seafloor spreading theory of lithospheric evolution that holds that the ocean floors are spreading outward from vast underwater ridges. First proposed in the early 1960s by the American geologist Harry H. Hess, its major tenets gave great support to the theory of continental drift and provided a conceptual base for the development of plate tectonics .

Discovery of the Mid-Ocean Ridges

Development of highly sophisticated seismic recorders and precision depth recorders in the 1950s led to the discovery in the early 1960s that the Mid-Atlantic Ridge, a vast, sinuous undersea mountain chain bisecting the Atlantic Ocean, was in fact only a small segment of a globe-girdling undersea mountain system some 40,000 mi (64,000 km) in length. In many locations, this mid-ocean ridge was found to contain a gigantic cleft, or rift, 20 to 30 mi (32–48 km) wide and c.1 mi (1.6 km) deep, extending along the crest of the ridge. The ridge itself does not form a smooth path, but is instead offset in many places. The offsets are called fracture zones, or transform faults . The ridge crest and its associated transform faults are the locus of nearly all shallow earthquakes occurring in mid-ocean areas. Continued study of the mid-ocean ridges is a major component of U.S. research in the global oceans.

Role of the Spreading Center

In 1962 Hess proposed that the seafloor was created at mid-ocean ridges, spreading in both directions from the ridge system. At the spreading center, liquid rock called basaltic magma rises from the earth's mantle as it upwells beneath the spreading axis. When the magma hardens, it forms new oceanic crust that becomes welded to the original crust. Spreading is believed to be caused by far-field stresses, and the upwelling of the mantle beneath the spreading axis is the passive response to plate separation. The oceanic trenches bordering the continents mark regions where the oldest oceanic crust is reabsorbed into the mantle through steeply inclined, earthquake-prone subduction zones. The pull of the deeply plunging lithosphere is one of the forces that may drive plate separation.

Supporting Evidence for Seafloor Spreading

Abundant evidence supports the major contentions of the seafloor-spreading theory. First, samples of the deep ocean floor show that basaltic oceanic crust and overlying sediment become progressively younger as the mid-ocean ridge is approached, and the sediment cover is thinner near the ridge. Second, the rock making up the ocean floor is considerably younger than the continents, with no samples found over 200 million years old, as contrasted with maximum ages of over 3 billion years for the continental rocks. This confirms that older ocean crust has been reabsorbed in ocean trench systems.

By the mid-1960s studies of the earth's magnetic field showed a history of periodic reversals in polarity (see paleomagnetism ). A timescale for "normal" and "reversed" polarity was established, showing 171 magnetic "flip-flops" in the past 76 million years. Magnetic surveys conducted near the mid-ocean ridge showed elongated patterns of normal and reversed polarity of the ocean floor in bands paralleling the rift and symmetrically distributed as mirror images on either side of it. The magnetic history of the earth is thus recorded in the spreading ocean floors as in a very slow magnetic tape recording, forming a continuous record of the movement of the ocean floors. Other supportive evidence has emerged from study of the fracture zones that offset the sections of the ridge.

Bibliography

See J. Coulomb, Sea Floor Spreading and Continental Drift (1972).

Sea-floor spreading

Earth's surface is composed of two kinds of crust , continental and oceanic. Most continental crust is over 3 billion years old, while virtually all oceanic crust is less than 180 million years old. Oceanic crust is young because it is continually destroyed in some places and created in others. Subduction is the process that destroys oceanic crust, and sea-floor spreading is the process that creates oceanic crust.

Sea-floor spreading is driven by crust formation along the mid-ocean ridges , meandering undersea mountain ranges that span Earth like the seams of a baseball. Oceanic crust is continually produced by magma welling up along the centerlines of the mid-ocean ridges. This new crust flows away from each ridgeline in two symmetric sheets, one on each side. The rate of sea-floor spreading resulting from this process is from 0.5 to 8 inches per year (1–20 cm/yr), depending on the particular mid-ocean ridge.

The Mid-Atlantic Ridge offers a particularly clear case of sea-floor spreading. About 165 million years ago, the Americas were matched to Africa and Europe like the pieces of a puzzle. Then, magma upwelling at the Mid-Atlantic Ridge began to produce oceanic crust, parting the continents to form the Atlantic Ocean. Today the Mid-Atlantic Ridge snakes down the center of the Atlantic all the way from Iceland to the Antarctic Plate and remains an active site of sea-floor spreading.

