A clock in the rocks: what cosmic rays tell us about Earth’s changing surface and climate

By Shaun Eaves, Te Herenga Waka — Victoria University of Wellington, Jamey Stutz, The Ohio State University, Kevin Norton, Te Herenga Waka — Victoria University of Wellington, Pedro Doll, University of Canterbury

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.

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.

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.

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