Evidence For Sea-floor Spreading Research Articles

PLATE TECTONICS IN PORTUGUESE AND SPANISH SCIENCE TEXTBOOKS: FROM THE 1960s TO THE 1980s

Plate tectonics caused a revolution within earth sciences which then was transposed into science textbooks. The main objective of this paper is to explore how plate tectonics influenced Portuguese and Spanish science textbooks published from the 1960s through the 1980s. For this purpose, a qualitative method based on the concept of didactic transposition is used. The didactic transposition of seafloor spreading evidence such as ridges, rifts and trenches, transform faults, seafloor sediments, the age of seafloor basaltic rocks, the magnetic anomalies on the seafloor, the Benioff zones and the subduction process, and also the didactic transposition of the formation of mountains ranges and island arcs, convection currents, plate tectonics concepts, boundaries and motion, and plate tectonics acceptance are studied in a comprehensive sample of science textbooks. The analysis of textbooks shows that the didactic transposition of seafloor spreading, and plate tectonics started mainly in 1970s Portuguese and Spanish textbooks and had a strong development in 1980s textbooks. No major differences were found between the approaches to plate tectonics in similar age Portuguese and Spanish textbooks. At the beginning of the 1970s, textbooks presented partial evidence for seafloor spreading, such as magnetic anomalies and the characteristics of ridges, rifts and trenches. They also addressed convection currents but only those that were related to geosynclines. In the mid 1970s and in the 1980s, textbooks presented more comprehensive evidence of seafloor spreading, by adding didactical transpositions of transform faults, seafloor sediments and the age of seafloor rocks. They also presented in more detail topics such as magnetic anomalies, the Benioff zones, orogenic processes and the tectonic significance of ridges, rifts and trenches. Plate tectonic theory was presented in major textbooks as widely accepted, and discussions about speculative facts or processes were rare.

The transformation of geology to earth sciences: an example of the appropriation of technology by science

The widespread impression that technology is derived from science has been demonstrated previously to be incorrect. The example often chosen to make the case for technology preceding science are drawn from optics and mechanics: ancient technologies and old science. This paper describes a much more recent example. An examination of textbooks over the 20 years since the adoption of plate tectonics changed ‘geology’ into ‘earth sciences’ shows that the technological advances in echo‐sounding and submarine detection that played such a vital part in providing evidence for seafloor spreading has been progressively downplayed: as plate tectonics theory is applied more widely to earth's processes the theory's technological antecedents fade. This is particularly revealed in 1972 and 1992 versions of an Open University text Understanding the Earth. This incorporation of technology into earth sciences is accompanied by language changes that reduce the sense of personal involvement both by authors and readers. While it may represent the transition of what has been described as scientists’ private understandings into public knowledge it serves to divorce science from technology.

Evidence for seafloor spreading in the Laxmi Basin, northeastern Arabian Sea

Marine magnetic anomaly data from the Laxmi Basin for the first time reveal the presence of fairly correlatable NNW-trending magnetic lineations. These magnetic lineations are symmetric about a central negative magnetic anomaly and the axis of symmetry coincides with a characteristic short-wavelength free-air gravity low. The anomalies are interpreted as representing a two-limbed seafloor spreading sequence which can be equated to the A28–A33 interval of the geomagnetic polarity reversal timescale. These results suggest that the Laxmi Basin is underlain by an oceanic crust. Evidence of seafloor spreading in this basin possibly implies a pre-A27 spreading episode in the evolutionary history of the Arabian Sea.

Palaeomagnetic evidence relating to the cenozoic drift of Greenland

An analysis is made of the available palaeomagnetic data for the Early Tertiary basalts of Greenland. The Early Tertiary pole for Greenland, calculated from stable remanent magnetic directions of 81 lava flows on Disko Island, is located at 67.5° N,165° W, with dp = 5° and dm = 6°. The Disko pole is widely separated from the pole obtained for contemporaneous lava flows on Baffin Island, Canada, and this suggests a significantly large separation between Canada and Greenland as early as 65 m.y. ago. The Disko pole is also displaced from poles of similar age in northwestern Europe (British Isles and the Faeroe Islands). This is apparently inconsistent with the idea that the split of Greenland from Europe occurred concurrently with the widespread Early Tertiary igneous activity on the continental borderlands of the northeast Atlantic. The main results of the palaeomagnetic comparisons between eastern Canada, Greenland and North Europe are discussed in the light of available evidence for sea-floor spreading in the North Atlantic.

