Cosmic clocks help keep time in Yellowstone

Editor’s Note: Yellowstone Caldera Chronicles is a weekly column written by scientists and staff at the Yellowstone Volcano Observatory. This week’s post is from Mark Stelten, research geologist with the US Geological Survey and deputy scientist in charge of the Yellowstone Volcano Observatory.

Geologists can use cosmic rays from space to determine the timing of past geological events such as volcanic eruptions and glaciations. But how does that work? It turns out that rocks, just like people, can get sunburned!

When you think about a place like Yellowstone, with its amazing geological features and unique wildlife, you can’t help but wonder how the landscape came to be and what the future holds. For example, when was the Yellowstone Caldera formed? When was the last volcanic eruption? When was the last time Yellowstone was covered by glaciers? To answer these questions, geologists turn to geochronology, the study of Earth’s history from the perspective of time.

There are many different types of geochronological techniques that can be used to determine the age of geological events. Most commonly, radiometric dating techniques are used to determine the timing of past volcanic eruptions by measuring the proportions of parent and daughter material remaining after the decay of radioactive atoms naturally present in the sample. Argon dating is one of the most important radiometric dating techniques and can be used to determine the age of rocks formed thousands to billions of years ago. Carbon dating is another example of a radiometric technique, but it is limited to samples less than 50,000 years old and contains carbon that is missing from many deposits.

However, sometimes geological events do not produce new material that can be dated using radiometric techniques, but rather displace rock already present on the surface to a new location. For example, glacial deposits are a mixture of rocks from different locations that were moved by advancing glaciers, and hydrothermal explosion deposits consist of fragments of pre-existing surface rocks that were ejected during a steam explosion (like the one that occurred at Black Diamond Pool on July 23, 2024). ). In these cases, geologists look to the sky for answers using a versatile dating technique called cosmogenic surface exposure dating. This technique determines how long a rock’s surface has been exposed to cosmic rays, which are high-energy particles that enter Earth’s atmosphere from space.

Cosmic rays consist primarily of protons, or atomic nuclei, traveling through space at nearly the speed of light. This cosmic radiation does not come from our sun, but from exploding stars outside our solar system. When these particles reach Earth, they collide with atoms and molecules in the atmosphere and break them apart, creating a cascade of secondary cosmic rays. Some of the cosmic rays reach the Earth’s surface, and when they collide with elements in surface rocks, they trigger a reaction that produces cosmogenic isotopes (where isotopes are atoms of the same element that have the same number of protons, but different numbers of protons ). neutrons).

The reactions that form cosmogenic isotopes like ³⁶Cl, ³He, ¹⁰Be, ²¹No, and ²⁶Al occurs at a known speed. The type and amount of cosmogenic isotopes produced depends on the type of material the cosmic rays hit. By measuring the amount of cosmogenic isotopes present in a sample, it is possible to determine how long that sample was exposed to the surface – for example, when glacial material or debris from an explosion was deposited. Dating cosmogenic surface exposures is similar to using the level of redness of a person’s skin to estimate how long they have been in the sun. The worse the burn, the longer they were in the sun.

One of the interesting and challenging aspects of cosmogenic dating is that the amount of cosmic rays hitting the Earth’s surface depends on a number of factors. For example, significant amounts of cosmogenic radiation are deflected from the Earth by the Earth’s magnetic field. Therefore, your location on Earth is an important factor in how many cosmic rays reach the Earth’s surface. Height is also an important factor. The higher the earth’s surface is, the stronger the cosmic rays hit the earth’s surface. Cosmic rays can also be affected by physical obstacles that act as a “shield” for cosmic rays, such as a large hill next to a sampling site. Things like snow cover or vegetation can also reduce the amount of cosmic rays reaching the surface. Conceptually, this isn’t all that different from our sunburn analogy. The risk of sunburn is lower when you are in the shade than when you are outdoors or when you apply sunscreen. To apply the technique of cosmogenic dating, geologists must make detailed observations of all of these factors before using an accelerator mass spectrometer to measure the abundance of cosmogenic isotopes in the sample.

Although complicated, cosmogenic dating offers the opportunity to determine the age of samples that cannot be dated using other methods. A prime example of this is the dating of Glacial deposits in Yellowstone. By sampling the surface of large boulders deposited by glaciers, geologists were able to determine the history of glacial advance over the Yellowstone region—specifically, that the last major glaciation began about 22,000 years ago, and deglaciation of the region was mostly complete about 14,000 years ago . Information like this allows geologists at Yellowstone to assess important issues such as past climate, Postglacial earthquake activity, how ice and volcanism might interact, and how Yellowstone became the geological wonder it is today.