Geo 730: May 20, Day 506: Lawsonite?

I'm really not sure what this pocket of minerals actually is, but lawsonite seems a reasonable guess. It's fairly common at Bandon's blueschist jetty, and should be easy to find for other visitors- probably not this same block, but there are plenty of similar samples. In reading over the wikipedia entry on lawsonite, the thing that jumped out at me- which I had either not known or had forgotten- was how hard that mineral is: 7.5-8 on the Mohs scale. This gives an easy, if not necessarily diagnostic, field test if you have or can find a bit of quartz lying around. A fragment of lawsonite would be able to scratch quartz, and there are relatively few minerals for which that's true. I need to remember that the next time I visit Bandon!

Photo unmodified. March 9, 2012. FlashEarth location.

Geo 730: May 19, Day 505: Busy in Blue

There's a lot going on in this block. First, there's the exquisite weirdness of blueschist, which never really gets old, no matter how much time you spend at Bandon's jetty. Next, it looks as if the rock was cataclastically shattered during metamorphism; the white slabs (quartz?) on the left can be sensibly reassembled into a more or less coherent whole. On the other hand, the clasts around and above the hammer's head look more chaotic. Finally, there are a number of what appear to be veins shooting through in a few places. The two that are clearest are running through the slabs on the left.

Photo unmodified. March 9, 2012. FlashEarth location.

Sunday Funnies: Fail Edition

Bits and Pieces

Bad Newspaper

Pleated Jeans

Wil Wheaton

Tastefully Offensive

Darius Whiteplume

What Would Jack Do?

Funny to Me

I hate it when the TP runs out. Nice to have some extra around, though it starts to smell funny. Bad Newspaper

See? Aren't you glad there's extra? Very Demotivational

Funny to Me

Tastefully Offensive

Fake Science

Very Demotivational

"Curse that chili bean burrito!" Senor Gif

Bits and Pieces

Wil Wheaton

Cyanide and Happiness

Bits and Pieces

Very Demotivational

Texts From TNG

Narcolepsy Incorprated

Julia Segal

Very Demotivational

Happy Place

Cheezburger

Bits and Pieces

Happy Jar

Texts From TNG

Poorly Drawn Lines

Married to the Sea

Blackadder

Blackadder

Bits and Pieces

Geo 730: May 18, Day 504: Patchy Blue Amphibole

Here we see a fairly common phenomenon in sheared coarse metamorphic rocks: a tendency to segregate into separate groups. I don't entirely understand this; it certainly isn't based on something as simple as color, though it can appear to have that effect. It's more likely due to some physical characteristic like susceptibility to plastic deformation. In this case, more plastic mineral groups such as chlorite end up somewhat segregated from less plastic minerals, like the glaucophane. Also, if you enlarge the photo to full-size, you can spot bright little red garnets all over the place, though for me, at least, they tend to stand out better from the lighter micaceous matrix.

Photo unmodified. March 9, 2012. FlashEarth location.

Saturd80's: Guadalcanal Diary Edition

"Where Angels Fear to Tread," searing satire of fundamentalist religionists from 30 years ago. I've been trying to find a recording of this for ages!

"Watusi Rodeo"

"Always Saturday" I've posted this before, but what the heck: it's a great song.

Geo 730: May 17, Day 503: Bluedinage

The correct term is actually boudinage, but, hey, it's in blueschist, so there's your geo-pun for the day. Boudinage forms when two components of a strained rock have different competencies, with one more plastic, the other less so. The more plastic component tends to deform more easily, shearing due to the stress it experiences, while the other tends to deform more brittley, breaking up into discrete blocks and pieces. There are a large number of boudins visible in the above photo, and I've annotated just a few below to help clarify the idea for those not familiar with it.

In this case, the more plastic component is the mica-rich regions, probably mostly chlorite. The more brittle component is the glaucophane-rich regions, which are dark blue.

Photo unmodified. March 9, 2012. FlashEarth location.

Geo 730: May 16, Day 502: Pile O' Schist

Pyrite and glaucophane stand out in this shot, but I'm not sure what the light colored minerals are. There are at least two different light minerals here- one looks a bit greenish to me, and may be mostly chlorite. The two patches, one above my finger tip, and the other above my finger in the lower left corner, look like they might be quartz. Albite can be a significant constituent of blueschist, but I'd expect better defined cleavage than I see here. Long and short, I'm just not sure. What I am sure of, though, is that this would make a stunning thin section. There are very few rocks as pretty in either plane polarized light or under crossed polars as a nice, coarse-grained blueschist!

