Fadl Raad – Université de Montpellier (France)

In 1770, wandering around his farm in eastern Scotland, James Hutton, today known to be the father of geology, noticed that the soil in his farm was being created from the erosion of the surrounding rocks and subsequently washed and carried away by rain, streams and rivers and re-deposited somewhere else. He realized that if such a process were carried to its natural conclusion then Earth would eventually be worn quite smooth. Yet everywhere around him there were hills. After several years full of research and observations, Hutton deduced a series of things that lead him to write a masterpiece that he named ‘A Theory of the Earth with Proofs and Illustrations’. Here is one of his conclusions:

‘’The world which we inhabit is composed of the materials, not of the earth which was the immediate predecessor of the present, but of the earth which, in ascending from the present, we consider as the third, and which had preceded the land that was above the surface of the sea, while our present land was yet beneath the water of the ocean.’’

It is no wonder that we don’t understand anything of what he wrote if we realize that even James Watt who was a friend of Hutton, did not understand a thing of Hutton’s manuscript. Indeed, Hutton had one serious problem: it was beyond him to verbalize his thoughts in a form that anyone could begin to understand. His words had meaning, but it would take a generation before scientists such as Charles Darwin would grasp the importance of his work. Later in the 18th century Alfred Lothar Wegener came out with the plate tectonics theory, and only then part of Hutton’s words became clear.

The concept of plate tectonics which was further developed in the 19th century is that Earth has a rigid outer shell (layer), known as the lithosphere, which is typically about 100 km thick and overlies a plastic (partially molten) layer called the asthenosphere. The lithosphere is broken up into seven very large continental- and ocean-sized plates, six or seven medium-sized regional plates, and several small ones. These plates move relative to each other, typically at rates of 5 to 10 cm per year, and interact along their boundaries, where they converge, diverge, or slip past one another.

Figure showing the boundaries (blue lines) of the major tectonic plates. Res arrows show the direction of the movement. The longer the arrows (vectors) the higher the velocity of the plate.

Such motions cause mountains to rise where plates push together, or converge, and continents to fracture and oceans to form where plates pull apart, or diverge. So now it is clear what Hutton meant back in the 17th century by saying that our present was yet beneath the water of the ocean.

Now let’s go back to what this blog is about, that is the Mediterranean Salt Giant. Looking at the distribution of the evaporitic sediments belonging to the Messinian Salinity Crisis, we notice that more than 90% of the Messinian deposits in the Mediterranean are located offshore below the see floor, whereas only a bit of them are lying onshore (inland) and are physically reachable (outcrops).

Relief map of the Mediterranean area with DSDP and ODP drill sites superimposed to the present-day spatial extent of the MSC markers, i.e. after Plio-Quaternary basin geodynamic evolution and possible crustal tectonics or salt tectonics. Sediments onshore are represented by a brown color. Green, yellow and blue colors represent different types of Messinian deposits all offshore. From Lofi J. (2018)-Seismic Atlas.

The explanation of this distribution between onshore and offshore Messinian sediments is now simple to comprehend, since the answer is all about plate tectonics. Thus, the areas where these sediments are outcropping underwent and/or are undergoing more tectonic movements and they are more active with respect to the other areas where the sediments are slightly disturbed.

It is now intuitive that in order to completely understand the formation and distribution of such deposits, we somehow need to access the deposits offshore because they represent the majority. One direct way is drilling wells allover the Mediterranean, but this requires a crazy big budget. That’s why an indirect alternative method is widely used to get images of the underground (subsurface) deposits, The Seismic Reflection method. Nothing complicated… We all, at least once experienced, the terrible joy of yelling in a valley and wait to hear our sound echoes back after few seconds. The first sound echo that we here is the one bounced back from the nearest obstacle, thus the one that traveled less distance and time, then the second and so on… In geophysics the same concept is used to explore the subsurface by creating seismic (sound) waves and wait to listen and register the echoes by special receivers. The early sounds received arrive from shallow sedimentary layers and late sounds come from deeper ones.

Figure demonstrating a seismic reflection experiment

The registered waves at the receivers are then subject to a complex processing and transformed into interpretable images called Seismic Sections.

Seismic interpretation is the last essential step in the research applied to geological studies, since it allows to give geological attribute to any geophysical data. In this step, we look to the seismic sections, detect the boundaries between sediments with different ages and highlight them to get their overall distribution and geometric relationships.

Figure showing a seismic section before (above) and after (below) interpretation of the Messinian sedimentary units. From Lofi J. (2018)-Seismic Atlas.

So, at the end of this entry we conclude 3 main things:

  1. The reason behind the distribution of the Messinian sedimentary deposits bla bla…
  2. Another stuff about how seismic reflection method works…
  3. The last and most important point is how essential it is to learn how to express our ideas in the clearest ways so that our audience don’t have to struggle and stop listening us, otherwise we end our career, and no one appreciate our work.

Hope that this article catches your attention and make it easy to understand some basic things about Geology, Geophysics and Messinian Salinity Crisis. If not, it means that, as a writer, I’m worse than Hutton and I shall work on it.

—————————————————————————————–

References:

  • Bill Bryson – A short history of nearly everything.
  • J. Brendan Murphy & Tjeerd H. van Andel. Plate Tectonics. Encyclopedia Britannica.
  • Lofi J. (2018). Seismic atlas of the Messinian salinity crisis markers in the Mediterranean Sea. Volume 2.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s