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2 November 2017
This morning on Twitter I was reminded of komatiites, those “extinct” ultramafic lavas that were relatively common eruptions during the Archean. I’ve actually got a good number of komatiite photos to share from my time last year in South Africa’s Barberton Greenstone Belt. But since time is short this morning, I’ll start with a short photo set of three images, all showing different aspects of how these ultramafic rocks weather …
22 September 2017
It’s the First Friday of Fall! Here’s a sort of fold to help you celebrate: a section through a ∧ shaped bend in a vesicular basalt flow from the eastern flanks of Mount Etna in Sicily. It’s due to volcanic lava flowing rather than ductile deformation of a pre-existing solid rock (our usual habit with this feature), but I think we can appreciate it regardless:
15 July 2017
I collected only a single rock on my summer travels in France and Italy. (Those of you who know me will realize how extraordinary this low number is!) It’s a flow-banded rhyolite from Vulcano, in the Aeolian Islands north of Sicily a few weeks ago. It contains porphyritic vesicular basalt xenoliths. I featured a similar sample on Twitter yesterday on the occasion (supposedly) of “International Rock Day”: For #InternationalRockDay , …
15 April 2017
My son and I hiked Compton Peak in Shenandoah National Park this morning, and saw these two lovely examples of xenoliths. The example above is small, but it shows clearly the difference between the coarse, felsic basement rock (Mesoproterozoic granitoid, comprising the xenolith) and the surrounding fine-grained dark green metabasalt of the Catoctin Formation (Neoproterozoic). Here’s another, bigger example: These two Blue Ridge examples both illustrate the principle of relative …
25 January 2017
A basalt flow in Iceland shows both enticing pahoehoe and fractures with a Y-shaped intersection pattern. Comparisons to bread loaves and east Africa suggest a reason why.
13 January 2017
On the western coast of Islay, Saligo Bay showcases turbidites of the Neoproterozoic Colonsay Group. The Smaull Graywacke shows Caledonian (late Ordovician) folding and cleavage superimposed on world-class graded bedding. There’s also a nice dolerite dike to examine.
11 January 2017
Since I showed off the 3D kimberlite intrusion breccias yesterday, I feel as if I owe you some other photos from that lovely exhibit at the IGC. I apologize for the poor quality of these photos – the gorgeous samples were behind glass and brightly lit, which made photography difficult. But the rocks are sooooooooo pretty, I think you’ll enjoy viewing them just the same. Let’s start with a gargantuan …
10 January 2017
While in Cape Town for the 35th meeting of the International Geological Congress in August/September, I was delighted at the “Messengers from the Mantle: Craton Roots and Diamonds” exhibit on kimberlites. It was a world-class collection of excellent specimens that traveled to the Congress from across the city at the University of Cape Town. I took some photos of two specimens to make 3D models from, and my student Marissa …
22 December 2016
Yesterday I blogged the stromatolites to be seen in northeastern Islay, south along the shore from the distillery at Bunnahabhain. The sharp-eyed among you may have noticed that in this GigaPan, there’s more going on than merely Neoproterozoic carbonates: [gigapan src=”http://gigapan.org/gigapans/189616/options/nosnapshots,hidetitle/iframe/flash.html” height=”250″ scrolling=”no” width=”100%”] Link 1.46 Gpx GigaPan by Callan Bentley There’s also a prominent dolerite dike, weathering out recessively. A photo, centered on the GigaPanned dike: This is but …
29 October 2016
In Shenandoah National Park, astride Virginia’s Blue Ridge, feeder dikes of Catoctin Formation (meta-)basalt cut across the Grenvillian-aged granitoid basement. Due to their mafic composition and columnar jointing, these feeder dikes generally weather more rapidly than their host rocks. I led a field trip in the park on Thursday for my son’s school, and my student Marissa was there the weekend prior, checking out the autumn leaves and geology with …
24 October 2016
Erik Klemetti posted today at Eruptions about komatiite, which is apropos, considering I just finished imaging some samples of that ultramafic volcanic rock. Have a look at three samples from Barberton Greenstone Belt here, each from the 3.27 Ga Weltevreden Formation: [gigapan src=”http://gigapan.org/gigapans/192621/options/nosnapshots,hidetitle/iframe/flash.html” height=”250″ scrolling=”no” width=”100%”] Link GigaPan by Callan Bentley [gigapan src=”http://gigapan.org/gigapans/192599/options/nosnapshots,hidetitle/iframe/flash.html” height=”250″ scrolling=”no” width=”100%”] Link GigaPan by Callan Bentley [gigapan src=”http://gigapan.org/gigapans/192337/options/nosnapshots,hidetitle/iframe/flash.html” height=”250″ scrolling=”no” width=”100%”] Link GigaPan by Callan …
11 October 2016
One of my favorite places in Northern Ireland is the east side of the peninsula that hosts the tourist town of Portrush. There, two early schools of geological thought engaged in a battle. The opposing sides were: the Neptunists, who thought all stratified rocks, and in particular basalt, must form from precipitation from the sea, and the Plutonists, who thought some rocks, including basalt, formed through intrusion of molten rock …
10 October 2016
Silly Iceland! Don’t you know you’re not a continent?
