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Backyard Geology

Scientists can tell a lot about the geologic history of Spirit and Opportunity's landing sites by examining the rocks they find. The same applies to your backyard. The rocks you see around you offer a record of ancient happenings in your patch of planet Earth. Over the next several months, Athena Student Interns from across the United States will present a glimpse of the geology that is native to their regions.

What's in YOUR backyard?

Heyworth High School – Heyworth, IL

One of the most commonly found rocks in our area is limestone. We chose to use a limestone pillar from the front of our school. It is approximately fifteen feet tall, and two feet in diameter. Limestone’s most common color is a pale gray, which is also the color of our sample. Because of its porous eroded surface, it has a rough, sandpaper like feel to it. However, it is neither a conglomerate of rock fragments, nor is it layered.

Illinois was once a shallow ocean and the limestone was formed by all of the small creatures over time after the ocean dried up laying fossils into it. That is where it gets its porous appearance. After the ocean dried up it steadily compressed and dried up itself and that is how we got the sedimentary rock that we know today as limestone.

Limestone is a big part of everyday architecture for in Illinois, and for many other places. In Illinois, limestone is commonly used to make concrete. Fortunately for us when our school was built in 1921 they provided solid limestone pillars and plaques on our building.

Limestone is not only important for building, it also helps to keep a handle on global warming. Limestone captures carbon, holding it in a reservoir. This capture process is an important “sink” in the carbon cycle. So, it is so important that we preserve all the limestone creation processes we can for as long as we can.

Limestone is made from a white mineral called calcite. While in the ground, the calcite dissolves out of the limestone and ends up in our water system, which causes problems with plumbing, hot water heaters, and water fixtures. This is known as hard water. In central Illinois most people use a water softener to combat the problem.

Things we tried with Limestone

There are ways to tell if a piece of rock is truly limestone or not. The most common test is to place a few drops of hydrochloric acid on it (If you try this, wear safety goggles and have an adult supervise). If it is truly limestone then it will bubble. The reaction should not take very long at all, but you may have to look closely to see the bubbles. Acid rain will also tend to eat away at the limestone over time, with exception to the parts that are protected from weather damage.

Erosion isn’t a major problem in limestone, but it does take its toll over time. Both wind and water eat at our pillars. Living on a prairie, there is nothing to protect the limestone from the tornado like winds or flood like rainstorms. There are various pockmarks on the limestone from the Aeolian processes. As scientists, we predict that Gusev Crater rocks will show some of these same types of pockmarks due to the Aeolian processes at work on Mars.

Our team is working on looking at rover tracks. One of the questions we had was, if the soil material on Mars is similar to the crushed limestone we find in Illinois, what would the tracks look like? To simulate this we rolled a similar wheel over a bed of crushed limestone cement mix. We also compared this to silica sand which is made from quartz. The lines left by the tread in crushed limestone were much cleaner and less clumped than those in the silica sand.