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.
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.
Cartoon illustrating cosmogenic nuclide exposure ages. A glacier transports an erratic boulder, and then recedes, exposing it to cosmic rays.
Trimlines can therefore also be used to reconstruct past ice sheet thickness.
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.
Beryllium-10 (Be) does not occur naturally in quartz, and once it forms following spallation it becomes trapped by quartz’s regular crystal lattice.
For a rock to be suitable for cosmogenic nuclide dating, quartz must occur in the rock in sufficient quantities and in the sufficient size fraction.