6 October 2016
Iceland does basalt really, really well. But there are a few non-basaltic igneous rocks to be found there, too. One of them is a green ignimbrite (pyroclastic conglomerate) that crops out in coastal Berufjörður, eastern Iceland. Check it out!
3 September 2016
It was five years ago when I first visited Sea Point, the outcrop on the coast of the Cape Peninsula where the Cape Granite (~540 Ma) intrudes the (meta-)sedimentary rocks of the Malmesbury Group. The outcrop is (a) beautiful and evocative, and (b) of historical importance, as Charles Darwin visited it while on the voyage of the Beagle, contemplating and confirming Lyell’s assertions of the validity of plutonism as he …
21 June 2016
At the opposite end of the beach at Cushendun, Northern Ireland, we found some outcrops of schist – I’ll be featuring some of them as Friday folds later this week. But cutting across the schist was a pink porphyry, with big well-formed potassium feldspars. I splashed some water from the Irish Sea onto it to increase the contrast: Here’s a handheld GigaPan image, so you can explore it for yourself. …
18 June 2016
Two 3D models for you today, both produced by my student Marissa Dudek, using photo sets I gathered in Iceland: [sketchfab id=”4f2b60fc7045452799ed226b7e0d04a9″ start=”0″ spin=”” controls=”0″] Photoscan model by Marissa Dudek (That one has paleomag holes drilled into it!) [sketchfab id=”294c341a817241ecbbdecbf8107cd257″ start=”0″ spin=”” controls=”0″] Photoscan model by Marissa Dudek (That one I’m particularly pleased with. Given the circumstances of image acquisition, this is a very good result!)
18 May 2016
Pillow basalts form when mafic lava erupts underwater. Here are several examples from the Miocene Columbia River flood basalts, a large igneous province in eastern Washington state.
15 April 2016
James Farrell is our newest Friday fold source. Today he shares a primary (not tectonic) fold – the fold is in the ropy texture of a pahoehoe flow: Those colors! What a gorgeous rock. Thanks for sharing, James! You, too, can share your folds here. Send me your images. I look forward to featuring them on the Friday fold!
2 April 2016
Here are a few new images I’ve been working on with my home-based Magnify2 imaging system from GIGAmacro. Strophomenid brachiopods from Mississippian Mauch Chunk Formation, West Virginia: [gigapan src=”http://gigapan.org/gigapans/185738/options/nosnapshots,hidetitle/iframe/flash.html” height=”250″ scrolling=”no” width=”100%”] Link Boninite from New Caledonia: [gigapan src=”http://gigapan.org/gigapans/185707/options/nosnapshots,hidetitle/iframe/flash.html” height=”250″ scrolling=”no” width=”100%”] Link Lepidodendron scale-tree bark from Poland: [gigapan src=”http://gigapan.org/gigapans/185689/options/nosnapshots,hidetitle/iframe/flash.html” height=”250″ scrolling=”no” width=”100%”] Link Potassium feldspar crystal, from a pegmatite: [gigapan src=”http://gigapan.org/gigapans/185688/options/nosnapshots,hidetitle/iframe/flash.html” height=”250″ scrolling=”no” width=”100%”] Link Catoctin Formation greenstone from …