A Discussion on volcanism and the structure of the Earth - Proposals concerning the variation of volcanic products and processes within the oceanic environment

1972 ◽  
Vol 271 (1213) ◽  
pp. 131-140 ◽  
Keyword(s):  
Structural Model ◽  
Early Stage ◽  
Volcanic Rocks ◽  
Fracture Zones ◽  
Sea Floor ◽  
Stage Of Development ◽  
Incompatible Elements ◽  
The Earth ◽  
Sea Floor Spreading ◽  
Low Pressures

An attempt is made to fit available petrochemical data on oceanic volcanic rocks into the structural model for the ocean basins presented by the plate tectonic theory. It is suggested that there are three major volcanic regimes: (i) the low-potassic olivine tholeiite association of the axial zones of the oceanic ridges where magmatic liquids are generated at low pressures high in the mantle, (ii) the alkalic (Na > K) associations along linear fractures where liquids generated at greater depth gain easy egress to the surface, (iii) those alkalic associations, rich in incompatible elements, of island groups, remote from fracture zones, where magmas created at depth proceed slowly to the surface and in consequence suffer intense fractionation. There are certain discrepancies in this pattern, notably that there is no apparent relation between rate of sea-floor spreading and degree of over-saturation of the axial zone basalts and that certain areas, such as Iceland, are characterized by excess volcanism. Explanation of these anomalies is sought by examining an oceanic area in an early stage of development—the Red Sea. It is tentatively suggested that the initial split of a contiguous continent might be brought about by the linking of profound fractures, caused by domal uplift related to rising isolated lithothermal systems, and that the present anomalies in oceanic volcanism may reflect the variation in rate of thermal convection within the original isolated lithothermal plumes.

The role of fracture zones in sea floor spreading

1973 ◽  
Vol 78 (32) ◽  
pp. 7776-7785 ◽  
Author(s):  
Christopher G. A. Harrison ◽  
Mahlon M. Ball
Keyword(s):  
Fracture Zones ◽  
Sea Floor ◽  
Sea Floor Spreading

scholarly journals The Temple of Apollo at Delphi

1888 ◽  
Vol 9 ◽  
pp. 282-322 ◽  
Author(s):  
J. Henry Middleton
Keyword(s):  
Early Stage ◽  
Pure Water ◽  
Stage Of Development ◽  
The Earth ◽  
The Mind ◽  
The Temple

In many respects Delphi and its varied cults possess an interest which is not to be rivalled by that of any other Hellenic site. The lofty precipices, the dark deeply-cleft ravines, the mysterious caves, and the bubbling springs of pure water, combine to give the place a romantic charm and a fearfulness of aspect which no description can adequately depict.Again Delphi stands alone in the catholic multiplicity of the different cults which were there combined.In primitive times it was the awfulness of Nature which impressed itself on the imaginations of the inhabitants.In an early stage of development the mind of man tends to gloomy forms of religion: his ignorance and comparative helplessness tend to fill his brain with spiritual terrors and forebodings. Thus at Delphi the primitive worship was that of the gloomy Earth and her children, the chasm-rending Poseidon, and the Chthonian Dionysus, who, like Osiris, was the victim of the evil powers of Nature. It was not till later times that the bright Phoebus Apollo came to Delphi to slay the earth-born Python, just as the rising sun dissipates the shadows in the depths of the Delphian ravines, or as in the Indian legend the god Indra kills with his bright arrows the great serpent Ahi—symbol of the black thunder-cloud.

Early Paleozoic Volcanism and Metamorphism of the Moretons Harbour–Twillingate Area, Newfoundland

1973 ◽  
Vol 10 (9) ◽  
pp. 1363-1379 ◽  
Author(s):  
D. F. Strong ◽  
J. G. Payne
Keyword(s):  
Oceanic Crust ◽  
Volcanic Rocks ◽  
Metal Sulfides ◽  
Early Paleozoic ◽  
Extensive Area ◽  
Sea Floor ◽  
Pillow Lavas ◽  
Intense Deformation ◽  
Sea Floor Spreading ◽  
Harbour Area

