A nice example of plumose structure, enhanced by a pronounced joint set which cross-cuts the be-plumed surface. Hammer for scale. White Mountain front, California, September 2009.

Labels: california, joints, structure

Here we have some nice little glacial striations exposed in the Grinnell Glacier cirque in Glacier National Park, Montana. These grooves were carved by pebbles and other clasts within the glacial ice as it flowed over this outcrop of the Mesoproterozoic Helena Formation (part of the Belt Supergroup). Perhaps some of the same pebbles you see in this photo were responsible for acting as carving tools -- though the 'hand' that wielded them, Grinnell Glacier itself, melted away from this point sometime since 1939.

Also of interest to me in this photo is the lingering stain of water around the joint set in the upper right. I'm fascinated at the interplay between physical and chemical weathering, and seeing stuff like this emphasizes how even a simple hairline fracture can help funnel water, with all its destructive effects, deeper into the heart of an outcrop. Weathering is focused on these areas, and in another century this outcrop may look quite different.

Labels: glacial landforms, glaciation, joints, montana, national parks, pleistocene, proterozoic, sediment, snow, weathering

My MSSE advisor John Graves (previously mentioned here) went on a float down the Green River in Utah last weekend.

This appears to be a huge set of concentric ribs (a.k.a. "arrest lines") on the face of a big joint in massive quartz-rich sandstone. Bedding runs ~horizontally across the image, though not to be confused with the perfectly horizontal "bathtub ring" waterstains from the river. John says, "My best guess from the guide book is that it's Entrada Sandstone, Carmel Formation & Navajo Sandstone top to bottom." The fracture appears to have started in the middle of the cliff and propagated downward and outward. Note how the ribs "flare" out at the far edge. I guess an alternative hypothesis is that this is some weird kind of dune cross-bedding in the Navajo Sandstone: the inside of a barchan dune, perhaps? (though barchans wouldn't form in the "sand sea" situation in which the Navajo was deposited)

Anyone else want to offer another interpretation for this? I think that's what it is.

Labels: joints, msse, structure, utah

I've discussed the phenomenon of jointing on this blog before, and how when rocks fracture, sometimes they leave behind structures we can see that tell us something about the jointing process. Where did it start? Where did it stop? To answer these questions, we turn to structures like plumose structure, arrest lines (concentric ribs), and hackle fringes.

On this past Sunday's field excursion out to the Massanutten Synclinorium (Shenandoah Valley), MSSE John Graves and I saw some more nice examples of these phenomena, and as usual, I took some photos of them.

Let's start with this one, which shows plumose structure (and thus joint propagation) starting at the right and heading to the left.



A closer-up shot of this same fracture surface (in the Ordovician Martinsburg Formation):



Here's another one (in the Devonian Needmore Formation):



Sorry -- no sense of scale in that (above) one -- it was a few feet above my head. Total width of the photo is about two feet (call it half a meter).

This one (also in the Needmore) shows some really wavy plumes:



At the end of joint surfaces, we find hackle fringes, these "rough edges" where the little ridges and valleys of the plumose "topography" flare up and out in a spiralling kind of shape. When you slice through this spiral shape, it appears as a series of little itty-bitty joints at an angle to the main joint. Here's some hackle fringes on a joint surface from the Martinsburg Formation:



Each of these represents the edge of the fracture at one point. But then stresses built up again past the rock's strength, and it cracked anew, extending the fracture and producing a new hackle fringe. A closer-up shot (rotated) of the above fringes:



And back to the Needmore again, for a lovely series of hackle fringes that I've shown you before, but I couldn't resist photographing again. But to mix it up a bit, this time I used a penny instead of a quarter for scale...



Contrastified version of the above, with annotations:



Lastly, remember that I showed you this photo on Monday, from the Billy Goat Trail?



Well, I think you can see some hackles there, too. Take a closer look...

Below, I've zoomed in on the far upper right of the previous photo, and rotated it 90 degrees. I've also transplanted the penny from another part of the photo to maintain a sense of scale, and drawn a quick sketch of the fractures:



I think the little itty-bitty fractures (again, infused with quartz, making them weather out in high relief) traversing the main left-right joint trace are hackle fringes associated with that joint. Anyone care to differ?

Labels: devonian, geology, joints, ordovician, quartz, structure, virginia

A couple days ago, I showed a photo of plumose structure here, a feature that sometimes forms when rocks fracture (i.e. a joint is formed). I invoked the image below to show the relationship between the plumose structures and the concentric "ribs" that sometimes show up on a joint (here labeled as "arrest lines"). The point was to show how they were mututally perpendicular.

But the diagram shows something else, too: that the delicate topography of the plumes becomes more exaggerated away from the main surface of the joint, and they grow into twisted "hackles" along the edge of the joint. Joints have ruffled edges! These hackle fringes can also be spotted on many rock surfaces, if you're looking for them.

