A funny coincidence transpired a couple of weeks ago. I posted about "Amygdules!" and so did Andrew. We were both so excited by these cool primary igneous structures that we added an exclamation point to our post titles. Over the weekend, I found some more. These are in Dark Hollow, in Shenandoah National Park, above the falls. Pretty sweet, eh?



I hereby give them two exclamation points. Let's see if anyone else can come up with two-exclamation-point-worthy examples of amygdules...

Labels: basalt, blogs, blue ridge, falls, igneous, primary structures, proterozoic, shenandoah

Amygdules (mineral deposits filling extrusive vesicles) in the Neoproterozoic-aged Catoctin Formation meta-basalt, Shenandoah National Park, Virginia.

Labels: basalt, blue ridge, igneous, primary structures, proterozoic

So, it's a month until my Rockies class starts. I've been encouraging all the students to get in shape, because the high elevations, rough terrain and multimile distances we'll be hiking in Montana and Wyoming could really kick an east-coast flatlander's arse. So we've scheduled a few training hikes to help everyone physically prepare for the Rockies experience. Last weekend, we did a 5.5-mile circuit up the steep Little Devil's Stairs trail in Shenandoah National Park. I was joined by five Rockies students + one of their kids. Here's a map of the loop we did:



Here's a few photos of the hike, and the geology we encountered along the way:



John poses next to some jointed columns in the Catoctin Formation, a Neoproterozoic rift-related series of flood basalts (subsequently metamorphosed during Alleghenian mountain building).


End-on view of one of the columns:


Overhanging cliff showing columns weathering out along jointed surfaces:


Bob poses next to a cliff, helping me demonstrate how difficult it is to take a well-exposed photo in the jungle of the Virginia hardwood forest:


A wiggle in some columns:


Jared thought these columns were better than the first ones he saw, at Old Rag Mountain.


Here's me with a fifteen-foot-long section of columns, indicating that the flow from which this boulder was derived must have been at least fifteen feet thick, maybe more:




But it wasn't all columns. There was also a lot of column-less massive Catoctin Formation, and some nice inter-flow conglomerates which are interpreted as stream deposits that developed atop a cooled flow before the next flow erupted. These conglomerates imply a reasonable amount of time passed between successive eruptions of the Catoctin flood basalts. The lichens obscure the rock, but note for instance the fingernail-sized chunk of greenstone an inch above my hand:


More chunks in the conglomerate:


And more:


Jared guards the way forward:


The view from the top:

Labels: basalt, iapetus, igneous, metamorphism, nova, primary structures, proterozoic, sediment, shenandoah, structure, teaching

Dear readers,

Here's a list of the samples I'd really like to have to show my students examples of the processes we discuss:

• A lava pillow (maybe a pillow basalt). Fresh would be best, so I could show the outer crust of obsidian, and the inner basalt. An ancient pillow would be second best.


• Boudinage. A nice hand sample of boudinage, maybe a granite dike in a shist? Or a sandstone stratum within a shale matrix? I feel like I should already have one of these, but I don't... All the good local examples are too big.


• Flame structures/ball-&-pillow soft sediment deformation.


• A komatiite sample.


• One of these. (Eubrontes track with radiating mudcracks, featured this morning on ReBecca's Dinochick Blogs)

I've been a good boy this year. Anybody got any spares they want to trade for some nice Skolithos-bearing cobbles? (...or something else we've got a local supply of?) I'll pay shipping!

If not, please alert Santa that I'd appreciate him filling my stocking with these goodies,

Callan

Labels: basalt, igneous, primary structures, structure, teaching

After the Garth Run high-strain zone and a night hanging out by the campfire at Heavenly Acres with the William and Mary Structural Geology class, the second stop on our Structural Geology trip was in Shenandoah National Park, looking at the deformed meta-basalt columns on the Limberlost Trail. Longtime readers of the blog have seen these unique (in my experience) columns before, in a post from last May.

This is an outcrop of the Catoctin Formation, a series of (mainly) basaltic lava flows that erupted sometime older than 565 Ma (only the youngest, rhyolitic layers have been dated, and evidence suggested that significant amounts of time may have passed between the eruption of each stratum of basalt deeper down in the stratigraphic stack). As the lava cooled, it developed cooling fractures that formed perpendicular to the isotherms. These fractures likely initiated at the top and the bottom of the flow, and propagated towards the middle over time.

Later, during Alleghenian mountain-building (~300 Ma to ~250 Ma, roughly), the rocks were subjected to greenschist-facies metamorphism, and were deformed. The basalt's consituent minerals re-equilibrated and reacted to become other minerals, most notably chlorite and epidote (both of which are green).

Here's John and Joe checking out the columns:


Exquisite! Even arrest lines on the side of each column are preserved. In an undeformed basalt column, these arrest lines would be perpendicular to the column edge. Here, they have a pronounced angular relationship, indicating the shearing of the overall column:


Bobby measures the angular shear along the length of the column:




Goofball professor poses with column:


Jay plays the column like an electric guitar:


We found some nice plumose structure too:


Finally, we evaluated the concentric rings of minerals filling amygdules (vesicles that had been infilled with mineral deposits after lithification) in an attempt to determine whether they could be used as strain markers, or whether they may have attained their ellipsoidal shapes due to stretching of the bubbles in the originial lava (i.e. like this) and then been infilled with minerals:




...and then we were off to Field Study Area #3...

Labels: appalachians, basalt, field trips, igneous, metamorphism, national parks, primary structures, proterozoic, shenandoah, structure, teaching

Almost a week after the field trip to Old Rag Mountain, and the Facebook-hosted pictures keep trickling in. Here's some shots by NOVA student Eileen Lodovichetti, and an ensuing discussion of feeder dikes and supercontinent breakup.

Here's a shot of the upper reaches of Old Rag, showing the characteristic spheroidal weathering of the Old Rag Granite and the relative lack of trees on top:

photo by Eileen Lodovichetti

...And here's a shot that Eileen took which shows the interior of one of the weathered-out feeder dikes we had to hike through on our way to the summit. You can actually see the classic geoprofessorial arm-waving caught in blurry motion!

photo by Eileen Lodovichetti

This is one of the coolest things about hiking Old Rag: after scrambling up on top of spheroidally-weathered granite domes, you drop into these tabular "hallways." The astute observer will note that the floor is made of a fine-grained, dark-green-colored rock, quite distinct from the light-colored, coarse-grained granite that makes up most of the mountain. These are dikes of metamorphosed basalt that intruded the granite during the breakup of the supercontinent Rodinia in the Neoproterozoic era of geologic time.

Here's one of my former Field Studies in Geology students, Mike Nelson, pointing out a similar dike along Skyline Drive, in the main part of the park:


Basically, the story goes like this: Around 1.2 to 1.0 Ga, continental fragments amalgamated into a supercontinent called Rodinia. In Virginia, this is recorded in the rocks of the Blue Ridge province, where the basement consists of granitoids (granites and related rocks) and metamorphosed granitoids (gneisses, mylonites). Among the youngest of these is the Old Rag Granite, which intruded the Pedlar Formation granite gness around 1.0 Ga.

Later, Rodinia broke apart, resulting in an extensional tectonic regime and mafic volcanism. Fractures opened up in the Old Rag Granite and funneled mafic magma towards the surface. Massive eruptions of basalt blanketed the landscape. The resulting layers of basaltic lava are known as the Catoctin Formation. At Old Rag Mountain, we can see some of the plumbing that led to these flood basalt eruptions: these are feeder dikes, because they "fed" the eruption above them.

