Daily Maverick Interview: Mauritia, the continental Lost and Found
- Marelise van der Merwe
- 03 Feb 2017 12:30 (South Africa)
Earlier this week, Wits University announced the discovery of a “lost continent” hidden under the island of Mauritius. MARELISE VAN DER MERWE spoke to geologist Professor Lewis Ashwal, lead author of the research paper, to find out exactly what this means for the rest of us.
Hidden under nine-million years’ worth of lava build-up in Mauritius lies a fragment of the supercontinent of Gondwana, an enormous land mass that dates back to some 200-million years ago and contained rock more than three-billion years old. Now, geologists say there are probably several more fragments of ancient continent waiting to be found – and they have an idea where to start looking.
Photo: Wits geologist, Professor Lewis Ashwal, lead author on the paper “Archaean zircons in Miocene oceanic hotspot rocks establish ancient continental crust beneath Mauritius”, published in the prestigious journal Nature Communications. (Wits University)
DM: Can you clarify what you found and its exact location?
Professor Lewis Ashwal: We have discovered a new piece of continent and we suggest that it is about 10-15km beneath Mauritius, so you can’t see it or touch it.
How did it come to be below?
Imagine that there’s a piece of rock or continent in the ocean and by chance it happens to find itself in a place where a new volcano is going to form. The volcano pours out lava onto the top and over time the lavas build up to quite a thickness so the continent is buried under this volcano.
Why is this discovery so significant?
It’s not every day that someone will discover a new piece of continent! It’s exciting from that point of view.
We did this by looking at some of the rocks in Mauritius, grinding them up and looking for special minerals called zircon. If you analyse it the right way, zircon can give you age information – when the zircon formed and therefore when the rock formed that contained it.
Mauritius is a young volcano; there is nothing older than about nine-million years. So these crystals of zircon … we found some grains enormously older than that. Our zircons are 2.5- to three-billion years old, something completely unexpected in a young volcano; completely unexpected in the oceans at all. The oldest rock that you find in the oceans is about 200-million years old, and the grains in this lava are much older than that. The only place you do find old rock is in the continents. A good example is in South Africa, in Barberton. That is some of the oldest rock in the world, at 3,600-million years old. But you never find this in the oceans, always in the continents. So to find these in a very young volcano, in Mauritius, how could they have got there? Really, the only way is for the magma that erupts on the surface to have passed through this piece of continent and picked up the zircon grains.
So what does this tell us about how the Earth and the continents as we know them were formed?
The question you should be asking is how can this have happened? How could a piece of continent have got under this volcano? The answer has everything to do with plate tectonics. This is how the Earth works now. The oceans are young because this is where new crust is being formed right now, that takes place at these ridges, these big mountains in the ocean. New crust is forming and spreading apart. The continents, which are old, are sitting on top of them like passengers.
Photo: Lead author Prof. Lewis D. Ashwal (left, red cap) sampling trachyte rocks in Mauritius. Such samples are about 6 million years old, but surprisingly contain zircon grains as old as 3000 million years. Colleague Dr. Stephanie Werner (right, University of Oslo) is recording sample information. (Photograph taken by Prof. Susan J. Webb, University of the Witwatersrand)
As the new crust spreads apart and the pieces move away from each other, they take the continents with them, sometimes very far distances. In this way, you can have continents moving around. If one of these ridge systems forms under a continent, that will split the continent apart and cause it to break into pieces. That’s exactly what’s happening in the East African rift: Somalia, Ethiopia, Kenya, Tanzania. There are big mountains and big earthquakes. This is a place that is going to fragment. Africa is going to split apart right there, and probably a new ocean will start to form.
So this is what we are looking at in the Indian Ocean – pieces of continent of variable sizes that split some time ago. We can reconstruct them to where they were in the past, when they were together. It gives us some framework to understand what is happening now, and what’s happening in the past.
Plate tectonics explains what is happening now – how far in earth history can we confidently say the same thing was happening? Did plate tectonics start right away or in some younger time? That’s important for the understanding of how planets work.
Lead author Prof. Lewis D. Ashwal studying an outcropping of trachyte rocks in Mauritius. Such samples are about 6 million years old, but surprisingly contain zircon grains as old as 3000 million years. (Photograph taken by Prof. Susan J. Webb, University of the Witwatersrand)
This whole story starts with the supercontinent of Gondwana which existed 200-million years ago and earlier. That entity consisted of Africa, India, Australia, Madagascar and Antarctica. All those continents were joined together and then plate tectonics acted to break it apart into pieces. Our work allows us to understand better how to reconstruct these things, like putting together a puzzle, and also tells us something about the processes involved in the breakup of continental masses. The answer is: it’s much more complicated than we thought. It’s not a simple, clean break.
