National Geographic’s recent article outlining the discovery of what geologists are calling a “microcontinent” that has been submerged for 85 million years sparked a small controversy on my Twitter feed. That being, what exactly makes a “microcontinent” different from any old island? If the definition of an island is a land mass surrounded by water, then a microcontinent almost surely seems to fit the description. So what is the difference?
Well, if you want a question like that answered, you have to ask a geologist. And if you’re going to ask a geologist, why not ask the geologist who happens to be a co-author of the study that started it all?
Bjørn Jamtveit of University of Oslo in Norway responded to my email request with the kind of stunning swiftness that has always made me love the Internet. The answer, he says, has less to do with islands and much more to do with continents:
Geologists talking about continents do however refer to something that is made up of continental crust (which is chemically and physically different from oceanic crust – which makes up most of the sea floor) and not size.
In other words, while islands can be any land mass surrounded by water, in order for something to be considered a continent, geologically speaking, it must be made of the same rock as other continents. The difference is that oceanic crust is made up of volcanic rock: heavy, dense rock called basalt. The oceanic crust is basically that part of our Earth’s crust which is left after volcanic eruption pushes the continents apart. Continental crust, however, is largely made up of granite.
Geologists refer to oceanic islands, meaning islands which have been formed by volcanic activity. Here again, we have a very specific definition and of course we would expect the oceanic island to have lots of basalt. In the case of Mauritius, Jamtveit and his team believe that the presence of both basalt rock and older, granite rocks and the mineral zircon indicate that part of an older, very small continental crust also forms the island.
Thus the research suggests that, while Mauritius has long been considered an oceanic island, the truth may be that it was formed on the rubble of an older shard of continental rock.