Because the dikes (which were metamorphosed to greenstone during ~300 Ma Appalachian mountain-building) weather more rapidly than the Old Rag Granite, they are typically recessed into the landscape. That's what makes the "hallways" in the photograph above. Here's two more images, showing these weathered-out feeder dikes:



Check out how there's moderately-developed columnar jointing extending across the dike. These columns form perpendicular to the cooling front, and the dikes would have lost their heat out the sides. In horizontal lava flows, the heat is lost from the top and bottom surfaces, so you get vertical columns. Here, a vertical dike produces horizontally-oriented columns. Hikers appreciate these "steps" as they squeeze through the dikes on their way up the mountain.

Here's a map of part of Shenandoah National Park:


Please ignore the "hover" instructions at the lower right. I've reproduced the "hoverable" image below. Key: the orange is the Pedlar Formation. The pink is the Old Rag Granite, and the green is the Catoctin Formation. Feeder dikes of the Catoctin are shown as green lines.

Now, let's take away the map, and just preserve the orientation of the feeder dikes. This will tell us the overall tectonic stretching direction:
Various plate reconstructions show either Amazonia or the Congo craton offboard of Virginia at the time Rodinia broke apart and the Iapetus Ocean began seafloor spreading. I've illustrated it here as the Congo, but that might be wrong.

So: the hike up Old Rag is great exercise, and offers scenic views, but for those willing to consider the rocks and how they got there, it's an insightful view into the tectonic past.

Lastly, here's a lovely, well-developed weathering rind on the Catoctin meta-basalt (greenstone). When the dark green rock adjusts to the conditions at the Earth's surface, it breaks down, resulting in the tan/"buff" color on the outside. You're watching the rock "rot" from the outside surface, working its way inward:


More on the geology of Shenandoah National Park can be seen at this page on my website.

Labels: appalachians, basalt, blue ridge, field trips, metamorphism, mountains, national parks, nova, plate tectonics, shenandoah, structure, weathering

Last weekend, I took a group of students, mostly from NOVA but also 3 from GMU, up to hike Old Rag Mountain in Shenandoah National Park.

Here's a Google Map showing the terrain (and trails, which is a cool new addition to the already cool Google Maps):


The crew discusses debris flow deposits in the forest on the way up the mountain:

photograph by Charlie Corrick

The first spot where we get a nice view out over the valleys below:

photograph by Charlie Corrick

Spheroidal weathering in Catoctin Formation greenstone:

photograph by Jared Fortner

Spheroidal weathering in granite (the Old Rag Granite, 1.0 Ga):

photograph by Charlie Corrick


photograph by Charlie Corrick

Student Jared atop a spheroidally-weathered boulder of the Old Rag Granite:

photograph by me

Grain-size differences in the Old Rag Granite (balanced atop my leg):

photograph by me

Non-foliated Old Rag Granite (showing lovely "blue quartz"):

photograph by me

And the foliated version of the Old Rag Granite:

photograph by me

Labels: appalachians, basalt, blue ridge, field trips, geology, granite, igneous, mountains, national parks, nova, structure, weathering

What happened to these poor hand samples?

My friend and colleague Pete Berquist shot this video of his (successful) attempt to make lava in his own backyard with an acetylene torch:

Note how the basalt makes runny lava, but the granite yields lava so viscous it doesn't even drip!

Pete works at Thomas Nelson Community College in Hampton, Virginia. He also posted some photos online here.

Labels: basalt, granite, igneous

Labels: antarctica, astronomy, australia, basalt, gsw, hadean, igneous, isotopes, meteors, minerals

The perpetually-interesting site Oddee hosted a series of satellite images of the Earth today, including this one from April of last year. Somehow I missed it then...

The image, originally from NASA's Earth Observatory (one of the finest websites I know of for those interested in Earth science), shows a collection of volcanoes in the western Arabian Peninsula. A large version of the image (unlabeled) is here.

The most spectacular thing about this image is the color contrast between the volcanoes on the left versus the volcanoes on the right. This spectacular contrast is indicative of the rock types involved in each volcano. On the left, felsic lava was erupting, which cooled into the extrusive rock rhyolite. On the right, mafic lava was erupting, which cooled into the extrusive rock basalt. Mafic igneous rocks like basalt have a higher proportion of the elements iron, magnesium, and calcium as compared to elements like silicon, potassium, and sodium. Felsic igneous rocks are, in a sense, distillates of mafic source rocks: they are made of minerals that are more easily melted.

Also worth noting is the way the basalt overlaps the rhyolite between Jabal Bayda' and Jabal Abyad tells us that the rhyolite came first, and the basalt came second, an example of relative dating. And these insights can be gleaned from space... or more accurately, from our computer screens, depicting an image from space. That's pretty incredible, when you think about it.

FYI, here's what NASA's William Stefanov wrote as the caption for this exceptional image:

The western half of the Arabian Peninsula contains not only large expanses of sand and gravel, but extensive lava fields known as haraat (harrat for a named field). One such field is the 14,000-square-kilometer Harrat Khaybar, located approximately 137 kilometers to the northeast of the city of Al Madinah (Medina). The volcanic field was formed by eruptions along a 100-kilometer, north-south vent system over the past 5 million years. The most recent recorded eruption took place between 600-700 AD.

Harrat Khaybar contains a wide range of volcanic rock types and spectacular landforms, several of which are represented in this astronaut photograph. Jabal ("mountain" in Arabic) al Qidr is built from several generations of dark, fluid basalt lava flows. Jabal Abyad, in the center of the image, was formed from a more viscous, silica-rich lava classified as a rhyolite. While the 322-meter high Jabal al Qidr exhibits the textbook cone shape of a stratovolcano, Jabal Abyad is a lava dome; a rounded mass of thicker, more solidified lava flows. To the west (image top center) is the impressive Jabal Bayda'. This symmetric structure is a tuff cone, formed by eruption of lava in the presence of water. The combination produces wet, sticky pyroclastic deposits that can build a steep cone structure, particularly if the deposits consolidate quickly.

White deposits visible in the crater of Jabal Bayda' and two other locations to the south are sand and silt that accumulate in shallow, protected depressions. The tuff cones in the Harrat Khaybar suggest that the local climate was much wetter during some periods of volcanic activity. Today, however, the regional climate is hyperarid - little to no yearly precipitation - leading to an almost total lack of vegetation.

Labels: basalt, blogs, igneous, middle east, satellite imagery, volcano, websites

Kilauea Iki is the name given to a lava lake that formed in Hawai'i Volcanoes National Park in 1959. It erupted from Pu'u Pua'i, the mound you see in the middle distance of this photograph:

The lava pooled in a pre-existing crater below to a maximum depth of about 400 feet, and has been solidifying ever since. Researchers have drilled though the cooling crust of Kilauea Iki to determine how fast the lava cools. By 1981, a good 200 feet of solid rock had formed at the top of the lava lake.

Here's a view into Kilauea Iki from a different angle, with me rotated about 90 degrees along the crater rim relative to the first photograph:



As you look down there, you'll see that Kilauea Iki does not display a nice smooth surface. Instead, it's fractured, and those fractures have a familiar shape: polygonal and relatively regularly-spaced. They look kinda like the tops of ginormous columns...


When you get down inside, it's pretty flat. You really get the feeling you're walking on a giant layer of soup scum:


...But it's not completely flat. There are cracks and crevices, buckles and upwarps:


Dynamics playing out in this mega-scum layer atop a roiling lava lake are thought to be human-scale analogues of the motion and dynamics of tectonic plates. Here, for instance, two "plates" of cooled lava have drifted towards one another. This meso-scale "convergent boundary" has raised up a mountain range fit for Lilliputians:


Elsewhere, "plates" of lava scum have drifted apart, opening up a "rift" between them. Here, I lie down to bridge the rift:


These cracks are utilized by plants because they offer a shaded nook where moisture isn't immediately evaporated by the sun:


Lastly, I thought I'd point out some neat mass wasting and structural geology I saw there. Here's a shot looking roughly westward across Kilauea Iki, towards the cinder cone of Pu'u Pua'i:

I know it's kind of washed out, but in this photo, you can see a big solidified lava flow that came over the lip of the crater, and then solidified, and then partially collapsed downward.

