21 October 2016

Friday fold: a miniature ramp and flat

This is kind of fun – a folded and faulted block of chert (lens cap for scale) as seen along R40 in Barberton Mountain Land, South Africa. This is likely the Archean-aged Msauli Chert (~3.3 Ga). This block shows a nice “ramp and flat” fold/fault structure, a common “structural ingredient” in many fold and thrust belts, including the Appalachian Valley & Ridge province.


The harsh sunlight made me take a second photo, where the entire sample was in even shadow. I’m not sure, however, which image I prefer.


Here’s an annotated copy, with bedding highlighted in yellow, and faults in black. White arrows show the kinematics on the main fault. Minor back-thrusts on the trailing edge of this “proto-nappe” may


A miniature ramp and thrust… Too bad it wasn’t a wee bit smaller – I could have brought it home with me!

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19 October 2016

Oddball Icelandic rocks, part III: conglomerate

At the Volcano Museum in Stykkishólmur, I learned that Iceland has fossils. Specifically, they have a display of bivalve (clam) fossils there, and when I asked where to find them, I was directed to a point further east on the Snæfellsnes Peninsula. The next day, I set off to find them.

Here was what I saw on a cliff high above me, at the spot nearest to where I thought I was supposed to be looking:



There were some suspicious looking outcrops there…


I climbed up. Here’s the view from on high…


Rotating, I faced the cliff.

In detail, the outcrop object of my quest showed inclined packages of conglomerate:

iceland-seds-foresetssmClick to make much, much larger

A few adjacent details:



This was the only outcrop of sedimentary rock I saw anywhere in Iceland. It was too coarse for delicate external molds of clams, but it was cool in its own right on account of its unique status in my experience.

There were also a few boulders showing rotated blocks of the stuff:





And here’s a cobble hosting a decent graded bed:


And there you have it. With the green ignimbrite and the granite intrusion, those are the only three rocks I saw in a week in Iceland that weren’t mafic volcanics.

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11 October 2016

A virtual field trip to Portrush, Northern Ireland

One of my favorite places in Northern Ireland is the east side of the peninsula that hosts the tourist town of Portrush. There, two early schools of geological thought engaged in a battle. The opposing sides were:

  • the Neptunists, who thought all stratified rocks, and in particular basalt, must form from precipitation from the sea, and
  • the Plutonists, who thought some rocks, including basalt, formed through intrusion of molten rock from deep within the Earth (potentially followed by its eruption from a volcano).

The Neptunists are exemplified by Abraham Gottlob Werner, and the Plutonists were represented by James Hutton. So why did these titans clash in Portrush, of all places?

This is a spot where ammonite fossils are found in dark-colored, hard, stratified rocks. The Neptunists said, “Aha! Here are sea creatures fossilized in basalt! Therefore basalt must be a rock precipitated by the oceans!” But then Hutton showed up and demonstrated that in fact the rock had been misidentified. The “basalt” was actually hornfels, a contact-metamorphosed mudrock. The strata in question were of the Lias, a latest Triassic / earliest Jurassic unit in British stratigraphy. The Liassic shale became Liassic hornfels when it was baked by the heat of a nearby gabbroic intrusion, the ~60 Ma Portrush Sill. So there is mafic igneous rock here, but it’s gabbro, not basalt. And Portrush therefore proves the Plutonists’ case, not the Neptunists’.

A diagram from an interpretive sign at the site:

img_7993(Note the error on the sign where they substitute “dolomite” for what surely was intended to be “dolerite,” another name common in the U.K., for what U.S. geologists might call “diabase.” In my opinion, the sill is coarse enough to qualify as gabbro. So I’m sticking with that.)

Let’s dig in. Here’s a Google Map of the place, showing that something odd must be up with the geology in order to have such a profound promontory on what’s otherwise a relatively smooth coast.

The Portrush outcrops look like this:

portrush-sm Click to embiggen



Interesting intrusive structures along the contact zone between the Lias and the gabbro:


My field assistant examples samples of the different rock types on the rocky beach…


…the sandy beach…


…and on the outcrop:


Here’s what we find in terms of the two main lithologies:




You kind of can’t blame the pre-petrographic-microscope Neptunists for mistaking the hornfels for basalt. It’s fine-grained and black and there’s a lot of basalt in this corner of Ireland. It’s reasonable for that to be their initial take.

Hornfels sample on gabbro background:


Here’s another sample that I felt looked a lot like a basalt:



Now for the ammonites:





Here is a gallery of 16 more specimens of ammonites from the outcrops of Portrush. Click through to see them all:

This slideshow requires JavaScript.

My favorite ammonite fossil is this big one, decorated as it is with modern barnacles:


Portrush is proud of its geologic heritage, and the lovely spiral forms of ammonites decorate many municipal structures:




This isn’t an ammonite, but it might be Aviculopecten or another bivalve:


Here are the GigaPans that I made of the site.

