Building an Understanding of Geological Time
Jeff Dodick, Science Teaching Center, The Hebrew University of Jerusalem, Givat Ram Campus
My research in science education has focused on both understanding the cognitive processes needed for understanding geological time, as well as developing instructional models for teaching this subject.
In my cognitive work, I discovered that thinking about geological time actually occurs on two different planes:
- Logic-based thinking is concerned with a subject's ability to apply logical principles used to order strata in time so that depositional events can be reconstructed. Of course, for those with a background in geology this is the same as relative time principles, such as superposition, taught in any introductory geology course.
- Event-based thinking is concerned with a subject's ability to connect specific important geo-biological events, such as the appearance of the dinosaurs, in "deep" time. Event-based thinking differs from logic-based thinking in that is connected for the most part to knowledge rather than to the use of logical principles from the earth sciences. (I say "for the most part" because certain events such as the "evolution of the first cells" can be deduced logically to occur before the "evolution of the first animals" for those with a basic knowledge of biology). Again, for those with a geology background, event-based thinking is closely associated with absolute / numerical time taught in any introductory geology course.
With regards to logic-based thinking, my research (Dodick and Orion, 2003 a, b) points to three factors influencing people's ability to reconstruct static strata into a temporally changing picture. The first is explained by psychologist J. Montangero's (1996) "diachronic thinking" model, which is concerned with the capacity to "represent transformation over time", such as when a child reconstructs a tree's life-cycle based on its appearance in a single stage of time. Montangero argues that there are three cognitive schemes1, which are activated when one thinks diachronically. These schemes were translated in my research into earth science principles that are used to reconstruct relative geological sequences:
- Transformation: This scheme defines a principle of change (qualitative or quantitative), and is correlated to the geological principle of actualism ("the present as key to the past.")
- Temporal Organization: This defines the sequential order of stages in a transformation. In geology this is translated into principles used to temporally order strata and the fossils that they contain, such as the principle of superposition.
- Interstage Linkage: This scheme deals with the connections between successive stages of a transformation; such stages are reconstructed in the earth sciences based on the use of actualism and causal reasoning.
Two factors seem to affect people's ability to activate the diachronic schemes:
- Empirical knowledge about the relationship between the environment and the fossil/rock type. Without such knowledge it is impossible to use actualism for reconstructing sequences in time.
- Spatial (visual) thinking: Recent work by psychologists and researchers in science education shows that temporal understanding is influenced by spatial reasoning (Longo and Lourenco, 2007; Matlock, Ramscar, and Boroditsky, L, 2005). So too, in geology the ability to correlate strata, temporally is influenced by a subjects' ability in spatial visualization (Dodick and Orion, 2003 a, b).
I actually used Montangero's diachronic model to help students (grades 9-11) reconstruct fossil sequences, in relative time in a curriculum I developed: From Dinosaurs to Darwin (Dodick and Orion, 2000).
Diachronic thinking is appropriate for small-scale events, where one wants to reconstruct local depositional sequences. However, the earth sciences also unravel large-scale events in "deep time" which requires the use of event-based thinking. There have been many studies on event-based thinking (i.e. how subjects understand "deep time"); however, there are far fewer (practical) educational materials for enhancing this ability. I will describe an instructional model that I use in From Dinosaurs to Darwin to help students to represent "deep time" better.
In the second chapter of this curriculum, the goal is to connect major evolutionary events with "deep time". This consists of four in-depth activities in which the guiding principle is to shape students' ability to manipulate the multiple, iconographic representations of evolution in "deep time". According to Kozma, et. al (2000), the ability to interpret representations is critical to scientists, as it permits them to organize information into conceptually meaningful patterns. They have shown that chemists have the ability to move flexibly between multiple representations so that they may better interpret chemical phenomena. Similarly, paleontologists must mediate between multiple representations, including phylogenetic trees, anatomical illustrations, and stratigraphic profiles to solve specific problems.
Thus, in From Dinosaurs to Darwin, students experiment with geo-biological representations that symbolize aspects of evolution in time. This material is scaffolded into a four-stage model linked by a series of bridging questions that induce the students to critique their representations at each stage of this activity, while linking them to the next representation in the activity. These 4 stages include:
- "infamous ladder of progress": In this activity the students construct a composite cross-section consisting of key features of evolutionary history uing the principles of biostratigraphic correlation. Concurrently, they list these features in a geological time scale which contains numerical dates indicating the origin of each feature. The cross-section built in this activity anticipates the misconception of "the ladder of progress": the understanding that evolution can be represented by a linear progression from bacteria to human, perpetuating the misconception that the history of life represents progress from primitive to complex (Gould, 1995). Thus, after completing this activity, the students tackle two questions challenging this misconception. The first asks them to critique this "representation of evolution in time", whereas the second asks them to suggest a "better representation." (To the latter question, most students suggest a branching tree-like icon which connects with the second activity).