This sequence resulted in the big talus pile you can see at center-right, but there are remnants of the original sheet (or "tongue") of basalt there.





















Zooming in and cranking up the contrast, let's label a few things:
Up at the top, we can see some fault scarps that have developed as the massive tongue of basalt pulled downward.

A major scarp marks the edge of the cliff, and then below it you see a big slab of basalt with an edge that's just barely in the sunshine, and a bunch of more fragmented pieces below that (marked "breakdown"). Another big slab is seen alongside the breakdown.

What really caught my eye, though, was the en echelon array of pull-apart fractures seen in between the arrows. Here, the stress of the main tongue of basalt sliding downhill sheared this slab of rock, causing it to develop fractures at a ~40 degree angle to the shearing direction. These pull-aparts therefore represent a big surface-condition analogue for tension gashes that can form in subterranean rocks experiencing shear stress.

Labels: analogies, basalt, hawaii, landslide, mass wasting, plate tectonics, structure, travel

It's the 50th anniversary of Chinua Achebe's Things Fall Apart, a reminder that things continue to fall apart. Like... rocks. ...and steel. Today, I'd like to share a "compare & contrast" of two kinds of fractures I saw on my Thanksgiving trip to Hawai'i. One is caused by a decrease in volume; the other is caused by an increase in volume.


Type 1: Columnar jointing (shrinkage fractures)









Columnar jointing results from the decrease in volume as hot lava crystallizes into cool rock. The overall shrinkage in the rock's volume is accomodated by fractures that (all else being equal) are oriented at 120-degree angles on the surface of the flow, and then propagate downward into the flow, perpendicular to the cooling front (isotherm of the critical fracturing temperature, which here is subparallel to the surface of the lava flow). Similar fractures form in drying mud, where the volume loss is due not to cooling but to the evaporation of water. Generally, these mud contraction fractures (a) don't go as deep, and (b) experience more volume loss, resulting in wider fractures. These are in the Mauna Lani resort area, on the western shore of the big island of Hawai'i.
_________________________________________________________________


Type 2: Rust blisters (expansion fractures)








Here, we see fractures forming not due to a loss of volume, but the opposite: an increase in volume! Here the metal (steel, presumably?) in the pole is oxidizing, and in completing that reaction, rust is forming. The layer of paint probably got nicked, water (probably saltwater?) got under it, and then the paint kept the water down there, facilitating the rusting reaction. As the rust formed, it swelled relative to the volume of the original metal. It expanded in the direction that offered the least resisting stress (out away from the surface of the pole). As the rust bumps grow, they impart a new stress on the metal/rust, and this causes fractures to form subparallel to the pole's surface. These are near Ka Lae ("South Point"), near the start of the hike to Green Sands Beach.

Labels: basalt, hawaii, structure, travel, weathering

Holy lava, geoblogosphere!

On my recent trip to Hawai'i, I saw a variety of different kinds of holes in the basaltic "lava rock" that makes up the majority of the island. The largest examples were lava tubes, like the Thurston Lava Tube near Kilauea Iki in Hawai'i Volcanoes National Park:

This is a conduit through which molten lava once flowed. Once the source of that lava ceased producing, though, the lava drained out and the tube was left empty, like a cave. (Caves, of course, are holes produced through an entirely different process.) The ceiling of this lava tube is about twenty feet high.

Not too far distant, there's a nice area where you can see tree molds:


These are holes left in the rock as the lava flowed around a tree. The heat of the molten rock burst the tree's cells, releasing water and quenching the lava in a cylindrical tube around the tree. The dewatered tree then burned up, leaving a hollow mold showing the shape of its (former) trunk:


The holes are kinda deep:


Inside the tree mold, you can see the texture of the (in this case, pahoehoe) lava that flowed around the tree trunk:


Looking up the invisible tree trunk, and out the hole towards Lily:


Here's a bigger hole, the Halema'uma'u Crater within Kilauea Caldera:

It's venting a lot of steam, hydrogen sulfide, and other gases.

Google Map for reference on how this hole relates to the even bigger hole that is the caldera:


The photo of Halema'uma'u above was taken from the Hawai'i Volcano Observatory adjacent to the Jagger Museum in the park. Stepping back a bit from the window, you can see that I'm not the only one taking this particular photo... This is the same spot where the Halema'uma'u Crater webcam is filmed. That's what all these cameras are doing in the foreground:


Janet Babb took some time out of her day to show us around the place (thanks, Janet!), and I made sure to sign into the guest book. There, I was pleased to see past visitors, including (I think) Ron Schott's crew from Fort Hays State University Lake Superior State University, the William and Mary crew, and most recently, the NOVA crew headed by my colleagues Ken Rasmussen and Nancy Chamberlain:


Janet let me hold a chunk of recently erupted basalt. This one erupted in early October, I think she said. It was about a month old when I held it -- that's my record for a really recent rock:

As noted in a previous post, this vesicular texture displayed by this sample is one more example of (smaller) holes in lava.

Labels: basalt, hawaii, travel, volcano

What's the tallest mountain on Earth?

Everest, right? Well, yeah: if you're measuring from sea level. If you're measuring from the top of the crust the mountain rises from though, it's Mauna Kea, Hawai'i. It's about ~13,800 feet above sea level, but it rises ~33,500 feet from the oceanic crust to the peak (that's compared to Everest's mere ~29,000 feet from base to peak. So... you could say that Mauna Kea is the tallest mountain on our planet... (you could!)

On Thanksgiving day, my friend Lily and I took a drive up to the top of Mauna Kea, and did a little hike up there at high elevation. Today, I'd like to share some photographs of that excursion. We saw some pretty cool geology.

On the drive up the mountain, we saw an animal which was apropos, considering the day:

Gobble, gobble, gobble. Watch out turkeys, we'll be back after we work up an appetite...

Here's Lily's jeep in the "saddle" between Mauna Kea and Mauna Loa, looking north (with Mauna Kea in the background and basaltic lava flows from Mauna Loa in the foreground):


Some cider cones (the Hawai'ian word for cinder cone is pu'u) in the saddle:


Turning the other way (looking south), you can see the bulky form of "the long mountain," Mauna Loa. What a classic shield volcano shape! I love the fact that it's so dang wide it makes a lousy photograph. You just can't capture its spread-out bulk in a photo; it's too massive:


This was the spot where I pretended to have my toes overrun by a pahoehoe flow:


As we drove up the road to the top of the mountain, I was amazed at the raw volcanic landscape, decorated with cinder cones like this one:


At one point, we passed a neat little angular unconformity on the roadside. Here it is, with a nickel (white dot left of center) for scale:


Here's a closer-shot of this small angular unconformity. Earlier layers of ash and lapilli were deposited at a steep angle, and then eroded (perhaps by glaciation? pure speculation there) before more ash and lapilli were deposited atop it, at a lower angle. There's not likely to be much time missing here, and so perhaps it's better to think of this as the top of a cross-bed, an advancing front of pyroclastic deposition moving down the mountainside, overrun by later eruptions, which may have scoured off the upper few inches (??? pure speculation) or so before deposition.

Really, the truncated tops of cross-beds are mini-angular-unconformities, when you think about it; just not with the same amount of time missing at a "real" angular unconformity (with millions of years missing) due to mountain building like the one at Siccar Point. (Video of cross-beds forming)

Here's something else which the clueless geologist might mistake for a sign of mountain building:
No, those aren't originally-horizontal strata that have later been folded. They're layers (again of ash and lapilli) deposited on the originally-rough topography of the mountainside, covering small ridges and filling small valleys. Where a given layer is exposed at higher elevation, I interpret to be a paleo-topographic high; where that same stratum is exposed at lower elevation, that's a paleo-topographic low. The roadcut reveals these layers have undulating shapes, but this is unlikely to be folding that results from tectonic compression: instead, I think it's showing us the lay of the ancient land surface.

