Do we really know where the Ocean is?

By Almudena Sanchez de la Muela Garzon

Believe it or not, scientists still can’t agree on where the continent ends, and the ocean begins. On the Atlantic coastlines of North and South America, or the western coasts of Europe and Africa, for example, we don’t know exactly where the real oceanic sea floor starts. I know, you may think, “who cares?”, but the location of the divide between continental crust and oceanic crust might determine whether they build an oil rig in front of your favourite beach in Portugal, can affect the rights of countries over the sea, and has an interesting scientific side, too.

Simplistic model of the transition of continental crust to oceanic crust along a rifted margin. There is still no unique model accepted globally. New models are being proposed and they vary geographically, depending on the margin they represent.
Image created by Almudena Sanchez de la Muela Garzon.

The Nature and Controversy of the COB

According to the Wilson cycle and the Plate Tectonics Theory, from time to time continents rift apart forming oceanic basins. Later, they can switch to a convergent phase in which they are pushed together again generating mountain chains as they collide. During the rifting of continents, the gap opened is filled with new oceanic crust formed by the material erupted in oceanic ridges. However, the complexity of this process makes it very difficult for scientists to trace the line that separates continental crust from the oceanic crust, known as the Continent-Ocean Boundary or COB.

The continental edges that underwent rifting but haven’t reached a reunification phase yet and remain tectonically still, are known as ‘passive’ or ‘divergent margins’¹. And it is on this type of margin that the search for the COB is focussed. As the map shows, these ‘passive margins’ are fairly widespread, appearing on the coasts of the eastern American continent, western and northern Europe, Russia, Africa, India, Australia and Antarctica.

Currently the most common type of crustal boundary on our planet are the passive or divergent margins
Image from Wikimedia commons, is in the public domain.

The continental crust tends to be lighter, older and thicker than the oceanic crust, which presents a different composition and tends to be more dense, but younger and thinner. The most popular methods used to measure these properties, and thus to map the COB are:

• Samples extracted from the sea bottom can be analysed to determine the age and composition of rocks.
• Seismic data are used to `image´ the geology under the surface, providing information on the depth, shape and composition of the crust.
• Anomalies in gravity data can be used to detect lateral density variations in the crust.
• Magnetic data maps show characteristic oceanic bands that help to identify the oceanic crust.

We count on several methods to find the COB, but when geoscientists have applied different techniques in the same area, they have obtained very different results. They tend to show a similar boundary shape, but the exact location can differ by hundreds of meters.

Map of the COBs and COTs compiled by Eagles and colleagues for the Gulf of Mexico (left) and the north Atlantic margins (right)
Images copyright Eagles et al.(2015)², used with permission.

This was recently examined by Graeme Eagles and colleagues by overlapping COB estimations from many different sources for a number of areas globally². They calculated the average of maximum differences between sets of COB location results for each area they studied, and got an alarming global value of 167 km. Such a difference is too high to be blamed on the observational uncertainty of the methods used. So, the disagreement must to be due to assumptions made in interpreting the data and/or a poor understanding of the origin of the crust transition.

Schematic cross section of a passive margin representing the idea of transitional crust as a mixed domain that separates continental crust from oceanic crust.
Adapted from a diagram by Joshua Doubek, under a CC-BY 3.0 license from Wikimedia commons.

Marine geology remained quite unexplored until just a few decades ago, when the study of the oceans first started to develop. As research continues, the term ‘COB’ is evolving to ‘COT’ (Continent-Ocean Transition). This has been spurred by recent publications showing that lying between the two crust types, there might be an intermediate region with mixed characteristics.

Scientific Impact

Illustration of the different phases of the Wilson Cycle.
Adapted from a diagram by Saphon, under a GNU Free Documentation License 1.2 from Wikimedia commons.

The exact location of the COB or COT might have a decisive role in the starting point of oceans closure. When two continents join, they must destroy the ocean between them. The oceanic crust must sink or ‘subduct’ underneath the continents, penetrating into the mantle below. The friction and rock melting involved in the subduction of the crust trigger major earthquakes and volcanic eruptions, such as have been seen in Japan in recent years.

It is thought that the COB/COT is one of the most likely places for the subduction process to start. In this sense, identifying the world’s current COBs/COTs would enable us to predict future subduction zones and seismically active regions, such as the coasts of Japan and western South America! How cool is that?!

Industry and Geopolitics

Rapid technological developments have led to a massive expansion of resource exploration into the deep waters. In their search for oil and gas, petroleum companies found that the COB was an important barrier. Extracting fossil fuels from the oceanic domain would mean drilling even deeper than they used to do, and the associated costs would be notably higher. But also, the expectation of finding oil and gas in sediments of the oceanic crust is lower than in continental crust, although these facts, too, are still hotly debated. In any case, this means that we won’t see oil rigs on coastlines where the oceanic crust begins just in or very close to the beach, at least for a while!

As mentioned, the COB could also have a role in determining the rights of countries and companies over the sea. As the end of the continental margin, it can define the breadth of the region in which the coastal State has exclusive rights to explore and exploit natural resources³.

UN Convention on the Law of the Sea³, Article 76: Definition of the continental shelf ‘The continental shelf of a coastal State comprises the seabed and subsoil of the submarine areas that extend beyond its territorial sea throughout the natural prolongation of its land territory to the outer edge of the continental margin… The continental margin comprises the submerged prolongation of the land mass of the coastal State, and consists of the seabed and subsoil… It does not include the deep ocean floor with its oceanic ridges or the subsoil thereof.’

The challenge now is for scientists to standardise the criteria to define such a critical boundary.

Maritime divisions defined in the UN Convention on the Law of the Sea
Image by Fisheries and Oceans Canada, UNCLOS.

References

1. Mann, P. (2014). Passive Plate Margin. In J. Harff, M. Meschede, S. Petersen, & J. Thiede (Eds.), Encyclopedia of Marine Geosciences (pp. 1–8). Dordrecht: Springer Netherlands.
2. Eagles, G., Pérez-Díaz, L. and Scarselli, N. (2015). Getting over continent ocean boundaries. Earth-Science Reviews, 151, pp. 244-265.
3. UN General Assembly, United Nations Convention on the Law of the Sea, 10 December 1982.