How do you determine age using zircon?
Radioactive decay. Zircon is a mineral that likes to accept uranium in its structure when it forms. Uranium is radioactive and we know the half-life, we know how long it takes to decay. Uranium eventually decays into lead. We can measure the amount of lead in the zircon that was produced by radioactive decay, and from there we can calculate the age.
It’s very precise. It’s about the best one you can use to date old things, even young things.
What size is this lost continent?
We don’t know how big it is, but probably it is either the same size as the island of Mauritius or smaller, but I’m thinking it’s about the same size as the volcano on Mauritius.
What’s the next step research wise?
Good question. Where do we take this further? We have looked at the Indian Ocean and there may be other pieces of continent littered throughout. We have suggested in the paper where some of them may be. Problem is that those places are hard to get to. A lot of them are submerged, and those that are sticking up are blanketed by coral reefs or sand or something.
Another thing we could do is go elsewhere to volcanoes, not only in the Indian Ocean. In fact, we have written about a very similar case in Iceland involving young volcanoes. We detected evidence geochemically to suggest that there is also a piece of stranded or lost continent under Iceland. This may not be that uncommon. I have a few ideas of where we could go – the Galapagos, maybe the Canary Islands or Cape Verde, where there are also young volcanoes. Is it possible that a piece of Africa lies under these young volcanoes?
This secondary ion mass spectrometer (SIMS) instrument, housed at GFZ Potsdam, was used to measure the ages of both young (5.7 million years) and old (2552 – 3030 million years) zircon grains in Mauritian trachytes. (Photograph taken by Prof. Michael Wiedenbeck, Deutsches GeoForschungsZentrum (GFZ), Potsdam, Germany)
A further thing we can do in Mauritius: we don’t know how deep this thing is or how big. A lot of people are asking that. “Where is it? Why can’t we see it?” Maybe we could do some kind of geophysical surveys or seismic studies across the island of Mauritius which might give us information about what its properties are. But a lot of this will cost a lot of money. Someone asked, “Why don’t you just get a drill?” But it’s 10, 15 kilometres deep!
That’s a heck of a drill.
It would cost a fortune. We are academics. We can’t afford to do those things. Oil companies, yes. But they’d have to have a good, convincing reason. They wouldn’t care that there’s a piece of granite under there.
What we’re very excited about is the media interest. A lot of people perceive science as boring. But when you start talking about lost continents, their eyes open. They think maybe you have found Atlantis.
What is important that I haven’t asked?
Part of the story that I didn’t really get to, although I mentioned it, is that we suggest there are other little pieces in the Indian Ocean. So we’ve tried to see what it would look like if we constructed them back in time, to a time in the past when they were all adjacent, connected.
Indian Ocean topography showing the location of Mauritius as part of a chain of progressively older volcanoes extending from the presently active hot-spot of Réunion toward the 65-million-year-old Deccan traps of northwest India. (Originally published in: Ashwal et al. (2016) A mantle-derived origin for Mauritian trachytes. Journal of Petrology, vol. 57, pp. 1645-1675. Oxford University Press.)
Say for instance there’s a gold mine in Mauritius today – you might want to ask: what was this gold mine sitting next to when it formed? These reconstructions can be used as an exploration tool today. Maybe that’s a long shot, but any information you can give to people trying to explore or exploit the earth is valuable, and you don’t know what the benefit will be tomorrow, but maybe in 10 or a 100 years it may be very useful.
We’ve reconstructed these pieces we think we’ve found into a microcontinent that must have existed, and we have given it a name: Mauritia. The lady who interviewed me this morning, from the BBC, said: “Why did you call it that?” We wanted to glorify Mauritius, where we made our discovery. She said, “Yeah, but why didn’t you call it Ashwaria, name it after yourself?” How do you answer that question? You don’t name continents after people. I really don’t want to have my name on this!
I’ve never had this [kind of response] in my life, ever. It’s very exciting but very stressful too. DM
This interview has been edited for clarity and brevity.
Main photo: Typical view of Mauritius beachfront with volcanic mountains in background. The basaltic lavas constituting these mountains formed no older than 9-million years ago. (Photograph taken by Prof. Susan J. Webb, University of the Witwatersrand)