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However, this can be difficult, as thermal boundaries within the ice sheet may mean that it is more erosive lower down than higher up, and that cold, non-erosive ice on the tops of mountains may leave in tact older landscapes.Cosmogenic nuclide dating can also be used in this context to understand past ice-sheet thicknesses and changes in subglacial thermal regime.Many mountains have on them, and are smoothed and eroded below the trimline, and more weathered with more evidence of periglaciation above the trimline.Trimlines can therefore also be used to reconstruct past ice sheet thickness.
Cosmogenic nuclides are rare nuclides that form in surface rocks because of bombardment by high-energy cosmic rays.
Cartoon illustrating cosmogenic nuclide exposure ages. A glacier transports an erratic boulder, and then recedes, exposing it to cosmic rays.
Spallation reactions occur in minerals in the rocks upon bombardment by cosmic rays.
Glacial geologists are often interested in dating the maximum extents of glaciers or rates of recession, and so will look for boulders deposited on moraines.
Once exposed to the atmosphere, the boulder will begin to accumulate cosmogenic nuclides.
The basic principle states that a rock on a moraine originated from underneath the glacier, where it was plucked and then transported subglacially.