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31 January 2020
Friday fold: tension gashes near Sunflower, Arizona
Reader John Christian shared these folds with me via email last week. They are quartz veins in slightly metamorphosed Precambrian igneous rocks found near Sunflower, AZ in the Mazatzal Mountains. The second photo is a close-up shot of the curviest, cleanest batch of folds from the first shot. These are beautiful examples of folds in similarly oriented quartz veins; we call them “en echelon” for the way you have a …
21 November 2018
Brittle overprinting ductile in the South Mountains metamorphic core complex, Phoenix, Arizona
Today, we take a look at the structural geology that reveals the deformation evolution (first ductile, then brittle) of the South Mountains metamorphic core complex, south of Phoenix, Arizona. Expect lots of photos of smeared-out rocks, broken by faults.
17 November 2018
Flow banding in pseudotachylyte, South Mountains, Arizona
Watch the flow of frictional melt in a “fossil earthquake,” frozen in time atop the South Mountains metamorphic core complex in Phoenix, Arizona.
22 August 2018
Q&A, episode 5
A reader asks: “What is foliation and what makes it so important to the structure of rock?”
Callan answers with a lot of images of beautifully foliated rocks.
25 May 2018
Friday fold: 3D model of intense folding
This is a block of rock I found in the rock garden at Northern Arizona University a few weeks ago: It’s intensely folded. Not sure what kind of rock it is, but it was quite dense.
12 January 2018
Friday folds: Hayden Butte (“A Mountain”), Tempe
In keeping with the Arizonarific theme of this week’s posts (thanks to my participation in the 2018 Structural Geology and Tectonics Forum), I thought I would wrap up my ‘geology of the Phoenix area‘ posts with a walk I took on my last day there. This was to what Google Maps calls “Hayden Butte,” but the locals call “A Mountain.” Not “a mountain,” but “the mountain called ‘A‘.” It has …
11 January 2018
Landslide breccias in Papago Park
A visit to Papago Park, north of Tempe, Arizona, reveals hanging wall rocks from the South Mountain detachment fault, a long way from South Mountain. Also, feast your eyes on these gorgeous red sedimentary breccias, interpreted to be landslide deposits from the Neogene.
10 January 2018
SGTF field trip
A virtual field trip to the Phoenix Mountains and South Mountains of Arizona, along with participants in the 2018 Structural Geology and Tectonics Forum.
10 February 2017
Friday fold: the Grand Canyon
Deep in the Grand Canyon of Arizona, Steve Mirsky found a Friday fold to share with you.
28 April 2015
Epidotized tuff, Tucson Mountains, Arizona
I was in Tucson this past weekend for a book project meeting, and my editor and coauthor and I took a hike on Sunday morning in the Tucson Mountains to Wasson Peak. Not far from the summit, we saw an epidotized tuff, where the fiamme and pumice blobs had undergone reactions to produce pods of epidote, giving the rock a look like a sick dalmatian: This is a cool rock …