INTRODUCTORY GEOSCIENCE STUDENTS' UNDERSTANDING OF ROCK TYPES

We investigated students’ views of the rock categories of igneous, metamorphic, and sedimentary rocks using semi-structured interviews of 10 students in introductory geology courses. The results of this investigation indicate that students do not associate the three rock types with three separate mechanisms of how rocks form, as professional geologists do, but instead blur the boundaries between rock types. For example, many students describe sediments as occurring in any rock type, identify igneous rocks by the presence of the color black, classify any light colored rock as felsic, and believe that any process that changes rocks results in metamorphic rocks. As a result, they often confuse rock types and misapply technical terms. Because of their confusion, students fail to identify the usefulness of the classification routinely employed by geologists.

Based on the results of the study, we believe that many of the difficulties students exhibit with the understanding and comprehension of the rock types relates to deeply held conceptual barriers that prevent them from gaining a full understanding of how rocks form. The conceptual barriers that affect how students view the rock types include the following: 1) the great length of geologic time, 2) the Earth is constantly changing, 3) the large scale at which things happen on Earth, 4) rocks form and exist as bedrock, 5) the properties of materials that make up rocks, 6) the processes that happen at the atomic scale, and 7) the scale and cause of pressure.

For example, although geologists commonly use granite as an example of an igneous rock, students describe granite as being composed of “sediments,” or pieces. Many of the conceptual barriers mentioned above can contribute to this misconception. Although students may identify black minerals in granite as originating from magma, they do not picture the entire rock as cooling from magma (Conceptual barriers: Atomic scale, Materials). Rather, students tend to describe the formation of granite in terms of pre-existing pieces (Changing Earth) coalescing over the course of years (Geologic time) to form a small handsample (Bedrock, Large scale), with or without the presence of magma (Atomic scale).