In the past couple of weeks, quite a few people have asked me for experiments on erosion. I am not sure if it is just a coincidence, but it is a good excuse for an experiment on geology. Geology was the subject that first got me interested in science, and it continues to be a favorite subject for me. This week, we are going to investigate chemical erosion from rainfall. This is a very slow process, that can take years to produce measurable results. Then how are we going to study it? For that you will need:
- several sheets of white paper
- some crayons, chalk, or charcoal
- a trip to a local cemetery
This may sound like a strange mix if things, but we need to find some pieces of rock that have been exposed to the weather for many years. We also need to be able to tell how long each rock has been exposed. The markers in a cemetery are perfect for this, as each has the date that it was placed carved into it. Look at the different markers, noticing the age of each, and how weathered the stone is.
The first thing you will notice is that several different kinds of stone are used in markers. Igneous rocks, such as granite, are very resistant to weathering and will not help us much unless you are looking at a VERY old cemetery. For our purposes, we need to look for stones made from either limestone or marble. Limestone markers will be gray and may resemble fine cement. Marble is usually white for markers, but can be almost any color. Both of these stones will show signs of weathering after several years.
Make rubbings of markers of different ages by placing a sheet of paper over the front of the stone and rubbing it with the side of a crayon or chalk. Be very careful not to mark on the stone itself. We do not want to cause any damage.
Once you have found a wide range of ages, compare the amount of weathering. You may also need to consider other factors, such as the impact of overhanging trees or variations in the hardness of the stone.
What is happening to the stones? They are being slowly dissolved by acids in the rain water. This acid may be the "acid rain" that is caused by industrial pollution, but it can also be natural carbonic acid that is formed as a rain drop falls through clean air. Some of the carbon dioxide in the air dissolves in the water, forming carbonic acid. This is the same acid that gives a carbonated drink its fizz, but in a much weaker concentration.