Backyard Geology: The Green Mountain Kimberlite

Unfortunately, I can’t provide very good directions to this one, and there’s a good reason for it. We drove up to Green Mountain (near Boulder, Colorado), got on one of the trails, and then at a random time just sort of bombed off into the underbrush. It involved going down and back up an extremely steep stream valley where there wasn’t even the hint of a track. Steep, like I’m clinging to trees to keep myself from tumbling down the slope steep. It was a very, very, very rough hike for someone with bad knees and an often embarrassing lack of balance. It’s only about a mile and a half, but it feels much, much longer.

About the best I can do right now is give you the lat/lon of the outcrop: 39º59.431’N, 105º18.09’W. These coordinates should have about a 20 foot accuracy if you believe the claims of the GPS unit’s manufacturers.

That said, the hike is very, very worth it. Big important note, though: the kimberlite is in park land. I honestly have no idea what trouble if any there could have been for us going off trail the way we did, but I know for certain that you’re not supposed to bring in a rock hammer and whack samples off the outcrop.

The kimberlite itself is very interesting. It intrudes through the Boulder Creek granodiorite, which is a holocrystalline intrusive rock with large crystals of quartz, feldspar, and mafic minerals. If you run across Boulder Creek outcrops, they have a distinctive “salt and pepper” appearance. In comparison, the kimberlite is a porphyritic extrusive rock where the ground mass is extremely dark. The samples we found contained large garnets, ilminites, and olivines. The weathered surface of the kimberlite is gray rather than black, with the chemically altered phenocrysts much more obvious by color difference.

The outcrop is mid to upper slope and stands out fairly well from the landscape. There are no trees growing in it. The outcrop itself is about 100 feet in diameter, though on the down slope it elongates into a teardrop-like shape due to the erosion of the slope.

So, a tough hike, but very cool rocks.

Kimberlite is actually one of my favorite igneous rocks, mostly because it’s very cool to look at in thin section. Much of the fine-grained ground mass in kimberlite isn’t actually silicate minerals – it’s calcite. This makes it incredibly colorful when looked at with crossed polars.

The story behind kimberlites is also very cool. They are effectively volcanic dikes, but rather unusual ones. Kimberlitic magma is produced when there’s a critical mass of volatiles in an area of the mantle, normally carbon dioxide and water. (The large amount of carbon dioxide present is the reason kimberlites contain so much calcite.) The volatiles lower the melting point of the surrounding mantle material, and with the sudden pressure on a body of volatile-filled magma, the results are explosive. The magma exits the mantle upward and comes exploding out of the crust, in some cases at the speed of sound. This incredibly explosive, violent eruption of magma under high pressure is what gives kimberlites their characteristic carrot or funnel-like shape.

Also, since the eruption of a kimberlite is so violent, they often carry significant chunks of everything they went through to get to the surface. This includes pieces of mantle peridotite – most of what we know about the mantle composition came from samples brought up in kimberlites. In certain areas, this also means that the kimberlite brings up pieces of old continental crust – most importantly, pieces of the remaining cratons from the Archean. And these craton bits are where diamonds come from. Kimberlites can be small (like the one on Green Mountain) or enormous, like the ones that are mined for diamonds in Africa.

And the best part? Technically speaking, we could get another one erupting out of the ground at any time. There’s no way of knowing. There’s just something cool about that thought, though I wouldn’t want to be standing on top of one when it made its appearance.