EARTH 520
Plate Tectonics and People

Allan Cox

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Figure 1. (Krauskopf 1997)

Biographical Information

Allan Cox was born on December 17, 1926.  He was a geophysicist that helped form our current understanding of plate tectonics. Allan Cox did not start out his academic career as a geologist.  Initially he  started college as a chemist.  He left school for a few years due to military service.  Upon his return , he took a summer job in Alaska.  This job convinced him that geology was the direction he wanted to pursue.  He died on January 27, 1987 in a bicycle accident.

Specific contributions to plate tectonic theory

He was very interested in the magnetic anomalies on the ocean floor.  He worked wih Richard Doell and Brent Dalrymple to date geomagnetic reversals.  They were the first to create geomagnetic polarity time scale.

Their work created a record of polarity intervals of the geomagnetic field over the past 5 Ma.  This record helped with the interpretation of the seafloor magnetic anomalies that were discovered to be parallel to mid ocean ridges.

The magnetism of the rocks on the seafloor can be measured in  a labratory or while at sea.  Ships are able to determine the polarity of the rocks by using a magnetometer (figure 2).

Figure 2. Left: Schematic drawing of the magnetic equipment used on ships.  The magnetometer is towed behind the ship. Right: Magnetometer (Missiaen, Feller 2008)

Allan Cox and his colleagues determined the magnetic orientation of continental volcanic rocks by using dating techniques based on isotopes of the chemical elements potassium and argon. 

Figure 3. Isotopes that Cox used for dating magentic polarity, Potassium and Argon (Cox 1969)

The potassium-argon method of rock dating recently began prior to Cox’s work on magnetism.  This new dating method was important to Cox because he needed to date rocks that ranged in ages from a few hundred thousand years to several million years.  Prior to potassium-argon there was not an accurate way to age rocks in this type of time range.

By calculating the age of sea-floor rocks, it became clear that rocks of the same age, regardless of their location, possessed the same magnetic polarity.  By using this dating method, Cox and his colleagues were also able to determine the duration for each particular polarity and when geomagnetic reversals occured. 

Rocks at mid ocean ridges generally show normal magnetic orientation, but there are belts parallel to the ridge that show a reverse direction (Figure 4).  As you measure the magnetism of the rocks going away from the mid ocean ridge, a stripped pattern occurs showing the polarity of the rocks.  As you get farther away from mid ocean ridges the age of the rocks increases.

The data generated from measuring the magentic orientations of the volcanic rocks created a stripped pattern of normal and reversed magnetic polarity on both sides of Mid Ocean ridges.

Their data suggested that there was a correlation between the ages of the Earth's magnetic reversals and the striping pattern.

Figure 4. Figure retrieved from (http://retosterricolas.blogspot.com/2012/03/seafloor-spreading-hypotheses-and.html).  The diagram shows the stripping pattern that is created as the magnetic polarity reverses.  The farther away the stripes get from the center, the older they are.

Other interesting scientific contributions

Allan Cox studied the effect that unrestricted logging can have on soil, streams, and watersheds, with a resulting increase in the danger of fires and landslides.  His research  helped create logging regulations which helped lessen the impact of logging on the local environment. 

Other cool stuff you should know

Allan Cox published two books on plate tectonics, Plate tectonics and Geomagnetic Reversals and Plate Tectonics, How it works. He also published over 100 scientific journal articles.

During his career he was elected to the National Academy of Sciences, American Philosophical Society, and the American Academy of the Arts and Sciences.  He was also the President of American Geophysical union. 

Throughout his career he won the Day medal of the Geological Society of America, Fleming medal of the American geophysical union, and the Vestlesen medal of Columbia university

Bibliography

Cox, A., (1969). Geomagnetic Reversals.  Science. January 17. Volume 163, Number 3864. 

Missiaen, T., Feller., P. (2008) Very high resolution seismic and magnetic investigations of a chemical munition dumpsite in the Baltic Sea.  Journal of Applied Geophysics.  Volume 65.  Issue 3-4, Pages 142-154

Krauskopf, K. B., (1997). A Biographical memoir: Allan V. Cox 1926-1987. National Academy of Sciences. National Academics Press. Washington D.C.

Watson, J.M., (1999). Magnetic Stripes and isotopic rocks.  Retrieved from (http://pubs.usgs.gov/gip/dynamic/stripes.html) on 2/1/13.