4,500 Ma must correspond to the existence of a magma
ocean in the Earth that may have been hundreds of kilometers thick, resulting in extremely hot conditions near the
surface of the early Earth. However, this state may not
have lasted for very long, as investigations of the oxygen
isotope compositions of the earliest rocks and minerals
(zircons) find that they formed in cool conditions in siliceous crustal rocks (that have since been destroyed) near
the Earth’s surface (Valley et al., 2002). Since the oldest
of these zircons are 4,404 Ma (Wilde et al., 2001), there
appears to be only 100 million years for the Earth to progress from extremely hot to cool conditions. The other
major simplification in the curve is that it represents the
whole Earth and implies a gradual, smooth evolution without any episodic or catastrophic developments. This contrasts with the red line superimposed on Figure 1, which
shows the rate of production of new continental crust as
gauged by the ages of igneous rocks on the same timescale
(Hawkesworth and Kemp, 2006). Although there is some
discussion as to whether this really indicates ages of
crustal growth rather than merely its survival, there appear
to be major periods of crustal formation in the late
Archean (3,000–2,500 Ma) and in the early Proterozoic
(2,000–1,750 Ma), with an additional pulse at around
1,100 Ma. There may have been a similar episodism in
plate movements and subduction (O’Neill et al., 2007).
These two uncertainties emphasize that the slope of the
curve and its exact form are debatable, and therefore, the
point marked for entry into the plate tectonic window at
around 3,000 Ma can only be approximate, although there
is increasing evidence for the onset of modern-style (i.e.,
steep and deep) plate tectonics at around 3,200 Ma
(Shirey and Richardson, 2011; Van Kranendonk, 2011;
Dhuime et al., 2012).
The other open question is that if plate tectonics did
begin at about this time, then what happened before that?
Was there some other form and movement of lithospheric
plates that is distinguishable from the modern style
(indicated by the light green band in Figure 1; Brown,
2007), or could small plates tip towards each other like
ice floes in the Antarctic, or was there a thick lid of crust
that was recycled into the mantle by eclogite dripping
from its base?
An outline of Archean geology
To adequately assess whether plate tectonics could have
operated during the first half of Earth history, it is important not just to ascertain the presence or absence of indicators for plate tectonics but also to explain the Archean
geological record. Archean rocks consist of about 80 %
high-grade gneisses, mostly summarized as the tonalitetrondhjemite-granodiorite (or TTG) suite, and 20 % greenstone belts with lower metamorphic grade. The gneisses
have igneous origins and can be explained by melting of
basaltic protolith material (Foley et al., 2002; Rapp et al.,
2003). Their deformation often indicates lateral compression, although in some regions, most notably Western
Australia, vertical movements may have been important
(Chardon et al., 1996). On the modern Earth, tonalitic
melts are formed by melting of basaltic material at subduction zones, and so a subduction environment is the most
commonly invoked option to explain them in the Archean.
Greenstone belts consist principally of volcanic rocks and
sediments wedged between domes of high-grade gneisses.
Their most famous constituents are komatiites, which are
magnesium-rich mafic volcanics that are almost exclusively Archean in age (Arndt and Nisbet, 1982). However,
the majority of volcanics in greenstone belts are more
“normal” basaltic to felsic rocks, and not komatiites. The
sediments are dominantly texturally immature, whereas
shelf sediments such as carbonates and mature sandstones,
although present back to 3.5 Ga, are much less common
(Eriksson et al., 1994). This would be expected on
a planet on which island arc environments and/or volcanic
plateaux are common but continents are rare; the first continents would be formed by the amalgamation of island
arcs, and all sediments that are preserved would be formed
close to these arcs. The rarity of shelf sediments may correspond to the rarity of continents. The relationship
between the high-grade gneisses and greenstone belts
ought, therefore, to be of paramount interest, but unfortunately, most contacts between them are the locations of
later tectonic movements. Examples exist, however, that
show basal unconformities, leading to the conclusion that
greenstones commonly do not represent ocean crust but
were deposited on continents (Bickle et al., 1994; Buick
et al., 1995).
There are also changes in rock types towards the end of
the Archean; following the peak in continental crust formation at 2.7 Ga (Figure 1), more potassic granites that
are typical for later periods in Earth history become most
common, often appearing to seal large sections of the
newly formed crust together into stable continents. Other
rocks remain notably absent – eclogites and blueschists –
which are indicators of high-pressure metamorphism at
low pressures, and do not appear until around 2,100 Ma.
