instrumental in formulating the hypothesis of seafloor spreading

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• Paleomagnetism, polar wandering, and continental drift
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• Hess’s seafloor-spreading model

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Hess ’s seafloor-spreading model

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The existence of these three types of large, striking seafloor features demanded a global rather than local tectonic explanation. The first comprehensive attempt at such an explanation was made by Harry H. Hess of the United States in a widely circulated manuscript written in 1960 but not formally published for several years. In this paper, Hess, drawing on Holmes’s model of convective flow in the mantle, suggested that the oceanic ridges were the surface expressions of rising and diverging convective mantle flow, while trenches and Wadati-Benioff zones, with their associated island arcs , marked descending limbs. At the ridge crests, new oceanic crust would be generated and then carried away laterally to cool, subside, and finally be destroyed in the nearest trenches. Consequently, the age of the oceanic crust should increase with distance away from the ridge crests , and, because recycling was its ultimate fate, very old oceanic crust would not be preserved anywhere. This model explained why rocks older than 200 million years had never been encountered in the oceans, whereas the continents preserve rocks almost 4 billion years old.

Hess’s model was later dubbed seafloor spreading by the American oceanographer Robert S. Dietz . Confirmation of the production of oceanic crust at ridge crests and its subsequent lateral transfer came from an ingenious analysis of transform faults by Canadian geophysicist J. Tuzo Wilson . Wilson argued that the offset between two ridge crest segments is present at the outset of seafloor spreading. As each ridge segment generates new crust that moves laterally away from the ridge, the crustal slabs move in opposite directions along that part of the fracture zone that lies between the crests. In the fracture zones beyond the crests, adjacent portions of crust move in parallel (and are therefore aseismic—that is, do not have earthquakes) and are eventually consumed in a subduction zone . Wilson called this a transform fault and noted that on such a fault the seismicity should be confined to the part between ridge crests, a prediction that was subsequently confirmed by an American seismologist, Lynn R. Sykes.

The Vine-Matthews hypothesis

In 1961 a magnetic survey of the eastern Pacific Ocean floor off the coast of Oregon and California was published by two geophysicists, Arthur D. Raff and Ronald G. Mason. Unlike on the continents, where regional magnetic anomaly patterns (that is, magnetic patterns that deviate from the common rule) tend to be confused and seemingly random, the seafloor possesses a remarkably regular set of magnetic bands of alternately higher and lower values than the average values of Earth’s magnetic field. These positive and negative anomalies are strikingly linear and parallel with the oceanic ridge axis, show distinct offsets along fracture zones, and generally resemble the pattern of a zebra skin. The axial anomaly tends to be higher and wider than the adjacent ones, and in most cases the sequence on one side is the approximate mirror image of that on the other.

The magnetic patterns that were observed on the seafloor defied explanation until the 1960s. Ironically, the clue for understanding these patterns came from the analysis of the magnetic properties of basaltic rocks on land. Basaltic lavas were extruded in rapid succession in a single locality on land and showed that the north and south poles had apparently repeatedly interchanged. This could be interpreted in one of two ways—either the rocks must have somehow reversed their magnetism , or the polarity of Earth’s magnetic field must periodically reverse itself. Allan Cox of Stanford University and Brent Dalrymple of the United States Geological Survey collected magnetized samples around the world and showed that they displayed the same reversal at the same time, implying that the polarity of Earth’s magnetic field periodically reversed. These studies established a sequence of reversals dated by isotopic methods .

Assuming that the oceanic crust is indeed made of basalt intruded in an episodically reversing geomagnetic field, Drummond H. Matthews of the University of Cambridge and a research student, Frederick J. Vine , postulated in 1963 that the new crust would have a magnetization aligned with the field at the time of its formation . If the magnetic field was normal, as it is today, the magnetization of the crust would be added to that of Earth and produce a positive anomaly. If intrusion of new magma had taken place during a period of reverse magnetic polarity, it would subtract from the present field and appear as a negative anomaly. Subsequent to intrusion, each new block created at a spreading centre would split, and the halves, in moving aside, would generate the observed bilateral magnetic symmetry. The widths of individual anomalies should correspond to the intervals between magnetic reversals.

