Earth & Space Systems - Pedagogical Connections: Impact Craters
This laboratory exercise is conducted to give pre-service teachers an example of a simple procedure allowing K-12 students to collect data relevant to developing a model of a space science concept. Impact cratering and its importance will have already been examined during the investigation of the planets and satellites of the solar system. Now students are asked to examine the craters themselves and determine what effects they might have on the bodies where they form. In this activity, students examine how the velocity and composition of bolides effect crater size and shape and how the surface impacted (solid vs. liquid) further alters the evidence left behind for us to study.
Students hypothesize about the role that increases in impact velocity will have on the size and shape of craters. For this experiment, hematite samples represent iron meteorites and a tray of sand represents the solid surface of a planet. Velocity is varied by increasing the height from which the "bolide" is dropped. The students hypothesize about the effect increasing the velocity will have on the size and shape of the crater before conducting multiple trials at several elevations. The results are used to support, reject or modify the hypothesis.
In this experiment, students keep impact velocity constant but alter the composition of the impacting bolide. With the iron meteorite serving as our standard, stony meteorites and comets are now tested by dropping similarly sized chunks of weak sandstone and crushed ice respectively into the sand tray. The weakness of these materials tends to result in shallower, less regular craters but with greater ejecta and secondary cratering of the sand surface. Student hypotheses often require modification to account for these secondary effects.
The last variable examined is the influence of the target surface on the crater formed. To limit the mess resulting it is performed as a demonstration. The students are asked what a bolide striking earth would be most likely to impact. A short discussion usually reminds the class that most of earth's surface is covered with water raising serious questions in their minds about what such an impact would do and what evidence, if any, it would leave behind.
A sand tray is flooded to create a shallow "continental shelf" environment with an adjacent "shoreline" before bolides of the various types are dropped by the instructor. Little if any evidence of cratering is left behind because wave action tends to fill in the hole left by impact. These "tsunamis" have a large effect on the shoreline areas.
Since movies have had a 'deep impact' on the students understanding in this situation it affords the instructor the opportunity to ask how the pre-service teachers would deal with the prior knowledge their students may have learned from Hollywood. Further, this activity leads to a detailed discussion of the role of bolides on evolution as recorded in the geologic record such as the Chicxulub impact crater dated to the KT boundary. The understanding gained from this lab is tested later in the semester in a quiz on which students must indicate what happened utilizing stratigraphic including data from the Chesapeake Bay impact crater.
An additional technique employed in this assignment is the use of a rubric (HTML File 7kB Jun6 05) for evaluating the hypothesis and results. A common error of students and teachers of science in following the scientific method is that insufficient attention is paid to checking the results against the hypothesis. Student labs are so universally designed to support a theory or concept that students and teachers often forget this most important step of the scientific method. Loss of potentially 20% of the points on what is otherwise a straightforward assignment tends to drive this point home for when these students begin teaching.