In the Moretons Harbour area, at the eastern end of the Lushs Bight terrane of central Newfoundland, the volcanic rocks of the "Lushs Bight Supergroup" are divided into two new groups, viz, the Moretons Harbour Group and the Chanceport Group. The former is separable into four formations, consisting primarily of variable proportions of basaltic pillow lavas and volcanoclastic sediments, with a composite thickness in excess of 6 km, or around 8 km including an extensive area of 'sheeted' diabase dikes. These formations are steeply dipping and face southwest; they are separated by the Chanceport fault from the Chanceport Group to the south. The latter consists of interbedded basaltic pillow lavas with graywackes and banded red and green cherts, all facing north and steeply dipping to overturned, with a composite thickness of approximately 3 km.The Moretons Harbour Group has been intruded by the Twillingate trondhjemitic granite–granodiorite pluton and abundant basic dikes intrude the granite, indicating that the mafic and felsic magmatism were coeval. Both have undergone intense deformation and the volcanics show a change from greenschist to amphibolite facies mineralogy within a distance of 2 km on approaching the pluton, a result of buttressing by the pluton during deformation, and not an intrusive effect.Base metal sulfides are common throughout the area, but the main occurrences of Cu, As, Sb, and Au are concentrated in the Little Harbour Formation, a 2600 m thick sequence of volcanoclastic rocks within the Moretons Harbour Group.The great thickness of volcanic rocks is interpreted as having formed in an island arc environment, although it is possible that the lowermost parts of the sequence represent oceanic crust. It is unlikely that the sheeted diabases of the Moretons Harbour area were produced by sea-floor spreading.

Geochemistry and origin of volcanic rocks from Tuzo Wilson and Bowie seamounts, northeast Pacific Ocean

1985 ◽  
Vol 22 (11) ◽  
pp. 1609-1617 ◽  
Author(s):  
Brian L. Cousens ◽  
R. L. Chase ◽  
J.-G. Schilling
Keyword(s):  
Mantle Plume ◽  
Mantle Source ◽  
Early Stage ◽  
Source Region ◽  
Volcanic Rocks ◽  
Major And Trace Elements ◽  
Spreading Ridge ◽  
Alkali Basalts ◽  
Stage Of Development ◽  
Juan De Fuca

The origin of the Tuzo Wilson Seamounts, 50 km south of the Queen Charlotte Islands, has been ascribed by various workers to either the Pratt–Welker mantle plume, which has formed the Pratt–Welker seamount chain, or the formation of a new segment of the Explorer–Juan de Fuca spreading ridge system. Abundances of major and trace elements in dredged alkali basalts from Tuzo Wilson and Bowie seamounts (360 km northwest of Tuzo Wilson Seamounts) are typical of alkaline volcanism on ocean islands associated with mantle plumes, but 87Sr/86Sr ratios (0.70252–0.70258) fall within the range of mid-ocean ridge basalts (MORB) from the Explorer and Juan de Fuca ridges. Geochronological and chemical data from the Pratt–Welker, Bowie, and Tuzo Wilson seamounts suggest that the Tuzo Wilson Seamounts are in an early stage of development as a result of activity of the Pratt–Welker mantle plume but that contributions from both a depleted and an undepleted mantle source are necessary to reconcile trace-element and Sr isotope values. Modelling of rare-earth behaviour during partial melting indicates that neither the Tuzo Wilson nor Bowie basalts could be generated from a mantle source similar to that of the Explorer or Juan de Fuca MORB, unless recent metasomatism has enriched the seamounts' source region in incompatible elements.

