Our Earth is constantly bombarded with high energy particles and cosmic rays.  These charged particles interact with the atoms in atmospheric gases, producing a cascade of secondary particles.  And you can use those for dating rocks! 

Surface Exposure Dating

Scientists have an array of geochronological techniques at their disposal for estimating the length of time a rock has been exposed at the Earth’s surface or buried near it.

Surface exposure dating is used to date all manners of geological events:

• • glacial advances and retreats,
  • erosion,
  • lava flows,
  • meteorite impacts,
  • landslides,
  • faults,
  • cave formations.

This technique is most useful for rocks which have been exposed to the surface for between 10³ and 10⁶ years.

Cosmogenic RadioNuclide Dating

The most common of these dating techniques is cosmogenic radionuclide dating or CRN.

Earth is constantly bombarded with primary cosmic rays, rich in 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 Earth’s atmosphere.

Isotopes

This cascade includes a small fraction of hadrons, including 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 rocks and materials of a similar density, most of the cosmic ray flux is absorbed within the first metre of exposed material in reactions that produce new isotopes called cosmogenic nuclides.

On the surface, most of these nuclides are produced by neutron spallation.

Spallation Reactions

Cosmogenic nuclides are produced by chains of spallation reactions.

Spallation describes the ejection of material from a target during impact by a projectile.

The reaction can occur with or without penetration of the target.

The production rate for a particular nuclide is a function of geomagnetic latitude, the amount of sky that can be seen from the point that is sampled, elevation, sample depth, and density of matter in which the sample is embedded.

Decay rates are given by decay constants of the radionuclides.

These equations can be combined to give a total concentration of cosmogenic radionuclides in a rock as a function of age.

Using certain cosmogenic radionuclides, scientists can discover:

• • how long a particular surface has been exposed,
  • how long a certain piece of material has been buried or
  • how quickly a drainage basin is eroding.

Half-Lives

The CRN dating method is based on the rate of accumulation of cosmic rays that stimulate the production and decay of radionuclides such as ¹⁴C, ¹⁰Be, ²⁶Al, and ³⁶Cl.

It takes the different rates of radioactive decay of multiple cosmogenic nuclides co-produced in a rock to determine the length of time that a previously exposed surface has been buried, and thus determine how long a rock sample has been shielded from cosmogenic nuclide production.

The half-life t^1/2 is the time required for a quantity to reduce to half its initial value.  It describes how quickly unstable atoms undergo, or how long stable atoms survive, radioactive decay.

However, a half-life usually describes the decay of discrete entities.

A half-life period is defined in terms of probability.

The basic principle is that these isotopes/radionuclides are produced at a known rate and also decay at a known rate.

Accordingly, 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, makes it possible to estimate how long the sample has been exposed to the cosmic rays.

Optically Stimulated Luminescence Thermochronometry

An Optically Stimulated Luminescence (OSL) signal is a function of trapped electrons in quartz (SiO₂) or feldspar.

Within the natural environment, these minerals store electron charges as they cool below their closure temperatures.

Irradiation of a quartz-like rock results from exposure to a radiation source, i.e. cosmic rays.

Natural Luminescence

Here geoscientists rely on the following rationale:

The luminescence is acquired in mineral grains through their exposure to ionizing radiation and the trapping of electrons.

The luminescence in the mineral grains is zeroed by exposure to sunlight due to erosion, but also transport.

With the cycles of burial and exposure to ionizing radiation, free electrons are stored in charge defects within grains crystal lattice.

Further light exposure of grains with erosion and transport zeroes the luminescence.

The grains are buried again and luminescence is acquired with exposure to ionizing radiation.

A “hole” is a virtual particle left behind when a valence electron leaves its position with a charge of e+.

Careful sampling without light exposure, and measuring of the natural luminescence, is followed by a normalizing test dose (Ln/Tn) compared to the regenerative dose to yield an equivalent dose (De).

