Saturday, January 25, 2014

Geo 730: Jan. 25, Day 390: Galice Slate Not-Fold

A point I'd considered bringing up yesterday, and which is actually better illustrated in that photo, is the apparent fold seen above the hammer in this picture. As you can see, the thin slabs of slate are weathered and broken up. What you can't see from this perspective is that this is a steep slope, around 45 degrees. The view is directly into it. Looking at yesterday's photo, you can see that the apparently unbroken exposure below and to the left of this area has a fairly consistent strike and dip of foliation (which is not necessarily the same as the original bedding- it most often isn't in outcrops of the Galice where I've been able to identify the bedding). There may be some minor flexure, but certainly not to the degree that seems to be presented above. So I believe what we're seeing here is simply creep: incremental downhill motion primarily under the influence of gravity, aided by freeze-thaw cycles, clay wetting/swelling-drying/shrinking cycles, and perhaps bioturbation. That creep is quickest in the area where the hammer is sitting, giving the loose rubble the outward appearance of a syncline.

Photo unmodified. May 8, 2013. FlashEarth Location. Indexed

Friday, January 24, 2014

Geo 730: Jan. 24, Day 389: Galice Slate

So far this year, I've been focused heavily on the igneous component of the Josephine Ophiolite. I'm pretty sure I read recently that the term "ophiolite" has been restricted to strictly the igneous sequence, with which I have no problem. However, as soon as the fresh sea floor is exposed- typically pillow basalt or breccia- it's subject to sedimentation. The sources of such sediment are numerous: near the ridge itself, metallic sulfides from black smokers can be significant. Accumulation of siliceous tests, for example from radiolaria and diatoms, in the absence of terrigenous sediments, can accumulate surprising thicknesses.
Image Credit: College of The Redwoods
In the case of the Josephine Ophiolite though, as a back-arc basin, it started receiving terrigenous (land-erosion-derived/terrestrial) sediment very soon after the sea floor was formed. In the above schematic, this sediment is labeled "pelagic sequence," and is named the Galice Formation. During deformation and compression, presumably associated with the accretion of the western Jurassic belt to North America, the Galice was metamorphosed to slate. This outcrop is at a little park just south of the major tunnel on 199.

BTW, the issue with the index I mentioned yesterday has been fixed, and based on several test clicks, last year's links are now working correctly.

Photo unmodified. May 8, 2013. FlashEarth Location. Indexed

Thursday, January 23, 2014

Geo 730: Jan. 23, Day 388: Serpentine Slickenside

Dana's photo, and Dana's hand over a very nice seam of serpentine at the Patrick Creek site. When you walk up to this outcrop, you may spot some small chips on the ground, but as a rule, you won't see anything like this on the surface. It takes some prying and digging to find the really pretty samples. Since the whole cliff is composed of crushed, broken rock, that prying and digging is quite easy, but requires a certain amount of caution: you don't want to bring a ton of it down on yourself. Pay attention to what is supporting what. As you pull a slab loose, watch to see what else is moving. Keep to one side or the other of your work area, rather than directly below it. I've never felt threatened or endangered at this site, in the sense of immediate peril, but danger is certainly present. Caution and common sense seem to be good ways to avoid the threats that are present.

(Update 6:30 PM: Problem described in this paragraph has been fixed. I think.) I noticed yesterday that all the links to last year's Geo 365 posts (though not this year's Geo 730 links) in the Geo 730 Index are, shall we say, messed up. This isn't a big problem, and I've already tested a method to fix it relatively quickly and easily. However, actually fixing it for once and for all will take some time, and will involve moving massive amounts of data back and forth, as well as making at least one or two precautionary back-ups so I don't accidentally lose many weeks' worth of work. In the meantime, here's a work-around. Taking as an example the very first link in the list, here's the URL that your browser will attempt to open if you click on it: "" Select and delete "," and that will leave you with the correct URL. I'm not sure how this happened, and in relative terms, it won't be too time-consuming to fix. But I want to make sure I'm at peak alertness and minimally distracted before I put in the hour or so of work it'll take to do it right, so it won't be happening right away.

Photo by Dana Hunter, unmodified. May 8, 2013. FlashEarth Location. Indexed

Wednesday, January 22, 2014

Geo 730: Jan. 22, Day 387: Barren Hillside

One of the things that jumps out at you, when you're in ultramafic rocks, especially those that have been serpentinized, is the ragged, spotty vegetation cover on them. There are a number of reasons for that: first and foremost is nickle toxicity, and to a lesser extent, chromium toxicity. While of vanishing importance in typical continental crust, these two elements are much more abundant in ultramafic rocks

The second reason is a little more complicated, but boils down to a single word: clay. Nearly a year ago, I wrote about the importance of feldspar and clays in dune deposits, and the issue raises its head here, again. In this case, the problem is that there is proportionally very little feldspar in these rocks, so when they weather, you get various metal oxides, hydroxides, and hydrates, but little clay. That in turn means that the soils don't do well at retaining water and plant nutrients.

The two factors of heavy metal toxicity and poor soils mean that plant communities on ultramafic rocks and serpentinite are limited, and often pretty marginal-looking, that is, they look as if they're barely hanging on. On the other hand, some plants have exquisitely adapted to these conditions, a topic that came up twice in the most recent Accretionary Wedge. Update: Dan McShane (of Reading the Washington Landscape) points out a couple of other stress factors for plants in this geological setting in the comments.

What all this means to geo-enthusiasts is that even doing 60-MPH reconnaissance geology, it's pretty easy to tell when you're going by these sorts of rocks.

Photo unmodified. May 8, 2013. FlashEarth Location. Indexed

Tuesday, January 21, 2014

Geo 730: Jan. 21, Day 386: Sepentine In Situ

Another one of Dana's photos, this shows how the serpentine appears in the outcrop. There are innumerable criss-crossing shear zones, and the serpentine fills those faults, to a greater or lesser thickness. Where it's thicker- say, on the order of a millimeter or more- one can often pry the translucent mineral away from its host rock. The host rock here is also mostly serpentine, but microcrystalline, and pretty dull.

Photo by Dana Hunter, unmodified. May 8, 2013. FlashEarth Location. Indexed

Monday, January 20, 2014

Geo 730: Jan. 20, Day 385: "It's...uh,'s green."

Dana's photo, my left hand. The seams of serpentine here are often translucent; while clearly green in reflected light (even to my eyes, which don't do well with reds and greens), in transmitted light, the color is beautiful. This is a small chip, but nice, nevertheless.

Anybody recognize the quote in the title? (Start at 1:15)

Photo by Dana Hunter, unmodified. May 8, 2013. FlashEarth Location. Indexed