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mean density of 2830 kgm-3 with an aver-
age thickness of 46 km for orogens and 30
km for extended crust. In contrast, oceanic
crust is typically 7 km thick, and, in its
lower part typically has a P wave velocity
of 7.5 km-1 (White et al., 1992).
From geophysical work, we know that
Zealandia has a continental crust velocity
structure, Vp, generally <7.0 km-1, and a
thickness typically ranging from 10 to
30 km throughout its entire extent to
>40 km under parts of South Island (Shor
et al., 1971; Klingelhoefer et al., 2007;
Grobys et al., 2008; Eberhart-Phillips et
al., 2010; Segev et al., 2012). Whereas most Nares Strait between northernmost
of Zealandia’s crust is thinner than the
30–46 km that is typical of most conti-
nents, the above studies show that it is
everywhere thicker than the ~7-km-thick
crust of the ocean basins. This result is
visible in the global CRUST1.0 model of
Laske et al. (2013) shown in Figure 3.
Collectively, the crustal structure results
show that the rock samples of Figure 2 are
not from separate continental fragments or
blocks now separated by oceanic crust, but
are from a single continental mass.
The thinnest crust within Zealandia is in
the 2200-km-long and 200–300-km-wide
New Caledonia Trough, where the water
depth varies from 1500 to 3500 m (Fig. 2).
This raises the question as to whether the
trough is floored by oceanic crust or is a
failed continental rift. Two wide-angle
seismic profiles across the trough near
New Caledonia (Klingelhoefer et al., 2007)
both show ~2–5 km of sedimentary cover
over 8.5 km of crustal basement that has a
velocity of ~7 km-1 throughout much of its
thickness. Klingelhoefer et al. (2007) noted
these profiles as atypical of normal oce-
anic crust. Sutherland et al. (2010) and
Hackney et al. (2012) interpreted the New
Caledonia Trough as continental crust that
was thinned in the Late Cretaceous and
re-deepened in the Eocene due to litho-
sphere delamination.
Limits and Area
Where oceanic crust abuts continental
crust, various kinds of continent-ocean
boundaries (COBs) define natural edges to
continents (Fig. 1; Eagles et al., 2015).
Despite its large area, Greenland is uncon-
troversially and correctly regarded as part
of North America (Figs. 1 and 4). This is
because, despite oceanic crust intervening
between southern Greenland and Labrador
and Baffin Island, North American
Figure 4. Areas and submergence of all of Earth’s geological con-
tinents (red symbols) along with microcontinents (brown symbols)
and intraoceanic large igneous provinces (LIPs, blue symbols)
shown in Figures 1 and 2. Note x-axis is log scale. Data mainly
after Cogley (1984) except Zealandia data from Mortimer and
Campbell (2014); microcontinents after Gaina et al. (2003) and
Torsvik et al. (2013). Emergent land area for Antarctica is the iso-
statically-corrected ice-free bedrock surface from Jamieson et al.
(2014). New Guinea and Greenland are arbitrarily given the same
submergence value as their parent continents. AP—Agulhas
Plateau; KP—Kerguelen Plateau; OJP—Ontong Java Plateau;
MP—Manihiki Plateau; HP—Hikurangi Plateau; N Am—North
America; S Am—South America.
continental geology is continuous across Mauritia, and Gulden Draak microconti-
nents (Gaina et al., 2003; Torsvik et al.,
Greenland and Ellesmere Island 2013; Whittaker et al., 2016). Discriminating
(Pulvertaft and Dawes, 2011). Tectonic between what is a continent and what is a
plate boundaries, with or without interven- microcontinent may be considered an arbi-
ing oceanic crust, provide the basis for trary exercise. Nonetheless, maps like
continent-continent boundaries between Figure 1 need labels. Therefore, following
Africa and Eurasia, and North and South Cogley (1984) and the vagaries of general
America (Fig. 1). Large area is an inherent conventional usage, we propose that the
part of the definition of a continent sensu name continent be applied to regions of
stricto (Neuendorf et al., 2005). Cogley continental crust that are >1 Mkm2 in area
(1984) defined Central America (1.3 Mkm2), and are bounded by well-defined geologic
Arabia (4.6 Mkm2), and greater India limits. By this definition India, prior to its
(4.6 Mkm2) as modern-day continents. collision with Eurasia, would be termed a
This schema has not been generally continent.
adopted, probably because Central
America (the Chortis block) is a piece of The edges of Australia and Zealandia
displaced North America, and Arabia and continental crust approach to within 25 km
India are transferring to, and are now across the Cato Trough (Fig. 2). The Cato
contiguous with, Eurasia and have clearly Trough is 3600 m deep and floored by oce-
defined COBs in the Red Sea and Indian anic crust (Gaina et al., 1998; Exon et al.,
Ocean (Fig. 1). The six commonly recog- 2006). The Australian and Zealandian
nized geological continents (Africa, COBs here coincide with, and have been
Eurasia, North America, South America, created by, the Cato Fracture Zone along
Antarctica, and Australia) are thus not only which there has been ~150 km of dextral
large but they are also spatially isolated by strike slip movement, linking Paleogene
geologic and/or bathymetric features. spreading centers in the Tasman and Coral
seas (Fig. 2; Gaina et al., 1998). This spatial
At the other end of the size spectrum, a and tectonic separation, along with inter-
number of continental crust fragments in vening oceanic crust, means that the
the world’s oceans are referred to as Zealandia continental crust is physically
microcontinents. Examples include the separate from that of Australia. If the Cato
Madagascar, East Tasman, Jan Mayen, Trough did not exist, then the content of this
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