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between Rb/Sr and crustal thickness is observed in subduction
settings (Dhuime et al., 2015), it implies that crustal thickness
may be a reasonable proxy for crustal volume at least over the
past 2.0 b.y.
Figure 4B (dotted blue curve) illustrates such a model for the
changes in crustal volumes through time. It is constrained by the
present volume of the continental crust, the presence of 70% of
that volume at 3.0 Ga, and the present-day distribution of crust
with different model ages (Condie and Aster, 2010). It starts at the Marchi et al., 2014), and so perhaps we are fortunate to have the
end of the heavy bombardment ca. 3.9 Ga; it assumes that the rate
of crust generation was greater before 3.0 Ga than subsequently,
and that the compositions of the crust generated before and after
3.0 Ga were different. Pre–3.0 Ga crust was preferentially recycled
from 3.0 to 1.5 Ga following the onset of subduction at 3.0 Ga.
Such models are not unique, but they indicate that changing
volumes of continental crust can be modeled with two types of
crust that are destroyed at different rates. The model outlined here plate tectonics resulted in collisional orogenies, supercontinent
predicts that nearly twice the present volume of the continental
crust has been recycled since the end of the heavy bombardment.
The increase in thickness at the sites of crust generation from
3.0 to 1.7 Ga indicates that more of the continental crust was
elevated above seawater (Flament et al., 2013), and hence suscep-
tible to sub-aerial weathering and erosion. This increase in crustal late Archean and the Proterozoic (Hawkesworth et al., 2013).
thickness is accompanied by a steady increase in the Sr isotope
ratios of seawater (Fig. 1; Shields and Veizer, 2002), implying
increased continental runoff, which would increase the amounts
of CO2 draw down due to continental weathering (e.g., Kramers,
2002). Preliminary models indicate that in the mid-Proterozoic
the volume of continental crust may have been up to 20% greater
than at the present day, and since 1.0 Ga the crust has become
thinner, and we infer that the crust decreased in volume (Fig. 4).
The rates of crustal growth appear to have decreased as Earth
cooled, and “cool” subduction began to dominate, which, on the
basis of preserved metamorphic and other rock units, is taken to
have commenced around 0.75 Ga (Brown, 2006; Cawood and
Hawkesworth, 2014), such that the rates of crustal recycling were
greater than the rates at which new crust was generated.
DISCUSSION
There appears to have been five stages in Earth’s evolution, with
the last four being recorded in the geology of the continental crust continental lithosphere was strong enough to be thickened and to
(Fig. 4). Stage 1 included the initial accretion of the Earth, core/ support the emplacement of large plutons into the crust, yet the
mantle differentiation, the development of a magma ocean, and of
an undifferentiated mafic crust. Most models suggest that a
magma ocean may have persisted on Earth for 5–10 m.y., and
continuing volcanism, along with deformation, would have
progressively thickened the initial mafic crust (Kamber, 2015;
Kamber et al., 2005). Once the crust was at least 15–20 km thick,
remelting could take place (Fig. 2; Kamber et al., 2005), and the
resultant felsic magmas represented the high silica component in
the distinctive bimodal silica distribution that characterizes the
Archean crust (Fig. 3). This second stage was marked by elevated
mantle temperatures compared to the present day (Fig. 4C) that
resulted in lithosphere weakened by the emplacement of melts
(Gerya, 2014; Sizova et al., 2010). This inhibited subduction, and
hence plate tectonics, and magmatism were driven by mantle
upwellings that percolated the lithosphere. These might have been
associated with deep-seated mantle plumes, but such models
remain difficult to test, and the upwellings may have been more GSA TODAY | www.geosociety.org/gsatoday
localized and likely originated at shallower levels, perhaps as in a
heat-pipe model (Moore and Webb, 2013). In today’s world, such
magmatism would be regarded as intraplate; it has no association
with subduction, but in a time before plates, such terminology is
arguably misleading. Destruction and recycling of early crust
occurred through a combination of delamination (Johnson et al.,
2013) and meteorite impact, the latter continuing until the end of
the late heavy bombardment ca. 3.9 Ga (Gomes et al., 2005;
few zircon grains that have survived from the Hadean.
The onset of stage 3 is taken to be the stabilization of Archean
cratons and the change in crustal growth rates ca. 3.0 Ga. It is
envisaged that Earth had cooled sufficiently for “hot” subduction
to take place (Sizova et al., 2010) from ca. 3.0 Ga to ca. 1.7 Ga
(Fig. 4). The continental crust became thicker and more evolved,
cycles developed, and there was increased erosion to the oceans
due to thickening and subaerial exposure of continental crust
resulting in an increase in Sr isotope ratios in seawater (Fig. 1B).
Models in which ~70% of the present volume of the continental
crust was present at 3.0 Ga also require that large volumes
of continental crust have been destroyed, presumably in the
However, the net growth of crust requires the rate of crust genera-
tion to exceed that of recycling (Fig. 4).
The fourth stage is from 1.7 to 0.75 Ga, referred to as the
“boring billion” (Holland, 2006), and more recently as Earth’s
middle age (Cawood and Hawkesworth, 2014). It is marked by a
paucity of preserved passive margins, an absence of significant
anomalies in the paleoseawater Sr isotope record and in Hf
isotopes in detrital zircon (Fig. 1), a lack of orogenic gold and
volcanic-hosted massive sulfide deposits, and an absence of glacial
deposits and iron formations (Cawood and Hawkesworth, 2015).
It appears to have been a period of environmental, evolutionary,
and lithospheric stability, which has been attributed to a relatively
stable continental assemblage that was initiated during assembly
of the Nuna supercontinent by ca. 1.7 Ga and continued until
breakup of its closely related successor, Rodinia, ca. 0.75 Ga. It is
also marked by abundant anorthosites and related rocks perhaps
linked with the secular cooling of the mantle. The overlying
underlying mantle was still warm enough to result in widespread
melting of the lower thickened crust and the generation of anor-
thositic magmas (Ashwal, 2010).
The termination of Earth’s middle age, and onset of stage 5,
corresponds with Rodinia breakup at 0.75 Ga and the develop-
ment of “cold” subduction. The latter is recognized by the onset of
high- to ultrahigh-pressure metamorphic rocks (Brown, 2006).
Falling mantle temperatures enabled deeper levels of slab breakoff
in collision zones and the resultant greater depths to which conti-
nental crust was subducted prior to exhumation, allowing the
development of the ultrahigh-pressure metamorphic assemblages
(Brown, 2006; Sizova et al., 2014). Stage 5 is marked by a strongly
episodic distribution of ages linked to the supercontinent cycles of
Gondwana and Pangea (Fig. 1). Oxygen levels in both the atmo-
sphere and deep oceans increased, phosphate and evaporate deposits
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