Looking south, we could see past Mauna Loa to the actively erupting steam vent coming out of Halemaumau Crater at Kilauea Caldera (source of the vog!):


Near the summit of Mauna Kea, there are a bunch of astronomical observatories:






On the summit is where you find those examples I mentioned the other day of hawaiite, a rock of basaltic composition that is very dense (ostensibly due to erupting beneath the extra pressures of a Pleistocene ice cap):


Here's me on the summit:


View to the north from the summit: More cinder cones...


Here's a YouTube video of me pointing stuff out from the summit (Kilauea, Hualalai, Mauna Loa, observatories, hikers, etc.). Unfortunately the wind makes it all but unintelligable, but I filmed it, doggone it, so I'm going to post it:



I found a beautiful example of a volcanic bomb up there:


After the visit to the summit, we went for a hike to a small supposedly-glacially-gouged-out lake below the summit (Lake Waiau):


Here's a Google Map, showing the lake's location:


I was surprised to see a thick biofilm on the bottom of the lake:


Encrusting the pebbles and cobbles there, it reminded me of Nora Noffke's modern and Archean biofilm photos in the recent GSA Today, as well as my "Life in Extreme Environments" class this past summer at Montana State University.


We saw some nice examples of structural geology on this hike. Previously, I've mentioned plumose structure, a branching pattern on the topography of fracture surfaces in fine-grained rocks. We saw some of that on blocks of basalt atop Mauna Kea, as in this example (again a repeat photo, but the other day I showed it to you for the vesicle; today I'm showing it to you for the plumose structure.)


A similar feature are arrest lines, which again are minute variations in the surface of a fracture. Like plumose structure, which branches from a source point (where the fracture initiated) and branches out in the direction of propagation, arrest lines tell us about the development of a joint. Unlike plumose structure, though, they are not parallel to the propagating fracture front. Instead, they form perpendicular to it, and record how the fracture propagates in small "steps." Each of these arrest lines is interpreted as being a spot where the fracture grew a little bit, then stopped ("arrested") and then grew some more. In this case, the fracture face we're looking at started at the bottom of the picture and grew towards the top of the photo. You can even see some less-discernible plumose structure backing this up:

Similar arrest lines can be seen in basalt images here and here...

We also saw some pretty spectacular xenoliths. Here's one of gabbro in basalt:


Here's one of peridotite in basalt:


And a few more:



My boots, with another volcanic bomb:


Driving back down the mountain afterwards, we got this nice view of the cinder cones (pu'us!) in the eastern part of the "saddle" between Maunas Kea and Loa:


This Mauna Kea excursion was one of my favorite things that I did on my all-too-brief trip to Hawaii. It was great to get up in the high country, where the air is thin (and vog free!) and the skies are deep blue, and the geology is surprisingly varied (at least it was surprising to me, and pleasantly so). The hike let us work up a good appetite, so we headed back down the mountain and straight to Thanksgiving dinner!

Labels: basalt, birdies, geology, glacial landforms, hawaii, igneous, mountains, msse, primary structures, structure, travel, unconformities, volcano, xenoliths

Contrary to what you may have heard, it's not all basalt. Even the basalt is astonishingly varied: the extrusive rock of a thousand faces... Here I'll share some pictures I took of rocks in Hawai'i:

There's pahoehoe:


...and there's a'a:


Here's a pahoehoe flow oozing over my boot (just kidding; it was cold when I did this):


Pahoehoe lobes can drain out, leaving only the outer skin as rock, but with a hollow center. These are lava tubes (nickel for scale):


Another one (nickel for scale):


Cool texture on the inside of this lava tube (nickel for scale):

...and zooming in a bit closer (it looks like wrinkled cellophane!):


A stack of cross-sectioned pahoehoe flows, showing their tubular (totally tubular, dude) shape:


Some Hawai'i basalt is massive, like this cobble...


...or like this cobble of hawaiite, a dense form of basalt found atop Mauna Kea (where it apparently erupted beneath Pleistocene ice caps):


But the majority of Hawai'i's basalts are vesicular, meaning they contain "Swiss Cheese" type holes that result from gas bubbles. When the lava erupts, it experiences less pressure at the Earth's surface than it was subjected to at depth. As a result, many gases (steam, CO2, sulfur dioxide, chlorine, argon, others) exsolve from the lava solution and make bubbles. If these bubbles don't get a chance to pop before the lava sets up into igneous rock, then they are preserved as vesicles. Sometimes the vesicles are small:


...and sometimes they are big:


Sometimes, they are really big. Here's one I could fit my entire Nalgene water bottle into:


When vesicles later get filled in with mineral deposits, we call them amygdules. Here's some vesicles that have gotten a light coat of a white mineral on their interiors: the first step to converting a vesicle into an amygdule:


Some of the vesicles show strain (almost certainly due to late-stage flow in the increasingly-viscous lava, getting stretched out like air bubbles in pouring honey). Surface tension on the bubble wants to make it spherical, and the lower the lava's viscosity, the easier it will be to attain that perfect spherical shape, minimizing the surface-area-to-volume ratio. So when we find them in cigar-shapes or pancake-shapes instead, that's a clue that they've been deformed. Deformed not by tectonic forces (ductile flow at depth in an orogen), but ductile flow as a result of their formation, in a sluggishly oozing blob of lava:


Another example of stretched-out vesicles:


A lonely vesicle in an otherwise massive basalt:


Not sure what's going on here, but it looks cool (popped vesicles in sticky lava?):


Another thing you see a lot of in these Hawai'ian basalts are phenocrysts of certain minerals. Here, for instance, is a cobble showing nice olivine phenocrysts:


...and another:


Here's one I showed you last week when we discussed Green Sands Beach:


Here's an outcrop which shows phenocrysts of plagioclase feldspar instead:


And a river cobble (also vesicular) bearing a healthy population of feldspar phenocrysts:


Holy feldspar, Batman! This rock has a huge proportion of feldspars (you'll note that it's still vesicular, though: in spite of the overwhelming volume of macroscopic crystals, this is still an extrusive rock):


Here's something else caught up in a finer grained (and yes, vesicular) basaltic matrix: another piece of basalt!

This is a xenolith of slightly-older basalt showing flow banding in its own trains of vesicles, that after solidification got broken off and included in younger flows of basalt. I'll post some additional xenolith photos later this week.

It's not all basalt, though. Here's a breccia made of basaltic cobbles (penny for scale):


And a closer shot of the same outcrop (penny for scale):


Finally, a rock I was surprised to see: an intermediate-composition extrusive igneous rock called benmoreite (nickel for scale, and note the rock hammer impact marks):


Benmoreite is way more felsic that anything else on the island. According to my volcanic advisor Jess, it's the result of late-stage partial melting of basaltic source rocks in the island's oldest volcano, Kohala. In other words, it's a distillation of basalt: concentrating the most felsic components in this decidedly-lighter-complected rock (nickel for scale):

Labels: basalt, geology, hawaii, igneous, national parks, pleistocene, primary structures, volcano, xenoliths

Yesterday, I showed you some sand, including some green sand from Green Sands Beach on the big island of Hawai'i. Today, I'll show you some more images from Green Sands. Let's start by orienting ourselves: We're on the south side of the island, just east of Ka Lae (a.k.a. South Point). Here's a Google Map of the cove (Mahana Bay) where Green Sands Beach (a.k.a. Papalakoa Beach) is located:


To get there from the Ka Lae parking area, you can either drive a four-wheel-drive vehicle over some very rough "roads" or you can hike about 2 miles along the coast. When I visited last week, we hiked. It's a pleasant walk, and there's plenty of green sand to be seen en route to the official Green Sands Beach. Here's the coast: basalt and grassy pastureland, with plenty of wind:


A view looking down into the cove where the green sand beach is located:


So, just why is the sand here green? It's full of olivine, which is weathering out from the local rocks. At first, I assumed the source was the local porphyritic basalt. The fine-grained basalt contains many large phenocrysts of olivine, and when the basalt breaks up, these dense grains tend to be concentrated together. Here's some of that olivine-rich basalt:


But apparently the major local source of olivine are some ash/lapilli layers that make up the prominent headland on the cove's eastern edge, as seen as the "backdrop" in this photo:


A close-up of the sand on the beach, with my fingertip for scale:


And a (repeated showing) of a handful of the stuff:


These green grains don't last especially long -- olivine isn't stable over geologic timescales at the earth's surface, and so it chemically degrades and weathers away. Thus, green sand beaches are extraordinarily rare on the planet Earth (according to Wikipedia, there are two: this one, and one in Guam). You've got to have a source of olivine right there, continually adding new green grains to the mix at a rate which matches or exceeds the rate at which they are being chemically broken down.

