demonstrating his theory. The paleontologists of the early
twentieth century clung to the concept of fixed geography,
and invented the theory of “continental bridges,” land ties
that would have linked the continents, to explain the
migration of fauna and flora. Nevertheless, some geologists
believed in continental drift and supported it, such as
Alexander Du Toit and Emile Argand. A decisive argument
in favor of Wegener’s theory was established by Arthur
Holmes. He was the first to establish the basics of thermal
convection of the Earth’s mantle (Holmes 1929). But here
again, this argument did not receive the expected positive
response from the Earth science community, and it was not
until the 1960s that Holmes was seen as a pioneer of plate
tectonics.
From the 1940s, exploration of the ocean floor would
revolutionize Earth sciences. The mapping of the morphology of the ocean floor, and the discovery of the Atlantic
Ridge under the initiative of Bruce Heezen and Marie Tharp
(Heezen 1962; Heezen and Tharp 1965), led Harry Hess
1 to
propose a theory of the expansion of the ocean floor (Hess
1962). Half a century after Wegener, Earth sciences were
experiencing a revolution, following the discovery of magnetic anomalies (Chap. 7). An American oceanographic
campaign mapped the magnetic field around the Juan de
Fuca Ridge, off the North American coast, in the eastern
Pacific Ocean. By subtracting the ambient magnetic field
from the magnetic data, alternately positive and negative
anomalies were found (Raff and Mason 1961). Based on
these observations, both Morley, and Vine and Matthews
(Vine and Matthews 1963), proposed the theory of the
renewal and expansion of the ocean floor in 1963.
These marine magnetic anomalies result from the acquisition of a thermoremanent magnetization by the iron and
titanium oxide particles in the oceanic crust subjected to the
Earth’s magnetic field, after the oceanic crust (basaltic lava),
emitted at the ocean ridges at a temperature of about 1100 °
C, begins to cool. When basalt reaches the Curie point (the
magnetic particle-dependent temperature, about 570 °C for
the ferromagnetic magnetite crystals contained in the basalt),
the direction of the field is fossilized by the magnetic carriers
of the oceanic crust. Above the Curie point, the material is
paramagnetic, and each magnetic carrier behaves like a small
compass that follows the direction of the magnetic field
without storing it. As soon as the temperature of the rock
passes below the Curie temperature, the magnetic carriers
aligned along the lines of force of the Earth’s magnetic field
are permanently frozen in this direction. The Earth’s
magnetic field can also be recorded in sedimentary rocks if
they contain magnetic particles (Chap. 7).
Sea floor spreading is the cornerstone of plate tectonics,
but to complete this theory, temporal constraints needed to
be integrated, enabling the understanding of the pace at
which ocean ridges opened. During this same period, the
geophysicists Cox and Doell, and the geochemist Dalrymple
established the first timetable of magnetic reversals for the
last 4 million years (Chap. 7), using a new technique of
isotopic dating with the potassium and argon elements (Cox
et al. 1964) (Chap. 5). By comparing the magnetization
polarity and the age of basalt samples taken at sea, it is clear
that the age of the oceanic crust increases with distance from
the line of the ridge. From the 1960s onwards, the DSDP
(Deep-Sea Drilling Project) and ODP (Ocean Drilling Program) scientific missions were launched with the aim of
drilling for sediment deposits on this oceanic crust. These
sedimentary cores allow the sequences of magnetic polarities
fixed by period to be established, thanks to the fossil content,
and thus, to also date the marine magnetic anomalies. The
speed of opening of the ocean ridges can then be determined.
In 1968, Heirtzler and his group quantified the speed of
opening of the South Atlantic Ocean by analyzing a marine
magnetic sequence dating back to the Pliocene (3.35 Ma). In
the following years, the kinematic parameters of all the
oceans were determined one after another, and the evolution
of the different ocean basins could then be traced. These
parameters reflect the movements of one lithospheric plate
relative to another one that is arbitrarily fixed. These
movements being defined on a quasi-spherical surface, they
can be expressed by an angle of rotation about an axis
passing through the center of the Earth and defined by the
longitude and latitude of its pole.
Moving on from the oceans to the land, in the early
1950s, some scientists studied the natural remanent magnetization of rocks. At all points of the globe, the magnetic
field is defined by a vector collinear to the field lines. The
magnetic field of the Earth originates from convective
movements within the outer core, which consists of liquid
iron (about 90 wt%), nickel (about 4 wt%), along with some
lighter elements, such as silicon, sulfur and oxygen. The
movements within the conductive core induce electric currents which, in turn, generate a magnetic field. The Earth’s
magnetic field functions like a self-excited dynamo. To
compensate for the energy losses associated with the electrical resistivity within the Earth’s core, energy input
obtained from the conversion of ohmic dissipation into heat,
from the gravitational energy and from the release of latent
heat during the crystallization of the inner core ensures the
thermal equilibrium of the milieu and permits the functioning of the geodynamo, as well as its continuity over geological time.
1
When B. Heezen presented their findings to Princeton in 1957, Harry
Hess stood up and said: “Young man, you have shaken the foundations
of geology!” (Yount 2009).
