A clock in the rocks: Cosmic rays and Earth science
Rocks act as geological archives and new techniques are helping scientists capture data about past events.
This article has been republished from The Conversation under Creative Commons licence CC BY-ND 4.0 and is written by Shaun Eaves, Jamey Stutz, Kevin Norton and Pedro Doll. It was originally titled: A clock in the rocks: what cosmic rays tell us about Earth’s changing surface and climate .
How often do mountains collapse, volcanoes erupt, or ice sheets melt?
For Earth scientists, these are important questions as we try to improve projections to prepare communities for hazardous events in the future.
Observing Antarctic rock surfaces
Scientists study rock surfaces in Antarctica to reconstruct past ice sheet change.
We rely on instrumental measurements, but such records are often short. To extend these, we use geological archives. And at the heart of this research is geochronology – a toolkit of geological dating methods that allow us to assign absolute ages to rocks.
In recent years, we have been using a state-of-the-art technique known as cosmogenic surface exposure dating which allows us to quantify the time a rock has spent on the surface, exposed to signals from outer space.
Using cosmic rays as a clock
Earth is constantly bombarded by high-energy charged particles, known as cosmic rays, coming from the depths of our galaxy. Most are intercepted by Earth’s magnetic field and atmosphere. But some are sufficiently energetic to reach Earth’s surface.
On impact, they break apart atoms of common elements in Earth’s crust, such as silicon and oxygen, to create new rare elements known as cosmogenic nuclides.
The presence of cosmogenic nuclides in rocks and sediments at the Earth’s surface is a clear indicator of atmospheric exposure. Their abundance tells us how long the rock has been exposed.
Creation of cosmogenic nuclides
This schematic shows how cosmic rays break apart atoms in the Earth’s crust to create new rare elements known as cosmogenic nuclides.
Cosmic rays were first discovered in the early 1900s, but it took almost a century until sufficiently sensitive particle accelerators became available to accurately count the small number of rare atoms produced when they hit Earth.
Today, cosmogenic surface exposure dating represents a primary technique for quantifying the rates and dates of several processes on Earth’s surface.
Timing mountain collapse
In southeast Fiordland, the Green Lake landslide is one of the largest landslides on Earth. Its large size is especially unusual given the relatively small stature of the mountains from which it came.
Green Lake landslide
The Green Lake landslide in the South Island is considered one of the largest in the world. The landslide started on the ranges above the lake. The middle area, now covered in trees, is the debris of the landslide.
Cosmogenic surface exposure dating has updated ideas about how the landslide occurred.
Previous research suggested the landslide was induced by the retreat of a large glacier that formerly supported the mountainside.
Given ongoing glacial retreat today, we sought to test this hypothesis by collecting boulders on the surface of the Green Lake landslide. These rocks had previously been shielded from cosmic rays in the mountain interior before becoming exposed by the landslide.
Our measurements yielded an exposure age of about 15,500 years, which postdates the end of the last ice age in the Southern Alps by 3,000 to 4,000 years. From this result, we conclude that deglaciation is unlikely to have been the primary cause of this spectacular mountain collapse. Instead, our findings point to an extremely large earthquake as the more likely trigger.
How often do volcanoes emit lava?
Lava-producing volcanic eruptions have built the large cone of Mt Ruapehu, the highest mountain in the North Island.
Despite some explosive episodes during the 20th century, there is no observational record of eruptions producing lava flows. Future lava-producing events could fundamentally reshape the volcanic cone, with potential implications for local infrastructure.
But how often do such eruptions happen?
Supported by the Resilience to Nature’s Challenges National Science Challenge, we tested whether cosmogenic dating could help us determine recurrence intervals of lava-producing eruptions on Mt Ruapehu over the past 20,000 years.
We found the mountain ejected lava in clusters of eruptive activity, which could last for millennia. The cosmogenic data also provided more precise dates for recent prehistoric eruptions, compared to those produced by other common volcanic dating techniques such as palaeomagnetic dating and radiometric dating methods.
Tracking the melting of ice
Before cosmogenic nuclide measurements, glacial geologists trying to determine the age of sediments relied on serendipitous discoveries of fossil plant material for radiocarbon dating. In alpine and polar regions, where most glaciers are, such matter is rarely available.
