Download Evolution of continents, cratons and supercontinents: building the

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Evolution of continents, cratons and
supercontinents: building the habitable Earth
M. Santosh*
School of Earth Science and Resources, China University of Geosciences, 29 Xueyuan Road, Beijing 100083, China
Modern concepts of plate tectonics provide new
insights into the processes of construction and
destruction of continents, cratons and supercontinents. Starting from the magma ocean stage at around
4.6 Ga, and evolving through a water-covered oceanic
realm in the early Archean with island arcs floating
on a double-layered convecting mantle, the dynamic
planet Earth commenced the building of continents
and cratons. These were then episodically assembled
into large supercontinents and broken apart. Plate
tectonics cleaned up the early toxic oceans, built
mountain chains and exposed these through sea-level
drop in the Neoproterozoic, enabling weathering and
transport of nutrients for evolving life. Drastic environmental changes influenced by both galactic and
terrestrial processes in the late Neoproterozoic finally
set the stage of making the Earth a habitable planet
for modern life.
Keywords:
continents.
Continents, cratons, plate tectonics, super-
Introduction
THE formation of continents, cratons and supercontinents
constitutes a focal theme of wide interest for not only
evaluating the geological and tectonic history of the
Earth, but also in understanding how our planet became
habitable through drastic changes in the surface environment. The early history of the Earth during Hadean is
shrouded in mystery with only traces of evidence preserved relating to the oldest continental crust. Granitic
crust is the main source of nutrient supply to evolve and
sustain life, and therefore the timings and relationships
between crustal evolution and life evolution are emerging
as an important theme in understanding the history of the
Earth. The formation of cratons through building cold,
rigid and thick rafts beneath ancient continents, and their
subsequent erosion are also topics of wide interest. The
episodic amalgamation of continents into large land
masses and their disruption, termed as supercontinent
cycle, have significantly influenced the biogeochemical
cycle and surface environmental changes. Above all, the
impact of all these processes in evolving modern life on
*e-mail: [email protected]
CURRENT SCIENCE, VOL. 104, NO. 7, 10 APRIL 2013
our planet and its implications on the search for life on
other planets are attracting considerable attention on a
broad and multidisciplinary scale1.
In this brief overview, I present a synopsis of some of
the recent concepts on the mechanism of evolution of
continents, cratons and supercontinents and its implications on the surface environmental changes and the dawn
of modern life.
Building continents
The Earth was born dry at around 4.6 Ga, and before this
‘naked’ planet was covered by the thin veneer of ocean
water, a hot magma ocean is considered to have prevailed
in the Hadean. One of the speculative models proposed
that all the Hadean anorthositic crust was deep subducted
and lost, dragged down to the bottom of the mantle where
it formed a major component in the D″ layer above the
core–mantle boundary2.
The birth of ocean might have marked the onset of
primitive plate tectonics, with water acting as the lubricant to drive the plates, as well as to promote melting and
magma generation. Among the two principal magma factories associated with plate tectonics, one is located at the
spreading centers where basaltic oceanic crust is generated in mid-oceanic ridge, and the other at subduction
zones in convergent margins where slab melting produces
a variety of magmas. Although the question of when plate
tectonics began on Earth is still being debated, the discovery of detrital zircons from the metasedimentary belt
in Jack Hills, Yilgarn Craton, Western Australia has
revealed the oldest preserved fragments of the Earth’s
crust. The Jack Hills zircons prove the existence of granitic continental crust and oceans on our planet from
almost 4.4 Ga onwards3. Although the production of
granitic crust signals the onset of the primitive plate
tectonics, the question whether abundant granitic crust
was present in the early Earth remains unanswered.
