Petrology in the Gravel Pit

Goals:

• Introduce students to a wide range of rock types.
• Reinforce prior (or subsequent) hand specimen petrology labs.
• Understand the range of rocks that fall into the major categories of sedimentary, igneous and metamorphic.

Skills:

• Rock description and identification
• Mineral recognition

This lab works best in an area with glacial sediments of mixed provenance, but even in materials that contain only one general type of rock, such as "granite," it may be useful for students to see the wide variety of ways that one rock type appears. Obviously, a good exposure such as a gravel pit or fresh road cut is necessary to be able to collect a variety of pebbles.

This field lab connects well with indoor labs about rock types, that might either precede or follow the field lab. A major advantage to a rock type lab in a gravel pit is the opportunity to see more than one type of a particular rock. Another advantage is that the rocks may be larger than those in the lab collection, and thus display differences in texture, structure and mineralogy that are unlikely to be evident in lab specimens.

A possible extension of this lab is to have each student return to campus with one to three interesting rocks that they can examine in more detail through the term (rather like a "pet rock" project). Depending on available equipment, students could cut and interpret thin sections, powder some of the minerals for X-ray analysis, and do a variety of other tests, culminating in a report or poster about the rock.

A good teaching strategy is to ask students or student groups to collect a hundred or so pebbles and to pile them into "igneous, metamorphic, sedimentary and 'not sure'" piles. In a discussion, each student could be asked to display and explain one rock he or she is confident about and one rock he or she isn't sure about, describing in each case the relevant rock features. You can also have students sketch particularly interesting boulders.

Inevitably, some of the rocks in gravel pits are real stumpers, even for experienced faculty. You can turn this particular complication into a "learning moment" by inviting students to "stump the prof" with particularly challenging samples.

If the catchment area for the glacial deposits is large with varied geology, you might consider having students and/or lab assistants construct a geologic map of that area in the floor of the pit, or in the lab, using the rocks that students have collected.

This lab can be assessed either by individual or group student reports, or by quizzes and examinations. For instance, in a subsequent class period, you could ask students to look at a series of rocks, identify them as sedimentary, igneous or metamorphic and describe the features that lead them to that conclusion.