Seafloor Magnetic Anomalies

Seafloor magnetic anomalies are “stripes of alternating high and low magnetic intensity running parallel to ocean ridges” (“Sea-Floor Spreading”). These linear anomalies end at fracture zones and have distinctive shapes that occur in predictable patterns across fracture zones as trends. Two scientists from Great Britain, Frederick J. Vine and Drummond H. Matthews, and one Canadian scientist by the name of Lawrence W. Morley were the first to propose an explanation for these anomalies.

Their theory rests on the assumptions that the magnetic field of the earth reverses polarity on a periodic basis, that seafloor spreading is a fact, and that the crust of the ocean experiences permanent magnetization as it is formed and as it cools at the centers of seafloor spreading. Hence, the crust of the ocean must include records of reversals of the magnetic field as it forms during spreading (“Ocean,” 2009).

There are two kinds of seafloor magnetic anomalies: the stripes of high intensity or positive seafloor magnetic anomalies and the stripes of low intensity or negative magnetic anomalies (“Sea-Floor Spreading;” “Ocean”). (See Appendix). Positive magnetic anomalies are produced when the oceanic crust experiences magnetization “in a ‘normal’ polarity parallel to the ambient field of the Earth” (“Ocean”). Low intensity or negative magnetic anomalies are the result of the crust being “‘reversely’ magnetized in an opposite sense” (“Ocean”). As the crust that has been magnetized moves down the sides of the ridge and away from the center of seafloor spreading, it remains magnetized – and permanently so – while carrying the stripes along (“Ocean”).

Because the patterns of reversal are maintained in seafloor magnetic anomalies, scientists have gathered that these stripes are “in principle correlatable back to the age of the oldest sea floor” (McElhinny & McFadden, 1999, p. 149). So, if the age of magnetic field reversals is known, it is theoretically possible to calculate the approximate age of the oceanic crust once the corresponding stripes have been mapped. In fact, scientists have already put together schedules of magnetic field reversals for the past four to five million years. They have done this by studying the approximate age in addition to magnetic polarities of flows of lava.

The rate of ocean floor spreading has also been measured. In the case of the Juan de Fuca Ridge in the United States, the first oceanic floor spreading is said to have occurred at 30 mm per year (“Ocean”). Vine and J. Tuzo Wilson, a Canadian geologist, calculated this rate by “piding the distance of an anomaly from the ridge crest by the age of the anomaly twice” (“Ocean”). This formula was used to calculate the current rate of seafloor spreading to boot. It has been found that the Juan de Fuca Ridge moves at approximately 15 mm per year from the crest of the ridge at present, and at around 60 mm per year from the segment of the crust that is on the side opposite to the crest (“Ocean”).

Thus, seafloor magnetic anomalies appear as extraordinary phenomenon to use in further exploration of the ocean. Scientists believe that lava layers in the upper crust are the most important contributors to these stripes. But, seafloor magnetic anomalies may also originate in the gabbros, in particular their upper layers (“Ocean”). Further research on seafloor magnetic anomalies may shed greater light, therefore, not only on the age of oceans but also the rocks and minerals that make up the oceanic crust.

References

• McElhinny, M. W., & McFadden, P. L. (1999). Paleomagnetism: Continents and Oceans. 2nd Ed. St. Louis, MO: Academic Press.
• Ocean. (2009). Encyclopedia Britannica. Retrieved Mar 11, 2009, from http://www.britannica.com/EBchecked/topic/424285/ocean/67133/Investigations-of-the-oceanic-crust#ref=ref540716
• Seafloor magnetic stripes: look again. (2008). Retrieved Mar 11, 2009, from http://www.newgeology.us/presentation25.html
• Sea-Floor Spreading. (2008, Sep 11). Retrieved Mar 11, 2009, from http://www.geo.umass.edu/courses/geo105/Lecture-4.pdf.