The history of continental growth through time is a
much debated topic, with end-member models presenting
diverse views, including bulk production of continental
crust in the early history of the Earth, gradual or episodic
growth through time, and dominant growth in the Phanerozoic (see Rino et al.4 for a discussion). The advent
of modern analytical tools, particularly zircon U–Pb
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Figure 1. a, Schematic illustration of the mechanism of building primitive continents through the parallel collision of arcs and formation
of composite arcs. The composite arcs then amalgamate to form embryonic continents (modified from Santosh et al.8). b, Tectonic framework of the North China Craton showing ancient continental blocks welded by Meso-Neoarchean greenstone belts (after Zhai and
Santosh12). c, Cartoon illustrating the growth of continents through the collision of intra-oceanic arcs with continental arcs.
chronology and Hf isotopes, has fuelled this debate in the
recent years5–7. On one side, the presence of voluminous
granitic crust in the early Earth has been negated based
on the history of supercontinents which suggests that the
first coherent supercontinent was built only in the Paleoproterozoic, after continental fragments grew into adequate size to be assembled into large land masses8. The
history of evolution of life on Earth identifies that nutrient supply to give birth to modern life requires granitic
sources and that such sources were not dominant in the
early Earth1. On the other hand, the wide occurrence of
granitic crust in Archean greenstone terranes and the
preservation of undisturbed and near-horizontal Archean
sedimentary sequences laid over buoyant granitic basement in some of the ancient continents do not support the
notion that granitic continental crust was dominantly
absent in the early Earth. There are also contrasting views
on the process of construction of continental crust, with
some models proposing that juvenile magma production
in convergent margins associated with supercontinent
building generates new continental crust, and others argu872
ing that convergent tectonics largely destroys the continental crust through subduction erosion, and that juvenile
crust production is dominantly associated with supercontinent break-up5,9. Recent estimates on the volume of
global crust generation and destruction along modern
subduction zones show that the rate of destruction equals
or exceeds the rate of production10. If this is true, the present volume of continental crust on the globe should have
been established 2–3 Ga ago7. Clearly, more studies are
required to understand the growth (and recycling) of continental crust through time.
If the early Earth was analogous to the present-day
Western Pacific as suggested in some studies8, then the
surface of the globe should have been dominated by
island arcs in an oceanic realm. The collision of intraoceanic arcs led to the formation of embryonic continents. Among the three modes of arc–arc collision such
as vertical, orthogonal and parallel, the parallel collision
of arcs is considered to be the most effective mechanism
to build composite arcs (Figure 1), since the other two
processes will partly or entirely destroy the arc through
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subduction8. Examples of composite arc bundles amalgamated to create micro-continents with evidence for island
arc to continental arc magmatism implying continental
collision is present in many Archean cratons such as in
the North Atlantic Craton of West Greenland11, and in the
North China Craton (NCC) that forms one of the important ancient nuclei in Asia12.
The growth of continents is promoted through vertical
and lateral accretion (Figure 2). Melting of the downgoing oceanic slabs in convergent margins generates juvenile magmas which result in the vertical growth. The
accretion of oceanic and trench material, sometimes
together with other exotic blocks, onto the continental
margin results in lateral growth. With continued arc–arc
and arc-continent collision, continents grow through time.
One of the best examples for continental growth is illustrated by the Central Asian Orogenic Belt (CAOB), representing the largest Phanerozoic accretionary belt on the
globe13–15. Lateral accretion of young arc complexes and
old micro-continents together with vertical growth
through underplating of mantle-derived magmas has been
clearly documented from the CAOB. Post-orogenic mantlederived magmatic intrusions also contributed to the
growth of the CAOB16.
Building cratons
Cratons are ancient continental nuclei and many of the
ancient cratons on the Earth, particularly those which are
older than 2.0 Ga, are underlain by thick subcontinental
lithospheric mantle (SCLM). The thickness of the lithospheric mantle varies from a few tens of kilometres
beneath rift zones to more than 250 km beneath some of
the ancient continents17. The relationship between the
lithospheric mantle and the denser asthenosphere beneath
is considered to be like that of an iceberg, buoyant but
partly submerged18. Most of the Archean cratons on the
globe are characterized by ancient, cool and thick lithospheric mantle, mainly composed of high-refractory
harzburgites and lherzolites17,19. In the absence of active
magmatism they remain dormant, except for sporadic
eruptions of kimberlites due to the thick lithosphere and
low geotherms. A well-known exception is NCC which
has lost most of its original cratonic character through
widespread magmatism, deformation and associated
metallogeny in the Mesozoic, providing a classic example
for cratonic destruction20–22.
