Chemical and Mechanical Weathering of Rock Lab

Lauren Templeton, Heather Mullen, Katherine Welmaker, Becca Gojda

2 October 2015

Abstract

This lab explores several different forms of weathering and the specific effects they have on rocks. Mechanical Weathering was found to reduce the weight of different types of rocks more efficiently than others. Geological Changes were found to make small physical changes to the exterior of the rock. Glacial Changes were found to affect the ground through the the sand the “glacier” moved around. Chemical Weathering was found to quickly dissolve the marble but not the granite. Oxidation was found to dissolve the pyrite very quickly and continue to dissolve it over the next 2-3 days. Organic Processes were found to have a major physical effect on the paste as it broke it apart.

Materials

Bean Seeds, 10 Magnifying glass
Calcium sulfate, CaSO4, 2 spoonfuls Marker
Granite chips, 20 g Paper towels Halite chips, 15 g Pipets, Beral, graduated
Hydrochloric acid solution, HCl, 1 M, 15 mL Plastic cups, small, 3 Ice cube Sample container
Local rock samples, 2 Sheet of white paper, 8 ½” x 11”, unlined
Marble chips, 25 g Spoon

Pyrite pieces, 2 Styrofoam tray

Sand, 6 spoonfuls Stopwatch

Balance, 0.1-g precision Water, tap

Dish, aluminum Weighing dish

Forceps or tweezers

Procedure

Part 1, Mechanical Weathering: 15 g of marble chips were placed into a sample container, and their mass was recorded in the Mechanical Weathering Data Table. Water was added to the sample container until it covered the marble chips, and the observations were recorded in the Mechanical Weathering Data Table. The lid was screwed onto the sample container, and the sample container was shaken for 3 minutes. After 3 minutes, the observations were recorded in the Mechanical Weathering Data Table. The lid was unscrewed and the water was removed carefully from the sample container without removing any of the marble chips. The marble chips were weighed and carefully observed with a magnifying glass, and the mass and observations were recorded in the Mechanical Weathering Data Table. The marble chips were covered in water, shaken for 3 minutes, and measured and observed 3 mores times. The whole process was repeated with the halite chips and the granite chips.

Part 2, Geological Changes: 2 local rock samples were obtained. The 2 rock samples were rubbed together over a white sheet of paper, and the observations were recorded in the Geological Changes Data Table.

Part 3, Glacial Changes: A small amount of sand was sprinkled onto a styrofoam tray. A paper towel was used to hold onto an ice cube. The ice cube was slowly moved over the sand while the ice cube was pushed down onto the styrofoam tray. The styrofoam tray was observed before and after the sand was removed, and the observations were recorded in the Glacial Changes Data Table.

Part 6, Chemical Weathering: 3 pieces of marble were placed into a small plastic cup. 3 pieces of granite were placed into another small plastic cup. A graduated pipet was used to add enough 1 M hydrochloric acid solution to cover the samples in both plastic cups. The effects after the samples sat for 20 minutes were recorded in the Chemical Weathering Data Table. The rock samples sat overnight, and the observations were recorded in the Chemical Weathering Data Table.

Part 7, Oxidation: 2 pieces of pyrite were placed into a small plastic cup and the observations were recorded in the Oxidation Table. A graduated pipet was used to add enough 1 M hydrochloric acid to cover the pyrite pieces. The cup was allowed to sit for 2 to 3 days. After 2 to 3 days, tweezers were used to removed the pyrite pieces from the cup and place them on a piece of paper towel. The pieces were carefully observed and the observations were recorded in the Oxidation Table.

Part 8, Organic Processes: 2 spoonfuls of calcium sulfate were placed into an aluminum dish. Water was added to the calcium sulfate until it formed a thin paste. 10 beans were added and submerged in the calcium sulfate. The beans and paste were observed periodically over the next 2 weeks and the observations were recorded in the Organic Processes Data Table.

Discussion

Part 1, Mechanical Weathering: The weight of all the rocks reduced after every 3 minute period of shaking except for the granite chips. The granite chips were shown to reduce then increase in weight which the lab group inferred to be due to human error. The rocks reducing in weight parallels with the effect that mechanical weathering has in real life.

Part 2, Geological Changes: Rubbing the rocks together resulted in particles of rock breaking off. In nature, rocks often rub together and create smaller particles; therefore, this outcome makes sense.

Part 3, Glacial Changes: The ice was found to push around the sand and create scratches on the styrofoam. This finding is logical because glaciers in nature move particles with them which results in those particles having a weathering effect on the ground below the glacier.

Part 6, Chemical Weathering: The marble was found to dissolve very quickly while the granite had no visible change. These findings were interesting to me because I expected both of the rocks to dissolve quickly.

Part 7, Oxidation: The pyrite quickly dissolved and then slowly dissolved over the following 2-3 days. I expected the hydrochloric acid to have this effect.

Part 8, Organic Processes: Over a 2-3 week period, the beans started to grow and break apart the “plaster”. In nature, plants often have a similar effect on the ground as they grow.

Conclusion

These many experiments helped to portray the effects of different kinds of weathering on rock. These experiments all proved to be accurate portrayals of weathering.

Post-Lab Questions

Part 1, Mechanical Weathering
The more weathering the rocks were exposed to, the more their weight decreased.
The mass of the marble chips decreased.
The mass of the marble chips would decrease tremendously.
The mass of the halite chips decreased tremendously.
The mass of the granite chips decreased slightly.
The granite seems to be the most resistant to mechanical weathering.

Part 2, Geological Changes
Small particles from each of the rocks broke off onto the paper.
This could simulate rocks rubbing together and creating soil in nature.

Part 3, Glacial Changes
The ice cube melted as it was rubbed on the styrofoam.
The styrofoam tray was scratched up by the sand.
If a glacier moved across the surface of land, the glacier would carry particles with it and weather away at the ground as its surface melted.

Part 6, Chemical Weathering
The marble was dissolved while the granite did not visibly change.
A chemical change did occur, shown by the marble fizzling and changing color.
The type of rock can affect the rate of chemical weathering.
The sample most resistant to chemical weathering is granite.

Part 7, Oxidation
The weathering seen is this activity seems to be chemical because of the rocks bubbling and changing color.
The pyrite changed to a green color.
The pyrite had a reaction with the hydrochloric acid.

Part 8, Organic Processes
The beans expanded and broke apart the simulated rock.
Two examples of organic soil disruption are plants creating cracks in concrete as they grow and roots growing up out of the soil and displacing it.