Magnetism, the Earth as a Magnet, and Seafloor Banding-How Much Magnetism Is Enough?

Article excerpt

ABSTRACT

Although seafloor banding patterns are widely taught as a key piece of evidence for plate tectonics, little research has been conducted on student understanding of magnetism in a geological context. This study was conducted in two introductory geology courses to identify the effect of students' understanding of basic magnetism on their ability to build a simple conceptual model of seafloor banding consistent with the scientifically accepted model. Students were asked open-ended questions to elicit their reasoning about magnetism, Earth magnetism, and seafloor banding. Results suggest that many students in the study had weak or alternate models of basic magnetism. These models led students to reason incorrectly about magnetic interactions of the Earth's magnetic field and Earth materials during rock formation. Even students with robust magnetic models had difficulty applying them correctly to account for the origin of seafloor banding.

INTRODUCTION

There is increasing evidence that the alternate conceptions that students bring to a course can have significant impact on what they learn. This has particular relevance to geology since many topics in geology are based on principles typically taught in introductory physics and chemistry. However, students at the university level often take introductory geology courses to fulfill university general education science requirements and so have little or no background in either physics or chemistry. Thus, in order to facilitate conceptual model development in introductory geology courses, instructors need to know what role alternate concepts about physics, chemistry, and geology play in students' concepts about the Earth.

The Earth magnetism model presented in introductory geology texts is that of a field generated by a simple bar magnet. Changes in the orientation of the bar magnet together with rock formation in the rift zone are then used to account for the origin of seafloor banding. Since magnetism provides a key to understanding the origin of seafloor banding and thus plate tectonics, we chose to investigate what effect students' understanding of basic magnetism have on their concepts of seafloor banding.

Little research has been conducted on student understanding of magnetism in a geological context. In one study of Portuguese high school students, about 35% thought that the Earth's magnetic field could not be recorded in rocks and many thought that the Earth's magnetic field does not change (Marques and Thompson, 1997). These pre-existing views of the Earth's magnetic field and rock magnetism are likely to inhibit student construction of scientifically accepted and commonly presented models of seafloor banding. Studies in physics suggest that difficulties understanding fields and electromagnetic interactions are common (for example (Maloney, 1985; Greca and Moreira, 1997). Alternate concepts of electromagnetism also may contribute to students' difficulties with explanations of seafloor banding based on the commonly presented, simple model of the Earth's magnetic field.

Most studies of students' concepts of electromagnetism focus on electricity, or the interaction between electricity and magnetism (McDermott and Redish, 1999) and are conducted in introductory college level physics courses. Despite the difference in the focus and level of material presented in introductory geology and introductory physics courses, familiarity with students' difficulties with electromagnetism can inform interpretation of students' alternate conceptions of the Earth's magnetic field. Studies suggest that students in introductory physics courses often have alternate concepts of electromagnetic interactions (Maloney, 1985; Maloney et al., 2001) coupled with incomplete mental models of fields (Galili, 1995; Greca and Moreira, 1997). Studies (Maloney, 1985; Maloney et al., 2001) confirm a common difficulty observed by physics instructors - students tested either before or after instruction reasoned that stationary charged electric particles will interact with a magnetic field, and that a moving electric charge will interact with a magnetic field regardless of the orientation of the field relative to the path of the charge. …