Jordan23 proposed the term 'tectosphere' for the highly
depleted and relatively low-density upper mantle layer,
and tomographic images beneath old cratons clearly show
high-velocity roots extending to at least 200 km depth,
and in some cases even up to 300 km (ref. 24). Tectosphere, also known as continental keel, is thus a rigid,
cold and chemically distinct raft that supports the continental crust. The marked difference in the thickness of
the tectosphere beneath continents of different ages can
be demonstrated from the significant variation in depth to
the lithosphere–asthenosphere boundary (LAB) among
Archean, Proterozoic and Phanerozoic continents as documented through seismic investigations and petrological
studies on mantle xenoliths (Figure 3).
The formation of tectosphere is a complex process and
diverse models have been proposed to explain the building of the cratonic root (Figure 4). The tectosphere is
considered in some models as a restite of mantle magmas,
predominantly komatiites. Extensive slab melting in the
Archean and extraction of felsic magmas which reacted
with olivine in the hanging wall are considered to have
generated the orthopyroxene-enriched mantle peridotites
which characterize the cratonic keel. Because of the high
degree of magma extraction from the primitive mantle to
build the early continental crust, the lithospheric mantle
beneath ancient cratons became highly depleted in basaltic components, resulting in lower density and higher
viscosity than that of the asthenosphere beneath25. The
contrast in density and viscosity probably explains why
most cratons have remained chronically stable over time.
Lithosphere formation, plume subcretion and
lithosphere accretion?
Figure 2. Schematic plate tectonic sketch illustrating convergent
margin architecture. Continents are built through vertical and lateral
growth.
CURRENT SCIENCE, VOL. 104, NO. 7, 10 APRIL 2013
Wyman et al.26, based on the co-existence of plumerelated and arc-related volcanic sequences in Neoarchean
greenstone terranes, proposed that migrating arcs capture
thick plume crust, which then becomes imbricated with
arc crust. According to this model, the low-density and
refractory buoyant residue from plume melting is coupled
to the crust generating the continental lithospheric mantle. In a recent study, Manikyamba and Kerrich27 applied
this model to explain the building of the Archean Dharwar Craton in India. In another recent study, Aulbach28
evaluated three major modes of craton building. (1)
Depending on the depth of initiation of partial melting,
the upwelling of plumes to shallow levels would leave
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behind a thick and highly depleted residue. (2) Subcretion
of plumes beneath a pre-existing lithospheric lid produces
less depleted residues owing to narrower melting intervals. (3) Subduction–accretion of oceanic or continental
lithosphere leads to lithospheric stacking. The crustal
nuclei comprising tonalite–trondhjemite–granodiorite
(TTG) and greenstone sequences formed from shallow
plate interactions are subsequently penetrated and subcreted by plume-derived melts, and following the delamination of dense eclogite and komatiites, TTG would be
trapped by buoyant plume residues to build the cratonic
mantle.
In recent years, seismic tomography has been used as a
tool to image the structure of the Earth’s crust and underlying mantle. One of the best regions to evaluate the
structure of the cratonic root is the NCC, where high
resolution and comprehensive geophysical data are available from recent studies. Chen et al.29 employed Sreceiver function (S-RF) technique and applied a wave
equation-based migration method on a combined S-RF
dataset collected from more than 200 broadband seismic
stations in the NCC to evaluate the subcontinental mantle
architecture. Their results show a thick lithosphere
(> 200 km) in the western part of the NCC, whereas the
lithosphere becomes considerably thin (60–100 km)
towards the eastern part (Figure 5). Santosh30 reinterpreted the seismic structure of the NCC and identified a
prominent layered nature and stacking of contrasting velocity domains, with the thickness (> 20 km) of the individual domains correlating well with the thick oceanic
crust of the Archean/Paleoproterozoic. These stacked layers would thus define the tectosphere beneath the NCC. A
possible interpretation for the layered structure with clear
anisotropy is that it represents repeated accretion of the
mid-oceanic ridge basalt (MORB) crust and mantle peridotite. The slabs were stacked because they could not
penetrate deep due to the double-layered convection prevailing in the Archean mantle31. A similar scenario of a
thick continental root with seismic anisotropic layering
has been observed beneath Archean cratons elsewhere on
the globe, including the Dharwar Craton in India (Figure
5), which might suggest that stacking of subducted oceanic lithosphere served as an important mechanism to
build the lithospheric keel beneath ancient continents.
