Conference Name: Geological Society of America Annual Meeting & Exposition 2010: Reaching New Peaks in the Geosciences
Educators have suggested using rock tumblers in introductory geology classes at the college level and in K-12 earth science courses to simulate weathering and erosion of pebbles or cobbles, but we suggest experiments using a tumbler are also useful in sedimentology courses for geology majors. Experiments can be used to demonstrate loss of mass as a function of rock type or degree of lithification as well as to determine effects on grain sphericity and roundness. Tests can be run for several days or weeks with periodic pauses for measurements. Groups of students may develop their own tests, or can be given a general area to study (e.g., grain shape) and then be required to plan the test (e.g., rock types, initial sizes and shapes). In addition to writing lab reports, the groups can give presentations in class. By doing the tests and presentations early in the semester, the instructor can refer back to the results when presenting a variety of topics including intra- versus extraformational origin of clasts, grain shape as a function of distance of transport, and the biased preservation of rock types in clastic rocks.
We found that loss of mass and increase in roundness are predictable when the rocks being compared vary significantly in composition and texture (e.g., granite versus limestone); however, significant variation is observed when comparing different sandstones and limestones due to variation in the degree of lithification. Differences in weight loss between duplicate samples can range from almost zero to several percent depending on the homogeneity of the sample. Using carborundum (SiC) rather than quartz sand for grit accelerates the tests. However, preliminary results indicate the amount of mass lost using the two grits may not be consistent for various rock types perhaps causing difficulty when equating distance of transport in the tumbler to a natural environment in which quartz sand dominates.
Advantages to using a tumbler in a sedimentology course are many: (1) the equipment is inexpensive and simple to operate; (2) the concept is easy to explain and takes little class time, (3) the tests are ideal for group work with each student charged with certain runs; (4) the results are commonly easily plotted and presented in class, and (5) the results can be used throughout the semester to help demonstrate sedimentological concepts.