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, flint, geology, glacial landforms, northern ireland

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