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Cosmogenic exposure dating

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Text by Carrie Patterson, MGS

Cosmogenic isotopes are created when elements in the atmosphere or earth are bombarded by high energy particles (-mesons and protons, collectively known as cosmic rays) that penetrate into the atmosphere from outer space. Some cosmic ray particles reach the surface of the earth and contribute to the natural background radiation environment. It was discovered about a decade ago that cosmic ray interaction with silica and oxygen in quartz produced measurable amounts of the isotopes Beryllium-10 and Aluminium-26. Researchers suggested that the accumulation of these isotopes within a rock surface could be used to establish how long that surface was exposed to the atmosphere. Assuming a constant rate of production, the number of atoms of Be-10 and Al-26 that accumulate in a rock surface will be proportional to the length of time the rocks were exposed to cosmic ray bombardment and the respective rates of radioactive decay for each isotope. An age determined by measurement of the amount of each nuclide would be an estimate of the minimum time that the particular surface had been exposed, but would not date the maximum age of the surface exposure, that is, the surface could have been exposed for much longer than the minimum calculated age. Theoretically, exposures of surfaces from between a few thousand to about 10 million years old can be dated by the measurement of the Be-10 and Al-26 isotopes.