Magnetic anomaly

The magnetic variation (geomagnetic reversals) in successive bands of ocean floor parallel with mid-ocean ridges was important evidence for seafloor spreading, a concept central to the theory of plate tectonics.

If some such contaminant is overlooked, it may show up as a sharp spike in the anomaly, so such features are treated with suspicion.

The survey generally involves making a series of parallel runs at a constant height and with intervals of anywhere from a hundred meters to several kilometers.

[2]: 166 [3]: 81–83 In shipborne surveys, a magnetometer is towed a few hundred meters behind a ship in a device called a fish.

This is a large-scale, time-averaged mathematical model of the Earth's field based on measurements from satellites, magnetic observatories and other surveys.

If a remanence is present, it is difficult to separate from the induced magnetization unless samples of the rock are measured.

An algorithm developed by Talwani and Heirtzler(1964) (and further elaborated by Kravchinsky et al., 2019) treats both induced and remnant magnetizations as vectors and allows theoretical estimation of the remnant magnetization from the existing apparent polar wander paths for different tectonic units or continents.

[14][15] Magnetic surveys over the oceans have revealed a characteristic pattern of anomalies around mid-ocean ridges.

They involve a series of positive and negative anomalies in the intensity of the magnetic field, forming stripes running parallel to each ridge.

The source of these anomalies is primarily permanent magnetization carried by titanomagnetite minerals in basalt and gabbros.

As magma rises to the surface and cools, the rock acquires a thermoremanent magnetization in the direction of the field.