- Representing evolutionary relationships in time: The second activity is connected to the first by requiring the students to build the preferred icon of evolution in time, the evolutionary tree. To do this, groups of students complete reports on a number of key features of the fossil record which they share with the class, orally, leading to the construction of a simple evolutionary tree. In building their tree, the students construct an association between biological events and geological time periods. This strategy is based on research which indicates that one of the symbolic modes involved in representing conventional time systems (such as months or weeks) is the associational network. Thus, Friedman (1982, p. 182) argues that individual months are often recognized by their linkage with "numerous personal or shared propositions (e.g., my birthday, cold, Halloween, etc.)." So, too, it might be possible to understand geological time by associating specific time periods with key evolutionary events.
- Comparing the magnitude of different timelines in biological and earth history: In this investigation, students compare different types of timelines, each with a different representative range that corresponds to a critical facet of time: geological time (4.6 Byr), cellular evolution (3.8 Byr), skeletal evolution (542 Myr), human evolution (2 Myr), civilization (5000 yrs) and personal time (80 yrs). This comparison teaches students that different temporally constrained disciplines, such as evolutionary biology and history, by necessity operate on different ranges of time which dwarf the human life span
- Understanding the rate and scale of evolution. This involves proportionally scaling an evolutionary tree on a geological time line. As this curriculum's focus is evolution, it was important to show that much of evolution, as seen in the fossil record, has occurred in the last 530 million years of geological time, beginning with the "Cambrian Explosion"; this represents (approximately) the last 12% of geological time. (In fact, cellular life began as much as 3.8 billion years ago but the fossil record is biased towards multi-cellular organisms with hard skeletons which evolved from the Cambrian) Thus, in this stage, students return to the evolutionary tree they completed in stage 2 and proportionally scale it along a geological timeline. Thus, they see that much of the evolution of animals with hard skeletons is compressed to the upper 12% of geological time. Moreover, they gain a new-found perspective into the antiquity (and diversity) of unicellular life.
This model uses a series of scaffolded, visual representations, each symbolizing a different aspect of evolution in time. In this way, students critically evaluate the representations they investigate, while constructing a more sophisticated understanding of evolution in time. Using this method, students learn that different time scales are appropriate for representing different phenomena, from our planet's birth to the evolution of the human species. Additionally, they also discover that different events develop at different rates. Such insight into differing scale and rate are not confined to evolutionary processes alone; thus, investigating evolutionary change within the framework of "deep time" serves as a starting point for discussing many other sciences, influenced by time spans of varying magnitudes. Obviously, this model is content specific to evolutionary / fossil history, so for those who are interested in teaching about geological history, a different set of pictorial representations will be needed, but I believe the principles will remain the same.
Dodick, J. T. & Orion, N. (2000) From dinosaurs to Darwin: Evolution from the perspective of "deep time."Rehovot, Israel: Weizmann Institute of Science.
Dodick, J. T. & Orion, N. (2003a). Cognitive factors affecting student understanding of geological time. Journal of Research in Science Teaching, 40,415-442.
Dodick, J. T. & Orion, N. (2003b). Measuring student understanding of "deep time." Science Education, 87,708-731.
Friedman, W. (1982). Conventional time concepts and children's structuring of time. In Friedman, W. (Ed.), The developmental psychology of time(pp. 174-205). New York: Academic Press.
Gould, S. J. (1995). Dinosaur in a haystack: Reflections in natural history. New York: Harmony Books.
Gruber, H.E. & Voneche, J.J. (1995). The essential Piaget. New York: Basic Books, Inc.
Kozma, R., Chin, E., Russell, J., & Marx, N. (2000). The roles of representations and tools in the chemistry laboratory and their implications for chemistry learning. The Journal of the Learning Sciences,9, 105-143.
Longo, M. R. & Lourenco, S. F. (2007). Spatial attention and the mental number line: evidence for characteristic biases and compression. Neuropsychologia, 45, 1400-1406.
Matlock, T., Ramscar, M. & Boroditsky, L. (2005). On the experiential link between spatial and temporal language. Cognitive Science, 29(4), 655-664.
Montangero, J. (1996). Understanding changes in time. London: Taylor and Francis.