QUESTION: I found this green rock while digging in my backyard. My yard to be a river bed long ago and thus there are a lot of rounded rocks. This one caught my eye as it isn't rounded and grey like the rest. Curious as to the kind of rock it is and if it may possibly by a Native American artifact as it's shape does not look natural. I live in the Willamette valley in Oregon.
ANSWER: Based on the geologic history of the Willamette Valley (I brew beer and frequently use the variety of hop by the way) I would say it is basalt.
The valley's history includes a period of flood basalts originating in the Cascades flowed down the Columbia Gorge towards the sea covering the sediments in the valley with basalts.
Much later during the end of the last ice age a glacial ice dam near Missoula Montana dammed up melt water that repeatedly flooded the vally up to 400 feet. Later the lake drained in a series of catstrophic floods that resulted in the rocky stream deposits that are now your yard.
I'll propose a few ideas about the rock and how it might have gotten there. It could be what in glacial terms we call an erratic, these are rocks that are ice rafted to a location. They get frozen in a glacier, the glacier snow entraps the rock, freezed and compacts, plucking it from the outcrop and carries it along to the end of the glacier where it calves off as an iceberg, and then floats it to a location where the ice eventually melts dropping the rock in a place it has no right to be. Usually these rocks are hundreds of miles from where the parent rock is located. Some large granitic boulders on the US east coast have been traced back to their origin in Canada by comparing mineralogy of the rock.
It could have been transported by the stream itself. This is doubtful due to the angular nature of the rock. The rounded rocks you describe are typical of what happens to rocks transported by saltation. They bounce along the bottom grinding and chipping one another into roundness. This happens on the large scale, all the way down to sand size. We can look at sand and tell how relatively old it is, and by the depositional environment by the level of roundness. Wind transported sand has a frosted, like your bathroom shower glass, look to it, where water transported sand does not. Your rock is too angular to have been transported that way.
I don't know where the closest basalt deposit is to you, but that is probably where it came from. Basalt is an extrusive volcanic rock. It occurs in a variety of types. Think of it as a family of rocks. The greenness of the rock and the porphyries, are probably due to it being high in Olivine a ferro magnesian mineral. That would make it an Olivine Basalt. These minerals are common in the darker rock forming minerals. Minerals like quartz and feldspar make up most of the rock forming minerals and the rocks are classified by the relative percentages of the minerals in them.
Basalt is usually dense and has a somewhat conchoidal fracture, that means it breaks in the scalloped, glassy way yours appears to. This could be due to the high content of Olivine. At first I thought it might be obsidian, but it being wet, makes me think that once dry it won't have the glassy appearance it does in the photo. If it is indeed very glassy, it could be obsidian, which from the tool making standpoint, can be flaked into a much sharper edge.
Having said all that I do indeed think you might have a very primative tool It looks similar to the ones used in the PALEOLITHIC (Not neolithic) period to dismember animal carcasses. Kind of a hand held axe. The rounded part would be held in the palm and the point would be used as the edge to crudely cut and dismember a kill after a hunt. Many of these have been found in the stampeed fall kill sights where heards of animals were stampeeded over cliffs in Montana by early man in order to kill them, and then were dismembered on sight. So it is likely that the rock was indeed dropped or left at the place you found it, after the stream deposit was formed by some early tribe of hunters.
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QUESTION: Thank you for the very informative answer! I didn't mean to attach the same picture twice. I attached a profile picture of the rock aswell that gives a better look at the color of the rock.
Okay, now the new picture opens up at least one other possibility. Do you know if there were any old iron or other types of smelters in the area? The rock definitely has some flow pattern to it, and looks a lot more glassy in this picture than in the first. The reason I ask about the smelters, is I come from an area of Virginia where iron was smelted in stone furnaces up till WWI. They used oak charcoal in place of coal, and limestone and iron rich sandstone for ore. This resulted in a lot of slag, a mix of quartz silicate and impurities that looked a lot like this rock. It had a dense glassy appearance, and I can see bubbles in the broken edge. They would run out the iron from the bottom of the furnace into sand molds that formed ingots, that looked like piglets nursing on a large central ingot, and that gave the product the name....wait for it....Pig Iron. Seriously. After the molten iron had been run out, they let the molten slag run out into the nearby creek where it cooled and fractured.
Similar stuff used to be pulled from the boiler furnaces of old steam locamotives too, but not big chunks, smaller clasts that can sometimes still be found in the ballast of long serving railroad track beds.
The mineralogy is probably as I said, a lot of Olivine and maybe even some Epidote. If no smelters were ever operated nearby, then it is probably an extrusive piece of basalt very high in olivine and epidote. There is a small chance, it would take more mineralogic analysis than I can give from a photo, that it could be a partially melted xenolith, that is, a rock carried from deep in the mantle up to the surface where it does not belong. These occur when they are plucked by the upwelling magma from the magma chamber wall, and transported to the surface similar to the way I described the glacier doing it with rocks. The magma flows out as basalt, or cools underground as gabbro, coarse grained similar to granite, but black and with the mineralogy of basalt. The xenoliths are preserved if they do not have enough time to melt and be incorporated back into the magma. So we get a snapshot of the rocks that exist deep in the earths mantle.