Cosmogenic nuclide burial dating
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.
These cosmic rays originate from high-energy supernova explosions in space.
Wherever we are on Earth, when we are outside, we are constantly bombarded by these cosmic rays.
Trimlines can therefore also be used to reconstruct past ice sheet thickness.
It is an excellent way of directly dating glaciated regions.
It is particularly useful in Antarctica, because of a number of factors: Cosmogenic nuclide dating is effective over short to long timescales (1,000-10,000,000 years), depending on which isotope you are dating.
Before sampling a rock, geologists must take detailed and careful measurements of the landsurface, and satisfy themselves that the rock is in a stable position, has not rolled, slipped downslope, been repeatedly buried and exhumed during periglacial rock cycling within the active layer (frequently a problem with small boulders), and has not been covered with large amounts of soil, snow or vegetation.
Geologists must also be sure that the rock has undergone sufficient glacial erosion to remove any rock that might have previously been exposed at the Earth’s surface, and so have accumulated cosmogenic nuclides.