Wooster Geologist in Idaho: Adventures in granite

July 26th, 2017

Gloria and I traveled out west this summer to see family and a bit of scenery. Of course, geologists are always looking for geological attractions, and we found a delightful one at City of Rocks National Reserve in southern Idaho. We visited this granite wonderland with my parents, Gary and Corinne Wilson. The weather was perfect and the rocks spectacular. Above is a view of Granite Peak on the left and Steinfells Dome on the right.

The City of Rocks (great name!) is along the California Trail of the 19th Century. Its distinctive geology is described in many emigrant journals, and several travelers left their names smeared in axle grease on some of the overhanging granite outcrops. These inscriptions are at Camp Rock.

The granite overhangs provide shelter for hundreds of mud nests constructed by American Cliff Swallows. Note the baby peeking out on the left.

There are two granites present in the City of Rocks reserve. Above is the porphyritic Green Creek granite, part of the Green Creek complex of granite, gneiss, and schist that is Archean in age, an astounding 2.5 billion years old. This is one of the oldest rocks exposed in the United States. The term porphyritic means there are large crystals interspersed with small crystals. The visible white crystals here are potassium feldspar. (Dr. Meagen Pollock, Wooster’s petrologist, will be very proud of me.)

Above is the Almo Granite, which has a more even grain size distribution. It was intruded into the Green Creek Complex only around 28 million years ago.

Here is a contact of the Almo Granite above and the Green Creek Complex granite below, with my Dad’s arm for scale. This is the igneous equivalent to an unconformity, with an almost 2500 million year difference in age between the two rocks.

A closer view of the contact. The angled dikes in the unit below are made of fine-grained quartz and feldspar, a mix called aplite.

The Almo Granite shows a peculiar kind of weathering called tafoni. The outer surface of the rock acquires a resistant crust through weathering, a process called case-hardening. Apparently in this case the hardening is due to the dissolution of silicate minerals, which reprecipitate as hard silica-rich minerals. The softer rock underneath is then dissolved by salty water (the salt probably coming through dust blown from evaporative salt flats), resulting in small cavities and caves like the one above.

The tafoni and case-hardening is also visible in this Green Creek granite outcrop.

The case-hardened layer can slip off rounded surfaces through exfoliation of the granite domes, sometimes leaving a small resistant remnant on the very top. These remnants are called pickelhauben because of the resemblance to the spiked World War I German helmets. This monolith is termed, in fact, Kaiser’s Helmet.

Another distinctive form of chemical weathering of the granite produces panholes. These have flat bottoms rather than the rounded bottoms of potholes.

Dr. Shelley Judge, Wooster’s structural geologist, will love the joints visible in this granite body. Joints are fracture sets along which there is no movement (thus they are not faults). The joints here are complex and probably related to thermal contraction as the granite pluton slowly cooled underground.

This is my favorite monolith in the reserve. It shows a combination of the spherical exfoliation of the granite dome and a consistent set of joints cutting through it.

Joints dominate the Almo Granite exposed at our picnic site.

Finally, the granite outcrops at City of Rocks National Reserve are a famous destination for rock climbers. In the center here is one of the best-known climbing sites, Morning Glory Spire. At the tippy top you might be able to see two climbers standing there enjoying the view and their accomplishment.

Thank you to my parents for suggesting this trip (and Dad for driving). It is a geological paradise, even if there isn’t a fossil for miles.


Wooster’s Most Beautiful Building Stones

January 31st, 2012

Wooster, OH – Volcanoclast is hosting the latest Accretionary Wedge, and since I have exactly 2 hours left until the end of January, I thought I’d post a last-minute entry. The theme is countertop geology, or more broadly, stones that are “decorative and completely detached from their origin.” My contribution is inspired by my weekly “Research Friday” routine.

Perhaps the most crucial countertop in my life is the one at the Old Main Cafe, where my Research Friday mornings begin. Photo courtesy of Matthew Gardzina.


While waiting on my caffeinated beverage, I admire their choice of countertop: a perthitic, alkali-feldspar-rich "red" granite.


Leaving Old Main, I pass the Kauke Arch (shown here packed with snow) and the Old Main patio (on the garden level below the arch), which are paved with anorthosite tiles. Photo courtesy of Matthew Gardzina.

If you look closely, you can see the striations and play of colors in the plagioclase crystals.

With my latte in hand, I make my way to the Timken Science Library.

Not only do I get to see this gorgeous granite at the Timken check-out desk, but also on the entrance floors and caps of the entryway walls.

I wonder…would my Research Friday routine be different if I weren’t an igneous petrologist?

Wooster Geologists in Sweden

July 11th, 2011

STOCKHOLM, SWEDEN–I had not realized how much water is in the city of Stockholm. Almost a third of the city’s area is water because the center is built on 14 islands connected by bridges and ferries. “The Venice of the North” some call it.

Rachel, Nick and I are here for a very short visit. We’ll spend tomorrow in the Naturhistoriska Riksmuseet (Royal Museum of Natural History), so for now we just explored the neighborhood around our hotel. It is called Östermalm and is one of the older parts of Stockholm.

The Nordiska museet (Nordic Museum) has a glorious array of building stones, all from Sweden. The simple blocks are sandstones and fine-grained metamorphic rocks, and the carved pieces are limestones.