Overviews of the baked Liassic outcrops:
Link GigaPan by Callan Bentley

Link GigaPan by Callan Bentley

Link GigaPan by Callan Bentley

Link GigaPan by Callan Bentley

Link GigaPan by Callan Bentley

This one looks particularly basaltic:
Link GigaPan by Callan Bentley

The gabbroic intrusion:
Link GigaPan by Callan Bentley

Link GigaPan by Callan Bentley

Link GigaPan by Callan Bentley

Link GIGAmacro by Robin Rohrback & Callan Bentley

The fossils:
Link GigaPan by Callan Bentley

Link GigaPan by Callan Bentley

Link GigaPan by Callan Bentley

Here’s a close look at a similar ammonite fossil in another shale, close-up.
Link GIGAmacro by Robin Rohrback

Lastly, here’s some sand from the beach nearby. It’s dominated by shells, not by either of the rock types discussed above:
Link GIGAmacro by Robin Rohrback

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10 October 2016

Oddball Icelandic rocks, part II: granite!?!?

Around the corner from the Hvalnes Lighthouse in eastern Iceland is the second non-basaltic igneous rock I saw in Iceland.


I couldn’t drive past something like that and not stop. Seriously – this was the only high-albedo rock I saw on the entire island!


(Note the lens cap for scale, as in all the photos in this post.)

Yes, that’s what it appears to be – here on the flanks of the Mid-Atlantic rift, partial melting of the mantle yields basalt and partial melting of the basalt has apparently generated a granite.


This was unexpected (to me), as I associate granites with more evolved crust – like that of the continents, where the material has been cycled through multiple rounds of “distillation” to pull out the most easily-melted components. (Felsic minerals melt at lower temperatures than mafic ones, so if you partially melt a given rock, it’ll segregate into a more-felsic liquid and a more-mafic solid residue.) Iceland’s apparently got a sort of “hot spot” underneath it (like Hawaii) as well as being situated on a divergent boundary (the Mid-Atlantic Ridge), so I would have expected nothing but basalt. (And indeed, that’s mostly what Iceland is made of!)

I wouldn’t have guessed any of the rocks here would have had the opportunity to get the granite sweated out of them. But: apparently it has happened, at least once.

The granite contains xenoliths of more mafic compositions:



And now for a few GigaPan views of this place:

Link GigaPan by Callan Bentley

Link GigaPan by Callan Bentley

Link GigaPan by Callan Bentley

Link GigaPan by Callan Bentley

Finally, a GIGAmacro view of a sample I collected at the site. You can explore its phaneritic (coarse-grained) texture and see its quartz content readily with this image:
Link GIGAmacro by Robin Rohrback

A granite in Iceland! Who’d have thunk it?

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7 October 2016

Friday folds: Dalradian schists at St. Ninian’s Isle

Remember St. Ninian’s Isle?

It is connected to Mainland Shetland by a tombolo. But it has rocks there, too. Here are some outcrops on the beach:


If you visit these schisty fins, you’ll find they are populated by a cavalcade of small folds.



Some of the folds are crisp things known as kink bands:


Annotated version:

And finally, as a lagniappe, here’s a bit of boudinage. This quartz vein has broken into segments, and the segments have separated from one another along the plane of foliation. Note how the foliation deflects into the boudin neck from each side:


Annotated version:

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6 October 2016

Oddball Icelandic rocks, part I: A green ignimbrite

Most of Iceland is basalt.

But there are a few less common rock types to be found. We stumbled on one in the eastern fjords region, on the northern margin of Berufjörður. It was a volcanic conlgomerate, maybe an ignimbrite, and it was G R E E N.

Here’s the lovely setting of the intriguing outcrop:


…And the cliff itself, spalling off bouders and cobbles galore:


A few example cobbles from the adjacent beach:




And now for three GigaPan views – two outcrops (I think the first one shows an unconformable contact in the lower right) and a cobble I brought home with me:

Link Handheld GigaPan by Callan Bentley

Link Handheld GigaPan by Callan Bentley

Link GIGAmacro by Robin Rohrback

I suppose after Eyjafjallajökull’s explosive eruptions in 2010, I shouldn’t be surprised to see such a rock in Iceland, but I was nonetheless. After nothing but black basalt, this green ignimbrite was a sight for sore eyes.

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5 October 2016

Hverir, Mývatn, Iceland

Today, I offer up a few photos and some video from the Hverir geothermal area on the east side of Mývatn National Park in Iceland – a rift zone astride the central Mid-Atlantic Ridge, and thus an area where you would expect to see a high heat flow through the crust. Heat interacting with meteoric water produces geothermal features: hot springs, geysers, fumaroles, and mud pots (where the groundwater is acidic).

It’s a harsh looking landscape. No placid pile of basalt flows here:


Craters galore dot the landscape, each home to a hissing vent of steam, or a pool of viscous mud:









Old mud dries out, and cracks in the loveliest way:




Thermophile microbes dwelling on the pile of rock:


Video to show the dynamism of the place:

Shades of Yellowstone, for sure…

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3 October 2016

A virtual field trip to Rathlin Island, Northern Ireland

Rathlin Island lies north of mainland Northern Ireland, a few miles offshore. I spent three lovely days there this past summer, investigating the geology and appreciating the wildlife (puffins and other sea birds, and seals). The geology is pretty straightforward: Paleogene basalt overlying Cretaceous “chalk” (really not so chalky here – technically, it’s the Ulster White Limestone). Here’s a suite of interactive imagery that you can use to explore Rathlin’s geology in a “virtual field trip.”

