Today’s deskcrop is a small piece of rhyolite with a bread crust texture. The bread crust texture forms as an extrusive rock cools, and resembles (wait for it)… bread crust. I have a much larger piece, somewhat more pumiceous, from the same locality, but I’ll save that one for another day.
I usually do a quick Google search before I post each deskcrop, just to make sure my understanding of the samples I post about isn’t contradicted by other, possibly more knowledgeable folks who have published on the web before me. Normally that means checking the Wikipedia article (if one exists) for the rocks I’m posting about. It’s not uncommon that there are other reputable scientific accounts of the rocks I post about, as well. You can then, perhaps, imagine my disappointment when I got a look at the results of the search term “blue calcite“. I wanted to find a concise scientific explanation for its color causing mechanism, not how it affects my chakra when placed between purple candles. (Though I’m tempted to test whether it can actually “help keep teenagers away from bad influences or company”.)
[Update: A reader e-mailed me to point out this Caltech website that says, "Radiation is associated with blue and amber colors of calcite. Natural radiation interacts with sheared calcite to produce blue colors. An interesting experiment is to break a colorless calcite crystal into chips upto 3 mm in size. When some of the are chips are exposed to ionizing radiation (such as gamma-rays) they turn amber colored. If some more of the same chips are put into hydrolyic press and squeezed (One can use a KBr pellet press such as are used in chemisty laboratories and pressurize the die to the same pressure used to prepare KBr pellets), they will remain colorless. If they are subsequently exposed to ionizing radiation, they will turn blue." Unfortunately, no reference is cited on the website.]
Blue Calcite from the Valentine Mine, Adirondack Mts., New York
Today’s deskcrop is a bit of a puzzle. I collected this cobble on a beach on Great Cranberry Island, Maine. What caught my eye were the igneous crosscutting relationships. Take a look and see how many episodes of magmatism are indicated by all of the crosscutting relations. Beyond the crosscutting relations, see if you can also figure out all of the other geologic events that this rock has experienced. If you’re up to the challenge, write the geologic history of this clast in the comments.
Because today is a holiday and I’m not teaching, I’m going to select an outcrop rather than a deskcrop. Although, to be honest, today’s outcrop might very well have become a deskcrop if it hadn’t been located in a very public area of a National Park.
Vesicular Basalt Ventifact
What you see above is a beautiful example of what happens when rocks are exposed to sustained winds over a long period of time. It is a small boulder of vesicular basalt that has been sculpted by sandblasting into a ventifact. And fittingly enough it’s locality is Ventifact Ridge, opposite the southern junction of Artists Drive and Badwater Road in Death Valley National Park, California.
Fortunately I had good weather the day I photographed this ventifact, but the day before when I arrived at Death Valley the wind was howling and the sand was blasting. Too bad I didn’t have a video camera at the time…
Last year about the last geological thing that I was blogging about on a regular basis was my trip back to Hays, KS from AGU in December 2008/January 2009. My last post on that trip was about the geology around Bisbee, Arizona. Today I’m going to select one of the GigaPans featured in that post for my outcrop of the day.
These are the supergene altered oxidized (orange) and primary altered quartz-sericite-pyrite (grey) rocks of the Sacramento Stock, which was host to the copper mineralization mined in the open pit opposite this face. You can’t quite zoom in far enough to make out the minerals here but there are a number of small scale structures visible if you look closely.
In the 2000-2001 school year I was a visiting assistant professor at Western Kentucky University, where I contributed to the first ever Geoblog run by Fred Siewers. In the spring of 2001 we made a joint structural geology/petrology field trip to the Ocoee Gorge and Ducktown, TN, on which I had my first opportunity to make the acquaintance of today’s outcrop.
Bedding-Cleavage Relationships in Slate and Limestone, Ocoee River Gorge, near Caney Creek, Tennessee
Above you can see an outcrop scale image (left) and a closer view (right) of the angular cleavage-bedding relationships at this locality. Cleavage dips to the east and bedding is nearly vertical. Note that the cleavage, which is well developed in the slate, is nearly absent in the limestone bed.
Geologists may also recall a more recent event that happened just a few miles upstream from this point last fall…
Where on (Google) Earth has been locked in a deep freeze for the last couple of weeks. But now that it’s been released from Antarctica’s icy grip, let’s see if we can’t shake things up a bit and get it rolling again.
For those of you who may be unfamiliar with WoGE I’d point out that it’s currently just shy of its third birthday – perhaps the longest continuously running social game in the geoblogosphere. The object is to search Google Earth until you find the tract of land pictured below. Once you’ve found it, identify its latitude and longitude in the comments to this post and do what you can to describe the geological significance of this area. The winner (first person to post the correct location and geology) will have the honor of hosting the next WoGE competition on their own Geoblog. If you haven’t won (recently) or have just been thinking about starting your own geology blog it’s a great chance to win a little exposure among your colleagues and the bragging rights that go with that.
In order to level the playing field a bit, we often invoke the Schott Rule, meaning previous winners (it’s fixed!) must wait one hour per previously solved WoGE before submitting an answer. I’ll invoke the Schott Rule this time with a posting time of 9:00 am CST (15:00 GMT).
We’re back to the Adirondacks to close out this week’s deskcrops. Today’s sample is one of my prized finds. The sample comes from the Tahawus mine dump on the south side of the High Peaks Region, not far from the Mount Marcy trailhead. Large labradorite crystals originating from the Marcy Massif Anorthosite here float in a matrix of magnetite-ilmenite ore. This suggests that the ilmenite-magnetite ore was a liquid phase, presumably immiscible and consanguinous – if not comagmatic – with the parent magma of the labradorite crystals. Elsewhere anorthositic and ferrodioritic dikes crosscut the ilmenite-magnetite orebody.
Plagioclase Megacrysts in Ilmenite-Magnetite Ore
Upon closer examination of this sample, one can also observe garnet coronas at the contact of the plagioclase and oxide minerals. These little garnet “necklaces” presumably formed during granulite facies regional metamorphism.
Garnet Coronas Surround the Plagioclase Pheno(Xeno?)crysts
Today’s small deskcrop comes from the shores of Batchewana Bay in Ontario, Canada. It’s a very well rounded pebble of epidote (probably of vein origin) with a little orange cap of granite. This rock’s beautifully contrasting palette is what makes unakite prized as a semi-pecious gemstone – I just picked it up because it was pretty. The rounding results from mechanical abrasion in the surf zone along a pebbly beach on the eastern shore of Lake Superior.
The polished slab you see below is a piece of Montello Granite. The economy of Montello, Wisconsin for many years was anchored by the granite quarry in the center of town. Renowned as one of the strongest and most beautiful granites from the region, it’s most prominent claim to fame is that it is the stone from which Ulysses S. Grant’s sarcophagus is carved in Grant’s Tomb. Today the quarry is flooded and serves as a municipal park.