2.3: Magnetic Stripes and Isotopic Clocks
Oceanographic exploration in the 1950s led to a much better understanding of the ocean floor. Among the new findings was the discovery of zebra stripe-like magnetic patterns for the rocks of the ocean floor. These patterns were unlike any seen for continental rocks. Obviously, the ocean floor had a story to tell, but what?
In 1962, scientists of the U.S. Naval Oceanographic Office prepared a report summarizing available information on the magnetic stripes mapped for the volcanic rocks making up the ocean floor. After digesting the data in this report, along with other information, two young British geologists, Frederick Vine and Drummond Matthews, and also Lawrence Morley of the Canadian Geological Survey, suspected that the magnetic pattern was no accident. In 1963, they hypothesized that the magnetic striping was produced by repeated reversals of the Earth's magnetic field, not as earlier thought, by changes in intensity of the magnetic field or by other causes. Field reversals had already been demonstrated for magnetic rocks on the continents, and a logical next step was to see if these continental magnetic reversals might be correlated in geologic time with the oceanic magnetic striping. About the same time as these exciting discoveries were being made on the ocean floor, new techniques for determining the geologic ages of rocks ("dating") were also developing rapidly.
An observed magnetic profile (blue) for the ocean floor across the East Pacific Rise is matched quite well by a calculated profile (red) based on the Earth's magnetic reversals for the past 4 million years and an assumed constant rate of movement of ocean floor away from a hypothetical spreading center (bottom). The remarkable similarity of these two profiles provided one of the clinching arguments in support of the seafloor spreading hypothesis.
A team of U.S. Geological Survey scientists -- geophysicists Allan Cox and Richard Doell, and isotope geochemist Brent Dalrymple -- reconstructed the history of magnetic reversals for the past 4 million years using a dating technique based on the isotopes of the chemical elements potassium and argon. The potassium-argon technique -- like other "isotopic clocks" -- works because certain elements, such as potassium, contain unstable,
parent
radioactive isotopes that decay at a steady rate over geologic time to produce
daughter
isotopes. The rate of decay is expressed in terms of an element's "half-life," the time it takes for half of the radioactive isotope of the element to decay. The decay of the radioactive potassium isotope (potassium-40) yields a stable daughter isotope (argon-40), which does not decay further. The age of a rock can be determined ("dated") by measuring the total amount of potassium in the rock, the amount of the remaining radioactive potassium-40 that has not decayed, and the amount of argon-40. Potassium is found in common rock-forming minerals, and because the potassium-40 isotope has a half-life of 1,310 million years, it can be used in dating rocks millions of years old.
Other commonly used isotopic clocks are based on radioactive decay of certain isotopes of the elements uranium, thorium, strontium, and rubidium. However, it was the potassium-argon dating method that unlocked the riddle of the magnetic striping on the ocean floor and provided convincing evidence for the seafloor spreading hypothesis. Cox and his colleagues used this method to date continental volcanic rocks from around the world. They also measured the magnetic orientation of these same rocks, allowing them to assign ages to the Earth's recent magnetic reversals. In 1966, Vine and Matthews -- and also Morley working independently -- compared these known ages of magnetic reversals with the magnetic striping pattern found on the ocean floor. Assuming that the ocean floor moved away from the spreading center at a rate of several centimeters per year, they found there was a remarkable correlation between the ages of the Earth's magnetic reversals and the striping pattern. Following their break-through discovery, similar studies were repeated for other spreading centers. Eventually, scientists were able to date and correlate the magnetic striping patterns for nearly all of the ocean floor, parts of which are as old as 180 million years.
Contributors and Attributions
W. Jacquelyne Kious and Robert Tilling (" The Dynamic Earth " via the U.S. Geological Survey )