Google Maps view of the island:

A photo to show the basalt overlying the chalk:

In places the chalk is rich in chert nodules:


The chalk contains beautiful belemnite fossils:






Here’s another fossil of some sort. Anyone able to identify it?



Lithophagus clams bore holes into the chalk, and dwell therein:



These are modern critters, occupying a Cretaceous substrate. They are not fossils.



On the west side of the island, sea stacks provide nesting places for a LOT of seabirds:






Okay, now that you’ve run your eyeballs over those photos, consider some GigaPans:

Gazing over the pastoral scene on the Roonivoolin Trail, you don’t really get a sense of the geology:
Link GigaPan by Callan Bentley

…But basically the island is made of Paleogene basalts slathered over Cretaceous “Chalk” (the Ulster White Limestone, technically), as the next two GigaPans show:
Link GigaPan by Callan Bentley

Link GigaPan by Callan Bentley

The basalt dominates the outcrops you’ll see walking around the island. See if you can find some laterite in this view:
Link GigaPan by Callan Bentley

The West Light bird sanctuary features some horrifically stinky seabird nesting colonies (puffins!!) atop sea stacks eroded into the basalt:
Link GigaPan by Callan Bentley

Link GigaPan by Callan Bentley

A weathered outcrop of basalt nearby the bird sanctuary visitor center:
Link GigaPan by Callan Bentley

Let’s head down to the shore next, and take a look at the beach. You might find some seals in this image:
Link GigaPan by Callan Bentley

The beach there hosts a nice variety of sediment. The contrast between the chalk and basalt cobbles is striking:
Link GigaPan by Callan Bentley

Link GigaPan by Callan Bentley

Here’s a Petri dish full of a slightly finer collection of sediment from the shores of Church Bay:
Link GIGAmacro by Robin Rohrback

And a finer, wind-sorted fraction of the sediment that accumulated higher up the berm:
Link GIGAmacro by Robin Rohrback

Here’s a cobble of the basalt, showing well-developed amygdules in its vesicles:
Link GIGAmacro by Robin Rohrback

Front and back of a cobble of the Ulster White Limestone, showing a cross-section through a belemnite:
Link GigaPan by Callan Bentley

The obverse really shows the radial arrangement of the calcite crystals in the belemnite’s rostrum:
Link GigaPan by Callan Bentley

Scanning electron micrograph (SEM) GigaPan of the Ulster White Limestone. You’ll find some coccoliths in there – the little plates surrounding the coccolithophore organism which is the source of chalk:
Link SEM GigaPan by Robin Rohrback

And finally, an SEM image of a foram from the fine sediment sample. Somewhere, you’ll find a diatom hiding in its interstices!
Link SEM GigaPan by Robin Rohrback

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30 September 2016

Friday fold: Torqued talc from the 2016 Biggs awardee

My friend Joshua Villalobos was celebrated this week at #GSA2016 as the recipient of the Biggs Award for Geoscience Teaching Excellence. I was his citationist (nominator), paying forward the effort and attention that Heather Macdonald and Bob Blodgett showed for me two years ago. Josh is a talented and accomplished educator on many levels, and an exemplary choice for this honor.


Now, as it turns out, I was able to get Joshua hooked on GigaPanning (he has over 60 images to his name so far), and I went out to El Paso in 2013 to do a brief field campaign with Josh and his colleagues and his students. One of those students, Jenn, turned up again at GSA. It turns out that not only is she now in graduate school at UTEP, but that GigaPanning campaign was inspirational, and she’s now working in high-resolution imagery and 3D modelling using drones. Yay!


Okay – so now you know the backstory. How about some folds?

Here are two views of quarry walls of the American Talc Company near Van Horn, Texas. The rocks are talc-bearing strata of the Precambrian Allamoore Formation (1.25 Ga). Much of the deformation seen in the Allamoore is considered to be the northernmost extent of a Grenville-age collisional belt along the southern margin of Laurentia. Happy fold exploration!

Link GigaPan by Joshua Villalobos

Link GigaPan by Joshua Villalobos

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28 September 2016

Virginia Museum of Natural History specimens in GIGAmacro view

One of the small sub-projects of my 2015-2017 Chancellor’s Commonwealth Professorship is to create some GIGAmacro images of cool fossil specimens from the Virginia Museum of Natural History in Martinsville. Curator of paleontology Alex Hastings was good enough to loan us a few specimens to image, and hopefully there will soon be more where they came from. Here are three examples: a fossil plant, and two fossil vertebrates:

Strobilus cone (Wise County, VA):
Link 2.8 Gpx GigaPan by Callan Bentley

Coelocanth tail from Solite Quarry, Dan River Basin, VA:
Link 1.6 Gpx GigaPan by Callan Bentley

Tanytrachelos (reptile) from Solite Quarry, Dan River Basin, VA:
Link 0.7 Gpx GigaPan by Callan Bentley

Here is a link to the whole collection (7 and counting).

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