On the back side of the beach, draped up against the outcrops of ash and lapilli, is a big slope of sand piled up at the angle of repose. I really liked the patterns made between the olivine and the dark grains as small "avalanches" flowed down the "slip face" of this pile:


I even made a pointless little movie showing these mini-avalanches of green sand:

...Or not pointless? Maybe the sandman, new on the geoblogoblock, can tell me more about what's happening here.

A poorly-lithified chunk of green sandstone (cemented with halite from seawater, apparently, as it crumbled readily in my hands):


Some green sand on a basalt cobble (which itself hosts plenty of olivine phenocrysts):


And now a closer look at some of these ash/lapilli layers which are supposedly the main source of all this olivine. These ash layers were erupted by a cinder cone called Pu'u Mahana, and apparently date to 49,000 years ago:


Some of these layers are better lithified (probably due to welding, a phenomenon that occurs when pyroclastics are erupted at higher temperatures and then deform around one another as the particles settle) and thus stand out as little 'shelves' that are more resistant to erosion by the waves and wind:


Close-up of the ash/lapilli layers, with my fingertip again providing a sense of scale:


After an hour of swimming and relaxing on the beach, we climbed back up to the plateau above the beach, where we noticed this contact between lower ash/lapilli layers and overlying basalt flows:

Notice also all the white stuff filling in fractures here. I'm betting it's calcite, especially considering the little stalactites hanging down, but I didn't have any acid with me, and I neglected to collect any to confirm that assumed identity once I got home. Mea culpa.

Hiking back along the coast to the west, we encountered more beautiful olivine basalt. Porphyritic and vesicular, this stuff just about made me cry, it was so beautiful:


I was delighted when we detoured along one of the little unnamed coves between the official Green Sands Beach and the car, and found this:


This green sand was greener than the official Green Sands Beach:


A reprise of yesterday's image of this beautiful stuff:


We noticed some footprints of a mongoose (introduced species) crossing the green sand:


Close-up of the mongoose tracks:


There was also a nice accumulation of basaltic cobbles (some porphyritic, some not, almost all vesicular) mixed in with chunks of coral:


Wow. What a cool place! Unique in my experience, and pretty close to unique in the world. If you're ever on the big island, you've got to check it out. As a geologist, visiting Green Sands Beach imparts big bragging rights: it will make all your friends green with envy!

Labels: basalt, hawaii, oceans, sediment, travel

I'm on the big island of Hawai'i for the Thanksgiving break; and I've really enjoyed trooping around and checking out the volcanic features. (Photos once I get back to DC...) The other night I saw Bela Fleck and the Flecktones perform in Waimea, and they were playing lots of Christmas tunes from their brilliant new album. The next day, hiking on Mauna Kea, the residual music mixed in my brain with the cool igneous geology I was seeing. The result? The Twelve Days of Volcanoes... Enjoy!

On the first day of Christmas my island sent to me:
a bunch of pahoehoe

On the second day of Christmas my island sent to me:
2 Pele's hairs
and a bunch of pahoehoe

On the third day of Christmas my island sent to me:
3 aa's
2 Pele's hairs
and a bunch of pahoehoe

On the fourth day of Christmas my island sent to me:
4 falling blocks
3 aa's
2 Pele's hairs
and a bunch of pahoehoe

On the fifth day of Christmas my island sent to me:
5 volcanoes
4 falling blocks
3 aa's
2 Pele's hairs
and a bunch of pahoehoe

On the sixth day of Christmas my island sent to me:
6 basalts flowing
5 volcanoes
4 falling blocks
3 aa's
2 Pele's hairs
and a bunch of pahoehoe

On the seventh day of Christmas my island sent to me:
7 tubes of lava
6 basalts flowing
5 volcanoes
4 falling blocks
3 aa's
2 Pele's hairs
and a bunch of pahoehoe

On the eighth day of Christmas my island sent to me:
8 steam explosions
7 tubes of lava
6 basalts flowing
5 volcanoes
4 falling blocks
3 aa's
2 Pele's hairs
and a bunch of pahoehoe

On the ninth day of Christmas my island sent to me:
9 green sand beaches
8 steam explosions
7 tubes of lava
6 basalts flowing
5 volcanoes
4 falling blocks
3 aa's
2 Pele's hairs
and a bunch of pahoehoe

On the tenth day of Christmas my island sent to me:
10 billion vesicles
9 green sand beaches
8 steam explosions
7 tubes of lava
6 basalts flowing
5 volcanoes
4 falling blocks
3 aa's
2 Pele's hairs
and a bunch of pahoehoe

On the eleventh day of Christmas my island sent to me:
11 craters glowing
10 billion vesicles
9 green sand beaches
8 steam explosions
7 tubes of lava
6 basalts flowing
5 volcanoes
4 falling blocks
3 aa's
2 Pele's hairs
and a bunch of pahoehoe

On the twelfth day of Christmas my island sent to me:
12 voggy lungfuls
11 craters glowing
10 billion vesicles
9 green sand beaches
8 steam explosions
7 tubes of lava
6 basalts flowing
5 volcanoes
4 falling blocks
3 aa's
2 Pele's hairs
and a bunch of pahoehoe

Labels: basalt, hawaii, humor, music, travel, volcano

Today: I offer some photos I took in Boulder Canyon, Colorado, in June. These are all igneous rocks exposed in the Precambrian 'basement' rocks, brought to the surface by the Laramide Orogeny.

Directions: Drive to Boulder; go west up the main canyon into the Rocky Mountain Front Range.

Location map:


Granite pegmatite:


Contact! Granite pegmatite meets granodiorite:


Contact! Granite dike cutting across granodiorite (with one small mafic xenolith):


Contact! Mafic xenoliths afloat in granodiorite:


Put the previous two pictures together, and what do you get? My favorite outcrop of the whole excursion... Contact contact! A granite dike cutting across mafic-xenolith-bearing granodiorite. This would be a good practice photo for introductory level students to establish relative ages of the three different rocks shown:


Contact! More prosaic, but high-contrast... Granite meets basalt:


Epidote vein (Without any good reason, I love the color of epidote):


My Prius parked on the side of Boulder Canyon Drive:

Labels: basalt, colorado, granite, primary structures, prius, travel, xenoliths

In December of 2005, I went out to The Sea Ranch, California, for Christmas. (The Sea Ranch is one of those towns that is officially called "The" something, kinda like The Plains, Virginia. Sorta weird, but there it is.) I want to share an experience I had there, because it gave me an important perspective on my own 'native' geology back in the mid-Atlantic region. It was a significant moment of understanding for me. Let me walk you through it...