Photo unmodified. March 9, 2012. FlashEarth location.

Geo 730: May 15, Day 501: (Blue) Schist Happens

Trying to get close-ups is problematic... I seem to have a difficult time getting the plane of focus actually aligned with the rock surface. The mica, likely mostly chlorite, is nice and sharp, but the glaucophane (dark blue) and pyrite (gold) are badly fuzzed out. That said, this is one of those amazing oddball rocks I love so much, because its mere existence tells an incredible story. Blueschist is composed of a suite of minerals that indicate extremely high pressure, but comparatively low temperature. Until the advent of plate tectonics theory, which came into its prime about 50 years ago, and broad acceptance and utility over the last 40 years or so, this rock was enigmatic in terms of its genesis. With the realization that subduction could transport great slabs of cold oceanic crust down into the mantle relatively rapidly- that is, carried into much higher pressure environments before heating up to expected higher ambient temperatures for those depths- came the understanding of how such a rock could form. The jetty here at Bandon was quarried from a larger pod of blueschist in the marine terrace on the Coquille River's south side. That pod, in turn, is a part of the Otter Point Formation, a classic melange correlative to the famous Franciscan Melange.

Now the problem with melange (which, incidentally, is French for "mixture") is that it is a complete jumble of rock types that have no apparent business being together. For example, the blueschist under discussion is juxtaposed in many places against poorly sorted- and unmetamorphosed- marine sandstones. Try as they might, geologists could find no mappable patterns within melanges, and it was explained at length to me as a first year student in historical geology that this had been considered a hair-yankingly nasty problem. It turns out, an explanation had been worked out only a few years before I started my degree: as oceanic plate subducts, the upper portion of the plate and the overlying sediments can detach and mix into the accretionary wedge. Essentially, the accretionary wedge behaves as an enormous horizontal cement mixer, churning the pile, carrying shallower rocks down, and deeper rocks back up again. This explains both the lack of any consistent stratigraphy within melanges, as well as the chaotic mixture of apparently unrelated rock types.

Photo unmodified. March 9, 2012. FlashEarth location.

Geo 730: May 14, Day 500: Glaucophane

Can you believe I'm on #500 already? Seems like only #495...

Above we see some nice isolated crystals of glaucophane, which along with lawsonite, are the diagnostic minerals for blueschist, or as it's sometimes called, glaucophane schist. The micaceous, light-colored mineral is probably mostly chlorite, but there may be some muscovite as well. I don't distinguish reds and greens well, so I can't tell if the mica's apparent green tint is real or an artifact of my poor color acuity. I also can't pick out the garnets easily at this magnification, but they're readily apparent to me, especially to the right of the lens cap, in the full-size image.

Photo unmodified. March 9, 2012. FlashEarth location.

Geo 730: May 13, Day 499: Complete BS

We left Coos Bay, and drove about half an hour south to Bandon, Oregon, where we had breakfast. I don't remember the name of the restaurant, but my omelette was awesome. Afterwards we went and checked out Bandon's famous BS jetty- blueschist, that is. Spectacularly coarse-grained, the mineral grains are large enough to identify, mostly, though I don't recall enough of the mineralogy of this metamorphic suite to be very confident. In this shot, though, pyrite, normally considered an accessory mineral, is surprisingly abundant. The lens cap is 52 mm in diameter.

Photo unmodified. March 9, 2012. FlashEarth location.

Geo 730: May 12, Day 498: Sheared Granite

This is a countertop in the hotel we stayed at in Coos Bay, a Super 8, after visiting Shore Acres and Sunset Bay State Parks on March 8, 2012. At a rough visual estimate, and taking the pinkish/buff minerals as alkali feldspar, the whiter/creamy minerals as plagioclase, and the darker grey as quartz, I'm putting the akali feldspar at ~25%, plag at ~40%, quartz at ~30%, with the remaining 5% or less as mafic minerals, probably mostly amphibole and magnetite. According to this QAP diagram (scroll down a bit past the rock photos and following block of text), that makes it a true granite, assuming I've got the mode more or less right. The interesting things here are that, first, the rock appears to have a distinct metamorphic fabric, with the mineral grains flattened roughly parallel to horizontal in this view, and second, there is a distinct right-lateral shear zone running vertically on the right side. I appreciate when nice rocks are used in the design of a living space, even if I only live there overnight.

Photo unmodified. March 9, 2012. FlashEarth location.