Five Things You Should Know
The Geologists' Corner
Photos and Timelines
Bill and Marty's Rock Garden (Extended Text Version)
By Bill Bonnichsen and Marty Godchaux (Bios)
Each rock has its own story, and we've selected a bunch of typical rocks from Idaho, kind of in chronological order [click on the thumbnails for larger images].
Dunite: This first rock here came from near New Meadows. It's a rock type called Dunite. It was formed when a piece of the sub-oceanic sea floor got pushed up onto the continent, and then it got heated as the Idaho batholith was emplaced. And the brown stuff in here got turned into this really fancy fibrous mineral which is anthophyllite. Later, as it cooled off, that was turned into talc; so this rock has a really interesting history, and it tells us a lot about how that part of western Idaho formed.
It's an interesting rock because it was originally mainly the mineral olivine, which is a very high temperature mineral; but after it was in its final resting place, the Idaho batholith was injected nearby and heated it up and added a little bit of water. So this nice fibrous mineral and anthophyllite formed at the expense of part of the olivine, and as that cooled off, more water was added; so the mineral talc partially replaces the anthophyllite.
Granite: And going almost to the other end of the state, from way up near Bonners Ferry, we have a nice piece of granite with a dark inclusion. That was probably the country rock into which the molten magma seeped, and it seeped around cracks and incorporated this piece of darker rock.
It's typical granite in that it's got these great big pink things that glint when you rotate them in the sunlight, and they are called potassium feldspar. They are characteristic of high level granites. There are some mafic minerals — the ones that are dark colored and have magnesium and iron in them — but not a whole lot, so this is a pretty typical granite intruded into the older rocks of northern Idaho.
Sedimentary rock: This next rock here is this sedimentary rock, part of the Paleozoic package of rocks in central Idaho. Some of the older part of the Idaho batholith came up under it, metamorphosed it, so it turned the original carbonate minerals into things like augite and scapolite and anorthite, and you see the very delicate banding in it preserved all the way through metamorphism.
If you look carefully, you see this really pointy thing. The thing may have been folded very tightly in sometime in its history before it got thoroughly metamorphosed. And then erosion dropped a chunk of this off in the stream systems, and it got rounded into the big river cobble that you see today.
Volcanic rock: This is certainly one of my favorite pet rocks. It comes from a Challis-age caldera in northern Idaho near where I live. I live in Moscow, and this comes from Deary, a small town east of Moscow. There's a unique feature called Potato Hill. Everyone asks if it is a volcano because it is kind of shaped that way, and the answer is . . . Yes and No.
It's not a present day volcano. It's about 50 million years old. It's a volcanic rock that has brought up with it great chunks of both Idaho batholith rocks and rocks of the sedimentary Belt Super group. The caldera is not perfectly preserved but one can see that this is one of the products probably fairly early in the eruption when water got into cracks and caused it to erupt explosively, so it had the carrying power to bring all these big pieces of pre-existing rock.
It's kind of a lone volcano up there, not clustered the way they are in central Idaho, but this is a 50 million year old Challis rock.
Rhyolite: Okay, this next guy here is one of my favorites. It's a piece of rhyolite; and rhyolite comes in many colors, sometimes black when it's still glassy and sometimes red. This has a nice purple tint, and then you see that it is layered.
This rock formed as huge eruptions of rhyolite laced with water exploded out of the earth, spread as a wide ash flow sheet; but it was so hot, it coalesced back to flowing material that behaved like lava. In the process of doing that, it became crystalline; but before that, flow layers in the flowing material were formed into tight folds. You can see these very tight isoclinal folds here. It's a typical rhyolite of the Snake River plain. It's a pretty piece of rhyolite.
Columbia River basalt: No pet rock display in Idaho could be complete without some piece of the Columbia River basalt, one of the famous formations in Idaho. And this is a piece of a basalt pillow where a flow of the Columbia River basalt was flowing up the valley of a fairly consequential stream with a lot of water in it. So it made pillows; it exploded and made this yellow stuff, which geologists call palagonite. It's kind of a clay that comes from the minerals of the basalt that are altered in hot water.
This was exposed in a road cut where on either side there are rocks of the Idaho batholith, so this was a tongue of a basalt flow that flowed up a valley — a steep-sided valley that was already cut in rocks of the Idaho batholith.
So, what happened was there was this big lake in southwestern Idaho — Lake Idaho — and sand and volcanic ash washed into the sides of it and made muddy banks. A lot of shell fish grew in that environment, including these turritellid gastropods that you can see here.
These shells grew in that, and then that sediment was buried, and later in time, solutions migrating through the area made a concretion; so the body of rock itself was a concretion cemented by calcite, and it has incorporated the shells in it, so it is preserved in that fashion.
Basalt: And the final pet rock for our display is one of the youngest rocks we ever hope to find in Idaho, and this comes from near Craters of the Moon. It is what we would call a cored bomb. The internal part is a slightly older basalt — older perhaps only by a thousand or two thousand years — and then there's this vesicular scoria coat around that; so a rock from one eruption gets caught up in the frothy gas-rich phase of an eruption that comes afterwards, and is blown out of the volcano to go through the air making a whistling sound that you've probably seen on nature programs about Kilauea.