The following collection of images are what I saw walking a mere 1 mile up and down the coast from the house where we were staying. I hope you will be struck by the incredible diversity of rock types seen here (as I was):

Conglomerate:




Siltstone and shale interbedded (vertical bedding):


Siltstone and shale interbedded (anticline):


Siltstone and shale interbedded (syncline):


Mudchip conglomerate (mud chips are "rip-up" clasts due to scouring of a muddy location by a sudden intense current, which carries much larger particles like the sand that now surrounds the darker, finer-grained mud chips):


Quartz-rich sandstone:


Graywacke (showing mouthwateringly beautiful graded bedding):


A zoomed-out shot of that graded bed:


Various sedimentary layers (sandstone, silstones, shale partings):


And a close-up of a few small faults that cut through them:


And it's not just sedimentary rocks. Here's some greenstone (metamorphosed basalt). Note the cluster of amygdules (infilled vesicles) in the center:


The greenstone is green due to a lot of chlorite, but it also shows some nice epidote:




Looking north up the coast from our rental house, you could see greenstone and conglomerate intermingled on the 10m-scale:


This is in the small cove directly in front of our rental. There are three different rock units seen here (greenstone, conglomerate, clayey sand), all indicating different things. Note the big clast of greenstone "hovering" in the clayey sand part:




So after taking a walk along the lovely coast there, and seeing all this stuff, I thought "Wow."

The tremendous diversity of rock types along this section of the Sonoma County coast was due to tectonic shuffling of rock types at a subduction zone. In the Mesozoic, this part of California was at a trench where the Farallon Plate was being subducted to the east underneath North America. Melting at depth produced magma, which resulted in the Sierra Nevada continental volcanic arc (excellently reviewed by Geotripper in his "Under the Volcano" series examining the Sierras). But at the trench itself, all the sediments at the edge of North America were being compressed and squeezed and mixed up with the sediments being scraped off the subducted oceanic slab. Some knobs and bumps of basalt even got scraped off the Farallon Plate and added into this jumbled mess. Altogether, this big pile of debris from the convergent boundary is referred to as an accretionary wedge. "Accretionary" because it got accreted, or added, onto the western edge of North America. "Wedge" because that's its overall shape in cross-section.

When subduction ceased (due to the subduction of the East Pacific Rise), the Farallon Plate was gone at this latitude, and the Pacific Plate and the North American Plate were now in direct contact for the first time. As time went by, the accretionary wedge reacted to now longer being dragged downward, and it began to isostatically rebound. It bobbed upward, and brought its 'melange' (French for mixture) to the surface. The uplifted accretionary wedge is the California Coast Ranges, a fantastic place for varied geology mainly because of the tectonic "shuffling" that happened here during the Mesozoic.

So, I mentioned that seeing all this diversity in so short a hike really impressed me. But the insight it gave me is that the same thing happened on the east coast. Where I live and work, in DC and Virginia, an accretionary wedge developed during the early Paleozoic, just like in California, with the exception that ours got subsequently squeezed and metamorphosed in a series of mountain-building events. It's a bit more difficult to recognize, partially due to that metamorphism and partially due to all the @#$%ing vegetation obscuring the underlying bedrock. But it's there: we have metagraywacke, with relict graded beds, metabasalt, quartzite, schist ("meta-shale") and metaconglomerate: it's everything I saw in California with a metamorphic overprinting!

"Wow," I thought again.

Here's some shots of DC-area rocks that are analogues for the ones I've already showed you in California:

Metamorphosed mud-chip conglomerate (near Chain Bridge, DC):


Metamorphosed quartz-rich sandstone (the Sugarloaf Mountain quartzite, MD):




Metagraywacke showing metamorphic chlorite, garnet, and pyrite (both from DC):




Graded bed preserved in metagraywacke (Billy Goat Trail, MD):


Metabasalt (amphibolite, again from the Billy Goat Trail, MD):


Metaconglomerate (Klingle Road, DC):




The experience comparing the two coasts greatly enriched my understanding of tectonics and subduction, and gave me perspective on DC's geologic history. Two different accretionary wedges, two coasts, two eras... but one underlying process. That's what really hit home. Geology repeats itself. It gave me a renewed interest in my local geology. Everyone always hears about what great geology California has (and it does), but doggone it, DC pulled that same trick millions of years earlier, and experienced a series of orogenies immediately afterwards (which California can't claim!).

If it's true that "the best geologist is the one who has seen the most geology," then I became a better geologist that day on the Sonoma coast.

PS - I think it's funny to note that I didn't put a sense of scale in any of the California pictures, but that most of the DC area pictures do have one. I think that says something about my development as a geologist and educator too...

Labels: basalt, plate tectonics, primary structures, sediment, structure

Today, some shots from my time in Yellowstone National Park last summer. Here's Mammoth Hot Springs:

Close-up of the travertine deposits at Mammoth:

Me advertising my brother's company at Mammoth:

Norris Geyser Basin, slime:

Norris Geyser Basin's loneliest tree:

More slime, this time two colors:

Nasty patch of slime. Looks like snot:

Bison herd:

Columnar jointing in basalt:

Me showing you where the columnar jointing is. (I'm pointing at it...)

Strata exposed in the Tower area:

And here they are again, labelled:

Lastly, heading north out of Yellowstone back to I-90 and Bozeman, here's a weathered-out Eocene dike in the Paradise Valley. The dike is more resistant to weathering than the rock it cuts through, so it stands up as a "wall"-looking feature.

Labels: basalt, field trips, hot springs, limestone, montana, primary structures, stratigraphy, travel, yellowstone

The Culpeper Basin is a Mesozoic (Triassic/Jurassic) rift valley in northern Virginia.

As Pangea was breaking apart, a series of normal-fault-bound basins stretched open in an NW-SE direction (giving them long axes that run NE-SW). Some of them connected together in a NE-SW direction, and kept spreading further and further open. Through continued seafloor spreading, these became the Atlantic Ocean basin. Some did not keep opening, and essentially filled in with dirt. Those are the ones that are still preserved up on the North American continent today, including the Culpeper Basin. These basins vary in size, but they run up and down the coast of eastern North America, from Newfoundland down at least into the Carolinas (presumably there are more buried beneath Coastal Plain layers even further south than that). Collectively, these basins are referred to as the Newark Supergroup. They are characterized by immature sedimentary rocks and mafic igneous rocks.

Here's an E-W cross section through the Culpeper Basin, by Chuck Bailey at W&M:

Structurally, then, the basin is a graben, bounded east and west by normal faults.

The igneous rocks in the Culpeper Basin are mostly diabase, but there are some basalt flows too. The sedimentary rocks are a motley mix, including arkose, red siltstones, and lake deposits including siltstones and anoxic black shales. Along the eastern and western boundary faults, we also find coarser sediments that have been lithified into conglomerates. Sediments flowed into the basin from source areas both to the east and west, so you would expect the conglomerates along each edge to look a little different. Indeed, they do!

A modern analogue for the Culpeper Basin is the Afar Triangle region of northeastern Africa (Ethiopia, Eritrea, and Djibouti). Note the sedimentary influx from both the east and the west. Note the lakes, and note the mafic extrusions:

Back to the Old Dominion: I've mentioned the Culpeper Basin's eastern boundary fault before, back in March, when I posted this picture of the conglomerate that outcrops in Clifton, Virgina. It is characterized by lots of clasts of highly-foliated metamorphic rocks (derived from the neighboring Piedmont).

...But I haven't talked about the western boundary fault much. And since I visited it yesterday, today's the day to talk about it.

One of these western Culpeper Basin conglomerates is kind of famous. It's the Leesburg Conglomerate, and it outcrops near Leesburg. It's mostly limestone cobbles and gravel, with some quartzite, too, set in a red matrix. It's a beautiful rock. Here's a couple of field photos taken on Route 15, a mile or two north of Leesburg proper:

The Leesburg Conglomerate was used in the awesome columns in the U.S. Capitol's Hall of Statuary (topped by the much less interesting Carrara Marble of Italy).