Cosmogenic nuclides solve this problem as glaciers quarry rocks from their bas e and transport them to the surface where they rest on hill slopes and moraines and begin accumulating their cosmic signal.
With support from the New Zealand Antarctic Science Platform, we have applied this technique to reconstruct the recent evolution of Byrd Glacier – a large outlet of the East Antarctic Ice Sheet.
Glacial cobbles, transported from the Antarctic interior and deposited on hillsides on either side of the flowing glacier, track how high the glacier was in the past.
Our study shows that the glacier thinned by at least 200 metres about 7,000 years ago during an interval of relative global climate stability. These results provide rare three-dimensional information that can be used to evaluate computer models used to simulate past, present and future ice sheet change.
Collecting samples for cosmogenic surface exposure dating
Dr Shaun Eaves and Dr Jamey Stutz collect rock samples at Byrd Glacier.
The yellow GPS device in the foreground records the precise location of samples – for example, latitude, longitude and elevation above sea level. Elevation is a key observation for the method as cosmogenic nuclide production rates increase with height (roughly 2 times for every 1 km).
Photo by Steve Grimes.
Rising sea level is one of the biggest challenges facing civilisation this century. However, the uncertain response of ice sheets to climate change currently hampers projections.
Cosmogenic nuclide specialists are now ambitiously attempting to recover rock samples from beneath sensitive portions of the present ice sheets. Testing them for cosmic signals will yield important insights about the potential of future melting of ice sheets.
Nature of science
Scientific knowledge changes over time as new techniques are developed and tested. New data, provided by cosmogenic surface exposure dating, disproved the hypothesis that the Green Lake landslide was caused by a receding glacier.
Related content
Geologists use several techniques to ‘date the past’. Look at the Dating the past – introduction and explore relative dating methods, and absolute dating methods in this article and interactive.
Read how different dating methods have been used to understand Whanganui rocks and climate cycles.
Learn more about radiometric dating methods: radiocarbon, potassium-argon and fission track dating.
To see examples of how scientists have used different dating methods in their work look at:
Date a dinosaur has a s on using microscopic fossils like pollen for relative dating.
In the video Shrinking tropical glaciers, a glaciologist explains how cosmogenic and other elements are used to map changes to glacier levels through history to understand what’s happening to ice through time because of increasing temperatures.
Learn more about the use of modern particle accelerators in The search for new elements.
Antarctica is a hub for research into past and future . Investigate more in: Antarctica and global climate change, Climate change, melting ice and sea level rise and Antarctic tipping points.
Discover some of the key ideas that underpin geology in The rock cycle and Earth systems and climate change.
Activity ideas
Build student knowledge of rock dating in Using absolute dating methods, Absolute dating rock layer quiz, Rock layers and relative dating and Which dating method?
To aid student understanding of concepts involved in different dating methods try the activities Radioactive decay, Fossil correlation and Build a timescale.
Useful links
Find out about some of the research mentioned in this article:
Introduction to cosmogenic nuclide testing from Antarctic Glaciers website.
The discovery of cosmic rays article from CERN.
The age and potential causes of the giant Green Lake Landslide, Fiordland, New Zealand
Green Lake Landslide and other giant and very large postglacial landslides in Fiordland, New Zealand
Cosmogenic 3He chronology of postglacial lava flows at Mt. Ruapehu, New Zealand
Inland thinning of Byrd Glacier, Antarctica, during Ross Ice Shelf formation Stutz, J., Eaves, S., Norton, K. et al. Earth Surface Processes and Landforms, Vol 48 (2023).
Read how GreenDrill is drilling through the Greenland Ice Sheet to recover rock samples from the bed beneath to obtain information about the history of the ice cover.
Resilience to Nature’s Challenges National Science Challenge funded some of the work detailed in this article.
Acknowledgements
This article was written by Shaun Eaves (Senior Lecturer in Physical Geography, Te Herenga Waka – Victoria University of Wellington), Jamey Stutz (Assistant Director Polar Rock Repository, Byrd Polar and Research Center, The Ohio State University), Kevin Norton (Associate Professor in Geochemistry, Te Herenga Waka – Victoria University of Wellington) and Pedro Doll (PhD candidate, University of Canterbury).
The article was originally published in The Conversation, 2 May 2024. Read the original article.
The Conversation
The Conversation is an independent source of news and views, sourced from the academic and research community and delivered direct to the public.
