Don't Take Limestone for Granite!

 

Okay, I know the title doesn't make much sense, since granite is an igneous rock and limestone is sedimentary. But! They are both rocks! So that's a similarity. Mainly I just wanted to make a pun of some sort. 

So, I searched for two days, digging through snowy fields and picking rocks out of flowerbeds, and trying to pry open the frozen ground around my apartment, and I got nothing. I found plenty of igneous rocks, tons of igneous rocks in fact, but no sedimentary rock. It was very frustrating, not to mention cold. Finally I gave up on that and curled up on my couch with a hot mug of cocoa, and read through a 1978 report on the geological makeup of the soil and reocks in Eastern Idaho and discovered that there really aren't that many deposits of sedimentary rock around here, but there are several limestone quarries! So I decided to write about limestone for today's geojournal.

What tells you that limestone is sedimentary? Honestly, I didn't know that limestone was a sedimentary

rock. When I look at it, it seems metamorphic to me on first inspection. Usually when I think of sedimentary rock, I think of sandstone: plenty of striation and visible strata, obviously made of sand, and very grainy in texture. Limestone, on the other hand, doesn't present obvious striation, and it seems to be relatively whole, and rather than being grainy it's quite smooth. Plus, whereas I can break sandstone with my hands, limestone is a harder rock, and more difficult to break. Of course, this all applies to limestone with which I have personal experience. So I didn't think of limestone as sedimentary, but it totally is! In fact, it turns out that as a classification of rock, about 10 percent of all sedimentary rock on Earth's surface is one or another form of limestone! 

Limestone is a carbonate rock, made up mostly of calcite and aragonite. Interestingly, even though many limestones are pure calcite, they are not considered a mineral, partially because it isn't formed of calcite in its pure form but rather from the calcite present in shells, coral, algal, and other debris, and also because it's formed through the laying down of sediment rather than any kind of chemical bonding--at least, that's according to the geology department at Cal State LA. Limestone is a fascinating rock, because it tends to form giant drainage caverns (which are apparently called karst landscapes) through chemical weathering, meaning that most major cave systems are found in limestone rock. Also, it has served as a popular building material for well over 4000 years. The pyramids are made of limestone, well over 5 million blocks between the three on the Giza plateau! Limestone has been a popular building material all over the world since humanity began building with stone and not just mud brick! It's a stone which has shaped our destiny, and I just find that supremely fascinating. So here's to limestone. Let us never take it for granite!












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