GRANITE PEAK PUBLICATIONS: Accompanying travelers to the national park since 2002

Yellowstone Rock Analysis 101

mystery rockNot long ago I received a picture of a rock that came from Yellowstone, sent to me by e-mail by a visitor who had picked it up this summer. So I asked my geologist husband to take a look at the picture. Every geologist gets asked frequently to help a friend or acquaintance figure out what some rock is. As you can see from what I wrote to this correspondent, it isn’t all that easy!

. . . I’ve consulted my geologist husband about your rock, and he and I came up with some ideas to pass on to you, but we cannot really answer your question.
I have to say I am very sorry you picked up this rock, since it really is against the law in all national parks to take anything natural out of the park. [Ed.: She later found out that they did not remove the rock from the park.] “Take only pictures, leave only footprints” applies to everyone. However, since you did take it, I’ll send a few comments.
1. If you can remember where you were when you picked up the rock, that would help a geologist determine what it is, since the entire park has been mapped for its geologic history.
2. Are you sure it’s a rock and not something manmade that is highly weathered? It is probably a rock if it’s heavy and dense. Note that a weathered surface may look entirely different from an unweathered one.
3. Were there a lot of rocks that looked like this lying about or only one or two? This would be a clue as to whether it was part of the bedrock of that area or brought there by a glacier or former stream.
4. To properly analyze a rock you have to break it open with something like a sledge hammer against a hard surface and separate rock fragments from anything else.
5. When broken, does the inside look the same as the outside, especially the color? Do the dark bands go through the entire rock?
6. Related to where you picked it up, it may be either a sedimentary rock (probably laid down in layers at the bottom of a sea or lake) or, more likely in Yellowstone, an igneous rock containing magnesium and iron that’s left from a long-ago volcanic eruption.
7. A far-out guess might be that the whitish parts might be plagioclase (igneous feldspar) and the greenish parts, pyroxene or olivine.
The main thing is that a geologist really must both know where it came from and be able to see and physically test it to determine what it is.