Check out these amazing structural photographs, available in high resolution too!

Labels: england, northern ireland, republic of ireland, scotland, structure

I just got an e-mail from Alan Watson, of Belfast, who read my article in Geotimes about geological travels in Northern Ireland. (This was one of my first topics on this blog, so newcomers may be interested in revisiting some of those old posts in the December and January archives.)

Anyhow, Alan clued me in to a new series from the BBC called "Blueprint," wherein they examine the natural history of the Emerald Isle. There's a cool interactive aspect to the website where you get a map of Ireland and a choice between "Plants/Animals," "Humans," and "Land." Choose "Land" and then select what you want to learn more about. Then you get a series of images, conversations, or videos about different aspects of Northern Ireland's geologic history. It's pretty cool -- there's a really enthusiastic dude (William Crawley) talking about the eruption of the Giant's Causeway, and also examinations of "the Chalk," graptolites, and granite gneiss. They even mention the Iapetus Ocean! (Which was a big focus of the field trip I led today!)

Labels: chalk, giants causeway, northern ireland, websites

Today my piece for Geotimes on geological travels in Northern Ireland went up on their website. You can check it out here.

Labels: northern ireland, websites

One final shot from Northern Ireland: me holding up a cloud. Sunset, December 30, 2007: Port Rush, County Antrim.

Labels: antrim coast, art, clouds, northern ireland

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

Geological travels in Northern Ireland, part II:

Mammatus clouds hanging over Lough Erne, in western Northern Ireland. Our friends Jodie and Rory have a caravan on this large lake. After our tour of the cathedrals of Armagh, Jodie drove us out here to have a hike at the lake (which was great in spite of ending in darkness and rain) and to rest up in their modish accomodations there.













This is Mount Slemish, an eroded volcanic neck in Northern Ireland near Antrim. This "basalt plug" was once the center of a volcano which erupted lava all over this vicinity. Because the massive basalt in the volcano's "throat" was tougher than the surrounding stratified rock layers, it stood up strongly to erosion, and now rises to 1,457 feet (437 m) in elevation, dominating the local landscape.

Labels: clouds, northern ireland, slemish, volcano

In the week between Christmas and New Years, my friend Casey and I took a trip to Northern Ireland. We stayed with her friends Jodie and Rory in Portadown, and on our first full day, Jodie took us out to Armagh (pronounced "Ar-maa"), where she teaches at a primary school. Saint Patrick apparently decided that Armagh was going to be the seat of Irish faith, and he decreed that the Archbishop of Armagh would have preeminence over the rest of Ireland. Of course, Northern Ireland is a land still strongly divided along religious lines. Though it's no longer violent, there is still strong "us and them" sentiment among the Northern Irish people I spoke to. Jodie took us to visit Armagh's two cathedrals: one Catholic, one Protestant. They occupy the two highest hills in town (of course!).







Flatscreen television monitors inside the Catholic cathedral, so that worshippers can see what's going on behind those massive columns.










I was delighted to note a bunch of geological details in the two buildings. This image is of the limestone that makes up the exterior of the Catholic cathedral. It's full of fossils. Here's some spiral-shelled creature. Not sure what exactly. Width of fossil is about 1 inch.












Fossil coral colony on the exterior of the Catholic cathedral. Pound coin for scale.











The Protestant cathedral (Church of Ireland) is made of a greater variety of stones. Most of it is sandstone, and the sandstone hosts deposits of iron oxide (hematite) in precipitated horizons called Liesegang banding. Though it looks strongly layered, the Liesegang banding is not sedimentary bedding. In this block, bedding is horizontal, and the Liesegang banding was deposited by groundwater at an angle to the bedding. Pound coin for scale.


The lower part of the Protestant cathedral is made of conglomerate/breccia. The large clasts are fairly angular, indicating that they did not travel far from their source area before they were deposited. This makes it more a breccia than a conglomerate. Unlike a lot of true breccias, however, this rock is pretty well stratified (layered), indicating that it was deposited by moving water: a characteristic of conglomerates. Pound coin for scale.



Here's one particular clast from the conglomeratic lower part of the Protestant cathedral is made of conglomerate/breccia. In it you can see fossil fragments, apparently of the same coral visible in the Catholic cathedral's stone. Pound coin for scale.






Of greatest interest to me was the fact that James Ussher was the head of the Church of Ireland (the full title is "Primate of All Ireland") from 1625 until 1656. As I mentioned earlier, this means that he was the Archbishop of Armagh. Ussher has a reputation as the most scholarly of the historial archbishops, and he is particularly known to geologists because he attempted to calculate the age of the Earth using the Bible. By estimating generational times and tracking geneaological lineages in Scripture, Ussher proposed that moment of the Earth's creation was the evening immediately before Sunday, October 23, 4004, B.C. It is from his work that the specific notion of a young Earth arose. According to Ussher and his subsequent legions of young Earth creationists, our planet is only 6000 years old (well, 6011 years, to be precise.) Of course, this caused some tension with geologists of the time like James Hutton, who realized that if the uniformitarian concept is correct, the Earth must be vastly older than 6000 years (or, to be precise, 5750 years old at the time Hutton himself was mulling it all over in the mid-1700s). Later discoveries by the many geologists inspired by Hutton, in particular that of radioactive decay, provide quantitative evidence that the Earth is in fact much older than 6000 years. Three different lead isotope systems, for instance, provide ratios of radiogenic lead to non-radiogenic lead that suggest the Earth is about 4.5 billion years old. That's approximately 6 million times older than Ussher calculated -- a vast, vast difference. In spite of the overwhelming physical evidence for an ancient Earth, I still find that many students enter my classes with a perception that the Earth is less than 10,000 years old. I have James Ussher to thank for that. It was a pleasant moment for me to visit his cathedral and ponder his lasting effects.

Labels: armagh, fossils, geology, northern ireland, ussher