How frequently do mountains collapse, volcanoes erupt, or ice sheets melt? These questions are of great importance to Earth scientists as they work to improve projections and prepare communities for future hazardous events. While instrumental measurements provide some information, they often have limited records. To overcome this, scientists rely on geological archives and utilize geochronology, a set of geological dating methods that assign absolute ages to rocks.
One such technique used in recent years is cosmogenic surface exposure dating, which quantifies the amount of time a rock has been exposed to signals from outer space while on the Earth’s surface. Earth is constantly bombarded by high-energy charged particles called cosmic rays, some of which reach the surface and break apart atoms in the Earth’s crust, creating rare elements known as cosmogenic nuclides. The presence of these nuclides in rocks and sediments indicates atmospheric exposure and provides information about how long the rock has been exposed.
Cosmic rays were first discovered in the early 1900s, but it took nearly a century for particle accelerators sensitive enough to accurately count the rare atoms produced by their impact to become available. Today, cosmogenic surface exposure dating is a primary technique for determining the rates and dates of various processes on the Earth’s surface.
One example of its application is in southeast Fiordland, where the Green Lake landslide is one of the largest landslides on Earth. Previous research suggested that the retreat of a large glacier induced this landslide. To test this hypothesis, scientists collected boulders on the surface of the landslide that had been shielded from cosmic rays in the mountain interior before becoming exposed by the landslide. Their measurements indicated an exposure age of approximately 15,500 years, which postdates the end of the last ice age in the Southern Alps by 3,000 to 4,000 years. This suggests that deglaciation was unlikely to be the primary cause of the mountain collapse, pointing instead to an extremely large earthquake as the more likely trigger.
Another example is the study of effusive volcanic eruptions on Mt Ruapehu, the highest mountain in the North Island. While there is no observational record of eruptions producing lava flows, future events could have significant implications for local infrastructure. Using cosmogenic dating, scientists found that the mountain ejected lava in clusters of eruptive activity that could last for millennia. This technique also provided more precise dates for recent prehistoric eruptions compared to other common volcanic dating methods.
Cosmogenic nuclide measurements have also been used to track the melting of ice, providing valuable information for understanding the response of ice sheets to climate change. In regions where fossil plant material is rarely available for radiocarbon dating, cosmogenic nuclides offer a solution. By studying glacial cobbles transported by glaciers and deposited on hillsides, scientists can reconstruct the past evolution of glaciers. For example, a study of Byrd Glacier in Antarctica showed that it thinned by at least 200 meters about 7,000 years ago during a period of relative global climate stability. These findings contribute to evaluating computer models used to simulate past, present, and future ice sheet changes.
In conclusion, cosmogenic surface exposure dating is a valuable tool for Earth scientists to improve projections and prepare for hazardous events. By quantifying the time rocks have spent on the Earth’s surface and utilizing cosmogenic nuclides, scientists can gain insights into the timing and causes of mountain collapses, volcanic eruptions, and ice sheet melting. This information is crucial for understanding and predicting future events and their potential impacts on communities.
