11th
March
2010
Today’s deskcrop comes from the Black Rock Desert of central Utah. It is a piece of snowflake obsidian. The Utah Geological Survey has a great write up of the origin of this rock and a field guide to the locality. You’ll notice the distinctive conchoidal fracture of obsidian catching the light nicely in the upper right portion of this sample. Also note that this sample comes from very near the spot where I collected the bread-crust rhyolite sample that was featured back in January.
Snowflake ObsidianExplore this and all of my 2010 Deskcrops and Outcrops in Google Earth!
posted in Geology, deskcrop |
10th
March
2010
Today’s deskcrop shares the textural designations hypocrystalline and porphyritic with yesterday’s, but I’ve chosen to identify today’s by another name: vitrophyre. I am much more certain of the locality where I collected this sample – it was from road grading material on Wyoming 89 near Lewis Falls in Yellowstone National Park – but it was not in place, so I’m uncertain whether it originated in the pyroclastic Lava Creek Tuff (in which case the obsidian would represent remelting of rhyolitic ash at the base of the pyroclastic deposits produced by the most recent caldera forming eruption at Yellowstone), or if it comes from the post-caldera obsidian/rhyolite domes that occur just upstream from this locality. In either case, its composition is more likely rhyolite than dacite, despite its similar texture and appearance compared to yesterday’s deskcrop.
Vitrophyre with Pumice InclusionExplore this and all of my 2010 Deskcrops and Outcrops in Google Earth!
posted in Geology, deskcrop |
9th
March
2010
One of my flaws as a geologist and rock collector is failing to label and clearly identify all of my samples. Today’s deskcrop is a great example of a rock with a hypocrystalline (partly glass, partly crystalline) and porphyritic (two distinct grain sizes – plagioclase phenocrysts in a glassy matrix) texture. I’m pretty sure this sample comes from Mount Lassen, which should make it dacitic in composition (despite its very dark color) – compare it to yesterday’s porphyritic hornblende dacite to see just how different rocks named “porphyritic dacite” can look! – but I’m just not certain because I never labeled it and it’s been separated from the rocks I originally collected it with. I have had the opportunity to hike to the summit of Lassen and I recall rocks there that looked very much like this, but whether this sample originates there or somewhere else in the Cascades, I cannot say for sure. In any case, it makes a useful teaching sample.
Porphyritic Dacite(?)Explore this and all of my 2010 Deskcrops and Outcrops in Google Earth!
posted in Geology, deskcrop |
8th
March
2010
Commonly cited as an example of andesite (I’ve done it myself), today’s deskcrop is actually a porphyritic hornblende dacite. The sample is from Black Butte, which will be familiar to travelers of Interstate 5 in northern California. Black Butte is a dacitic dome immediately east of the interstate and just west of Mount Shasta. Black Butte can be seen on the left side of this GigaPan by Rich Gibson (misidentified in the title as a cinder cone) – Shasta is the snow capped peak on the right.
Porphyritic Hornblende DaciteExplore this and all of my 2010 Deskcrops and Outcrops in Google Earth!
posted in Geology, deskcrop |
7th
March
2010
Continuing our tour of igneous exposures of the Black Hills, today’s can genuinely be called an outcrop. And what an outcrop it is! Devils Tower, the hero of Steven Spielberg’s Close Encounters of the Third Kind is a commonly cited example of a volcanic neck (questionable), and a spectacular example of columnar jointing (unquestionable).
Suddenly I have a craving for mashed potatoes…
Devils Tower, Morning Light
Fallen Column (foreground) and Intact Columns (above)
Porphyritic Phonolite (closeup)Explore this and all of my 2010 Deskcrops and Outcrops in Google Earth!
posted in Geology, Outcrop |
6th
March
2010
We’ve seen a streak of sedimentary deskcrops and outcrops and I’m feeling the strong urge to bring back some igneous goodness. We’ll start with a weekend of outcrops from the Black Hills. Well, not technically an outcrop today, since the exposure is not exactly naturally occurring, but hey, how often does nature make a spectacular exposure of intrusive igneous relations on her own? (Callan found a spectacular one, but that one’s a bit farther afield than I’ve had the opportunity to venture, at least to this point in my life.)
The Homestake Open Pit Mine in Lead, South Dakota beautifully exposes a lovely set of Tertiary rhyolite dikes which intruded the Precambrian metabasic rocks of the northern Black Hills, initiating a hydrothermal system that concentrated gold in sufficient quantities for humans to blast this massive hole in the ground. I’ve got both a standard photograph and a GigaPan of the view from the observation deck of the little museum perched on the edge of the pit. Zoom in on that 2.91 gigapixel Gigapan and be amazed at the detail that you can see!
Rhyolite Dikes, Homestake Open Pit (standard photo).
Explore this and all of my 2010 Deskcrops and Outcrops in Google Earth!
posted in Geology, GigaPan, Outcrop |
6th
March
2010
It didn’t take me long to find the Goat Paddock Crater in Western Australia that Michael Cohen featured in Where on (Google) Earth #190. Impact craters are really easy to spot, particularly with the help of compilations like the Consolidated Impact Crater Database. I recognized the color of the landscape as Australia right off the bat and it didn’t take long to find this distinctive hole in the ground. That’s why it’s called the Schott Rule…
It’s also really good to see WoGE getting competitive again and bringing in new winners. In fact, Michael’s new geoblog, From Saturn, With Love, is only the most recent to be inspired by winning a WoGE contest. Here’s hoping it will grow steadily in both content an visitors!
Since I had so much fun stumping the geoblogosphere with a river last time I hosted WoGE, I figure it’s only fitting to see if you’ve gotten any more familiar with fluvial landscapes since then. And since I like the wide-open race to discover these, I’m not invoking the Schott Rule this time. Just find the image below in Google Earth, and comment below with its latitude and longitude and a little about the geological significance of the site. Simple as that…
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Happy exploring!
posted in Geology, Google Earth, Where on (Google) Earth? |
5th
March
2010
We’ll end the work week with a deskcrop from high in the Colorado Rockies: an arkosic arenite (sandstone) from Vail Pass, Colorado. This rather immature sedimentary rock is rich in both K-feldspar and micas, giving it its distinctive reddish color. The rock has a Pennsylvanian depositional age, and was formed in association with the uplift of the Ancestral Rockies.
Arkosic AreniteExplore this and all of my 2010 Deskcrops and Outcrops in Google Earth!
posted in Geology, deskcrop |
4th
March
2010
Today’s deskcrop is a small fragment of a fossil of an orthocone cephalopod, from the Ordovician limestones of St. Joseph Island, Ontario, Canada. And because fossils are decidedly not my specialty, that’s about all I can tell you about it.
Orthocone Cephalopod FragmentExplore this and all of my 2010 Deskcrops and Outcrops in Google Earth!
posted in Geology, deskcrop |
3rd
March
2010
Today’s deskcrop comes from a small knob that may no longer exist. The sample consists of drusy quartz that fills vugs in the Mississippian Leadville Limestone. The vugs were formed when the Leadville Limestone was exposed to surface weathering and developed a karst topography. The hill from which this sample was collected is thought to have been a paleokarst tower. Unfortunately, when I last visited the locality a few summers ago it was on the verge of being blasted away to make room for a campsite.
Drusy Quartz in Limestone Vugs.
Explore this and all of my 2010 Deskcrops and Outcrops in Google Earth!
posted in Geology, GigaPan, Outcrop, deskcrop |
