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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.Ages of rock avalanche deposits throughout Norway cluster in the first few thousand years after deglaciation, however ages throughout the entire Holocene have also been obtained.The principles of dating sliding surfaces are more explained in the publication on the Oppstadhornet slide on Otrøya. 16.6 to 14.2 kyrs ago and past long-term displacement rates are in the order of 2 mm/yr.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.This explains their higher abundance in cosmic rays as compared with their ratios and abundances of certain other nuclides on Earth.This also explains the overabundance of the early transition metals just before iron in the periodic table; the cosmic-ray spallation of iron thus produces scandium through chromium on one hand and helium through boron on the other.

In NGU's projects on unstable rock slopes in Norway, TCN dating is used to date 1) rock-avalanche deposits and 2) sliding surfaces.Beryllium dating is used to estimate the time a rock has been exposed on the surface of the Earth, as well as erosion and sedimentation rates. Like carbon-14, most of it is formed in the earth’s upper atmosphere.After formation, beryllium-10 binds to atmospheric dust particles or dissolves in atmospheric water vapor.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 age of the surface exposure, that is, the surface could have been exposed for much longer than the minimum calculated age.

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