Barriers to Teaching or Learning About Rates and Time
This summary was compiled by Carol Ormand, Science Education Resource Center.
The Difficulty of Authentic Assessment
It's challenging, particularly in large classes, to measure the depth of students' understanding of and ability to apply the geologic time scale and related concepts. Even if students can manipulate the large numbers associated with Deep Time and can use the vocabulary of the geologic time scale, it's difficult to assess how well they grasp the vastness of millions or billions of years, the rates of geologic processes, and the uncertainties inherent in dating geologic events. Students may also be reluctant to divulge that the concept of Deep Time conflicts with their religious beliefs.
The Depths of Deep Time
Geologic time involves magnitudes that are difficult for the human mind to grasp. We do not have direct experience with millions or billions of anything, so these numbers are not very meaningful to most students. For them, anything that happened before recorded human history was "a long time ago." Yet, as humans, we are accustomed to having an intuitive understanding of time in our lives. So students may also experience metacognitive dissonance about their own inability to intuitively grasp Deep Time. Finally, a significant proportion of the U.S. population does not "believe" that the Earth is 4.6 billion years old.
The incomprehensibility of the large numbers involved in geologic time make it difficult for many students to distinguish between events that occurred tens of thousands of years ago, millions of years ago, and billions of years ago. Compounding the cognitive challenges of understanding these numbers in terms of the magnitudes of time involved, many students struggle with the more fundamental challenge of understanding the numbers themselves and their proportional relationships. (See this video explaining the relationship between millions and trillions, focusing on the U.S. federal budget.) Students with weak mathematics backgrounds may not know the relationships between thousands, millions, and billions. Some students who understand the magnitudes of these numbers still struggle to read mathematically scaled time lines, to place events on time lines (via algebraic interpolation), and to understand the relationships between time lines with different scales. As a result, they may ignore the scales of time lines, thus leading to a lack of understanding of what events took place when.
The Duration of Geologic Events and the Rates of Geologic Processes
Related to the difficulty of imagining the vastness of Deep Time are the twin challenges of (a) imagining "events" that take place over time intervals much greater than the entire length of human history and (b) imagining similarly vast (or greater) stretches of time between such events. Students tend to think of events as occurring instantaneously, like the events they hear about on the daily news, rather than as the impossibly slow processes that happen between disasters. Yet understanding the Earth's history (and predictions about its future) depends on a fundamental understanding of how processes we can observe within our lifetimes (plate motion, erosion, natural selection) produce profound changes over geologic time scales: the formation and destruction of mountain ranges, for example, or the development and extinction of species. Plotting such events on a time line also leads to the misconception that the events have instantaneous beginnings and endings. In addition, related to the lack of understanding of the scale of time lines, students who know a sequence of events in geologic history may think of them as occurring back to back, although the length of time between events may be immense.
The Uncertainty of Geologic Dates and Ages (and Rates)
We date geologic events using both "absolute" and "relative" methods, and students struggle with the inherent uncertainties of both. Students have a hard time understanding the mathematical uncertainty of any scientific measurement, such as radiometric dates. This may be related to a poor understanding of the underlying processes, including radioactive decay and other processes that may affect the numbers of parent or daughter isotopes present in a sample. In addition, some students struggle with the idea of bracketing the age of a rock unit or the date of an event. In both cases, the idea of uncertainty goes against many students' preconception that science offers definitive, simple answers to questions.
The Unfamiliarity of Geologic Processes
Our intuitive sense of time is based on our personal experiences of its passage. Trying to imagine the passage of vast stretches of time may be made more challenging if the processes occurring during those intervals are unfamiliar. For example, students who have no direct experience of sedimentary deposition struggle to visualize the processes of deposition, compaction, and lithification that lead to the formation of stratigraphic sequences at all scales. Without a sense of the time required to form a thin sedimentary deposit, students have no frame of reference for understanding the time required to form thicker sequences, such as the bedrock of the Midwest or the walls of the Grand Canyon.
The Unfamiliarity of the Geologic Time Scale
Geologists think of geologic time as subdivided into eons, eras, periods, and epochs, each with names and dates that are unfamiliar to the novice. For geologists, these subdivisions are meaningful, defined by important events in geologic history, and the names have become familiar through years of use. For the students, learning these names and dates can present an additional barrier to understanding the geologic time scale: we expect students to associate names that don't mean anything (to them) with numbers that also have very little meaning (to them). Without some contextual information about why the geologic time scale is useful, this may seem like a pointless exercise in memorization.