One of the most important measuring tools of geology might be frequently misread, scientists at Queensland University of Technology, Australia have argued, and the consequences could be serious. If we are getting the age of geological features wrong, we might not just be misunderstanding the story of life on Earth, but distorting our estimates of risk for volcanic eruptions.
Zircons are usually small silicate crystals common in granites. They have a couple of distinctive features that make them great for establishing the dates of rocks. They exclude lead when forming, but include radioactive elements that slowly decay to lead with time, providing an indication of how long this decay process has been running. They are so tough that some have survived 4 billion years.
However, Dr Scott Bryan thinks we are making a mistake in jumping from the age of zircons to the age of the rocks they comprise. In Earth-Science Reviews, Bryan presents evidence that zircons from locations as diverse as Queensland, New Zealand, and Hawaii, have inspired misreadings, and suggests how geologists can do better.
“One of the assumptions being made is that the composition of the zircons and the rocks in which they have formed give an accurate record of the magmas and conditions at which the zircons and magmas formed,” Bryan said in a statement. “From this, we then estimate the age of the event that caused them to form."
"But some zircon crystals may not be related to their host rocks at all," he added. "They may have come from the source of the magma deep in the Earth’s crust or they may have been picked up by the magma on its way to the surface.”
When pre-existing zircons are treated as being the same age as the rocks that hold them, the antiquity of eruptions is overestimated, sometimes by millions of years.
This may sound as though getting accurate ages for rocks is impossible, but Bryan disagrees. Instead, he discusses techniques to distinguish between zircons that formed with rock and pre-existing crystals incorporated during an eruption. Unfortunately, these don't work in every case, with uranium and thorium-rich zircons being resistant to such differentiation.
Problematic as inaccurate dates can be – for example when estimating species' ages from volcanic rocks sandwiching fossil specimens – there are bigger dangers.
Recent rumblings in the supervolcano beneath Yellowstone National Park reminded the world of the danger such events pose. But to know how likely a civilization-threatening eruption is, it is important to know how frequently, and at what intervals, such events have occurred in the past. For that we usually rely on zircons, using the assumptions Bryan and his co-authors criticize.