In 1966, correlation of magnetic traverses from different oceanic ridges demonstrated an excellent correspondence with the magnetic polarity-reversal timescale established by Cox and Dalrymple on land. This reversal timescale went back some 3 million years, but since then further extrapolation based on marine magnetic anomalies (confirmed by deep-sea drilling) has extended the magnetic anomaly timescale far into the Cretaceous Period (145 million to 66 million years ago) Subsequent research in the 21st century suggests that the oldest oceanic crust may have been formed during the middle of the Jurassic Period (174.1 million to 163.5 million years ago).

About the same time, Canadian geologist Laurence W. Morley, working independently of Vine and Matthews, came to the same explanation for the marine magnetic anomalies . Publication of his paper was delayed by unsympathetic referees and technical problems and occurred long after Vine’s and Matthews’s work had already firmly taken root.

ENCYCLOPEDIC ENTRY

Seafloor spreading.

Seafloor spreading is a geologic process in which tectonic plates—large slabs of Earth's lithosphere—split apart from each other.

Earth Science, Geology, Meteorology, Geography, Physical Geography

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Seafloor spreading is a geologic process in which tectonic plates —large slabs of Earth's lithosphere —split apart from each other. Seafloor spreading and other tectonic activity  processes are the result of mantle convection . Mantle convection is the slow, churning motion of Earth’s mantle . Convection currents carry heat from the lower mantle and core to the lithosphere . Convection currents also “recycle” lithospheric materials back to the mantle .

Seafloor spreading occurs at divergent plate boundaries. As tectonic plates slowly move away from each other, heat from the mantle’s convection currents makes the crust more plastic and less dense . The less-dense material rises, often forming a mountain or elevated area of the seafloor. Eventually, the crust cracks. Hot magma fueled by mantle convection bubbles up to fill these fractures and spills onto the crust. This bubbled-up magma is cooled by frigid seawater to form igneous rock . This rock ( basalt ) becomes a new part of Earth’s crust.

Mid-Ocean Ridges

Seafloor spreading occurs along mid-ocean ridges—large mountain ranges rising from the ocean floor. The Mid-Atlantic Ridge , for instance, separates the North American plate from the Eurasian plate, and the South American plate from the African plate.

The East Pacific Rise is a mid-ocean ridge that runs through the eastern Pacific Ocean and separates the Pacific plate from the North American plate, the Cocos plate, the Nazca plate, and the Antarctic plate.

The Southeast Indian Ridge marks where the southern Indo-Australian plate forms a divergent boundary with the Antarctic plate.

Seafloor spreading is not consistent at all mid-ocean ridges. Slowly spreading ridges are the sites of tall, narrow underwater cliffs and mountains. Rapidly spreading ridges have a much more gentle slopes. The Mid-Atlantic Ridge, for instance, is a slow spreading center . It spreads 2-5 centimeters (.8-2 inches) every year and forms an ocean trench about the size of the Grand Canyon. The East Pacific Rise, on the other hand, is a fast spreading center . It spreads about 6-16 centimeters (3-6 inches) every year. There is not an ocean trench at the East Pacific Rise, because the seafloor spreading is too rapid for one to develop!

The newest, thinnest crust on Earth is located near the center of mid-ocean ridges—the actual site of seafloor spreading. The age, density, and thickness of oceanic crust increases with distance from the mid-ocean ridge.

Geomagnetic Reversals

The magnetism of mid-ocean ridges helped scientists first identify the process of seafloor spreading in the early 20th century. Basalt, the once- molten rock that makes up most new oceanic crust, is a fairly magnetic substance, and scientists began using magnetometers to measure the magnetism of the ocean floor in the 1950s. What they discovered was that the magnetism of the ocean floor around mid-ocean ridges was divided into matching “stripes” on either side of the ridge. The specific magnetism of basalt rock is determined by the Earth’s magnetic field when the magma is cooling. Scientists determined that the same process formed the perfectly symmetrical stripes on both side of a mid-ocean ridge. The continual process of seafloor spreading separated the stripes in an orderly pattern.

