eminent scientists such as Khain and Khalilov (2008) and
Lev Zoneshain (2013) to push for the acceptance of the
new ideas.
Only some 35 years ago, it was still possible for geologists to think that the sediments on the deep-sea floor
might contain the entire Phanerozoic record and lead us
back even into the Precambrian. Today, there would be
few indeed who would harbor such fond hopes. The most
ancient sediments recovered from the seafloor are about
180 million years old, which is less than 5 % of the age
accorded to fossil-bearing sedimentary deposits on land.
Where, then, is the debris which must have washed into
the deep sea, for the several billion years that continents
have existed? According to the hypothesis of seafloor
spreading, all sediments accumulating on the ocean bottom are swept toward the trenches, as on a tectonic conveyor belt. Here, some of the sediments are subducted
into the Earth’s crust; others are scraped off against the
inner wall of the trench. Thus, the ocean floor is cleaned
of sediments by constant renewal and destruction.
This remarkable idea did not exactly find a warm welcome when first proposed by Hess (1962) and Dietz
(1961). Somewhat similar ideas had been advanced much
earlier (f. e. by Alfred Wegener) and had likewise been
discounted as premature speculations. Gradually, however, as more facts became available to test the hypothesis,
opposition weakened, and by 1970 there were only a very
few defenders of tradition who challenged the concept of a
moving seafloor.
The opposition to large-scale horizontal movement on
Earth’s surface had a long history. It all began with continental drift, an idea that arose early in the present century
and challenged the prevailing assumption of fixism with
the new arguments of mobilism. The most serious challenge, as already mentioned, came from the German geophysicist A. Wegener, who proposed drastic changes in
the distribution of continents and ocean basins within the
last 200 million years. He also proposed that the continents actively plowed through the magma which carries
them, an idea that proved to be wrong. In addition, he
had a timetable for the drifting of certain land masses
which was quite unrealistic. Skeptical geophysicists recognized these weaknesses in Wegener’s proposal and
argued forcibly for the rejection of the theory of continental drift. Thus, despite the support from geologists familiar
with the incredible similarities of ancient rocks and fossils
in South America and South Africa, Wegener’s hypothesis
did not find general acceptance before the 1960s.
In essence, evidence for continental drift was dismissed
because the proposed mechanism was wrong. Only in the
late 1950s, through the work on geomagnetism and polar
wandering by E. Irving and S. K. Runcorn (Runcorn,
1967), did it become possible again to talk about continental drift without being thought ignorant of physical principles. The most compelling evidence, however, came from
the magnetism of the seafloor itself (Vine and Matthews,
1963), and final proof came from deep-sea drilling, almost
a decade later.
Toward the end of the 1960s, the new hypothesis of seafloor spreading had metamorphosed into the theory of
plate tectonics. The theory is based on the mapping of
magnetic anomalies and earthquakes, by which it is possible to define large regions of the Earth’s surface that move
as units (plates), with earthquakes occurring at the boundaries of these units. The mathematical tool allowing efficient description of the movement of the plates is a
theorem of the Swiss mathematician Leonhard Euler
(1707–1783), which states that uniform motion on a
sphere is uniquely described by defining rotation about a
pole. The path of migration of any point on a plate appears
as a portion of a circle about that pole. Bullard and associates introduced Euler’s theorem to global tectonics, in
1965, to produce a new fit of the continents bordering
the Atlantic. In the same year, J. T. Wilson explained the
nature of what he called transform faults as boundaries
between “rigid plates,” where there is lateral motion generated at a spreading center; in subduction zones they
can end at a trench.
For the present, and at any time in the geologic past,
plate motions on the surface of Earth are defined by giving
the outlines of the dozen or so plates, the geographic location of the poles of rotation, and the associated angular
velocities. The principles of this approach were presented
by W. J. Morgan early in 1967 and by D. P. McKenzie and
R. L. Parker also in 1967. Using this methodology, X. Le
Pichon, in 1968, made a global map of the relative motions
of the major plates. The same concepts and methods
allowed the large-scale reconstruction of the past geographic configurations of continents and ocean basins,
back into the Cretaceous period (cf. Le Pichon, 1968;
Allégre, 1988; Lobkovsky et al., 2013, 2014).
Deep-sea drilling: discovering new dimensions
of ocean history
The emergence of the new paradigm greatly stimulated the
beginning of a major venture in marine geology: the systematic exploration of the deep-sea floor by drilling. Up
to 1968, before the drilling vessel GLOMAR Challenger
set out on her first cruise (from Galveston in Texas),
knowledge of sediments older than 1 million years was
entirely based on cores taken in regions of greatly reduced
sedimentation and erosion, where younger sediments had
been removed. Such a core would typically represent a
short stretch of time somewhere within the long history
of the ocean. Detailed comparison with other cores, from
other regions, was hardly ever possible, because of the difficulty of exact age assignment. The task of reconstructing
a global ocean history for the pre-Quaternary could only
be seriously attempted when more or less continuous
sequences of samples became available, through drilling,
from many different regions. The earliest 23 legs of DSDP
were actually dominated by marine geophysicists who
spot cored the sites because they wanted mainly to date
seismic reflectors. Only during Leg 24 in the NW Indian
Ocean, the principle of continuous coring was adopted
442
MARINE GEOSCIENCES: A SHORT, ECCLECTIC AND WEIGHTED HISTORIC ACCOUNT
Lev Zoneshain (2013) to push for the acceptance of the
new ideas.
