In a classroom setting, students are well exposed to different cycles of matter and energy through systems. Indeed, this is mandated in the current language of the National Science Education Standards. Yet despite having had a parade of cycles throughout school, starting with the water cycle in elementary schools, the rock cycle in middle school, and the carbon cycle in high school, relatively little of this knowledge either retained or capitalized on by students, as was shown by Assaraf & Orion's research. In an effort to impact this difficulty, I've used a simple activity with classes that not only demonstrates the interactions between cycles but also allows for a demonstration of the dynamic nature of these interactions.

Students should be familiar with basic matter cycles in the Earth system

This is an introductory activity in a capstone discussion of biogeochemical cycling, in the context of complex Earth systems.

Students will be required to deeply investigate and be able to communicate to others the nature of their assigned matter cycle.

Called the "Web-o-Cycles," groups of students are each assigned a different matter cycle to become deeply familiar with not only the internal components and interactions, but also possible connections to other cycles. For example, volcanic activity in the rock cycle also discharges sulfur into the atmosphere, which turn interacts with the water cycle in cloud formation. Connections such as these are made between posters of the cycles using colored yard, hooked on the appropriate nodes on each cycle and labeled by the nature of the interaction with the note cards hung on the yarn. In a short period of time, the classroom is a web of yarn, connecting each cycle to the others.

The next element of this activity attempts to capture elements of complex Earth systems, especially the concepts of equilibrium, hysteresis, power law relationships, and sensitive dependence. All lines connecting the cycles are held taut, representing an equilibrium condition. Small shifts in one cycle are compensated for by consequent shifts in other cycles. Selecting one of the interconnecting strands, tension is in introduced, first in small pulls which accumulate to imbalance and shift the cycles slightly. A single large pull in one strand, to the point of breaking the yarn, causes some lines to slacken, perhaps to the point that they cannot be easily restored to tautness without dramatic shifts in the connected cycles. Re-tightening the connections causes a shift in the cycles, which takes place quickly and assumes a slightly different but at least familiar pattern. Having students then share their observations of the process of pattern description-imbalances-shifts-new equilibrium allows them to recognize the dynamic nature of Earth systems interactions as well as to seek deeper understanding of hidden elements within the Earth system.

Materials needed:

• At least four posters depicting detailed graphical representations of matter cycles, such as water, carbon, rock, nitrogen, sulfur, phosphorous, mounted on cardboard or another rigid material. Students should have available to them information on each cycle, depicting relative volumes of material in each cycle phase, residence times of the material in those phases, and the processes that drive changes from one phase to another;
• At least one ball of yarn, in a different color, for each poster;
• Note cards on which students will write a description of the individual processes used to link cycles;
• Paper clips to hang the note cards on these connective strands.

This activity is intended as an introductory exercise as a part of a capstone or summary experience. Thus, assessment is not formal, but should be framed on the basis of students abilities to discuss short- and long-term temporal components of the overall system that has been modeled, how positive and negative feedbacks impact other components of the system in a cumulative fashion, and evaluate predictions of emergent system patterns in a "what-if" manner. Used previously with in-service science teachers and pre-service Earth science teachers, the activity has provided the necessary added dimensions that bring to light the hidden, dynamic nature of the Earth system, empowering these teachers to share the same appreciation with their students.

Assaraf, O. B., & Orion, N. (2005). Development of system thinking skills in the context of Earth system education. Journal of Research in Science Teaching, 42(5), 518-560.

Matter cycle diagrams:

• Matter Cycles (more info)
• Hydrologic Cycle