Surface exposure dating

Surface exposure dating is a collection of geochronological techniques for estimating the length of time that a rock has been exposed at or near Earth's surface. Surface exposure dating is used to date glacial advances and retreats, erosion history, lava flows, meteorite impacts, rock slides, fault scarps, and other geological events. It is most useful for rocks which have been exposed for between 10 years and 30,000,000 years.

The most common of these dating techniques is Cosmogenic radionuclide dating. Earth is constantly bombarded with primary cosmic rays, high energy charged particles — mostly protons and alpha particles. These particles interact with atoms in atmospheric gases, producing a cascade of secondary particles that may in turn interact and reduce their energies in many reactions as they pass through the atmosphere. By the time the cosmic ray cascade reaches the surface of Earth it is primarily composed of neutrons. When one of these particles strikes an atom it can dislodge one or more protons and/or neutrons from that atom, producing a different element or a different isotope of the original element. In rock and other materials of similar density, most of the cosmic ray flux is absorbed within the first meter of exposed material in reactions that produce new isotopes called cosmogenic nuclides. At Earth's surface most of these nuclides are produced by neutron spallation. Using certain cosmogenic radionuclides, scientists can date how long a particular surface has been exposed, how long a certain piece of material has been buried, or how quickly a location or drainage basin is eroding. The basic principle is that these radionuclides are produced at a known rate, and also decay at a known rate. Accordingly, by measuring the concentration of these cosmogenic nuclides in a rock sample, and accounting for the flux of the cosmic rays and the half-life of the nuclide, it is possible to estimate how long the sample has been exposed to the cosmic rays. The cumulative flux of cosmic rays at a particular location can be affected by several factors, including elevation, geomagnetic latitude, the varying intensity of the Earth's magnetic field, solar winds, and atmospheric shielding due to air pressure variations. Rates of nuclide production must be estimated in order to date a rock sample. These rates are usually estimated empirically by comparing the concentration of nuclides produced in samples whose ages have been dated by other means, such as radiocarbon dating, thermoluminescence, or optically stimulated luminescence.

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