Only some 35 years ago, it was still possible for geologists to think that the sediments on the deep-sea floor
might contain the entire Phanerozoic record and lead us
back even into the Precambrian. Today, there would be
few indeed who would harbor such fond hopes. The most
ancient sediments recovered from the seafloor are about
180 million years old, which is less than 5 % of the age
accorded to fossil-bearing sedimentary deposits on land.
Where, then, is the debris which must have washed into
the deep sea, for the several billion years that continents
have existed? According to the hypothesis of seafloor
spreading, all sediments accumulating on the ocean bottom are swept toward the trenches, as on a tectonic conveyor belt. Here, some of the sediments are subducted
into the Earth’s crust; others are scraped off against the
inner wall of the trench. Thus, the ocean floor is cleaned
of sediments by constant renewal and destruction.
This remarkable idea did not exactly find a warm welcome when first proposed by Hess (1962) and Dietz
(1961). Somewhat similar ideas had been advanced much
earlier (f. e. by Alfred Wegener) and had likewise been
discounted as premature speculations. Gradually, however, as more facts became available to test the hypothesis,
opposition weakened, and by 1970 there were only a very
few defenders of tradition who challenged the concept of a
moving seafloor.
The opposition to large-scale horizontal movement on
Earth’s surface had a long history. It all began with continental drift, an idea that arose early in the present century
and challenged the prevailing assumption of fixism with
the new arguments of mobilism. The most serious challenge, as already mentioned, came from the German geophysicist A. Wegener, who proposed drastic changes in
the distribution of continents and ocean basins within the
last 200 million years. He also proposed that the continents actively plowed through the magma which carries
them, an idea that proved to be wrong. In addition, he
had a timetable for the drifting of certain land masses
which was quite unrealistic. Skeptical geophysicists recognized these weaknesses in Wegener’s proposal and
argued forcibly for the rejection of the theory of continental drift. Thus, despite the support from geologists familiar
with the incredible similarities of ancient rocks and fossils
in South America and South Africa, Wegener’s hypothesis
did not find general acceptance before the 1960s.
In essence, evidence for continental drift was dismissed
because the proposed mechanism was wrong. Only in the
late 1950s, through the work on geomagnetism and polar
wandering by E. Irving and S. K. Runcorn (Runcorn,
1967), did it become possible again to talk about continental drift without being thought ignorant of physical principles. The most compelling evidence, however, came from
the magnetism of the seafloor itself (Vine and Matthews,
1963), and final proof came from deep-sea drilling, almost
a decade later.
Toward the end of the 1960s, the new hypothesis of seafloor spreading had metamorphosed into the theory of
plate tectonics. The theory is based on the mapping of
magnetic anomalies and earthquakes, by which it is possible to define large regions of the Earth’s surface that move
as units (plates), with earthquakes occurring at the boundaries of these units. The mathematical tool allowing efficient description of the movement of the plates is a
theorem of the Swiss mathematician Leonhard Euler
(1707–1783), which states that uniform motion on a
sphere is uniquely described by defining rotation about a
pole. The path of migration of any point on a plate appears
as a portion of a circle about that pole. Bullard and associates introduced Euler’s theorem to global tectonics, in
1965, to produce a new fit of the continents bordering
the Atlantic. In the same year, J. T. Wilson explained the
nature of what he called transform faults as boundaries
between “rigid plates,” where there is lateral motion generated at a spreading center; in subduction zones they
can end at a trench.
For the present, and at any time in the geologic past,
plate motions on the surface of Earth are defined by giving
the outlines of the dozen or so plates, the geographic location of the poles of rotation, and the associated angular
velocities. The principles of this approach were presented
by W. J. Morgan early in 1967 and by D. P. McKenzie and
R. L. Parker also in 1967. Using this methodology, X. Le
Pichon, in 1968, made a global map of the relative motions
of the major plates. The same concepts and methods
allowed the large-scale reconstruction of the past geographic configurations of continents and ocean basins,
back into the Cretaceous period (cf. Le Pichon, 1968;
Allégre, 1988; Lobkovsky et al., 2013, 2014).
Deep-sea drilling: discovering new dimensions
of ocean history
The emergence of the new paradigm greatly stimulated the
beginning of a major venture in marine geology: the systematic exploration of the deep-sea floor by drilling. Up
to 1968, before the drilling vessel GLOMAR Challenger
set out on her first cruise (from Galveston in Texas),
knowledge of sediments older than 1 million years was
entirely based on cores taken in regions of greatly reduced
sedimentation and erosion, where younger sediments had
been removed. Such a core would typically represent a
short stretch of time somewhere within the long history
of the ocean. Detailed comparison with other cores, from
other regions, was hardly ever possible, because of the difficulty of exact age assignment. The task of reconstructing
a global ocean history for the pre-Quaternary could only
be seriously attempted when more or less continuous
sequences of samples became available, through drilling,
from many different regions. The earliest 23 legs of DSDP
were actually dominated by marine geophysicists who
spot cored the sites because they wanted mainly to date
seismic reflectors. Only during Leg 24 in the NW Indian
Ocean, the principle of continuous coring was adopted
442
MARINE GEOSCIENCES: A SHORT, ECCLECTIC AND WEIGHTED HISTORIC ACCOUNT