Submerged continents: Mauritia and Zealandia

As a cartographic lover, it was sad to discover that the Earth’s surface is perfectly drawn today thanks to the advanced technology tools that we have, specially the spatial satellites. I would have liked to live in the Discovery’s Era, when there were left unexplored regions yet, waiting for being showed in a map.

But it seems that I was wrong. So how happy I am.

In February, they have appeared some news about two ancient continents submerged beneath the Indic and Pacific oceans. Both would be part of a massive continent called Gondwana, the southern split of the super-continent Pangaea 200 million years ago, fragmented by the tectonic plates. At the end, this tectonic process would organise the current position of the now well-known continents (Asia, America, Europe, Africa, Oceania and Antarctica).

1. Brief Earth’s geologic evolution. Source: science20.com

The microcontinent of Mauritia

Mauritia owns its name to the islands where was discovered: Mauritius Islands. However, its extension goes much further, taking up the Mascarene archipelago and its adjacent plateau, forming an arch from Mauritius Islands and Reunion Islands, in the southernmost, to the north of the Seychelles Islands in the northernmost.

During the Triassic Period, Mauritia was part of the vast continent of Gondwana, until the division of India and Madagascar, when Mauritia was finally submerged into the ocean, 85 million years ago.

But, what does difference Mauritia for the rest of the areas under sea level? Here is where researchers from the Oslo, Liverpool and Witwatersrand universities have shed light on that subject.

All started with a research which had as its objective to confirm the hypothesis of that the volcanic islands should show evidence of the existence of lost continents. The Mauritius Islands were an ideal destination, as they are relatively young, and thus the geologic thickness in the island is smaller; this would make easier to find remainders that (would) prove the presence of another nature that was not volcanic, as it happened.

The scientist found zircon minerals, one of the most plentiful in the continental crust, aged in more than 9 million years. That would prove that the Mauritius magma forged its way through a lost continent, and not directly from the oceanic floor, that is, the oceanic crust. The latter case would be, for example, the Canary Islands.

2. Location of Mauritia, forming an arch from Mauritius to Seychelles. Source: researchgate.net

However, the controversy came when they realised that the crushing equipment contained rest of zircon from other places in which these equipment were used, what detracted the evidences already found and contaminated the sampling area.

Few years later, some team members came back to the islands in order to collect new samples in two different beaches from the previous one, and the results obtained confirmed the first hypothesis, pointing that the zircon found was much older than the Mauritius’ lavas, and being consistent with the ages of known continental rocks in Madagascar, Seychelles, and India.

Even then, far from clearing up doubts, some experts, such the geologist Jérome Dyment, declared that they were sceptical by this discovery, as the zircon evidences could be transported to the islands by other means, as a result of anthropological action. In fact, Dyment himself pointed out that if there were real evidences of an ancient lost continent beneath Mauritius, it would have been already discovered by the French and German experiment (called RHUM-RUM), which takes place in Reunion Islands, and who are investigating the Earth’s mantle around the islands.

Nevertheless, according to Conall Mac Niocaill, geologist at the University of Oxford, there is not a natural source of zircon in Mauritius, and it seems rather unlikely that the mineral had reached the islands by other means, natural or human. So the most obvious conclusion, according to Mac Niocaill, is that the volcanic eruption swept along the zircon from the depth to the islands’ surface, where the lava was solidified, containing the zircon minerals.

To conclude, the Mac Niocaill’s theory has proved to be true, as a new study published in the British Nature Communications’ magazine confirms the existence of zircon in volcanic rocks, up to 3 billion years old, and not only in the beach’s sand, which clears up any doubt about the zircon’s origin, and shows how Mauritia was one of the pieces that formed Gondwana, but was sunk almost entirely in the Indic Ocean waters.

The continent of Zealandia

The etymology of Zealandia, for its part, results from the largest country which top its Surface over the sea level: New Zealand. It is a continent whose 94% of its territory is submerged, and takes up 4,9 million of km².

This landmass was formed after the Gondwana’s break-up, while Mauritia vanished beneath the Indic waters, 85-30 million years ago, just like scientists of GNS Science of New Zealand have published.

