Seafloor Spreading and Plate Tectonics: the New Paradigm
7
Seafloor Spreading and Plate Tectonics: the New Paradigm
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 (for stratigraphic terms see Appendix A3). Today, there
would be few indeed who would harbor such fond hopes. The most ancient sediments
recovered from the sea floor are about 150 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 sea-floor spreading, all sediments accumulating on
the ocean bottom are swept towards the trenches, as on a conveyor belt (Fig. 1.20).
Here, some of the sediments are subdue ted into the mantle, 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
H. H. Hess (1960) and by R. S. Dietz (1961). Somewhat similar ideas had been
advanced earlier (Fig. 0.3 a, b) 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 sea floor.
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 time table 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
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. Runcom, 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 sea floor itself,
and final proof came from deep-sea drilling, as we shall see in Chapter 1.
Toward the end of the 1960s, the new hypothesis of sea-floor 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 posible 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
7
Seafloor Spreading and Plate Tectonics: the New Paradigm
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 (for stratigraphic terms see Appendix A3). Today, there
would be few indeed who would harbor such fond hopes. The most ancient sediments
recovered from the sea floor are about 150 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 sea-floor spreading, all sediments accumulating on
the ocean bottom are swept towards the trenches, as on a conveyor belt (Fig. 1.20).
Here, some of the sediments are subdue ted into the mantle, 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
H. H. Hess (1960) and by R. S. Dietz (1961). Somewhat similar ideas had been
advanced earlier (Fig. 0.3 a, b) 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 sea floor.
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 time table 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
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. Runcom, 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 sea floor itself,
and final proof came from deep-sea drilling, as we shall see in Chapter 1.
Toward the end of the 1960s, the new hypothesis of sea-floor 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 posible 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