As the previous GeoCurrents post argued, the division of the terrestrial world into a handful of continents derives in part from the division of the Earth’s crust into tectonic plates. But there is another geological factor equally pertinent to the concept of “continent.” It refers not to individual landmasses but rather to a particular kind of crust – the outermost rocky layer of the planet – regardless of whether it extends above sea level. If continents are defined based on continental crust, an entire “continent” could conceivably be well below sea level.
As the diagram posted below show, the Earth has two kinds of crust: oceanic and continental. Oceanic crust is relatively thin and dense, rich in iron and magnesium. Less-dense continental crust, rich in aluminum silicates, is much thicker. Being light and thick, continental crust generally extends above sea level. But the continental shelves that skirt most coastlines of continents are below sea level even though they are composed of continental crust. Several isolated fragments of continual crust are almost entirely submerged. The largest of these inundated “continents” is Zealandia, which covers approximately 4.9 million km2 (1.9 million sq mi). Only about seven percent of Zealandia is currently above sea level, mostly the two main islands of New Zealand and New Caledonia. Once connected to the massive southern continent of Gondwana, Zealandia began to rift away roughly 85 million years ago. As this occurred, its crust stretched and thinned, resulting in a lower elevation than the major continents.
Although oceanic crust is generally well below sea level, exceptions occur. Extensive volcanic eruptions associated with hotspots can generate exceptionally thick oceanic crust, creating sizable island. Important examples include the Hawaiian Islands, the Mascarene Islands (Reunion and Mauritius) in the Indian Ocean, and, to some extent, Iceland. Iceland is a complicated case, however, as it is essentially oceanic but does contain a few fragments of continental crust. Kerguelen, in the southern Indian Ocean, is another such “composite” island. It is associated with a hotspot and as a result is mostly covered by rock of oceanic origin. But these rocks sit above a continental fragment that rifted away from Gondwana roughly 100 million years ago. As a result, Kerguelen is depicted as a microcontinent in the map posted below.
During glacial periods, when the sea dropped due to the massive amount of water in continental ice caps, most continental shelves were above sea level. The continents were therefore larger than they are today, with a much closer correspondence between continental crust and dry land. Zealandia, however, had subsided to such a degree that even at the Last Glacial Maximum (LGM) around 20,000 years ago, only around ten percent of it extended above sea level. But that was enough to join the islands of New Zealand together, forming a significantly larger landmass than what exists today.
As I was not able to find an adequate map of continental crust, I made my own, which must be regarded as a crude approximation (posted above). Several difficult judgement calls were necessary, as many island chains located tectonically active areas are composed of complex mixtures of oceanic and continental crust. Most, however, are dominated by oceanic crust, and as a result do not appear on this map (examples include the Andaman and Nicobar islands and the Kuril and Aleutian archipelagos). The Ryukyu Archipelgo, on the other hand, is composed mostly of continental crust and thus appear on this map as a narrow peninsula extending south from Japan.
Needless to say, it is difficult to derive the standard seven-continent model from this map of continental crust. A six-continent model, however, can be construed, albeit one quite different from the conventional six-continent model. Its constituent elements would be North America-Eurasia, Africa, South America, Antarctica, Australia, and Zealandia.