General features of magnetic anomalies
The earth's field reverses periodically. When this happens the north and south magnetic poles switch locations. The process takes place rapidly (in geological terms), in under about 5000 years. On average there are about 2-3 reversals per million years, but the rate varies considerably, from about 30 ky to 40 my for the long Cretaceous Quiet Zone with normal polarity.Anomalies are numbered :
When the magnetic anomalies are plotted on a map with dense ship tracks, the patterns reveal the ages of the seafloor in long linear strips parallel with the ridges. As profiles, we see positive anomalies over normally magnetized ocean crust (where they add to the intensity of the earth's field), and negative anomalies over the reversely magnetized crust (where they subtract from the intensity of the earth's field). These anomalies provided the convincing evidence for plate tectonics and the resulting revolution in the earth sciences.
In addition to the intensity of the field, we can measure two angular components of the field vector:
Measurement of the inclination and declination requires an oriented sample, and is not typically done for marine surveys. If the rocks are tilted, they must be restored to horizontal before analysis, and any inferences about location are good for the time the rock cooled below the Curie temperature. Plate motions may have moved the location significantly since the magnetism was locked in.
The marine magnetic time scale is relative. The anomalies are picked based on the reversals, without any knowledge of the absolute times. If later dating changes the times of the reversals, it does not change the relate order of the anomalies. If additional anomalies are discovered, as happened in the initial development of the time scale, it only changes the duration of the intervals by inserting new anomalies, but does not otherwise change the relative time scale.
|Eltanin 19 cruise track highlighted in black. This cruise collected one of the first profiles that convinced geoscientists that the plate tectonic model worked. The vessel sailed generally east to west. The other tracks in the region are shown in blue.|
|Magnetic anomalies for the entire cruise, over 4000 km long. The anomalies on the vertical axis are typically a few 100 nT.|
|Magnetic anomalies for the last quarter of the cruise, which proved conclusive to the acceptance of the role of magnetic anomalies to document sea floor spreading.|
|Long straight sections (such as 1000-1300 km, and 1400-2100 km) likely indicates gaps in recording and should be ignored.|
|EW9008 cruise (black) on the Mid Atlantic Ridge SW of Iceland, with a typical "mowing the lawn" survey pattern. Interpretation would require looking at each segment individually. Since the sailing direction also reverses, you would want to reverse alternate segments.|
|The results from a number of cruises can be combined into a grid. This requires filling in smaller holes, but allows profiles to be generated in any direction. Reds and magentas mark the highest positive anomalies, and blues the largest negative anomalies. A ridge runs diagonally through the map from the upper right to the lower left. Note that in addition to the pattern of stripes, symmetrical about the ridge, the intensities also vary in a systematic manner. This happens due to the varying intensity of magnetization and alteration, as well as differences in depth. The ridge crest, shallower in depth, produces a larger anomaly as sea level.|
|Note the long linear patterns running almost east-west in the left part of the map, and about 150º--340º in the right part. They join at about a 120º angle, typical of the angle at which three ridges would join in a triple junction such as that found in the Indian Ocean. The third ridge would have had an orientation about 30º, but is not visible in this area because this is older crust that moved away from the other two ridges.|
|Individual readings plotted on the map for cruise EW9008 on the Mid Atlantic Ridge SW of Iceland. Colors are for the anomaly value.|
Magnetic anomaly data sources
Marine magnetic profile interpretation. Includes time scale and anomaly names.
Quick Profile Exploration
Last revision 5/9/2016