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Not a geologist but I do have something I have been wanting to ask a geologist about. I will get some photos posted but I came across a river rock this summer up in Alaska. It was a typical egg shape except it has somehow been split in half in a perfectly straight fracture. On both of these exposed sides it appeared that rust has formed although there was no indication of rust on the outside of the stone. So, my assumption is that there was iron in the rock which had rusted and been worn away long ago on the outside but the fracture exposed new iron inside the rock which was now exposed to the elements and oxidizing. Am I on the right track?
I'm a civil engineer, so we're somewhat in the same arena.....
Post up some photos, that'd be good. Does the whole inside appear "rusty", or are there veins of "rust" running through it? Either way, you're probably just looking at an iron-rich sedimentary rock. Veins of iron rich material may have intruded the sedimentary unit, or the whole composition may be heavy in iron, causing the whole thing to appear that way. Both are common in the lithification of sed rocks.
The egg shape indicates it's been weathered a lot, probably down a river like you said The first thing to change as a rock weathers in this fashion is the outward appearance. Crack open a river rock from here in NC and you will see quartz crystals and color variations that are just beautiful, but the rock was pretty dern ugly on the outside. In geology, there is always more than meets the eye![]()