Geographic Features

Oceanic crust slowly moves away from mid-ocean ridges and sites of seafloor spreading. As it moves, it becomes cooler, more dense, and more thick. Eventually, older oceanic crust encounters a tectonic boundary with continental crust . In some cases, oceanic crust encounters an active plate margin . An active plate margin is an actual plate boundary, where oceanic crust and continental crust crash into each other. Active plate margins are often the site of earthquakes and volcanoes . Oceanic crust created by seafloor spreading in the East Pacific Rise, for instance, may become part of the Ring of Fire , the horseshoe-shaped pattern of volcanoes and earthquake zones around the Pacific ocean basin .

In other cases, oceanic crust encounters a passive plate margin . Passive margins are not plate boundaries, but areas where a single tectonic plate transitions from oceanic lithosphere to continental lithosphere. Passive margins are not sites of faults or subduction zones . Thick layers of sediment overlay the transitional crust of a passive margin. The oceanic crust of the Mid-Atlantic Ridge, for instance, will either become part of the passive margin on the North American plate (on the east coast of North America) or the Eurasian plate (on the west coast of Europe).

New geographic features can be created through seafloor spreading. The Red Sea, for example, was created as the African plate and the Arabian plate tore away from each other. Today, only the Sinai Peninsula connects the Middle East (Asia) with North Africa. Eventually, geologists predict , seafloor spreading will completely separate the two continents—and join the Red and Mediterranean Seas.

Mid-ocean ridges and seafloor spreading can also influence sea levels . As oceanic crust moves away from the shallow mid-ocean ridges, it cools and sinks as it becomes more dense. This increases the volume of the ocean basin and decreases the sea level. For instance, a mid-ocean ridge system in Panthalassa—an ancient ocean that surrounded the supercontinent Pangaea —contributed to shallower oceans and higher sea levels in the Paleozoic era . Panthalassa was an early form of the Pacific Ocean, which today experiences less seafloor spreading and has a much less extensive mid-ocean ridge system. This helps explain why sea levels have fallen dramatically over the past 80 million years.

Seafloor spreading disproves an early part of the theory of continental drift . Supporters of continental drift originally theorized that the continents moved (drifted) through unmoving oceans. Seafloor spreading proves that the ocean itself is a site of tectonic activity.

Keeping Earth in Shape

Seafloor spreading is just one part of plate tectonics . Subduction is another. Subduction happens where tectonic plates crash into each other instead of spreading apart. At subduction zones, the edge of the denser plate subducts, or slides, beneath the less-dense one. The denser lithospheric material then melts back into the Earth's mantle. Seafloor spreading creates new crust. Subduction destroys old crust. The two forces roughly balance each other, so the shape and diameter of the Earth remain constant.

Triple Junctions Seafloor spreading and rift valleys are common features at “triple junctions.” Triple junctions are the intersection of three divergent plate boundaries. The triple junction is the central point where three cracks (boundaries) split off at about 120° angles from each other. In the Afar Triple Junction, the African, Somali, and Arabian plates are splitting from each other. The Great Rift Valley and Red Sea (a major site of seafloor spreading) are the result of plate tectonics in the Afar Triple Junction.

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Seafloor Spreading

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Seafloor spreading is the mechanism by which new oceanic lithosphere is created at and moves away from the divergent plate boundaries known as mid-ocean ridges. The seafloor spreading hypothesis led to one of the most important paradigm shifts in the history of the Earth sciences, the plate tectonics scientific revolution.

Introduction

The revolutionary seafloor spreading hypothesis improved and subsumed the continental drift hypothesis, and rapidly culminated in what is now known as plate tectonic theory. First hypothesized by Harry Hess in a 1960 preprint and paper (Hess 1962 ), he considered so speculative he called it “an essay in geopoetry,” and named in another influential early paper (Dietz 1961 ), it offered a simple explanation for many problems with the prevailing paradigm that the Earth was a mostly static, slowly contracting planet, with fixed continents and old ocean basins, and no large-scale horizontal displacements. This paradigm had previously been...

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Hey, R.N. (2021). Seafloor Spreading. In: Gupta, H.K. (eds) Encyclopedia of Solid Earth Geophysics. Encyclopedia of Earth Sciences Series. Springer, Cham. https://doi.org/10.1007/978-3-030-58631-7_171

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