MODULE: Weathering – Instructor’s Guide

CASE STUDY: Granite or Marble?

A.      Learning Outcomes: After completing this module, students will be able to:

1. Identify marble and granite based on the physical and chemical properties of each rock.
2. List the mineral or minerals commonly found in each rock.
3. List the products of chemical weathering of each of the minerals in each of these rocks.
4. Observe and measure the effects of chemical weathering and mechanical weathering in a humid climate on headstones of granite and marble, each of different ages.
5. Draw a conclusion as to which rock type is more resistant to weathering in a humid climate.
6. Provide logic and evidence to support this conclusion.
7. Determine whether headstones made of granite and marble would weather differently in an arid climate versus a humid climate.
8. Draw a conclusion as to which rock type is more resistant to weathering in an arid climate.
9. Provide logic and reasoning to support this conclusion.

B.   Materials:

1. Access to a cemetery with headstones dating back from recent to at least ~90 years.
2. A bottle of dilute hydrochloric acid.

C.      Format:

1. Students and instructor visit a nearby cemetery (with permission).
2. Instructor shows students how to recognize and distinguish between granite headstones and marble headstones, using dilute hydrochloric acid.
3. Working in small groups, students find headstones of each rock type of three ages: recent (within last 10 years), mid-range (40-50 years), and old (90-100 years).
4. Students observe and record the amount of chemical weathering on stones of each age. They must use their sense of touch to feel the pitting from chemical weathering and their sense of sight to determine the amount of rounding of certain engraved letters.
5. Students also observe and record the amount of mechanical weathering (mostly from lawn maintenance equipment) using their senses of touch and sight to determine the amount of chipping of each headstone.
6. Students, working individually, go to the Internet to gather climate data for two locations, one in a humid climate and one in an arid climate. This data should include average monthly rainfall, average annual rainfall, and average high and low temperatures for each month.
7. Students then complete the handout including the required recommendations.
8. Finally, students provide a written summary of evidence and reasoning that support why they made those recommendations in an understandable and persuasive manner.

D.      Assessment:

1. Recording of observational data from headstones done as a group activity should be weighted at ~30% of total grade.
2. Answers to questions, including conclusions and supporting evidence for these conclusions, done on an individual basis, should be weighted at ~70% of total grade.

E.       Answers:

1. It is important for the students to establish that the primary minerals in granite are quartz and orthoclase, which are silicates, and that the primary mineral in marble is calcite, a carbonate.
2. It is also important for the students to mention that quartz is very resistant to chemical weathering caused by carbonic acid in rain, and that orthoclase is somewhat resistant to the same type of chemical weathering, although over time, orthoclase will eventually decompose to form clay minerals.
3. Students should also mention that calcite, is very susceptible to chemical weathering by carbonic acid, and undergoes solution weathering to form calcium ions and carbonate ions.
4. One additional fact that students like to mention is that both quartz and orthoclase, with a hardness of 7 and 6 respectively, will be more resistant to the mechanical weathering (scratching and chipping) caused by lawn maintenance equipment than calcite, which has a hardness of 3.
5. The students should then conclude that granite is more durable and will last longer than marble in a humid, temperate climate.
6. Students should use the information and data in #’s 1-4 above as evidence to support their conclusion in #5 above.
7. Students should recognize that in an arid, temperate environment, e.g. El Paso, Texas, chemical weathering will not have the same effect on marble as it would in a humid climate, because most chemical weathering is associated with rainfall and the carbonic acid it contains. Less rain means less carbonic acid and less carbonic acid means less chemical weathering.
8. Finally, extra credit might be given if either of the following is mentioned:
   1. In an arid, temperate climate, mechanical weathering of marble and granite from lawn maintenance equipment would still be approximately the same as in a humid, temperate climate. However, because grass typically grows more slowly in an arid climate, the overall amount of mechanical weathering from equipment might be less than in a humid climate.
   2. In an arid, temperate climate, mechanical weathering in the form of abrasion from windblown sand might be more prevalent than in a humid, temperate climate. This would lead to more mechanical weathering of marble than of granite because of the differences in the hardness of their constituent minerals.