2 The Changing Face of the Earth Throughout the Ages
25
twentieth century clung to the concept of fixed geography,
and invented the theory of “continental bridges,” land ties
that would have linked the continents, to explain the
migration of fauna and flora. Nevertheless, some geologists
believed in continental drift and supported it, such as
Alexander Du Toit and Emile Argand. A decisive argument
in favor of Wegener’s theory was established by Arthur
Holmes. He was the first to establish the basics of thermal
convection of the Earth’s mantle (Holmes 1929). But here
again, this argument did not receive the expected positive
response from the Earth science community, and it was not
until the 1960s that Holmes was seen as a pioneer of plate
tectonics.
From the 1940s, exploration of the ocean floor would
revolutionize Earth sciences. The mapping of the morphology of the ocean floor, and the discovery of the Atlantic
Ridge under the initiative of Bruce Heezen and Marie Tharp
(Heezen 1962; Heezen and Tharp 1965), led Harry Hess
1 to
propose a theory of the expansion of the ocean floor (Hess
1962). Half a century after Wegener, Earth sciences were
experiencing a revolution, following the discovery of magnetic anomalies (Chap. 7). An American oceanographic
campaign mapped the magnetic field around the Juan de
Fuca Ridge, off the North American coast, in the eastern
Pacific Ocean. By subtracting the ambient magnetic field
from the magnetic data, alternately positive and negative
anomalies were found (Raff and Mason 1961). Based on
these observations, both Morley, and Vine and Matthews
(Vine and Matthews 1963), proposed the theory of the
renewal and expansion of the ocean floor in 1963.
These marine magnetic anomalies result from the acquisition of a thermoremanent magnetization by the iron and
titanium oxide particles in the oceanic crust subjected to the
Earth’s magnetic field, after the oceanic crust (basaltic lava),
emitted at the ocean ridges at a temperature of about 1100 °
C, begins to cool. When basalt reaches the Curie point (the
magnetic particle-dependent temperature, about 570 °C for
the ferromagnetic magnetite crystals contained in the basalt),
the direction of the field is fossilized by the magnetic carriers
of the oceanic crust. Above the Curie point, the material is
paramagnetic, and each magnetic carrier behaves like a small
compass that follows the direction of the magnetic field
without storing it. As soon as the temperature of the rock
passes below the Curie temperature, the magnetic carriers
aligned along the lines of force of the Earth’s magnetic field
are permanently frozen in this direction. The Earth’s
magnetic field can also be recorded in sedimentary rocks if
they contain magnetic particles (Chap. 7).
Sea floor spreading is the cornerstone of plate tectonics,
but to complete this theory, temporal constraints needed to
be integrated, enabling the understanding of the pace at
which ocean ridges opened. During this same period, the
geophysicists Cox and Doell, and the geochemist Dalrymple
established the first timetable of magnetic reversals for the
last 4 million years (Chap. 7), using a new technique of
isotopic dating with the potassium and argon elements (Cox
et al. 1964) (Chap. 5). By comparing the magnetization
polarity and the age of basalt samples taken at sea, it is clear
that the age of the oceanic crust increases with distance from
the line of the ridge. From the 1960s onwards, the DSDP
(Deep-Sea Drilling Project) and ODP (Ocean Drilling Program) scientific missions were launched with the aim of
drilling for sediment deposits on this oceanic crust. These
sedimentary cores allow the sequences of magnetic polarities
fixed by period to be established, thanks to the fossil content,
and thus, to also date the marine magnetic anomalies. The
speed of opening of the ocean ridges can then be determined.
In 1968, Heirtzler and his group quantified the speed of
opening of the South Atlantic Ocean by analyzing a marine
magnetic sequence dating back to the Pliocene (3.35 Ma). In
the following years, the kinematic parameters of all the
oceans were determined one after another, and the evolution
of the different ocean basins could then be traced. These
parameters reflect the movements of one lithospheric plate
relative to another one that is arbitrarily fixed. These
movements being defined on a quasi-spherical surface, they
can be expressed by an angle of rotation about an axis
passing through the center of the Earth and defined by the
longitude and latitude of its pole.
Moving on from the oceans to the land, in the early
1950s, some scientists studied the natural remanent magnetization of rocks. At all points of the globe, the magnetic
field is defined by a vector collinear to the field lines. The
magnetic field of the Earth originates from convective
movements within the outer core, which consists of liquid
iron (about 90 wt%), nickel (about 4 wt%), along with some
lighter elements, such as silicon, sulfur and oxygen. The
movements within the conductive core induce electric currents which, in turn, generate a magnetic field. The Earth’s
magnetic field functions like a self-excited dynamo. To
compensate for the energy losses associated with the electrical resistivity within the Earth’s core, energy input
obtained from the conversion of ohmic dissipation into heat,
from the gravitational energy and from the release of latent
heat during the crystallization of the inner core ensures the
thermal equilibrium of the milieu and permits the functioning of the geodynamo, as well as its continuity over geological time.
1
When B. Heezen presented their findings to Princeton in 1957, Harry
Hess stood up and said: “Young man, you have shaken the foundations
of geology!” (Yount 2009).
2 The Changing Face of the Earth Throughout the Ages
25