Day #73 Outcrop: Glacial Striations on Slate

Today’s outcrop takes us to Vermont for a look at some glacial striations on slate. This exposure is indeed a natural outcrop, though it sits at the rim of a slate quarry. I feel confident in identifying these as striations rather than tool marks of quarry equipment because they have an appropriate north-south alignment and the entire surface of the outcrop has a glacial polish. In the foreground of the first image below, you can see a bit of the slate’s cleavage dipping steeply to the right.

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Glacial Striations on Slate

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Follow the Striation

The area around Castleton and Poultney, Vermont was the final field mapping project area for Colgate’s Geology Summer Field Camp (The “OC”) when I was an undergrad there. As you can see from the quarry “tailings” pile below, there are no lack of samples to collect in this region. I’ve assembled a group of them for this week’s deskcrops, so if you enjoy slate stay tuned…

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Slate “Tailings”

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Day #72 Outcrop: Lamprophyre Dike

Kimberlites are not the only rocks that are diamond-bearing. Lamprophyres are another mantle-derived mafic igneous rock that occasionally hosts diamonds. Today’s outcrop is located beside the Trans-Canada Highway north of Wawa, Ontario, Canada. There was a small prospect not far from this locality that was being evaluated for possible diamond production.

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Lamprophyre Dike

Taking a closer look, you can see xenoliths of mantle material entrained in the lamprophyre.

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Xenolith in Lamprophyre

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Where on (Google) Earth #193?

As much as I had folks roaming the globe looking for greenery in the far corners of the earth in WoGE #191, P├ęter seems to have perplexed searchers with his colorful cenotes in WoGE #192. And although I beat everyone else to it, I’ve got to admit that even though I spent a lot of time searching during my Schott Rule waiting period, I didn’t ultimately find the spot tucked away in a corner of the Yucatan Peninsula until many hours after my wait was up. Although I recognized the flat topography and likely sinkhole origin of the feature pretty quickly, I scanned the Yucatan at least twice before finally getting down close enough to find the spot in one of the few cloudless gaps that I had previously overlooked.

Needles in the haystack may be where the WoGE challenges need to go in order to avoid repetition or obviousness, but it can be a fine line between choosing a locality that’s challenging, but findable in light of the geologic evidence, and one that is so obscure that everyone searching throws their hands up in frustration. So far, we’ve generally been able to steer clear of the latter situation, though I know that from time to time I’ve been sorely tempted to highlight a postage stamp-area that has geologic significance that may only be obvious to me. I, for one, thrive on challenges like that, so I’m in no way complaining.

For WoGE #193, I’m choosing a small area, but one that has a fairly distinctive geologic feature (I think), so I’m going to invoke the Schott Rule – wait an hour for each WoGE win before answering, please. Post time: 3/13/2010, 18:50 Central Standard Time (USA).

Where on (Google) Earth #193.

Have fun!

Day #71 Deskcrop: Kimberlite Breccia

We’ll end the work week with a deskcrop of kimberlite breccia. Kimberlites, of course, are the type of rock that most commonly hosts diamonds. Unfortunately, I don’t think I’ve got any in this sample. This deskcrop comes from the Lake Ellen Kimberlite in Michigan’s Upper Peninsula. To the best of my knowledge no macroscopic diamonds have been recovered from this locality, however diamonds are known from the glacial drift south of here in Wisconsin. As I recall, a few xenoliths of garnet-bearing peridotite were found by our group at this locality, suggesting a depth of origin suitable for diamonds, but they were relatively scarce, and I’m pretty sure most of the breccia fragments in this sample are of a crustal origin.

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Kimberlite Breccia

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Day #70 Deskcrop: Snowflake Obsidian

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.

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Snowflake Obsidian

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Day #69 Deskcrop: Vitrophyre with Pumice Inclusion

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.

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Vitrophyre with Pumice Inclusion

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Day #68 Deskcrop: Porphyritic Dacite(?)

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.

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Porphyritic Dacite(?)

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Day #67 Deskcrop: Porphyritic Hornblende Dacite

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.

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Porphyritic Hornblende Dacite

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Day #66 Outcrop: Devils Tower

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…

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Devils Tower, Morning Light

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Fallen Column (foreground) and Intact Columns (above)

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Porphyritic Phonolite (closeup)

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Day #65 Outcrop: Rhyolite Dikes, Homestake Open Pit

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!

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Rhyolite Dikes, Homestake Open Pit (standard photo)



Rhyolite Dikes, Homestake Open Pit (GigaPan)
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