According to that group and thanks to the submarine sensors, it has been proved that Zealandia fulfil the necessary requirements to be considered a continent:

-To have a high elevation related to the regions settled over the oceanic crust.

-To have a wide rank of igneous, sedimentary and metamorphic rocks.

-To have a crust thicker than the oceanic crust.

-To have well-defined boundaries and an area large enough to be considered a continent instead of a microcontinent or a continental fragment.

3. Location and extension of Zealandia. Source: rtve.es

Nick Mortimer, geologist and leader of the research, says that Zealandia is the thinnest and smallest continent which has been discovered so far, and the fact that 94% of its area is submerged but not fragmented makes it especially useful for exploring the cohesion and disintegration of the continental crust. At the same time, he suggests a new context for evolutionary biology investigations, which could explain the provenance of the both endemic New Zealander’s flora and fauna.

Apparently, Zealandia will be the last submerged continent that we find, since the data obtained by the satellites and research vessels that map the Earth globe, dismiss the possibility of finding a new one. What it would be possible is to find microcontinents, especially in the Indic Ocean, as it happened with Mauritia.

Now, Zealandia must find its place among the international scientific community, as there is not a list nor an official guideline to include this new proposal. Not even the geologists agree with what is the number of continents on Earth today, so some asseverate there are four (Antarctica, America, Oceania and Eurasiafrica) and others talk about seven (Antarctica, North America, South America, Oceania and Europe, Asia, and Africa).

Differences between them

Why are we talking about Zealandia as a continent and Mauritia as a microcontinent? It is a tricky question, which lead us to the fourth requirement, generally accepted, to consider a territory as a continent: “To have well-defined boundaries and an area large enough to be considered a continent […]”.

Doubtlessly, this criterion is broader and more inaccurate that might be expected for a scientific definition. In fact, apart from J. Graham Cogley (1984), it has almost never been discussed, since it was thought that all continents in the world were already known.

Continents are, by themselves, the biggest surface of solid land on Earth. On the Cambridge Dictionary is defined as “one of the seven large land masses on the earth’s surface, surrounded, or mainly surrounded, by sea and usually consisting of various countries.”, but the Glossary of Geology is more accurate: “one of the Earth’s major land masses, including both dry land and continental shelves.” (Neuendorf, 2005).

4. Map of continents (coloured), and microcontinents. Source: Zealandia: Earth’s Hidden Continent

Come back to the subject, the extension of Zealandia is 4.9 Mkm², significantly larger than Mauritia, which is 408,000 km² (12 times larger), and that can make us realise what scale we are talking about. Neuendorf holds that a great area is inherent in a continent’s definition, and Cogley had already suggested in 1984 Central America (1.3 Mkm²), Arabia (4.6 Mkm²), and India (4.6 Mkm²) to be considered as new continents.

However, this criterion is not conclusive by itself, as the well-known greatest continents have not only a vast area, but are spatially isolated by geologic or bathymetric features.

Then, microcontinents are nothing more than fragments of the continental crust scattered in the world’s oceans. The clearest examples are Madagascar, East Tasmania Plateau, Jan Mayen, Mauritia, and the Golden Dragon Plateau (Gulden Draak Knoll).

Even so, distinguishing microcontinents from continents can be considered as an arbitrary practice. For this reason, the New Zealander research team of GNS Science has proposed to name as continent these regions of continental crust that have more than 1 Mkm² of area, and that are within clearly defined geologic limits. As per this explanation, India would have been considered a continent before its collision with Eurasia.

About Zealandia, to be separated from Australia in 25 km of oceanic crust, to have an area larger than 1 Mkm², and to have geologic and geographic limits well defined, it would be considered with no doubt as a new continent; but Mauritia, yet meeting the previous requirements, would be excluded from being considered as such due to its area, far from the 1 Mkm² threshold, and remaining thus like a microcontinent or continental fragment about what was once Gondwana.

The Earth keeps hiding its secrets, and this time we have found, beneath the marine depths, ancient lands which once were bathed by the sun. Maybe seems ironic how much we can discover from places so far as the surface of Mars, and the little we are aware of oceanic depths of our own planet. Luckily, the nervous human mind still containing the questions about the unknown, and we will try to broaden the knowledge of our home and its history, to learn to live with it.