Ocean Margin Systems
Orogeny
Seamounts
Seismogenic Zone
Subduction
Subduction Erosion
Wadati-Benioff-Zone
Wilson Cycle
ANCIENT PLATE TECTONICS
Stephen F. Foley
Department Earth and Planetary Sciences, ARC Centre of
Excellence for Core to Crust Fluid Systems, Macquarie
University, North Ryde, NSW, Australia
Definition
Plate tectonics
Plate tectonics is a theory which attempts to explain all
forms of geological and geophysical observations of the
Earth through time in terms of the movement of rigid lithospheric plates that form the upper few tens of kilometers
of the Earth. These observations include rock types and
their spatial and temporal relationships to each other, their
structures and the formation of mountains, the chemical
and physical properties of rocks, as well as seismicity, volcanism, and even the distribution of fauna and flora. These
lithospheric plates are made up of the crust and the uppermost layers of the mantle (the mantle lithosphere) that are
mechanically coupled and can therefore move as a unit on
top of convection currents within the mantle. Although
moving at rates of only a few centimeters per year, these
convection currents explain the movement of continents
across the surface over geological time. The crust involved
varies from thin ocean crust (around 7 km) to thick continental crust (25–100 km, averaging 35 km). Many plates,
such as the South American plate, are topped by extensive
tracts of both. When mantle lithosphere is included, some
plates may reach total thicknesses of 250 km in the older
cores of the continents.
The fact that all geological processes on the modern
Earth are thought to be explainable by plate tectonics does
not necessarily mean that this was the case for earlier
periods in Earth history when the planet was significantly
hotter. In deciding whether this was the case, we have
a major problem concerning the preservation of rocks
from the first half of Earth history. The production and
consumption of ocean crust is axiomatic to the operation
of plate tectonics, and yet the oldest ocean crust on the
Earth’s surface today is less than 180 million years old,
which is equivalent to only 4 % of the age of the Earth.
Linked to this uncertainty is the growth rate of the continental crust, which is thought by most geoscientists to
have been gradual or episodic over the 4.57 Ga age of
the Earth, with perhaps less than half of it in existence at
the end of the Archean period 2,500 myr ago.
History of plate tectonics
The term plate tectonics first emerged at the end of the
1960s, following a turbulent decade in which many
branches of the geological sciences reorganized their
understanding of geological processes to fit this single
all-encompassing theory. Its forerunner was continental
drift, which followed centuries-old observations such as
the similarity of coastline form between Africa and South
America, and hypothesized the lateral movement of continents over thousands of kilometers. Continental drift,
most commonly associated with the work of Alfred Wegener, remained controversial for several decades owing to
the lack of an explanatory mechanism. It received support
from the then new science of paleomagnetism in the
1950s, which demonstrated the movement of continents
around rotation poles (Runcorn, 1959), but it was the
introduction of marine geology and geophysics in the late
1950s that led to the breakthrough. Magnetic stripes were
discovered on the ocean floor (Raff and Mason, 1961) and
eventually proved to be symmetrically distributed across
mid ocean ridges (Pitman and Heirtzler, 1966). The concept of sea-floor spreading was born (Vine and Matthews,
1963), once again linked to rotation around poles that also
explained the newly discovered transform faults between
ridge segments (Wilson, 1965). However, if sea-floor
spreading occurs commonly on a planet of constant size
(there were a few who denied this; Carey, 1975), then
the ocean crust produced must also disappear again, and
this led to the concept of subduction zones (Le Pichon
1968), where sinking ocean crust is delineated by seismically active zones below island arcs (Isacks et al., 1968).
Once the mathematical explanation of the rotation of rigid
lithospheric plates around poles and the catalog of possible triple junctions was in place (McKenzie and Parker,
1967; McKenzie and Morgan, 1969), all that remained
was to fit regional geology including mountains, rifts,
and continental margins into the model to explain the geological history of Earth through time (Dickinson, 1970;
Dewey and Bird, 1970). For the last 40 years, plate tectonics has been the single complete geological theory into
which all geological and geophysical observations are
expected to fit.