The characteristics of plate tectonics
From theoretical expectations, we may consider how we
can decide if plate tectonics operates on a planet and then
deliberate about whether these features are relevant for the
early Earth. Table 1 lists the main features by which the
operation of either continental drift or plate tectonics can
be recognized on the modern Earth. The problem in
ascertaining the relevance of plate tectonics for the
Archean and Proterozoic Eras is that most of these criteria
depend either on geophysical measurements that only provide a picture of the Earth today or on good preservation of
rock associations and geological structures. Much of the
acceptance and detail of plate tectonics is due to seismological and other geophysical surveys that provide information on all levels of the deep Earth, documenting
seismic discontinuities that delineate the size of core and
mantle, that the outer core is molten (Fowler, 2005), and
ANCIENT PLATE TECTONICS
15
ocean in the Earth that may have been hundreds of kilometers thick, resulting in extremely hot conditions near the
surface of the early Earth. However, this state may not
have lasted for very long, as investigations of the oxygen
isotope compositions of the earliest rocks and minerals
(zircons) find that they formed in cool conditions in siliceous crustal rocks (that have since been destroyed) near
the Earth’s surface (Valley et al., 2002). Since the oldest
of these zircons are 4,404 Ma (Wilde et al., 2001), there
appears to be only 100 million years for the Earth to progress from extremely hot to cool conditions. The other
major simplification in the curve is that it represents the
whole Earth and implies a gradual, smooth evolution without any episodic or catastrophic developments. This contrasts with the red line superimposed on Figure 1, which
shows the rate of production of new continental crust as
gauged by the ages of igneous rocks on the same timescale
(Hawkesworth and Kemp, 2006). Although there is some
discussion as to whether this really indicates ages of
crustal growth rather than merely its survival, there appear
to be major periods of crustal formation in the late
Archean (3,000–2,500 Ma) and in the early Proterozoic
(2,000–1,750 Ma), with an additional pulse at around
1,100 Ma. There may have been a similar episodism in
plate movements and subduction (O’Neill et al., 2007).
These two uncertainties emphasize that the slope of the
curve and its exact form are debatable, and therefore, the
point marked for entry into the plate tectonic window at
around 3,000 Ma can only be approximate, although there
is increasing evidence for the onset of modern-style (i.e.,
steep and deep) plate tectonics at around 3,200 Ma
(Shirey and Richardson, 2011; Van Kranendonk, 2011;
Dhuime et al., 2012).
The other open question is that if plate tectonics did
begin at about this time, then what happened before that?
Was there some other form and movement of lithospheric
plates that is distinguishable from the modern style
(indicated by the light green band in Figure 1; Brown,
2007), or could small plates tip towards each other like
ice floes in the Antarctic, or was there a thick lid of crust
that was recycled into the mantle by eclogite dripping
from its base?
An outline of Archean geology
To adequately assess whether plate tectonics could have
operated during the first half of Earth history, it is important not just to ascertain the presence or absence of indicators for plate tectonics but also to explain the Archean
geological record. Archean rocks consist of about 80 %
high-grade gneisses, mostly summarized as the tonalitetrondhjemite-granodiorite (or TTG) suite, and 20 % greenstone belts with lower metamorphic grade. The gneisses
have igneous origins and can be explained by melting of
basaltic protolith material (Foley et al., 2002; Rapp et al.,
2003). Their deformation often indicates lateral compression, although in some regions, most notably Western
Australia, vertical movements may have been important
(Chardon et al., 1996). On the modern Earth, tonalitic
melts are formed by melting of basaltic material at subduction zones, and so a subduction environment is the most
commonly invoked option to explain them in the Archean.
Greenstone belts consist principally of volcanic rocks and
sediments wedged between domes of high-grade gneisses.
Their most famous constituents are komatiites, which are
magnesium-rich mafic volcanics that are almost exclusively Archean in age (Arndt and Nisbet, 1982). However,
the majority of volcanics in greenstone belts are more
“normal” basaltic to felsic rocks, and not komatiites. The
sediments are dominantly texturally immature, whereas
shelf sediments such as carbonates and mature sandstones,
although present back to 3.5 Ga, are much less common
(Eriksson et al., 1994). This would be expected on
a planet on which island arc environments and/or volcanic
plateaux are common but continents are rare; the first continents would be formed by the amalgamation of island
arcs, and all sediments that are preserved would be formed
close to these arcs. The rarity of shelf sediments may correspond to the rarity of continents. The relationship
between the high-grade gneisses and greenstone belts
ought, therefore, to be of paramount interest, but unfortunately, most contacts between them are the locations of
later tectonic movements. Examples exist, however, that
show basal unconformities, leading to the conclusion that
greenstones commonly do not represent ocean crust but
were deposited on continents (Bickle et al., 1994; Buick
et al., 1995).
There are also changes in rock types towards the end of
the Archean; following the peak in continental crust formation at 2.7 Ga (Figure 1), more potassic granites that
are typical for later periods in Earth history become most
common, often appearing to seal large sections of the
newly formed crust together into stable continents. Other
rocks remain notably absent – eclogites and blueschists –
which are indicators of high-pressure metamorphism at
low pressures, and do not appear until around 2,100 Ma.
The characteristics of plate tectonics
From theoretical expectations, we may consider how we
can decide if plate tectonics operates on a planet and then
deliberate about whether these features are relevant for the
early Earth. Table 1 lists the main features by which the
operation of either continental drift or plate tectonics can
be recognized on the modern Earth. The problem in
ascertaining the relevance of plate tectonics for the
Archean and Proterozoic Eras is that most of these criteria
depend either on geophysical measurements that only provide a picture of the Earth today or on good preservation of
rock associations and geological structures. Much of the
acceptance and detail of plate tectonics is due to seismological and other geophysical surveys that provide information on all levels of the deep Earth, documenting
seismic discontinuities that delineate the size of core and
mantle, that the outer core is molten (Fowler, 2005), and
ANCIENT PLATE TECTONICS
15