Day #30 Outcrop: Hyaloclastite Boulder

Close by the base of a series of Recent rhyolitic lava domes near South Sister Volcano in the central Oregon Cascades is an outcrop of hyaloclastite. You can make out a number of large glassy clasts in an altered palagonitic matrix, presumably the result of either a sub-lacustrine or subglacial eruption in the area.

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Hyaloclastite Boulder, central Oregon Cascades

Day #29 Deskcrop: “Salt & Pepper”* Sand

Concluding the geomeme of sand for this week… if you’re up to the challenge, comment on what you think the composition of the sand is and where it’s from. We’ll find out who knows their sand… ;-)

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“Salt & Pepper” Sand (Macro)

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“Salt & Pepper” Sand (15mm microscopic)

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“Salt & Pepper” Sand (microscopic closeup)

*Not actual salt and pepper.

[Update: Okay, I cheated a bit here. This is not in fact a natural sediment - rather it is the sand size separate from a mechanical crush of fresh Half Dome Granodiorite, originating from near Olmstead Point in Yosemite National Park. If I had collected actual sand from this area I doubt the biotite grains would be quite so fresh and unweathered. By the way, did you spot that little wedge of sphene (titanite) in the most magnified view?]

Day #28 Deskcrop: White Sand

Continuing the geomeme of sand for this week… if you’re up to the challenge, comment on what you think the composition of the sand is and where it’s from. We’ll find out who knows their sand… ;-)

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White Sand (Macro)

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White Sand (15mm microscopic)

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White Sand (microscopic closeup)

[Update: Another easy one. This is, of course, gypsum sand from White Sands National Monument, near Alamagordo, New Mexico. Not an easy one to photograph well, as it turns out.]

Day #27 Deskcrop: Multicolored Sand

Continuing the geomeme of sand for this week*… if you’re up to the challenge, comment on what you think the composition of the sand is and where it’s from. We’ll find out who knows their sand… ;-)

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Multicolored Sand (Macro)

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Multicolored Sand (15mm microscopic)

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Multicolored Sand (microscopic closeup)

*Although this post has Wednesday’s date on it, I’m actually posting it and the other “sand series” posts late Thursday evening.

[Update: No doubt this one seems to have turned out to be the hardest nut to crack in the bunch. As it turns out, this is dune sand from Great Sand Dunes National Park in Colorado, collected just east of Medano Creek - the famous pulsating river. Despite the compositional immaturity of the grains, they are dominantly well rounded, and texturally well sorted.]

Day #26 Deskcrop: Green Sand

Continuing the geomeme of sand for this week*… if you’re up to the challenge, comment on what you think the composition of the sand is and where it’s from. We’ll find out who knows their sand… ;-)

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Green Sand (Macro)

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Green Sand (15mm microscopic)

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Green Sand (microscopic closeup)

*Although this post has Tuesday’s date on it, I’m actually posting it and the other “sand series” posts late Thursday evening.

[Update: There really wasn't much suspense about this one at all. Any geologist who's visited the Big Island of Hawaii or knows another who did has probably heard of the Green Sand Beach at Pu'u Mahana, Hawaii. The green color is, of course, imparted by the high abundance of olivine grains in the pocket beach below this eroding littoral cone near the southern tip of the Big Island.]

Day #25 Deskcrop: Black Sand

It seems to be “Sand Week” in the geoblogosphere*. Ian Stimpson has been featuring deskcrops of local sandstones, and Brian Romans has reviewed Michael Welland‘s new book Sand: The Neverending Story (which just went into my Amazon shopping cart) and conducted a Q&A with Michael over at Clastic Detritus (how apropos). Callan Bentley gets to host the roving virtual book tour next week, followed by David Williams and Andrew Alden. (Kansans don’t read books, I guess, or maybe it’s just that sand is still a bit of a sore subject here. Anyhow, I know when I’ve been told to pound sand.)

So for this week’s deskcrop series, let’s see how much the experts know about their sand. Four distinctive sands on Monday thru Thursday, and then an evil stumper on Friday. If you’re up to the challenge, comment on what you think the composition of the sand is and where it’s from. We’ll find out who knows their sand… ;-)

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Black Sand (Macro)

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Black Sand (15mm microscopic)

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Black Sand (microscopic closeup)

*Although this post has Monday’s date on it, I’m actually posting it and the following four “sand series” posts late Thursday evening.

[Update: As many of you were able to surmise this is indeed black sand from a Hawaiian beach. The sand is dominated by coarse, angular, well-sorted sand composed primarily of basaltic glass, with minor olivine. The locality of this sample is the Punalu'u Black Sand Beach, on the south shore of the Big Island of Hawaii.]

Day #24 Outcrop: Wind River Meander Bends

I’m loosening up the definition of “outcrop” today to feature some field shots of Quaternary geologic landforms. It’s not that I lack bedrock photos, but I want to include a little geomorphology while I’m taking you through my field expeditions.

Today we’ll gaze upon two lovely meander bends on the upper reaches of the Wind River, upstream of Dubois, Wyoming.

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Meander Bends, Wind River, Wyoming

As pretty and educational as they are, there are even more river features to be seen in a wider GigaPan view from this vantage point…

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Day #23 Outcrop: Ogallala Caprock, Chalk, Mudcracks & Raindrop Impressions

It’s interesting that my most recent posts in the Deskcrop/Outcrop series have been getting more commentary on Facebook than here on my blog. I’d like to find a way to import those comments back here to the WordPress blog, but I can’t seem to properly configure a Plugin to do the job. Harrumph!

I’m still playing catchup, so short writeups for the next few…

Today’s outcrop is a relatively local one. This escarpment in southwest Rooks County, Kansas is formed where a caprock of Tertiary Ogllala Formation conglomerate (of aquifer fame) unconformably overlies chalk beds of the Cretaceous Niobrara Formation (Smoky Hill Member). Elsewhere in western Kansas this same stratigraphic exposure is exposed with badlands topography developed in the chalk. But not here.

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Ogallala Conglomerate Capping Niobrara Fm (Smoky Hill Member) Chalk, SW Rooks County, Kansas

The other interesting geologic feature at this locality was a beautiful set of raindrop impressions in the mudcracked Quaternary layers on the quarry floor.

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Raindrops & Mudcracks

I believe I’m going to begin to use more Quaternary landforms for some of my upcoming outcrops…

Day #22 Deskcrop: Mt. Mazama Pumice

I’ve fallen a bit behind in posting, but I’ll get caught up.

Today’s deskcrop was formed in the violent climactic (ultra?)plinian caldera-forming eruption of Mt. Mazama some 7700 years ago. It is a large pumice ball, with a diameter of almost a foot. I collected it from the forest floor just east of the boundary of Crater Lake National Park. As lightweight as it is, it’s still amazing to me to imagine the power of an eruption that could toss a fragment of rock this size such a distance from the vent. The entire region is covered by a blanket of ash and pumice fragments from this catastrophic eruption – it’s easy to recognize its buff color in roadcuts and borrow pits.

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Mt. Mazama Pumice