The continental crust has low rigidity and an average
thickness of only 35 km. The tectosphere acts as a rigid
raft on top of which the thin granitic continental crust is
mounted. Therefore, Santosh et al.8 proposed that the tectosphere plays a key role in amalgamating wandering
continents and assembling them into supercontinents. The
extreme indentation of India during its collision with
Asia, a process that is still continuing, may also reflect a
thick tectospheric keel beneath the Indian subcontinent.
The volume of tectosphere beneath Archean cratons
was gradually reduced through thermal and material erosion by rising plumes during the break-up of supercontinents and by the subsequent intrusion of high-temperature
magmas. The NCC provides one of the classic examples
for destruction of the tectosphere, with substantial
erosion of the cratonic keel in its eastern part. Extensive
hydration and erosion through the subducting Pacific
plate from the east, coupled with thermal and material
erosion by rising magmas have destroyed the constitution
and architecture of the subcontinental mantle beneath this
craton30.
Supercontinents
Figure 3. a, Variation in depth to lithospheric–asthenospheric boundary (LAB) in Archean, Proterozoic and Phanerozoic terranes (after
Poudjom Djomani et al.43). The figure illustrates that the thickest subcontinental mantle (tectosphere) occurs beneath ancient cratons. b, Cartoon sketch of a cratonic keel (after King44) where the material down to
175 km depth is considered as compositionally distinct from the mantle, forming a chemical boundary layer (CBL). Below 175 km depth is
the more fertile mantle that is cooler than the average upper mantle at
the same depth. This forms the bottom of the thermal boundary layer
(TBL).
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The process of assembly of continental fragments into
supercontinents and their subsequent break-up is termed
as supercontinent cycle, which significantly influenced
the course of the Earth’s geologic, climatic and biological
evolution5,32–36. In contrast to the rather simple ‘opening
and closing’ mechanism envisaged in the Wilson cycle
involving the production and destruction of the ocean
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Figure 4. Schematic illustration depicting the formation of subcontinental mantle lithospheric root beneath cratons. a and b, The model of
Wymann et al.26 (modified from Manikyamba and Kerrich27) proposing the capture of plume crust by migrating arcs. c and d, Aulbach’s28 model of
plume subcretion. e, Subduction–accretion model generating stacked oceanic lithosphere. See text for discussion.
floor, supercontinent cycles involve more complex processes to assemble multiple continental fragments of various ages in different ways, including the suturing of
ancient cratons with accreted terranes. The Y-shaped
topology of triple junctions in major supercontinental assemblies prompted Santosh et al.8 to propose two types of
subduction zones on the globe: the Circum-Pacific subduction and the Tethyan. In the scenario where the subduction is double-sided, the triangular regions with Yshaped topology would selectively refrigerate the underlying mantle and turn down the temperature in these
domains compared to the surrounding regions. These
domains also accelerate the refrigeration through larger
amounts of subduction and thus promote stronger downwelling. Once this process is initiated, a runaway growth
of cold downwelling will be initiated which soon develops into a large zone of super-downwelling, pulling
together the continental fragments on the surface into a
tight assembly. The western Pacific region illustrates an
ongoing example of double-sided subduction (Figure 6),
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and is considered as the frontier for the future supercontinent37. Multiple subduction zones promote the rapid
amalgamation of continental fragments into supercontinents and also act as major zones of material flux into the
deep mantle transporting substantial volume of trench
sediments and arc crust through sediment subduction and
tectonic erosion8,33,38. Due to buoyancy, the subducted
TTG material is stacked in the mid-mantle region and
may not sink down to deeper levels, where they are
speculated to form the ‘second continent’ in the mantle
transition zone39. The high heat-producing elements in
the granitic components of the subducted TTG material
would account for the generation of plumes rising from
the mantle transition zone. On the other hand, the deep
subducted material drops down to the bottom of the mantle and forms ‘slab graveyards’ at the D″ layer near the
core–mantle boundary. Coupled with heating from the
core, such material (postulated to be recycled oceanic
lithosphere) could be viewed as a potential trigger of,
and contributor to, the superplumes rising from the
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Figure 5. A part of the Ps migrated section from Ramesh et al.45 for the Dharwar Craton (a), compared with the migrated
S-receiver function image for the North China Craton (b) from Chen et al.29. Note the striking similarity in the topology of the
subcontinental mantle down to 250 km beneath the two cratons. A possible interpretation of this crustal structure is that it
represents the frozen image of Archean subduction.