Mechanisms of plate tectonics
In order to decide whether plate tectonics functioned
either differently or even at all early in Earth’s history, it
is necessary to consider how plate tectonics is thought to
work and what characteristics lead us to decide in favor
of it. Some of the features described earlier in favor of continental drift, such as the similarity of coastlines between
continents, the movement of continents across the surface
of the globe, and polar wander curves defined by paleomagnetism, are compatible with plate tectonics but do
not require it. The underlying mechanism lies in the distinction between the rigid upper lithosphere and the ability
of the asthenosphere below it to flow and requires convection currents in the mantle below the lithosphere. Forces
ANCIENT PLATE TECTONICS
13
Orogeny
Seamounts
Seismogenic Zone
Subduction
Subduction Erosion
Wadati-Benioff-Zone
Wilson Cycle
ANCIENT PLATE TECTONICS
Stephen F. Foley
Department Earth and Planetary Sciences, ARC Centre of
Excellence for Core to Crust Fluid Systems, Macquarie
University, North Ryde, NSW, Australia
Definition
Plate tectonics
Plate tectonics is a theory which attempts to explain all
forms of geological and geophysical observations of the
Earth through time in terms of the movement of rigid lithospheric plates that form the upper few tens of kilometers
of the Earth. These observations include rock types and
their spatial and temporal relationships to each other, their
structures and the formation of mountains, the chemical
and physical properties of rocks, as well as seismicity, volcanism, and even the distribution of fauna and flora. These
lithospheric plates are made up of the crust and the uppermost layers of the mantle (the mantle lithosphere) that are
mechanically coupled and can therefore move as a unit on
top of convection currents within the mantle. Although
moving at rates of only a few centimeters per year, these
convection currents explain the movement of continents
across the surface over geological time. The crust involved
varies from thin ocean crust (around 7 km) to thick continental crust (25–100 km, averaging 35 km). Many plates,
such as the South American plate, are topped by extensive
tracts of both. When mantle lithosphere is included, some
plates may reach total thicknesses of 250 km in the older
cores of the continents.
The fact that all geological processes on the modern
Earth are thought to be explainable by plate tectonics does
not necessarily mean that this was the case for earlier
periods in Earth history when the planet was significantly
hotter. In deciding whether this was the case, we have
a major problem concerning the preservation of rocks
from the first half of Earth history. The production and
consumption of ocean crust is axiomatic to the operation
of plate tectonics, and yet the oldest ocean crust on the
Earth’s surface today is less than 180 million years old,
which is equivalent to only 4 % of the age of the Earth.
Linked to this uncertainty is the growth rate of the continental crust, which is thought by most geoscientists to
have been gradual or episodic over the 4.57 Ga age of
the Earth, with perhaps less than half of it in existence at
the end of the Archean period 2,500 myr ago.
History of plate tectonics
The term plate tectonics first emerged at the end of the
1960s, following a turbulent decade in which many
branches of the geological sciences reorganized their
understanding of geological processes to fit this single
all-encompassing theory. Its forerunner was continental
drift, which followed centuries-old observations such as
the similarity of coastline form between Africa and South
America, and hypothesized the lateral movement of continents over thousands of kilometers. Continental drift,
most commonly associated with the work of Alfred Wegener, remained controversial for several decades owing to
the lack of an explanatory mechanism. It received support
from the then new science of paleomagnetism in the
1950s, which demonstrated the movement of continents
around rotation poles (Runcorn, 1959), but it was the
introduction of marine geology and geophysics in the late
1950s that led to the breakthrough. Magnetic stripes were
discovered on the ocean floor (Raff and Mason, 1961) and
eventually proved to be symmetrically distributed across
mid ocean ridges (Pitman and Heirtzler, 1966). The concept of sea-floor spreading was born (Vine and Matthews,
1963), once again linked to rotation around poles that also
explained the newly discovered transform faults between
ridge segments (Wilson, 1965). However, if sea-floor
spreading occurs commonly on a planet of constant size
(there were a few who denied this; Carey, 1975), then
the ocean crust produced must also disappear again, and
this led to the concept of subduction zones (Le Pichon
1968), where sinking ocean crust is delineated by seismically active zones below island arcs (Isacks et al., 1968).
Once the mathematical explanation of the rotation of rigid
lithospheric plates around poles and the catalog of possible triple junctions was in place (McKenzie and Parker,
1967; McKenzie and Morgan, 1969), all that remained
was to fit regional geology including mountains, rifts,
and continental margins into the model to explain the geological history of Earth through time (Dickinson, 1970;
Dewey and Bird, 1970). For the last 40 years, plate tectonics has been the single complete geological theory into
which all geological and geophysical observations are
expected to fit.
Mechanisms of plate tectonics
In order to decide whether plate tectonics functioned
either differently or even at all early in Earth’s history, it
is necessary to consider how plate tectonics is thought to
work and what characteristics lead us to decide in favor
of it. Some of the features described earlier in favor of continental drift, such as the similarity of coastlines between
continents, the movement of continents across the surface
of the globe, and polar wander curves defined by paleomagnetism, are compatible with plate tectonics but do
not require it. The underlying mechanism lies in the distinction between the rigid upper lithosphere and the ability
of the asthenosphere below it to flow and requires convection currents in the mantle below the lithosphere. Forces
ANCIENT PLATE TECTONICS
13