The local bedrock is 1750-1900 million years old, formed during the Svecofennian (Svecokarelian) orogeny. The outcrops I saw, like this example of “living stone” at the base of a building, are metavolcanics (metamorphosed volcanic rocks, usually basalt). Apparently the bedrock of Stockholm is an engineering geologist’s dream because of its stability, moisture repelling capabilities, and uniform strength — great for bridge abutments and subway tunnels.

I spent my Stockholm afternoon in the museums found in an easy walk around our hotel. I was impressed with the Viking runestones on display in the Historiska muséet (History Museum), and I was touched by this one. The runes are translated as: “Una/Unna had this stone raised in memory of her son Eysteinn who died in christening robes. May God help his soul.” They are carved in a glacial granite boulder, the kind of rock we saw scattered across the Estonian western islands. Note the dark xenoliths.

A little something for igneous petrologists

June 29th, 2011

Like a lonely little onion in a petunia patch, a boulder of red granite sits on the cobblestone beach off Soeginina Cliff, western Saaremaa, Estonia.

KURESSAARE, ESTONIA–Hard-rock geologists sometimes complain that I flood this blog with too many sedimentary rocks and fossils (and just wait until I get to the Estonian wildflowers!). There are actually quite a few igneous and metamorphic rocks on Estonia — just like there are in Ohio — in the Pleistocene glacial till. They show up well on the beaches here in contrast to the sedimentary rocks around them.

A closer view of the above rock, just to show it really is granite.

Granite in action! A granitic vein through some unfortunate rock.

Back to granite on Cima Dome

March 17th, 2011

A granite exposure near Teutonia Peak on Cima Dome. Note our jackets and hands in pockets!

ZZYZX, CALIFORNIA–Our last stop of the rapidly-cooling day was on the huge Cima Dome east of Zzyzx in the Mojave National Preserve. The dome is so large (about 70 square miles) that it is impossible to detect when you are actually on it, but easily visible from miles away. It apparently is the eroded root of a granitic intrusion formed during subduction in the Jurassic to Cretaceous. The alkali granite exposed here is very similar to that of the Granite Mountains we saw yesterday.

Potassium feldspar crystals in the coarse alkali granite of Cima Dome.

The soil of Cima Dome is derived almost entirely from the underlying alkali granite.

Igneous delights at Amboy Crater and in the Granite Mountains

March 16th, 2011

Greg Wiles on the rim of Amboy Crater.

ZZYZX, CALIFORNIA–This afternoon the Wooster geologists studied two very different magma products in the southern part of our field area. After lunch we drove to Amboy on historic Route 66 and then hiked up to the rim of Amboy Crater. Here we saw the extrusive, mafic rock basalt at its finest in the cinder cone itself and the lava flows across the valley.

Desert iguana on the basalt near Amboy Crater.

After Amboy, we traveled north to the Granite Mountains and examined wonderful alkali granites weathering into rounded boulders. The feldspar crystals in these intrusive felsic rocks were extraordinarily large and numerous, and there were many xenoliths scattered throughout. The countryside here was lush with desert vegetation that made our all-too-brief stop most enjoyable.

An exposure in the Granite Mountains, California.

The feldspar-rich alkali granite in closer view.

The Åland Islands

June 20th, 2009

There is an exquisite archipelago of thousands of  islands between Finland and Sweden.  I took a six-hour ferry ride this morning from Turku in southwestern Finland to Mariehamn, capital of the Åland Islands, where I am spending a day and an evening.  Most of the islands are formed of a brilliant red Precambrian granite, polished smooth on the top by glaciation.  Scraggly pine trees grow in the cracks they can find in the bedrock, reminding me of the terrain in the High Sierra just below tree line.

N 60.08276°, E 19.93422°

N 60.08276°, E 19.93422°

A close look at the granite shows a wonderful mix of potassium feldspars, clear quartz, and flecks of biotite.  The weathered surfaces are thickly covered with colorful lichens, which are symbiotic associations of algae and fungi.


These islands have an unusual political history.  They are populated almost entirely by Swedish-speakers, yet are a province of Finland.  Various international agreements in the 19th and 20th Century gave the islanders autonomous status within Finland and, most famously, completely demilitarized the islands, a status they managed to retain through two world wars and the Cold War.  They have their own government with a premier and parliament, they fly their own flag, and they print their own stamps.

And there you see the attraction Åland has for a geologist.  Any place that features geology on its stamps deserves a visit!

A Wooster Geologist in Helsinki

June 18th, 2009

I’m between geological field trips right now. I left Russia by train through Karelia to Helsinki, Finland. It was a remarkable trip through woods and villages, with the swampy environs of St. Petersburg giving way to higher and drier ground where ribs of granite occasionally showed. Next week I go to Spitsbergen in the Arctic to look at a Jurassic sequence for a few days, and then I meet two Wooster students, Palmer Shonk and Rob McConnell, and an Ohio State University paleontologist, Bill Ausich, in Estonia where we will work in the Silurian with our Estonian colleague Olev Vinn.

Helsinki is, of course, a highly cultured city with many attractions. I don’t want to minimize those, but since this is a geology blog, we must note the gorgeous granite mounds which dot the city.
They were polished smooth on their upper surfaces by Pleistocene (and Holocene) glaciers, which left classic striations showing the direction of ice movement. The Finnish Parliament building is in the background.

Tomorrow I leave by train for the ancient city of Turku on the southwestern coast of Finland, and then the Aland Islands for a quick look. It’s a tough job, but someone has to do it.