Figure 6. a, Schematic illustration depicting the process of double-sided subduction, a process that is considered to promote the rapid
amalgamation of continental fragments during the assembly of supercontinents (after Santosh et al.8). b, Numerical model of multiple
subduction zones using mobile, deformable continents (after Yoshida and Santosh 40). c, Ongoing example of double-sided subduction in
the western Pacific region, considered to represent the frontier of future supercontinent (after Maruyama et al.37).
core–mantle interface to the uppermost mantle, penetrating the mantle transition zone and eventually giving rise
to ‘hot spots’37. Thus, the superplume acts as a giant pipe
that connects the Earth’s core to the surface and beyond
into the planetary space (Figure 7).
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The process of assembly of supercontinents induces a
temperature increase due to the thermal insulating effect.
This would lead to a planetary-scale reorganization of
mantle flow and results in longest-wavelength thermal
heterogeneity in the mantle, termed as degree-one conCURRENT SCIENCE, VOL. 104, NO. 7, 10 APRIL 2013
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vection in 3D spherical geometry. The rifting and breakup of supercontinental assemblies may be caused by
either tensional stress due to the thermal insulating effect,
or large-scale partial melting due to the flow reorganization and the resultant temperature increase beneath the
supercontinent. Recent numerical study allows the modelling of mobile, deformable continents, including oceanic
plates, and successfully reproduces the processes and
timescales envisaged in the supercontinent cycle, including multiple subduction zones and reorganization of
mantle thermal structure during supercontinent amalgamation and disruption40 (Figures 6 and 7).
Surface environment and life
During the 4 billion year history of its evolution, the
Earth has gone through extreme climatic perturbations,
when global glaciations alternated with warm periods
which were accompanied by atmospheric oxygenation
(Figure 8). Enhanced weathering of huge mountain belts
erected through the collisional assembly of continents
and incorporated within supercontinents stripped CO2
from the atmosphere, initiating a runaway cooling that
resulted in continental glaciations35,36. The ice cover prevented weathering, subsequent build-up of CO2 in the
atmosphere, finally leading to the melting of the ice
sheets. The Earth thus switched from snowball to spring
ball. The intervals between the glacial cycles were dominated by the flushing of nutrients into oceans by the
weathering of granitic crust, stimulating photosynthetic
activity and starting the atmospheric oxygen pump.
Figure 7. a, Schematic illustration of the re-organization of the mantle thermal structure during supercontinent amalgamation and disruption (after Yoshida and Santosh40). b, Cartoon illustrating superplume
rising from the core–mantle boundary, and leading to disruption of
supercontinents (after Santosh et al.46). See text for discussion.
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Maruyama and Santosh41 presented a synopsis of
events in the late Proterozoic during which time two
‘snowball Earth’ glaciations occurred – in the Sturtian
(715–680 Ma) and Marinoan (680–635 Ma) – following
which large multicellular animals of the Ediacara faunaflourished as a prelude to the Phanerozoic world. The
geochemical bridge to trigger the evolution of modern life
in the Cambrian was in place when atmospheric oxygen
levels were elevated and nutrient supply was enhanced
into lakes that developed within continental rifts where
hydrothermal systems in the granitic basement created the
chemical environment for the birth of modern animals.
