Rocks and weathered rocks

Goals

• Connecting rock and mineral formation with the changes rocks and minerals undergo at the earth's surface.
• Observing changes produced by weathering and relating these observations to chemical processes such as oxidation, hydrolysis, etc.

Skills

• Observation
• Outcrop and hand specimen description
• Quantitative skills (if lab is extended to analysis of chemical data)

This field lab best follows the section of the introductory geoscience course that covers mineral formation in igneous rocks (Bowen's reaction series) and after a lab or labs in which students learn to identify the minerals in various kinds of igneous and metamorphic rocks.

The minimum time required is two to four hours, depending on the number of outcrops that need to be visited to cover the range of fresh to weathered rocks.

Minimal equipment is required: Rock hammers, trowels or shovels (if really weathered rock will be encountered), sample bags (if follow up lab description and/or chemical analysis is involved).

This lab can be done in a variety of settings, from a single roadcut, to a comparison of rocks in a gravel pit, to a series of exposures that show a transition between fresh and weathered rocks.

This lab requires an outcrop or outcrops where the transition from fresh to weathered rock can be observed. Because the lab deals with mineral weathering, the ideal rock type will be an igneous or metamorphic rock with several different minerals (e.g. quartz, two feldspars, one or two mafic minerals). If obtaining fresh rock in the field is impossible (for instance in areas with thick saprolite), you may provide fresh samples from a quarry or other source for students to observe and describe in the field or in an earlier or later lab period. Similarly, if only fresh rock and slightly weathered rock are exposed in outcrop, you may provide samples (e.g. of kaolinite) to cover the very weathered end of the spectrum.

This lab can easily be extended with indoor components and additional outdoor components. For instance, thin sections can be made of samples collected in the field (or you can have these available to students) to observe the microscopic signs of weathering. You may also extend this field lab with chemical analyses of fresh and weathered rocks (using an AA spectrophotometer or other instrument) to show the relative gains and losses of various elements. Lab assistants could help students with the wet chemistry, or do the chemistry themselves, and then give the introductory students analytical results to interpret. Alternatively, you can give students oxide results from other published studies (such as Goldich, 1938 - see resource list for this lab). If the lab is extended in this way, students will gain experience analyzing quantitative data.

This field lab may be assessed by

• grading a written lab report.
• asking questions about mineral weathering on an exam.
• asking students in the next class period to write or sketch their overall results.

A good, basic reference is this 1938 article by Samuel S. Goldich:
Goldich, Samuel S., 1938, A Study in Rock Weathering: Journal of Geology, v. 46, p. 17-58.

Goldich examined igneous and metamorphic rocks from several areas to determine which minerals weathered first, second, third, etc. He also explores the chemical processes of weathering and the elemental changes in rock chemistry as weathering proceeds. Goldich concludes that minerals formed at high temperatures (near the top of Bowen's reaction series) weather faster than minerals near the bottom. Subsequent research suggests that biotite weathers more quickly than predicted from its position in Bowen's series, but the major elements of Goldich's work has held up.