A dramatic proof of sea-floor spreading was discovered in the mid 1960s when data revealed alternating stripes of magnetic orientation on the sea floor, parallel to the mid-ocean ridges and symmetric across them—that is, a thick or thin stripe on one side of the ridge is always matched by a similar stripe at a similar distance on the other side. This mirror-image magnetic orientation pattern is created by steady sea-floor spreading combined with recurrent reversals of Earth's magnetic field . Iron atoms in liquid rock welling up along a mid-ocean ridge align with Earth's magnetic field. When this magma solidifies into crust, its iron atoms lock into position. This solid crust flows away from the mid-ocean ridge in both directions, carrying its original magnetic orientation with it. Eventually Earth's magnetic field reverses. Previously solidified crust retains its original field state, but crust just forming along the ridge is locked into the new orientation. As crust feeds steadily and symmetrically away from the ridgeline and Earth's magnetic field reverses over and over again, a symmetric striped pattern of magnetism is created.

See also Geographic and magnetic poles; Lithospheric plates; Magnetic field; Mantle plumes; Mapping techniques; Ocean trenches; Paleomagnetics; Plate tectonics

sea-floor spreading Sea-floor spreading is the process whereby new oceanic lithosphere is created as a pair of plates diverges at a mid-ocean ridge spreading centre. It was originally proposed by R. S. Dietz in 1961, and was an important component of the theory of plate tectonics that developed shortly afterwards.

At a mid-ocean ridge, two plates diverge, probably driven mainly by the pull of subducting slabs and the gravitational sliding of the ridge flanks (see plate tectonics). As the plates separate, ductile upper mantle rises from the underlying asthenosphere, and undergoes adiabatic decompression (decompression without loss or gain of heat) and consequent partial melting. The melt rises toward the surface, where it solidifies to form new oceanic crust, while the residual ductile mantle is drawn along with the diverging plates. As the residual mantle cools, it becomes more brittle and is effectively accreted to the lithosphere, gradually thickening it.

The newly formed oceanic crust (especially the extrusive basalts at its top) acquires a thermal remanent magnetization as it cools; and as the Earth's magnetic field episodically reverses direction, strips of crust are formed with alternating polarities of magnetization. These give rise to alternating positive and negative magnetic anomalies that are symmetrical about the spreading centre and can be used to date the oceanic crust, as was first suggested by Vine and Matthews in 1966.

Roger Searle

sea-floor spreading The theory that the ocean floor is created at the spreading (accretionary) plate margins within the ocean basins. Igneous rocks rise along conduits from the mantle, giving rise to volcanic activity in a narrow band along the mid-ocean ridges. As these cool, the basaltic lavas and dykes form the upper part of the oceanic crust, and the underlying magma chamber solidifies to form layer 3 of the oceanic crust. The newly formed oceanic crust spreads perpendicularly away from the ridge, probably in response to mantle convective motions (see PLATE TECTONICS). As the basalts originally cooled, they became magnetized by the ambient geomagnetic field. As this field reverses polarity, oceanic crust formed at different times is characterized by oceanic magnetic anomalies that are parallel to the ridge at which they originally formed (Vine and Matthews, 1963). These anomalies allow the dating of the oceanic crust and the determination of its past relative motion. The creation of new ocean floor was implicit in the previous concept of continental drift, but is mainly characterized by the narrowness of the zone within which the new ocean floor is formed. It is now a fundamental concept within the platetectonic theory.

sea-floor spreading The theory that the ocean floor is created at the spreading (accretionary) plate margins within the ocean basins. Igneous rocks rise along conduits from the mantle, giving rise to volcanic activity in a narrow band along the mid-ocean ridges. The newly formed oceanic crust spreads perpendicularly away from the ridge.

seafloor spreading Theory that explains how continental drift occurs. It proposes that the ocean floor is moved laterally as new basalt rock is injected along mid-ocean ridges, and so the ocean floor becomes older with increasing distance from the ridge. See also plate tectonics