The world is split up into continents, there are seven in all.
And if you get the gist, we’re going to make a list,
From biggest to small …
Your kids may have come home from school singing this infectious ditty, or another like it. But are there really seven continents?
Anyone with a map can see that Asia and Europe are connected. They are often called Eurasia for that reason. The divide is pretty arbitrary, more culturally than scientifically defined. So, is it fair to say that there are actually only six continents?
That is just the first slippery step on a well-oiled slope. What about North America and Asia?
They are connected by the Bering Sea Shelf, once dry land crossed by humans and flooded only in the geologically recent past. Technically speaking, that makes Asia, North America and Europe all one continent. Does that mean there are only five?
Other experts contend that five, six and seven are wrong and argue in favor of eight continents. There are even those who go as far as to say there are only two.
Hiding within the simplicity of the song, there is an illusion of general agreement about the number of continents.
The dispute arises in part because there are really two types of continents: Those recognized by cultures around the world, and those recognized by geologists. Cultures can define a continent any way they want, while geologists have to use a definition. And geological research in recent years has made defining continental boundaries less simple than it might have once seemed as researchers find evidence of unexpected continental material.
“This triggers a lot of interest because there are significant implications for our understanding of the mechanisms of continent separation, ocean formation and plate tectonics,” said Valentin Rime, a geologist at the University of Fribourg in Switzerland. He added, “But after the excitement comes rigorous checking and debate to make sure the evidence is solid.”
Geologically speaking, to be a continent, a bit of the planet needs to have four things:
-
A high elevation relative to the ocean floor.
-
A wide range of igneous, metamorphic and sedimentary rocks rich in silica.
-
A crust thicker than the surrounding oceanic crust.
-
Well-defined limits around a large enough area.
The first three requirements are found in just about every geology textbook. But not so with the fourth. What is “large enough,” or how “well-defined” the limits of a potential continent need to be, are matters that are less often discussed, unless a geologist is studying bits of the planet that are on the cusp of being continental.
“Anything big enough to change the map of the world is important,” said Nick Mortimer, a geologist with the New Zealand government-owned GNS Science research institute. “Labeling and identifying part of the Earth as a continent, even a small, thin and submerged one, is more informative than just leaving a map blank.”
This creates problems for numbering continents.
Consider Iceland, which sits atop a rift that stretches around Earth, the Atlantic branch of the mid-oceanic ridge. Volcanic activity there slowly separates the tectonic plates on which North America and Europe rest. Most of the ridge lies deep beneath the ocean. But in Iceland, it sits above sea level.
Another enigma is that the volcanoes there often spit out lava made of molten continental crust, even though Iceland is thousands of miles from any continents. Some geologists therefore suspect Iceland is not a lonely island at sea, but actually part of a continent (although as to which one, that can get complicated, too).
That notion finds support off Africa’s eastern coast.
A mid-oceanic ridge in the Red Sea is dividing Africa from Asia. This divorce is happening at the rate at which fingernails grow. Along most of the ridge, the separation is straightforward. But the separation is much messier where the Red Sea meets the Gulf of Aden. In place of an obvious point of thinning where ocean crust is forming, the continental crust between Africa and Asia is splintering into hundreds of pieces. In that location, there is no obvious point where Africa ends and Asia begins.
“It is like very strong and very thick toffee getting stretched out, but not snapping,” said Gillian Foulger, a geologist at Durham University in England.
Dr. Rime and colleagues recently published a study in the journal Geology showing that Iceland, too, has heavily stretched toffee beneath its surrounding seas. Instead of a clean break between North America and Europe, there appears to be a complex mix of magma and continental crust fragments scattered in a path between the two land masses that runs right through Iceland. Just like the point where the Red Sea and the Gulf of Aden meet, there is no clear point where North America ends and Europe begins.
Of course, if Africa is still attached to Asia through the Gulf of Aden and North America is still attached to Europe, this kind of messes up the “Seven Continents” song. If the crust between these continents is stretching but not snapping, is it well-defined enough to identify the two land masses as separate continents? Or are they still one?
Then there is New Zealand, which really wrecks things for our kids.
Although New Zealand and Australia are often grouped together, they are not on the same continent. While Australia is widely considered to be a continent in its own right, the notion that New Zealand is part of its own continent, Zealandia, is a newer argument.
Submerged shelves that rise high above the ocean floor stretch for miles far beyond the tiny island nation. Along the edges of those shelves, there is deep water and oceanic crust that is thinner than the crust beneath the shelves. Drill cores, seabed dredge samples and rocks collected from the region show that the giant mass that New Zealand sits on is composed of igneous, metamorphic and sedimentary rocks rich in silica, just as is the case with all other continents.
While few think of Zealandia as a continent in cultural terms, “it is increasingly being recognized as a geological one,” Dr. Mortimer said.
But not everyone agrees, and they point to that meddlesome fourth criterion ignored in most textbooks. The crust making up Zealandia is between 10 and 30 kilometers thick, making it thicker than the seven kilometers of most ocean crust. But it is not as thick as other continents’ crust, typically between 30 and 46 kilometers. This makes the boundaries between Zealandia and the ocean less well defined and more difficult to discern. Size creates issues too. At 4.9 million square kilometers, the proffered Zealandia is much smaller than Australia, which itself is only 7.7 million square kilometers.
Then there is the fact that Zealandia is mostly submerged. Being above water is not part of the geological definition of a continent, but it does seem to matter culturally, because people are used to thinking of continents as dry land.
Geologists are still arguing what these discoveries about continental and oceanic crust mean for the number of continents. What is certain is that the research is revealing that there is more than one way for two continents to split apart, and that the split is not always clean or even complete.
“There are basically only two major continents,” Dr. Rime said. “Antarctica and everything else, since South America is connected to North America through Panama, North America is connected to Asia through the Bering Strait, and Asia is connected to Europe, Africa and Australia through the Urals, the Sinai and Indonesia, respectively.”
Dr. Mortimer doesn’t agree.
“Zealandia is separated from Australia by a 25-kilometer-wide, 3,600-meter-deep ocean trough,” he said. “Based on Valentin’s logic, that would mean there are actually three continents.”
But he acknowledged some uncertainty, adding, “Unless the trough is found to be very deep continental crust, like the Iceland situation, in which case Zealandia would be a part of Australia.”
And then there is the possibility that Iceland is sitting on top of its own large chunk of free-floating crust as perhaps continent No. 9.
“If true, ‘Icelandia’ would join a growing fellowship of continents large and small,” Dr. Foulger said.
Culturally, this research and the arguments it provokes change nothing — which is a good thing, because if there were only two continents, there might not be an infectious tune for your children to sing.
Content Source: www.nytimes.com