Are there really seven continents?
Anyone with a map can see that Asia and Europe, often called Eurasia, are connected. The divide is pretty arbitrary, more culturally than scientifically defined.
What about North America and Asia?
They are connected by the Bering Sea Shelf, once dry land crossed by humans and flooded in the geologically recent past. Technically speaking, that makes Asia, North America and Europe all one continent.
There are really two types of continents: Those recognized by cultures around the world, and those recognized by geologists. 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.
Geologically speaking, to be a continent, it needs to have four things:
n A high elevation relative to the ocean floor.
n A wide range of igneous, metamorphic and sedimentary rocks rich in silica.
n A crust thicker than the surrounding oceanic crust.
n 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.
“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 where volcanic activity 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. 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).
Then there is New Zealand.
While Australia is widely considered to be a continent, 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 kilometers far beyond the tiny island nation. Drill cores, seabed dredge samples and rocks show that the giant mass that New Zealand sits on is composed of igneous, metamorphic and sedimentary rocks rich in silica.
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. Because Zealandia’s crust is not as thick as other continents’, the boundaries between Zealandia and the ocean are more difficult to discern. And at 4.9 million square kilometers, Zealandia is much smaller than Australia, which itself is only 7.7 million square kilometers.
Geologists are still arguing what these discoveries mean for the number of continents. “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.
This article originally appeared in The New York Times.