A Geophysical Study of Lake Kivu, East Africa

Summary The geomagnetic field over Lake Kivu, an East African rift valley lake, is generally characterized by gentle gradients, and small, isolated anomalies of 50-100 gammas amplitude. Lineated anomalies do not occur except in association with Cenozoic volcanics. In the Northern Basin, which is probably Pliocene in age, the well-bedded sediments average about 300 m in thickness, exceeding 500 m in places. Elsewhere in the lake, the sediment thickness averages only about 100 m. Sonobuoy refraction studies show that the basement underlying the sedimentary column is composed of material having a compressional wave velocity of 5.3–5.4 km s−1, probably representative of sialic plutonic or metamorphic rock. Gravity measurements show that the western branch of the East African rift is approximately the surface trace of a transition in deep structure across Africa. Bouguer gravity anomalies in the immediate vicinity of Lake Kivu show a strong negative correlation with elevation. The Bouguer anomaly versus elevation relationship, however, is rather different from that for the Gregory Rift or from East Africa in general, thus implying differences in structure. The geophysical data in Lake Kivu is consistent with the idea that it is at a stage of volcanic and hydrothermal activity following graben formation. Evidence for sea-floor spreading is not observed, but this tectonic process may be in a nascent stage of development.

Evidence for sea-floor spreading in the Cayman Trough

New seismic reflection and bathymetric data reveal that the rough topography in the central quarter of the Cayman Trough consists predominantly of north-south oriented ridges and valleys. Ridges with a relief of 200–2,000 m lie on either side of a north-south trending V-shaped valley which extends from the north to the south wall of the trough. We suggest that this valley and the flanking ridges represent an axis of sea-floor spreading for which we propose the name Mid-Cayman Rise. Although overall depths are greater for the Mid-Cayman Rise (rift valley, 5,500–6,000 m; flanking ridges, 3,500–4,000 m) than are typical of the mid-oceanic rise, striking similarities in morphology and sediments are noted. Both rises exhibit an axial rift valley, rough sediment-free basement near the axis, increasing depth and increasing burial by sediments with distance from the axial rift, and symmetry in basement morphology about the rift. Earthquake epicenters are clustered near the ends of the rift, where it intersects linear deeps along the north and south walls of the trough. The rift in the Cayman Trough is approximately equidistant from the two ends of the trough, suggesting that the Mid-Cayman Rise may have been spreading symmetrically for a long time, in response to relative motion between the Americas and Caribbean plates. Our interpretations impose important constraints on tectonic models for the Cenozoic history of plate motions in the Caribbean region.

GEOLOGY AND GEOPHYSICS IN THE DISCOVERY OF GIANT OIL FIELDS

The paper gives the case histories of discovery in three major oil field areas-Iran, northern Alaska and the North Sea. These areas differ in their regional features and in the consequent requirements for geological and geophysical investigation and delineation. In Iran the earlier discoveries were based entirely on surface geology; geophysics became important with the need for deep survey. In northern Alaska structures mapped at surface gave only minor shows. Seismic surveys following geological deductions on reservoir development led to discovery of the giant Prudhoe Bay field near the Arctic coast. In the North Sea, in the much more complex extension of Permian-Mesozoic basins already well known on land, location of structures has been entirely by seismic survey.There is no close relation between the new Global Tectonics and the location of major oil field belts. The factors controlling the latter are multiple and complex. Resolution of the relative importance of vertical (epeirogenic) displacement as against the effect of lateral plate movement is nevertheless critical, particularly in Alaska.There is at present a large unresolved discrepancy between the times at which some continental margins developed (North Atlantic, East and West Indian Ocean) and with the physical evidence of sea floor spreading. In the three areas quoted, breakdown of the continental margins by vertical movements started in the Permian (—280 million years), but spreading is dated as beginning only in the late Cretaceous (—70 million years). It is the earlier movements, unexplained by current global theories, which have most relevance for petroleum exploration.

Progressive Weathering of Submarine Basalt with Age: Further Evidence of Sea-Floor Spreading

Densities of layer 2 basalt recovered during the Deep Sea Drilling Project have been found to decrease steadily with age, a finding ascribed to progressive submarine weathering in the context of sea-floor spreading. The least-squares solution for 52 density measurements gives a rate of decrease in density of Δρ/Δ t = −0.46 × 10 −2 g per cm 3 m.y. = −16 percent per 100 m.y., which is in excellent agreement with earlier estimates based on observed chemical depletion rates of dredged oceanic basalt. Weathering of sea-floor basalt, should it penetrate to any considerable depth in layer 2, will decrease layer 2 seismic refraction velocities, act as a source of geothermal heat, and substantially influence the chemistry of sea water and the overlying column of sediment.