For example, he asks if the ground movement was stronger in one direction than in other, or if the tide rose or fell just before the tsunami waves arrived. Today, we can reconstruct with rigor what happened that day because of the incredible vision of this man.

 

The center of Lisbon today. The statue of Marquis of Pombal facing the reconstructed downtown. Credit: Wikipedia.

 

Coming back to Lisbon. If you visit the old city by foot, you will realize that houses on the hills are closely packed, separated by narrow streets and passages, while in the flat downtown streets are wide and orthogonal. The hilly parts of Lisbon are an heritage of the Moorish and Medieval times. Mouraria and Alfama are the ideal neighborhoods to visit. The organized downtown was the area that was totally floored during the earthquake, due to ground liquefaction and the impact of the tsunami, and was rebuilt using a modern architecture (see Terreiro do Paço and the downtown area in the first figure in the top). The Grand Liberty Avenue is clearly inspired by the style of the Champs-Élysées. Going up the Liberty Avenue, from the downtown, you will find the statue of the Marquis of Pombal (see figure above). And if you are already planning to visit (or revisit) Lisbon, you should definitely stop by the Carmo Archeological Museum, a ruin left to remind us all of what happened on that day of 1755, and the Lisbon Story Centre.

The hills of Lisbon, with the Castle in the top left and the 25 de Abril bridge in the background. Credit: Flickr.

Rebuilding plan after the 1755 earthquake. Credit: Wikimedia Commons.

The 1755 Great Lisbon Earthquake was however not the only earthquake that hit the city. On the 28th of February 1969, another major quake, with a magnitude of 7.9, struck 200 km off the cost of Portugal, at 2 am in the morning. The earthquake generated a small tsunami but luckily, given the late hours, did not caused any casualties. This event also occurred in a particular point in history: The time of plate tectonics. The paper that inaugurated plate tectonics had been published only 4 years before, by Tuzo Wilson. And in 1969, geoscientists already realized that some continental margins were passive and did not generate major earthquakes, such as the margins of the Atlantic, while others were active and fustigated by major earthquakes, such as the margin of the Pacific (Dewey, 1969). It was somewhat strange that this Atlantic region was producing such big earthquakes, which therefore immediately resulted in scientists coming to study this area (see map below).

Fukao (1973), studied the focal mechanism of the 1969 earthquake and concluded that it was a thrust event. Purdy (1975), suggested that this could result from a transient consumption of the lithosphere, and Mckenzie (1977) proposed that a new subduction zone was initiating here, along the east-west Africa-Eurasia plate boundary (see the thinner segment of the dashed white line in the eastern termination of the Africa-Eurasia plate boundary, map below), SW of Iberia. Later on, in 1986, António Ribeiro, professor at the University of Lisbon, suggested that instead, a new north-south subduction zone was forming along the west margin of Portugal (yellow lines in the map), a passive margin transforming into an active margin. This could explain the high magnitude seismicity, such as the Great Lisbon Earthquake of 1755.

 

Map showing the main tectonic features in the SW Iberia margin. The Eurasia-Africa plate boundary spans from the Azores-Tripe Junction (on the left) until the Gibraltar Arc (on the right, with its accretionary wedge marked in grey). The yellow lines mark a new thrust front that is forming and migrating northwards away from the plate boundary and along the west Iberia margin. The smaller yellow line marks the approximate location of the 1969 earthquake. The 1755 Great Lisbon Earthquake might also have been generated in this region (see Duarte et al., 2013 for further reading on the tectonic setting of the region; the figure is adapted from this paper).

 

Today, we know that the SW Iberia margin is indeed being reactivated (Duarte et al., 2013). Whether this will lead to the nucleation of a new subduction zone is still a matter of debate, and we will probably never know for sure. Nevertheless, subduction initiation is one of the major unsolved problems in Earth Sciences, and the coasts off Lisbon might constitute a perfect natural laboratory to investigate this problem. It may be the only case where an Atlantic-type margin (actually located in the Atlantic) is just being reactivated, which is a fundamental step in the tectonic conceptual model that we know as the Wilson Cycle (see also Duarte et al., 2018 and this GeoTalk blog). In any case, we know that there are two other locations where subduction zones have developed in the Atlantic: in the Scotia Arc and in the Lesser Antilles Arc. How they originated is still being investigated; which is precisely what we are doing now in Lisbon. That is however a topic that deserves its own blog post.

 

Written by João Duarte

Researcher at Instituto Dom Luiz and Invited Professor at the Geology Department, Faculty of Sciences of the University of Lisbon. Adjunct Researcher at Monash University.

 

Edited by Elenora van Rijsingen

PhD candidate at the Laboratory of Experimental Tectonics, Roma Tre University and Geosciences Montpellier. Editor for the EGU Tectonics & Structural geology blog

 

For more information about the Great Lisbon Earthquake of 1755, check out these two video’s about the event: a reconstruction of the earthquake and a tsunami model animation

 

References:

Dewey, J.F., 1969. Continental margin: A model for conversion of Atlantic type to Andean type. Earth and Planetary Science Letters 6, 189-197.

Duarte, J.C., Schellart, W.P., Rosas, F.R., 2018. The future of Earth’s oceans: consequences of subduction initiation in the Atlantic and implications for supercontinent formation. Geological Magazine. https://doi.org/10.1017/S0016756816000716

Duarte, J.C., and Schellart, W.P., 2016. Introduction to Plate Boundaries and Natural Hazards. American Geophysical Union, Geophysical Monograph 219. (Duarte, J.C. and Schellart, W.P. eds., Plate Boudaries and Natural Hazards). DOI: 10.1002/9781119054146.ch1

Duarte, J.C., Rosas, F.M., Terrinha, P., Schellart, W.P., Boutelier, D., Gutscher, M.A., Ribeiro, A., 2013. Are subduction zones invading the Atlantic? Evidence from the SW Iberia margin. Geology 41, 839-842. https://doi.org/10.1130/G34100.1

Fukao, Y., 1973. Thrust faulting at a lithospheric plate boundary: The Portugal earthquake of 1969. Earth and Planetary Science Letters 18, 205–216. doi:10.1016/0012-821X(73)90058-7.

Kant, I., 1756a. On the causes of earthquakes on the occasion of the calamity that befell the western countries of Europe towards the end of last year. In, I. Kant, 2012. Natural Science (Cambridge Edition of the Works of Immanuel Kant Translated). Edited by David Eric Watkins. (Cambridge: Cambridge University Press, 2012).

Kant, I., 1756b. History and natural description of the most noteworthy occurrences of the earthquake that struck a large part of the Earth at the end of the year 1755. In, I. Kant, 2012. Natural Science (Cambridge Edition of the Works of Immanuel Kant Translated). Edited by David Eric Watkins. (Cambridge: Cambridge University Press, 2012).

McKenzie, D.P., 1977. The initiation of trenches: A finite amplitude instability, in Talwani, M., and Pitman W.C., III, eds., Island Arcs, Deep Sea Trenches and Back-Arc Basins. Maurice Ewing Series, American Geophysical Union 1, 57–61.

Purdy, G.M., 1975. The eastern end of the Azores–Gibraltar plate boundary. Geophysical Journal of the Royal Astronomical Society 43, 973–1000. doi:10.1111/j.1365-246X.1975.tb06206.x.

Ribeiro, A.R. and Cabral, J., 1986. The neotectonic regime of the west Iberia continental margin: transition from passive to active? Maleo 2, p38.

Wilson, J.T., 1965. A new class of faults and their bearing on continental drift. Nature 207, 343– 347