Recently there has been a push to identify the continent of Zealandia as the 8th continent, and the one that New Zealand is a part of. Politically, New Zealand is called a part of Oceania, or, sometimes, Australasia. These are mostly terms of convenience, to club New Zealand, Australia and the Pacific island countries into one group.
What qualifies a landmass to be a continent? Before we get into the definition of continents, it might help to quickly understand the layout of the earth. The outermost layer of the earth is called the crust. The crust is divided into continental crust and oceanic crust, defined by the kind of rock each is composed of.
The crust itself is broken up into tectonic plates. These plates are constantly moving over the inner layers. Earthquakes are manifestations of plate movement. A plate could cover both oceanic and continental crust.
It is on these plates that the continents lie. So the African plate holds the African continent, as well as parts of the Atlantic and Indian Oceans. Islands are merely those parts of tectonic plates that are visible above land. They could be continental crust sticking out – like Sri Lanka, Ireland etc. or they could be oceanic crust sticking out, like volcanic islands.
There is no rule of thumb, nor does any consensus, on how to strictly define a continent. But conventionally, the commonly accepted criterion for a landmass to be called a continent is that it should be on its own tectonic plate. To this extent, research shows that Zealandia is on a tectonic plate, stretching under the Pacific, from New Caledonia to New Zealand.
It is interesting to note that most of the conventional wisdom on continents is actually derived from convention itself, rather than updated research. We identify seven continents – North America, South America, Africa, Europe, Asia, Australia and Antarctica. But Europe and Asia are actually on the same tectonic plate – Eurasia. They’re treated as separate purely because of historical practice of distinguishing between the East and the West.
Similarly, India is actually on its own plate, yet is considered a part of Asia. Several people wonder why a landmass as massive as Greenland is an island, not a continent. Greenland sits wholly on the North American plate, and is therefore an island.
The debate over definitions is an ongoing and evolving one.