To demonstrate how weathering changes rocks over time, we did two hands on activities using chalk. The first activity showed how physical weathering affects existing rocks and also works to create new rocks, gravel, soil, and sand. They were instructed to hit two different colored pieces of chalk together over a black piece of paper. The kids took to this with vigor. It's not every day that they are encouraged to be destructive. As they banged their chalk together, the original pieces slowly changed shape. There were also large chunks of chalk that had been broken off (smaller rocks), as well as lots of chalk dust.(soil or sand). Over time this sediment is covered with new layers of sediment and eventually becomes new sedimentary rock.
The second activity showed how chemical weathering affects existing rock. To do this, students placed a piece of chalk into a clear cup and then poured vinegar over the rock. Slowly, the acid in the vinegar started to eat away at the chalk, creating what looked like tiny eruptions. This part was a test in patience. Some of the kids really wanted to stir the chalk and speed up the reaction. Stirring only made the water murky. As they watched, tiny pieces of chalk fell off the large piece and floated to the bottom of the cup. When I pointed this out, and also asked them if they had been picking at their rock, they then saw that the acid was moving the pieces and changing the existing rock. The subtlety of this activity was also a good lesson. It helped to show them that weathering doesn't change things overnight, but takes years and years. We ended by looking at images of the Hoodoos, many found in Bryce Canyon, after one of my parent volunteers asked me if I had seen them. Pretty Amazing! (see l