1.2.5 Sea-floor spreading

One of the principal stumbling blocks of continental drift was the inability to explain the mechanism by which drift took place. Wegener had invoked forces related to gravity and the Earth's rotation, which were demonstrably much

Constructive and destructive plate margins may consist of many segments linked by horizontal faults. A crucial step in the development of plate tectonic theory was made in 1965 by a Canadian geologist, J. Tuzo Wilson, who rec­ognized that these faults are not conventional transcurrent faults. They belong to a new class of faults, which Wilson called transform faults. The relative motion on a transform fault is opposite to what might be inferred from the off­sets of bordering ridge segments. At the point where a transform fault meets an oceanic ridge it transforms the spreading on the ridge to horizontal shear on the fault. Likewise, where such a fault meets a destructive plate margin it transforms subduction to horizontal shear.

The transform faults form a conservative plate margin, where lithosphere is neither created nor destroyed; the boundary separates plates that move past each other horizontally. This interpretation was documented in 1967 by too weak to drive the continents through the resistant basaltic crust. A. Holmes proposed a model in 1944 that closely resembles the accepted plate tectonic model (Holmes, 1965). He noted that it would be necessary to remove basaltic rocks continuously out of the path of an advancing continent, and suggested that this took place at the ocean deeps where heavy eclogite "roots" would sink into the mantle and melt. Convection currents in the upper mantle would return the basaltic magma to the con­tinents as plateau basalts, and to the oceans through innu­merable fissures. Holmes saw generation of new oceanic crust as a process that was dispersed throughout an ocean basin. At the time of his proposal the existence of the system of oceanic ridges and rises was not yet known.

The important role of oceanic ridges was first recognized by H. Hess in 1962. He suggested that new oceanic crust is generated from upwelling hot mantle material at the ridges. Convection currents in the upper mantle would rise to the surface at the ridges and then spread out laterally. The continents would ride on the spreading mantle material, carried along passively by the convec­tion currents. In 1961 R. Dietz coined the expression "sea-floor spreading" for the ridge process. This results in the generation of lineated marine magnetic anomalies at the ridges, which record the history of geomagnetic polarity reversals. Study of these magnetic effects led to the verification of sea-floor spreading.

1.2.5.1 The Vine-Matthews-Morley hypothesis

Paleomagnetic studies in the late 1950s and early 1960s of radiometrically dated continental lavas showed that the geomagnetic field has changed polarity at irregular time intervals. For tens of thousands to millions of years the polarity might be normal (as at present), then unaccountably the poles reverse within a few thousand years, so that the north magnetic pole is near the south geographic pole and the south magnetic pole is near the north geographic pole. This state may again persist for a long interval, before the polarity again switches. The ages of the rever­sals in the last 5 million years have been obtained radio-metrically, giving an irregular but dated polarity sequence.

A magnetic anomaly is a departure from the theoretical magnetic field at a given location. If the field is stronger than expected, the anomaly is positive; if it is weaker than expected, the anomaly is negative. In the late 1950s magnetic surveys over the oceans revealed remarkable striped patterns of alternately positive and negative magnetic anomalies over large areas of oceanic crust (Fig. 1.13), for which conventional methods of interpretation gave no satisfactory account. In 1963 the English geophysicists F. J. Vine and D. H. Matthews and, independently, the Canadian geologist L. W. Morley, formulated a landmark hypothesis that explains the origin of the oceanic mag­netic anomaly patterns (see also Section 5.7.3).

Fig. 1.13 Symmetric striped pattern of magnetic anomalies on the Reykjanes nt of the Mid-Atlantic Ridge southwest of Iceland. The positive anomalies are shaded according to their age, as indicated in the vertical column (after Heirtzler et al., 1966).

Observations on dredged samples had shown that basalts in the uppermost oceanic crust carry a strong remanent magnetization (i.e., they are permanently magnetized, like a magnet). The Vine-Matthews-Morley hypothesis integrates this result with the newly acquired knowledge of geomagnetic polarity reversals and the Hess-Dietz concept of sea-floor spreading (Fig. 1.14). The basaltic lava is extruded in a molten state. When it solidifies and its temperature cools below the Curie temperature of its magnetic minerals, the basalt becomes strongly magnetized in the direction of the Earth's magnetic field at that time.

Along an active spreading ridge, long thin strips of magne­tized basaltic crust form symmetrically on opposite sides of the spreading center, each carrying the magnetic imprint of the field in which it formed. Sea-floor spreading can persist for many millions of years at an oceanic ridge. During this time the magnetic field changes polarity many times, forming strips of oceanic crust that are magnetized alternately parallel and opposite to the present field, giving the observed patterns of positive and negative anomalies. Thus, the basaltic layer acts like a magnetic tape recorder, preserving a record of the changing geomagnetic field polarity.

Fig. 1.14 Upper, observed and computed marine magnetic anomalies, in nanotesla (nT), across the Pacific-Antarctica ridge, and (lower) their interpreted origin in terms of the Vine-Matthews-Morley hypothesis (after Pitman and Heirtzler, 1966).