Saturday, April 24, 2010

Adirondack Geology: Mysteries of Rocks and Minerals

Geology – it’s the backbone of this planet, and one area in which I find myself deficient in knowledge. It’s not that I don’t like rocks – I had quite a large collection as a kid, and even today I am drawn to rock shops. It just seems that geology is something my mind refuses to hang onto. Oh, the broad strokes are easy enough to remember (like how the Adirondacks are built of some of the oldest rocks on Earth, but the mountains are fairly young), but the little details, well, those I always have to relearn. So, I thought I’d put myself through a small crash course in rocks and minerals, and on the off-chance that others out there are similarly confounded, I decided to write about my findings.
First, what is the difference between a rock and a mineral? A mineral, according to Discover Nature in the Rocks by Rebecca and Diana Lawton and Susan Panttaja, is “the solid phase of a non-living, naturally occurring substance.” Minerals are composed of atoms arranged in a crystalline structure. Now, these atoms could be all of one kind, like they are in native metals. For example, in silver all the atoms are silver atoms. Other native metals include copper and gold. Some minerals, however, are composed of molecules of more than one kind of atom. Many multiple atom minerals are familiar, like table salt, which consists of sodium and chloride atoms. Chemically we call table salt sodium chloride, while its mineral name is halite. Minerals found, and some even mined, in the Adirondacks include mica, quartz, garnet, and pyrite.

Rocks, on the other hand, are all part of the Earth’s crust. From the small pebbles that get into our shoes to the glacial erratics and mountains that make the Adirondacks what they are, all these rocks originated in the thin layer that covers the molten and semimolten mass that forms the basis of this planet. Rocks come in three flavors: igneous, sedimentary and metamorphic.

Anyone who went through Earth Science in high school should be familiar with these terms. Igneous rocks are formed from once-molten material. The majority of igneous rocks were made by minerals that crystallized out of molten materials (magma) from deep below the Earth’s surface. Here in the Adirondacks, every time we see a piece of granite, we are looking at an igneous rock. Granites are rich in several minerals: silica, potassium, sodium, quartz, and alkali feldspars.

Sedimentary rocks are created when particles (or sediments) come to rest after being moved by wind and/ or water. Their final resting place is usually underwater, but some end up in deserts or on sand dunes. Sandstone and shale are two examples of common sedimentary rocks.

Metamorphic rocks were formed under either great heat or great pressure. They started life as either sedimentary or igneous rocks, or maybe even as a another metamorphic rock, but then they underwent a change. A simple example would be a slab of shale that gets squashed by a heavy weight (say, a thick layer of additional sediment) and ends up compressed into slate. Marble is an example of a metamorphic rock that was created when a chunk of limestone underwent extreme heat. Grenville marble forms the bedrock under Rich Lake here in Newcomb. Because of its limestone origin, it has provided a natural buffering agent to the lake, protecting it from the effects of acid deposition. A third type of metamorphic rock is formed when mineral-rich water is heated to extreme temperatures. This superheated liquid moves through the rock around it and either changes the rock’s structure or its mineral composition.

Contrary to popular belief, rocks are not static. They are constantly changing, but few of us witness this change because it happens in geologic time; in other words, very slowly. Still, the observant naturalist can see this change if he or she looks for it. A great place to start, and one easily found here in the Adirondacks, is at a glacial erratic. These boulders are found lying about the woods, far from their native homes. The term “glacial” tells us that they were deposited by the glaciers that passed over this area over 10,000 years ago. The word “erratic” means they came from somewhere else. Here at the VIC in Newcomb we have a wonderful glacial erratic sitting next to the Rich Lake Trail. In almost ten years of passing this rock, I’ve noticed that the crack that runs down its face has widened. Water gets into this crack and alternately freezes and thaws, each year making the crack a little bit wider. Eventually, I suspect the slab will fall away from its parent rock, but probably not within my lifetime.

Some rocks and minerals are fairly easy to identify, like mica and pyrite and granite. Others may require more in-depth investigations. I recall from a soils class I took in college that we had to test various rocks with chemicals to get positive IDs. I find, however, that the easiest route to take is a visit to the local rock shop, where the proprietor is often quite happy to help me out with any geologic conundrum I pick up. I have discovered that rock-hounds are a lot like birders – they are intensely “into” their subject and know a great deal. The Adirondack Park is fortunate to have more than one rock shop. I know of two within about a half-hour’s drive from where I live: one in Long Lake, and the other at Natural Stone Bridge and Caves in Pottersville. Kids love rocks and these shops can be a lot of fun for serious rock-hounds and novices alike.

The problem with being a JOAT (jack-of-all-trades) is that I find too many things to be interesting. Here I was happily buzzing along thnking that insects were my latest “thing,” but now I find myself wanting to know more about rocks and minerals – geology. I’ll take some time to finish reading my Discover Nature in the Rocks book and see if I can’t commit some of it to memory. But before that happens, I wouldn’t be the least bit surprised if something else came up and my curiosity swept me away down another tangent. Ah – the joys of being a naturalist!

Ellen Rathbone

Ellen Rathbone

Ellen Rathbone is by her own admission a "certified nature nut." She began contributing to the Adirondack Almanack while living in Newcomb, when she was an environmental educator for the Adirondack Park Agency's Visitor Interpretive Centers for nearly ten years.

Ellen graduated from SUNY ESF in 1988 with a BS in forestry and biology and has worked as a naturalist in New York, New Jersey, and Vermont.

In 2010 her work took her to Michigan, where she currently resides and serves as Education Director of the Dahlem Conservancy just outside Jackson, Michigan.

She also writes her own blog about her Michigan adventures.


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