Yesterday, NOVA adjunct geology instructor Chris Khourey headed out to Thoroughfare Gap (see map below) to check on a couple of field sites. Thoroughfare Gap is a water gap in the eastern limb of the Blue Ridge Anticlinorium, and it's also the western boundary of the Culpeper Basin. Both Interstate 66 and Route 55 pass through this striking landscape feature:

Note how different this looks as compared to the Leesburg Conglomerate. One thing that immediately jumps out at you when you see an outcrop of it is the large proportion of the cobbles that are pieces of the Catoctin Formation basalt (see more photos of the Catoctin in Monday's post on rocks of Shenandoah National Park). Here's a couple of close-up shots of such cobbles, bearing distinctive amygdules (filled-in vesicles):

But there's also plenty of limestone cobbles and gravel in there too, as this photo shows:

As with the Leesburg Conglomerate, the Waterfall Conglomerate's limestone inclusions are likely coming from the Cambrian & Ordovician carbonates exposed today in the Shenandoah Valley and other valleys of the Valley and Ridge province. More on that later this weekend, when I'll post some shots from the Massanutten Synclinorium.

Labels: basalt, culpeper basin, limestone, mesozoic, sediment, virginia

This weekend, I took a backpacking trip in Shenandoah National Park. Thought I would share a few photos today: scenery first, geology second...

Here's the view looking east from Skyline Drive:


The temperature difference due to elevation was striking. It was still early spring up on the top of the mountains, on Skyline Drive:


...But down below, it was green and lush (and sodden with pollen!):


I camped out for two nights near Corbin Cabin, and did a day-hike around Thorofare Mountain on Saturday, visiting this waterfall at lunchtime:


The geology of Shenandoah National Park is interesting: it records the assembly of the early supercontinent Rodinia at about a billion years ago, and then the breakup of Rodinia about 600 million years ago. The first event recorded is the generation of granite gneisses and granites due to the Grenville Orogeny. The oldest unit in the park is the 1.1 Ga Pedlar Formation, a granite gneiss. There's a slightly younger granite which intrudes it called the Old Rag Granite (~1.0 Ga), but I didn't see any outcrops (or float blocks) of it, so I'll not mention it further. There's a thin, patchy sedimentary cover called the Swift Run Formation deposited directly atop the granite gneiss and granite, providing a nonconformity surface. Atop that is a series of volumnious tholeiitic basalt flows: these mafic extrusions record the breakup of Rodinia and the opening of a new ocean basin: the Iapetus. In many places in the park, you can see "feeder dikes" of the Catoctin cutting through the older plutonic and metaplutonic rocks (see image below). There are also some sedimentary rocks layered atop the Catoctin (the Chilhowee Group), recording the transgression of the Sauk Sea on the North American platform. But I didn't encounter any good outcrops (or float blocks) of them on this trip, so I'll stick to the tectonic story: the Pedlar Formation shows us Rodinia getting put together, and the Catoctin Formation shows us Rodinia breaking apart. Later metamorphism due to Appalachian mountain-building resulted in changes in both of these rocks (development of "blue quartz" in the Pedlar, and the Catoctin metamorphosed to greenstone).

Here's a massive dike (possibly a "feeder dike" feeding surface lava flows) of the Catoctin basalt cutting through the Pedlar Formation granite gneiss, just north of the Marys Rock Tunnel. Note the columnar jointing extending perpendicular to the walls of the dike:


Having covered all that, I now propose to spend the rest of this blog post showing you the variety of cobbles and boulders in my campsite. I camped at the little wedge of land above the confluence of two streams. One stream's catchment basin was Catoctin, and the other drained outcrops of Pedlar. As a result, the "float" in my camp was all either Pedlar Formation or Catoctin Formation. I'll just run through them one after another so you get a sense of the range of variety in each formation.

You'll notice that the Pedlar is sometimes coarse, sometimes fine, sometimes well foliated, sometimes not so much. You'll also notice that the Catoctin varies a lot in terms of its extrusive texture: sometimes aphanitic (fine-grained), sometimes amygdular (formerly vesicular), sometimes it even runs to volcanic breccia. All of these original lithologies have been metamorphosed to various degrees in the Catoctin, which here can be seen by comparing the amount of green in the rock. This green comes from two metamorphic minerals: chlorite and epidote. Enjoy!

Pedlar Formation:



















Catoctin Formation:

Labels: appalachians, basalt, blue ridge, granite, iapetus, metamorphism, national parks, primary structures, proterozoic, shenandoah, virginia

Last week on one of the many field excursions, I found a nice cobble of amygdular basalt. Amygdules are vesicles (bubbles in degassing lava that didn't get the chance to pop before the lava solidified into igneous rock) that have been filled in with mineral deposits. In the mid-Atlantic, most amygdules are found in the Neoproterozoic lava flows of the Catoctin Formation, from which my cobble was presumably derived. The amygdules are typically filled in with zeolites, quartz, and jasper. This one doesn't show any jasper, but the basalt still appears to be basalt, too -- whereas the Catoctin typically is metamorphosed to greenstone / greenschist. I've noticed an association between jaspery amygdules and epidote formation in the metaingeous rock.

As with Skolithos-bearing Antietam Formation quartzite cobbles, clasts of the Catoctin deposited in the river gravels atop the Piedmont/Coastal Plain unconformity indicate a Blue Ridge provenance for the cobbles, and therefore a eastward-flowing river to deposit them 100 million years ago.

I took the cobble back to the lab and sliced it open on the rock saw. The brown circle in the background is a penny for scale.

Here's what the sawn surfaces look like after I sanded them down a bit and then scanned them:

Right purty, ain't it?

Labels: basalt, blue ridge, coastal plain, primary structures, sediment, virginia

Geological Travels in Northern Ireland, Part VII:

Ground moraine being used (quite appropriately) as a golf course, east of Port Rush.

An old quarry south of the road between Bushmills and Port Rush. This is easily accessible from the parking area for White Rocks, a popular surfing beach. (Yes, they surf in December in Northern Ireland!)

Well-exposed here is the unconformity between the Cretaceous-aged "Chalk" (the Ulster White Limestone) and the overlying "Lower" Basalts (Paleogene in age).

The ancient topography is revealed in the undulations of the unconformity surface: prominently featured here is an ancient valley that was topped off with basaltic lava during the eruption. Valley depth in this photo is about 80 feet.

The limestone ("Chalk") here was quarried for lime. Lime is the binding agent in cement and mortar, and it is produced from the burning of limestone. Disused kilns from the burning process were still situated in the quarry. The area was lousy with flint nodules, like the one here. I collected a beautiful one that looked like a cross between a sausage and a powdered donut, but security confiscated it from my carry-on luggage on my flight back home.

Labels: antrim coast, basalt, chalk, concretions, flint, geology, glacial landforms, northern ireland, unconformities

Geological travels in Northern Ireland, Part VI:

The word "joint" in geology refers to any fracture in a rock unit along which movement has NOT occurred. (If movement DOES occur along a fracture, that makes it a fault.)

The Giant's Causeway in County Antrim, Northern Ireland, shows jointing of a particular pattern: the intersection of the joints divide the rock into column-shaped pieces, shaped roughly like an un-sharpened pencil.

This is an image of two of the "Causeway basalt" layers exposed in a gorge east of the Giant's Causeway itself. Note their difference in size: slower cooling produces larger columns. Faster cooling produces smaller columns. Therefore the lower flow cooled off more rapidly than the upper flow.



Lava, when hot, takes up more volume than cold igneous rock. As it cools, the solidifying lava contracts. Because the whole volume of rock is contracting, evenly-spaced centers of contraction develop. Cracks open up to accomodate that contraction. This makes a honeycomb-style pattern, because 3 crack orientations is the minimum number necessary to allow contraction in every direction. These three orientations meet at an average angle of 120º.