Experimentation Method

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 isotopes produced in samples whose ages have been dated by other means, such as radiocarbon dating.

To calculate the age of a sedimentary deposit, a rock sample is collected, prepared and analysed to determine the luminescence of the quartz-rich extract and the sediment radioactivity.

While the outer few centimetres of the sediment are discarded, the rest of the sample is transported to the laboratory where it can be sieved and treated with an acid to extract the quartz grains.

Analysis of the sample is carried out in the dark.

Finally, a luminescence detector measures the dose absorbed during the burial stage.

Environmental Constraints

Of course, the cumulative flux of cosmic rays at a particular location can also 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.

The excess relative to natural abundance of cosmogenic nuclides in a rock sample is usually measured by means of accelerator mass spectrometry.

Beryllium (¹⁰Be) and Aluminium (²⁶Al)

The two most frequently measured cosmogenic nuclides are Beryllium-10 and Aluminium-26.

The Beryllium-10 isotope is created in the atmosphere by galactic cosmic rays.  Because the flux of such cosmic rays is affected by the intensity of the interplanetary magnetic field carried by the solar wind, the rate at which Beryllium-10 is created changes with solar activity.

These two radionuclides are particularly useful to geologists because they are produced when cosmic rays strike isotopes of Oxygen-16 and Silicon-28, respectively.

Production from Quartz (SiO₂)

The parent isotopes ¹⁶O and ²⁸Si are the most abundant of these elements and they occur commonly in the Earth’s crust, whereas the radioactive daughter nuclei ¹⁰Be and ²⁶Al are not naturally produced by any other processes.

As oxygen-16 is also common in the atmosphere, the contribution to the beryllium-10 concentration from material deposited rather than created in situ must be taken into account.

¹⁰Be and ²⁶Al are produced when a portion of a quartz crystal SiO₂ is bombarded by a spallation product.  The oxygen of the quartz is transformed into ¹⁰Be, and the silicon is transformed into ²⁶Al.

Finding Ratio and Production Depth

Each one of these two radionuclides is produced at a different rate.

Both can be used individually to date how long the material has been exposed at the surface.

But since there are two radionuclides decaying, the ratio of concentrations of these two nuclides can also be used without any other knowledge to determine an age at which the sample was buried past the production depth, typically between 2 – 10 metres.

Solar Winds

A more active Sun results in lower beryllium concentrations. Since the atmospheric residence time for beryllium is not more than a few years, it is also possible to resolve the solar magnetic cycle in beryllium concentrations.

Beryllium measurements, such as these, are the best evidence that the solar magnetic cycle did not cease even during the period with no evident sunspots.

Chlorine-36 (³⁶Cl) Dating

Chlorine-36 nuclides are also measured to date surface rocks.

The properties of ³⁶Cl make it useful as a proxy data source to characterize cosmic particle bombardment and past solar activity.

The half-life of this isotope makes it suitable for geological dating in the range of 60,000 to 1 million years.

This isotope may be produced by cosmic ray spallation of calcium-40 or potassium-39.  It can also be produced as a result of thermal neutron absorption of Chlorine-35.

Following 90,000 Year Old Footsteps

In 2023, archaeologists found the foosteps of a group of humans in rock sediments, on the northwestern Moroccan ocean shore.

Such a discovery is extremely rare.  The track preservation through time relied on a set of geological circumstances written into the very history of the local terrain.

In this case, the 85 human footprints were found in a sandy area on a rocky part of the shoreline in Larache, Morocco.

As often, it was a chance discovery.

Using the OSL dating  method, the scientists were able to determine that the footprints had been made during the late Pleistocene, approximately 90,000 years ago.

Previously, no human footprint sites were known to date back any earlier than the Holocene.  The Larache footprints are, therefore, the oldest attributed to Homo sapiens in Northern Africa and the Southern Mediterranean.

So yes, dating rocks.  Dating really rocks!