AGE OF THE EARTH: DEEP TIME, MASS EXTINCTIONS AND RADIOMETRIC DECAY

One distinguishing feature of the geological sciences is the very large temporal scale at which Earth events take place. Students’ developmental ability to comprehend and internalize the timing and duration of geological events presents challenges on its own and is often confounded by views that, whether deeply-held or previously unexplored, present barriers to acceptance or understanding of Earth formation and evolution. As students mature and their conceptual framework for deep time improves, it is important to scaffold prior learning with more advanced contextual discussion to help students reconcile divergent views in a scientific and respectful manner.

Radiometric dating of meteorites is commonly introduced as the primary vehicle by which we know the age of the Earth. To students unfamiliar with either planetary formation theory or the strengths and weaknesses of absolute dating, this information can be anything but compelling because it requires faith in events that cannot be replicated in the classroom. One observation commonly presented to cast doubt upon absolute dating is that different isotopic measurements carried out on a single sample can yield different ages. If the analytical method gives more than one result, how can any of them be believed? In this context, presenting not just the algebraic manipulation of radiometric decay series but the significant role of temperature-dependent diffusion is crucial.

By contextualizing the geographic significance and geological history of type localities named in the geological time scale, students can begin to construct a picture of Earth evolution that has meaning and that introduces the timing and duration of geological events. The age of the Earth is often presented through visual or graphical analogues that reinforce the enormity of geological time but lack integration with the relative time scale constructed from global fossil evidence for both coherent assemblages and extinctions; both of these approaches reinforce lower order skills over conceptual understanding. By linking a series of activities that gradually add complexity to the understanding, we encourage students to evaluate authentic geological evidence as they begin to explore and eventually accept deep time.