Bad News, Good News, and Really Good News
A trip to the repair shop leads to insights on geologic formations and thermal expansion valves.
By Bill Langer
This morning, I got up bright and early and took our 2004 mini-van to a little repair shop in North Phoenix — the one just down the street from the Volkswagen Bug perched atop a pole outside the shop. Our air conditioning was acting up, and I hoped it only needed a recharge. Ken, the mechanic, said, “I have some bad news and some good news.” The bad news was that the thermal expansion valve (TXV) was stuck, and it would take about six hours labor to replace it. “The good news,” he said, “is that I might be able to fix the valve rather than replace it.” Ken then described a novel (to me) approach he has used to free the valve. I heard Ken is a top notch mechanic so, what the heck, I gave him the go ahead.
While I was waiting to find out if Ken’s idea would work, I perused some of the hot rod magazines in the shop waiting room. An ad for some rubbing compound caught my eye. It was a picture of a rock sitting on a double yellow line in the middle of the road. The caption was something like, “So you are millions of years old, you are still just a rock waiting to dent my car.”
Just a rock? Phooey. My mind raced through some of the rocks I have seen and how different they can be, even when located in the same geologic formation. There is more than one type of rock waiting to dent a car.
Within one igneous body, I have seen rock with a specific gravity of 2.7 within 100 feet of another rock with a specific gravity of 3.2. This is because, when magma solidifies, each mineral begins to crystallize at a specific temperature and pressure. If the crystals are more dense than the liquid, they sink. If they are less dense than the liquid, they float. The process is called magmatic differentiation by crystal fractionation. So seeing light rocks overlying heavier rocks in a body of igneous rock is no surprise.
I have also seen sedimentary rocks of dramatically different properties overlying one another. This is because sediments are sorted as they are transported and deposited. Water with a higher velocity carries sediments that make conglomerates and sandstones. Slower moving water carries sediments that make siltstone and shale. Some types of limestone form in very still water with specific chemical properties. So seeing limestone inter-layered with sandstone or shale is quite common.
There are many more geologic processes that affect rock properties such as metamorphism and tectonics, but you get the point. That said, I have some bad news and some good news. The bad news is that I saw all the rocks described above in active quarries. While homogeneous rocks can lead to consistent rock properties, varied rocks commonly lead to varied rock properties. This can greatly complicate the job of producing aggregate of a consistent quality. Indeed, the specific gravity example caused quite a stir.
The good news is that a good geologist can anticipate changes in rock properties in your quarry. That knowledge may ultimately save you time, money, and headaches by allowing you to prepare for changes rather than be surprised by them.
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