The Earth and all the living things it supports have been evolving over more than four and a half billion years of history. How do geologists measure that immense span of time?
The Geological Survey of Canada (GSC) of Natural Resources Canada has a group of scientists that specialize in geological time, piecing together Earth’s long history. One of those “geochronologists” is research scientist Dawn Kellett, who explains how she and her colleagues are putting the puzzle together.
The process starts by selecting rocks from across Canada that contain elements like potassium, uranium, and lutetium. These elements have one thing in common, an unstable isotope (or two in the case of uranium!) that decays, or breaks down, at a very slow rate to form a new isotope: potassium becomes argon, uranium becomes lead and helium, and lutetium becomes hafnium. These parent-daughter pairs are measured using mass spectrometry, and since we know their rates of decay, we can calculate the ages of the minerals and rocks hosting those elements.
Due to the wide range of parent-daughter pairs with geologically-slow decay rates, and the wide range of minerals containing those parent elements, geochronologists are able to measure the age of the Earth and many of the rocks within it. This information has been used to construct a Geologic Time Scale of Earth history. Within that framework, the geological causes of past global mass extinctions can be determined by precisely relating events in the rock and fossil record at different locations around the Earth. We are able to study the time scales over which major past climate changes have occurred. Geochronologists can reconstruct the evolution of Earth’s giant mountain belts, even those that formed so long ago that the mountains themselves are long gone and only the deformed and metamorphosed roots remain.
As laser and mass spectrometer technology continues to rapidly improve, geochronologists focus less on “how old”-type questions, and more on “for how long”, “at what rate of change”, “since when” and “how many times”. NRCan geologists depend on geochronology to aid several GSC programs, such as: to investigate where and how Canada’s metal and energy resources have formed (Targeted Geoscience Initiative); to better understand the geological and climate evolution of our continent (Geo-mapping for Energy and Minerals Program), and; to determine Canada’s ocean frontiers (United Nations Convention on the Law of the Sea Program). The study of geological time has become the Rosetta Stone for all reconstructions of Earth’s history.