The same phenomenon can be seen at Devils Tower, Wyoming.

The weird columnar jointing patterns at the Giants Causeway were used on the cover of Led Zeppelin's album Houses of the Holy (1973). While I was there, I thought about re-creating the album cover with geologists (clothed!) in the same positions as these kids, but I forgot to bring along the album as a reference. Tragic, isn't it?

The overall loss of volume of the (hot versus cold) rock can be estimated with a photograph like this. Divvie the photo into equal units of area, and then count up how many are solid rock and how many are empty air. About 1% shrinkage is seen here -- more than in other places I've seen columnar jointing.

Once formed, these joints allow water to penetrate into the lava flow. Water encourages both physical and chemical weathering of the basalt, enlarging the size of the fractures. Water, being the universal solvent, helps catalyze many chemical reactions. Basalt is a rock that is stable under certain conditions in the Earth's interior, but it is not stable at the Earth's surface, where conditions of temperature, pressure, and humidity encourage it to break down. These break-down chemical reactions start on the surface of the column and work their way inward, like a thousand mice nibbling on the exterior of a large block of cheese. Physical weathering takes place when the water freezes. When water becomes ice, it expands in volume by about 9%. This "wedges" open the cracks even more. Once widened, they can accomodate more liquid water, which can then freeze again, widening the cracks further.
The end result of these physical and chemical weathering processes is to break down the rock, from the outside in. Rotten rock sloughs off in sheets, exposing fresh rock from the interior for weathering to attack. This produces an overall "onion skin" effect. An original polygonal chunk of rock become spheroidal over time, as weathering reduces it in size and volume. Pound coin for scale.

Labels: antrim coast, basalt, geology, giants causeway, northern ireland, structure, weathering

Geological travels in Northern Ireland, part V:

A hike east from the Giant's Causeway on the "Shepherd's Trail" takes you along the edge of a steep escarpment, where you can look down and see all kinds of cool things.

Here, I was struck by how plainly the sequence of geologic layers was revealed. The oldest exposed layer here is the sequence of lava flows known as the "Lower basalts." (I mentioned this layer earlier, in my post about the Antrim Coast.)

Atop them is a laterite layer. Laterite is a tropical soil, red in color due to the presence of oodles of oxidized iron. Of course, basalt is a mafic rock, meaning it is very rich in iron. When that iron-rich rock is exposed to warm, wet conditions, a lateritic soil develops atop it. The laterite layer therefore represents a time of relative calm in County Antrim, a time between eruptions, when the land was in a tropical latitude & climate.

Finally, atop the laterite is another series of basalt flows. These are sometimes called the "Interbasalt" layers, or more commonly "The Causeway basalts" since they are typified by columnar jointing of the type exposed at the Giant's Causeway. Here, you can see multiple layers exhibiting strong columnar jointing. (The stratum directly above the laterite layer is the one that filled the paleo-valley that is exposed today as the Giant's Causeway itself.) The Causeway basalts have been dated to about 60 million years ago, in the early Paleogene (about 5 million years after the extinction of the dinosaurs). Their tectonic cause was the rifting of Laurentia, separating Greenland from Europe. These basalts are part of a larger basaltic province, the Thulean Plateau, which can also been found in Scotland, the Faroe Islands, Iceland, and parts of Norway, as well as the eponymous area of Thule, Greenland.

Labels: antrim coast, basalt, geology, giants causeway, laterite, northern ireland

Geological travels in Northern Ireland, part IV:


"The Giants Causeway" is the name of this peninsula of land sticking out into the North Sea. Note the people on it for a sense of scale. Admittedly, it doesn't look too impressive from a distance. But when you get closer, an interesting pattern emerges...



The Causeway is made of thousands of columns of basalt. Oriented a few degrees shy of vertical, these columns formed when an ancient lava flow cooled down and contracted. Cracks developed on the top of the flow (the coolest part) and propagated downward, dividing the rock into these uniformly-shaped chunks.





Viewed from above, each column's shape becomes apparent: they are polygonal: mostly 6-sided, but there are also 5-sided, 7-sided, 8-sided, and 9-sided columns. There is a one-pound coin placed on the middle column in this photo to provide a sense of scale.

Casey sits in a natural "throne" made by the columns as they have been weathered by the pounding waves. You can see here that they are not quite vertical on the west side of the Causeway -- but instead are plunging steeply to the west.







On the east side of the Causeway, a tall outcrop of columns shows them plunging steeply in the opposite direction -- to the east. In between the two sides (down the middle) of the Causeway, the columns are approximately vertical. Note also the ~horizontal joints which divided each column into a series of cake-like stacks. You can tell that these joints came later, because they do not continue uniformly across columns (look at the lack of alignment at the bottom of these columns, for instance).












The overall sequence in the events of the formation of the Causeway would look something like this diagram, shown in cross-sectional view.

First, the "Lower Basalts" were eroded, and a valley was carved out.

Second, the "Causeway Basalts" were erupted, filling the valley. Columnar jointing began at the top of the flow and propagated downward.

Third, the "Causeway Basalt" lava had completely solidified, with columnar jointing dividing up the igneous rock into subterranean columns. Note the radial "splay" of columns in the paleo-valley. On the eastern side, they plunge to the east. On the western side, they plunge to the west.

Fourth, erosion attacks the landscape, removing some material. The Causeway itself pokes up above sea level.


Tourists clustered on the tip of the Causeway.

Labels: antrim coast, basalt, geology, giants causeway, northern ireland, structure

Geological travels in Northern Ireland, part III:

After a brunch in the village of Moira with my old friend Andrew and his newly pregnant wife Nadine, Casey and I drove up the coast of County Antrim. Her friend Jodie had loaned us her Audi and arranged for us to stay at a condo in Port Rush. Road trip!

This is the view south from an area called Garron Point.




I stopped and poked around amongst the boulders on the shore. Note the boulders are two colors: black basalt and the white chalk.








Here's Casey staring out across the North Channel at the Mull of Kintyre (Scotland), only 12 miles distant at the closest point.









Awesome, awesome, awesome. There's so much going on in this picture, I don't know where to start! Very prominent (and annotated with a dotted line) is the contact between the light-colored chalk and the overlying dark-colored basalt. This chalk layer is really a white limestone at this locality. Unlike the same layer where it famously outcrops at Dover (England), here the chalk has been compressed by heavy overlying lava flows. These basalt layers are called "lower" because they are the bottom of a three-part stack of igneous eruptions. The layers are all tilted here at Garron Point because they have slumped: large blocks of strata have slipped downward and outward, sliding along an underlying clay layer, the Lias. Conveniently, the Lias is Triassic in age, the overlying chalk is Cretaceous, and the basalts here are Paleogene: one formation per period. It's worth noting that the word "Cretaceous" itself comes from the Latin word creta, or "chalk." The entire Cretaceous period is named for this brilliant white layer of rock, which also extends across southern Britain and into France. This chalk is made up of gazillions of little coccolithophores, like I mentioned in an earlier post about ocean acidification.

Here's an image from a tourist sign at Garron Point which may make the geology a bit clearer. Note the sketch in the upper right of the slumped blocks.





Large grey nodules of flint that are present in the chalk exposed at Garron Point. These nodules probably form diagenetically -- after the sediment is deposited and the component bits were organizing themselves into rock. Smaller bits of silica (possibly from siliceous sponge spicules) dissolved and reprecipitated in these concentric nodules. Flint breaks conchoidally, like glass, and so these nodules were a terrific local source of arrowhead & axe tools for Stone Age peoples in Ireland. Pound coin for scale.

Lastly, here's a shot of sunset from the Torr Road, which is a crazy twisty little road that runs along the northeastern Northern Irish coast.

Labels: antrim coast, basalt, chalk, northern ireland, travel