Here's a photo I took a couple of weeks ago, in the Silurian Needmore Formation (exposed in the Massanutten Synclinorium between Waterlick, VA and Seven Fountains, VA). It shows a series of hackle fringes parallel to one another, showing the growth of the fracture surface over time.

Here it is again, with the Photoshop "contrast" dial turned up to 11:

The high-contrast view helps bring the hackles into high-relief, and also illuminates the subtle plumose structure. Looks like this surface formed from the top, down. As I read it, this joint started on the right side of the image and propagated leftwards as time went by.

(The hematite nodule at left is a bonus feature.)

Labels: concretions, joints, structure

Here's a photo one of my Audubon students (Albert) took this past Saturday on the Berma Road, in C&O Canal National Historical Park. The lighting was just right, so that when we passed by this outcrop of metagraywacke, we saw an illuminated example of plumose structure:

Plumose structure is something that forms when rock breaks. The fracture starts at one point, and then grows, propagating thorough the rock and leaving behind a telling signature of its growth. In this case, the fracture (also known as a joint) started at point A and propagated through the rock to point B (central 'shaft'), expanding laterally (feathery 'plumes') at the same time.

Sometimes, concentric 'ribs' form, perpendicular to all these feathery plumes, showing the actual leading edge of the growing fracture surface. An example most people are probably familiar with is the "clamshell" shape of a classic conchoidal fracture. Check out this image to see how the two relate to one another.

When we saw this lovely example, I pointed out to the students that if we had been there fifteen minutes earlier or later, this subtle topography would either have been obscured totally in shadow, or washed out in full light. It was only because the light was at juuuuuust the right angle relative to these mm-scale variations that we noticed it.

Labels: joints, structure

This is a fault in a quarry near Ensenada, Baja California, Mexico. My friend Annie Kammerer of the NRC sent me the image, and then I annotated it.

First off, notice that the rock is light-colored, with grey and pink tones. I suspect it's a granitoid of the Coast Ranges Batholith (one of many batholiths of the Mesozoic west coast, like the Sierra Nevada and much of Idaho.) The Coast Ranges Batholith extends from "mainland" Mexico to the Baja Peninsula, and up into southern California.

Second, a prominent cross-hatching pattern is seen in the rock. These fractures are two intersecting joint sets. Joints are fractures in rock along which there has been no movement. If the rock on opposites sides of the fracture does move, then it's not a joint; it's a fault. Joints are caused by stresses the rock experiences. Because tectonic stresses are often distributed over a large volume of rock, the rock often develops many joints in the same orientation. A bunch of parallel joints is called a joint set (here's an example from Utah). Joint sets are much more interesting than mere joints because (let's face it) joints are extremely common in rock: they are the most common geologic structure. Joint sets, on the other hand, speak about larger forces and bigger patterns.

The third and final reason for enjoying this photo is that it shows a fault well. Running right down the middle is a prominent fault. Note that the fault is wider than the joints: it's filled with some sort of pulverized goo. This is a material called fault gouge. Faults may or may not have fault gouge in them. It's essentially ground-up rock: any bits that stick out get crushed as the fault grinds over them: like a mortar and pestle smashes up spices. Sometimes when the fault is more planar, the rock rubs directly against its neighbor, producing slickensides. Sometimes, asperities (knobs & bumps on the fracture surface) get snapped off, but not ground into pulp: this produces a fault breccia (like this celebrated example in Death Valley).

I'm struck by how vertical this fault is. Faults come in all sorts of orientations, but there are four really common ones: (1) high-angle normal faults which dip at an average of 60º into the Earth, (2) low-angle reverse faults which dip at an average of 30º into the Earth, (3) extremely-low-angle thrust faults, which can be close to horizontal, and (4) vertical strike-slip faults, which dip at 90º, or close to it. This appears to be the latter. Annie, the photographer, tells me that there was a substantial (~100 m) offset along this fault. If you looked down on it from a bird's-eye view, you might be able to tell that, but it's impossible to gauge the offset from this cross-sectional view.

Final observations: the fault is oriented the same way as one of the joint sets. It's likely that the rock was jointed first, and then when tectonic stresses required strike-slip faulting, one of those joints (a pre-existing plane of weakness) was utilized as the site of movement. Note too up in the upper-left another "joint" seems filled with fault gouge, meaning it's really another (parallel) fault. That would be entirely expected if a jointed rock was being tectonically smeared out. The vertical "slices" of rock migrate past one another like a sheared loaf of (sliced) bread. Some of the slices adhere along their cut surface, whereas others move. Some of the joints become faults, but once they start moving, the stress is accommodated, and there's no reason for every joint to become a fault. The weakest link takes all the strain.

Labels: faults, joints, mexico