Introductory remarks

1980 ◽  
Vol 297 (1431) ◽  
pp. 137-138
Keyword(s):  
Mantle Convection ◽  
Plate Tectonics ◽  
Sharp Change ◽  
Indirect Approach ◽  
Sea Floor ◽  
Wave Velocities ◽  
The Earth ◽  
Magnesium Silicates ◽  
Sea Floor Spreading ◽  
Seismic Discontinuity

The substratum of the Earth, as Arthur Holmes originally described it, now generally known as the mantle , is the envelope, mainly of magnesium silicates, surrounding the fluid metallic core. It is separated from the continental and oceanic crusts which overlie it by the Mohorovicic seismic discontinuity, where there is a sharp change from earthquake wave velocities less than 7.2 km s -1 above to 7.8-8.1 km s -1 below. The thickness of the envelope is of the order of 2900 km, compared with about 4 km for ocean crust and 30 km for unthickened continental crust. Much attention has been devoted by geophysicists to the properties of the mantle, particularly in the course of the Geodynamics Project of I.U.G.G./I.U.G.S., during which important conclusions regarding sea floor spreading, plate tectonics and mantle convection have been reached. The fact that the overwhelming bulk of the mantle is not, and never will be, accessible for direct collection has perhaps resulted in less interest so far from the geochemical side. Accepting, however, that a partly indirect approach is inevitable, the time is now ripe for a thorough examination of the contribution that geochemical techniques can make.

A discussion on the structure and evolution of the Red Sea and the nature of the Red Sea, Gulf of Aden and Ethiopia rift junction - Introductory remarks

1970 ◽  
Vol 267 (1181) ◽  
pp. 5-7 ◽  
Keyword(s):  
Red Sea ◽  
World Ocean ◽  
The Other ◽  
Rift System ◽  
Gulf Of Aden ◽  
Sea Floor ◽  
The Earth ◽  
The World ◽  
The Many ◽  
Sea Floor Spreading

The Red Sea Discussion Meeting originated in the desire of the other organizers to bring together as many as possible of the earth scientists who have been working recently in that area to examine the latest evidence and ideas on its structure and origin, to see how they accord with modern continental and sea-floor spreading concepts. The Red Sea, Gulf of Aden and Afar crustal depressions, now known to be continuous with the extension of the world ocean rift system, have been claimed as a manifestation of crustal separation, but some Earth scientists still consider that the evidence can be explained by less drastic crustal rifting. Definite solutions to the many outstanding problems were not expected but discussions would clearly assist further researches.

scholarly journals A Geologist Reflects on a Long Career

2018 ◽  
Vol 46 (1) ◽  
pp. 1-20 ◽  
Author(s):  
Dan MKenzie
Keyword(s):  
Plate Tectonics ◽  
Continental Drift ◽  
Temperature Structure ◽  
Earth’S Gravity Field ◽  
Plate Motions ◽  
Sea Floor ◽  
Long Wavelength ◽  
Geological Processes ◽  
The Earth ◽  
Sea Floor Spreading

Fifty years ago Jason Morgan and I proposed what is now known as the theory of plate tectonics, which brought together the ideas of continental drift and sea floor spreading into what is probably their final form. I was twenty-five and had just finished my PhD. The success of the theory marked the beginning of a change of emphasis in the Earth sciences, which I have spent the rest of my career exploring. Previously geophysicists had principally been concerned with using ideas and techniques from physics to make measurements. But the success of plate tectonics showed that it could also be used to understand and model geological processes. This essay is concerned with a few such efforts in which I have been involved: determining the temperature structure and rheology of the oceanic and continental lithosphere, and with how mantle convection maintains the plate motions and the long-wavelength part of the Earth's gravity field. It is also concerned with how such research is supported.

The structure of the oceanic lithosphere

1971 ◽  
Vol 268 (1192) ◽  
pp. 619-619
Keyword(s):  
Oceanic Crust ◽  
Upper Mantle ◽  
Oceanic Lithosphere ◽  
Ocean Floor ◽  
Mass Motion ◽  
Sea Floor ◽  
Ocean Ridges ◽  
Velocity Gradients ◽  
The Earth ◽  
Sea Floor Spreading

Sea-floor spreading requires that new ocean floor be generated at mid-ocean ridges and that along with the underlying oceanic crust it move laterally away from its site of generation. In so far as it is unlikely that the 5 km thick oceanic crust moves independently of the underlying upper mantle, the horizontal mass motion associated with spreading extends at least some way into the mantle. The lithosphere is the crust and that part of the upper mantle to which it is mechanically coupled; together they form the brittle and relatively ‘strong’ outermost part of the Earth; velocity gradients within the lithosphere are negligible.

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