Introduction

By the late 1950s much evidence for continental drift had been assembled, but the theory was not generally accepted. Up to this time, work had concentrated upon determining the pre-drift configurations of the continents and assessing their geologic consequences. The paths by which the continents had attained their present positions had not been determined. In order to study the kinematics of continental drift it was necessary to study the regions now separating once juxtaposed continents. Consequently, at this time interest moved from the continents to the intervening ocean basins.

Any kind of direct observation of the sea floor, such as drilling, dredging, or submersible operations, is time consuming, expensive, and provides only a low density of data. Much of the information available over oceanic areas has therefore been provided by geophysical surveys undertaken from ships or aircraft. One such method involves measuring variations in the strength of the Earth's magnetic field. This is accomplished using either fluxgate, proton precession, or optical absorption magnetometers, which require little in the way of orientation so that the sensing element can be towed behind the ship or aircraft at a sufficient distance to minimize their magnetic effects. In this way total field values are obtained which are accurate to ±1 nanotesla (nT) or about 1 part in 50,000. Magnetometers provide a virtually continuous record of the strength of the geomagnetic field along their travel paths. These absolute values are subsequently corrected for the externally induced magnetic field variations which give rise to a diurnal effect, and the regional magnetic field arising from that part of the magnetic field generated in the Earth's core. In theory the resulting magnetic anomalies should then be due solely to contrasts in the magnetic properties of the underlying rocks. The anomalies originate from the generally small proportion of ferromagnetic minerals (Section 3.6.2) contained within the rocks, of which the most common is magnetite. In general, ultramafic and mafic rocks contain a high proportion of magnetite and thus give rise to large magnetic anomalies. Metamorphic rocks are moderately magnetic and acid igneous and sedimentary rocks are usually only weakly magnetic. A full account of the magnetic surveying method is given in Kearey et al. (2002).

On land, magnetic anomalies reflect the variable geology of the upper continental crust. The oceanic crust, however, is known to be laterally uniform (Section 2.4.4) and so unless the magnetic properties are heterogeneous it would be expected that marine magnetic anomalies would reflect this compositional uniformity.

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