With cosmic radiation exerting a significant control on
mutation, modern life bloomed on the Earth. Thus,
although the biological blueprint of modern life involving
bilateral symmetry was established probably in the early
Neoproterozoic, the drastic evolution of the biosphere
had to wait until end Neoproterozoic, illustrating a galactic to genome-level link41.
The stepwise increase in the concentration of oxygen in
the Earth’s atmosphere has also been linked with the
processes associated with the amalgamation of Earth’s
land masses into supercontinents. Campbell and Allen42
suggested that the continent–continent collisions associated with the formation of supercontinents produce vast
mountain belts (Figure 9), the rapid erosion of which
releases large amounts of nutrients such as iron and
phosphorus into the oceans, leading to an explosion of
algae and cyanobacteria and enhanced production of O2
through photosynthesis. Simultaneously, the increased
sedimentation promotes the burial of organic carbon and
pyrite, thereby preventing back-reaction with free oxygen
and maintaining sustained increases in atmospheric oxygen. In a recent synthesis, Maruyama et al.1 traced the
processes which might have led to the birth of early life
on Earth and its aftermath, finally leading to the evolution metazoans. They identified several key factors such
Figure 8. Relationship between Proterozoic supercontinents and major climatic perturbations through Earth’s history (after Young36). Relative changes in solar luminosity and mantle temperature and inferred
changes in atmospheric oxygen content are also shown. The end Neoproterozoic is marked by major perturbations, including global glaciations.
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Figure 9. a, Schematic illustration showing how supercontinental assembly builds vast mountain belts, the weathering of which
supplies abundant nutrients. b, Sea-level drop leading to the exposure of continental crust, promoting weathering, erosion and
nutrient supply (modified from Maruyama et al.1). c, Nutrient supply leading to the creation of habitable environment in the shallow sea. Modified from Papineau47.
as: (1) the source of nutrients, (2) chemistry of primordial
ocean, (3) initial mass of ocean, and (4) size of rocky
planet. The primordial ocean was extremely acidic and
toxic, inhibiting the birth of life. Plate tectonics probably
acted as a cleaner to generate the blue ocean. Even if
granitic continental crust was present in the early Earth,
this was largely submerged, which inhibited weathering
and nutrient supply. The Neoproterozoic witnessed the
initiation of return-flow of sea water into mantle, enabled
by the cooling of the planet, leading to the emergence of
a huge land mass above level, its weathering and distribution of nutrients on a global scale. The oxygen pump has
also played a critical role in maintaining high oxygen levels
in the atmosphere since then, leading to the emergence of
the ozone layer as a roof to shield radiation, and thereby
enabling animals and plants to finally invade the land.
Conclusions
Granitic crust is the fundamental source of life-building
nutrients. The timescales and rates of production (and
destruction) of continental crust are equivocal. Arc–arc
parallel collision built primitive continents which subsequently grew through vertical and lateral accretion. Continental mass fragments were episodically assembled and
reassembled into supercontinents. The formation of cratons through the construction of thick keels beneath
ancient continents is principally linked to convergent
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margin processes involving either plumes or subducted
oceanic lithosphere, or both. It took nearly 4 billion years
for the Earth to build a habitable environment for life,
involving cleaning up of the oceans, building granitic
crust, bringing down the sea level to enable weathering
and nutrient supply, and starting the oxygen pump, among
other factors, finally leading to the bloom of modern life in
the dawn of Phanerozoic.
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ACKNOWLEDGEMENTS. I thank Prof. P. Balaram for his kind
invitation to contribute this article. Comments from Dr G. Parthasarathy
and an anonymous referee helped improve the manuscript. This work is
supported by the ‘1000 Talents Award’ grant to me from the Government of China. I also thank all my collaborators for their support and
patronage.
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