Today’s imagery put me in mind of two hands reaching for one another:
What do you think? Shocking coincidence? Or a bit of a stretch?
Ice… serpentine… halite… What do they all have in common?
I’ve discussed mineral “ghosts” here before — really, those are only pseudomorphs, where one mineral’s chemistry becomes unstable due to a change in conditions, and then a new mineral forms in the same space. I’ve also brought up the issue of clasts of minerals which are unstable over the long term (ice).
Last night, at the final meeting of the Geological Society of Washington for the spring season, Bob Hazen of the Carnegie Institution of Washington gave the Bradley Lecture. Bob discussed his ideas about mineral evolution, and gave a compelling talk.
One of the key implications about thinking about minerals evolving over time is that new mineral species can evolve when conditions change and permit their growth, but so too can old mineral species go ‘extinct’ when conditions change and no longer promote their growth.
This got me thinking about that ice-clast breccia again (link above), and how that would be interpreted by future geologists, assuming the ice itself has melted away. Consider the geologic record of a superwarm planet, where temperatures never dip low enough to form ice. Would we be imaginative enough to invoke ice as the cause of glacial landforms, of striations and deposits of till? How would we explain dropstones and ice wedges if ice were an “extinct” mineral on Earth?
And so after the talk was over, I went up to Bob and introduced myself and asked him if he could think of (or imagine) other minerals which could profoundly affect the geologic record, yet disappear after they have done their work. As we were talking, it occurred to me that halite in the form of salt domes could perturb the local stratigraphy, then the salt diapirs could rise up to the surface and be eroded (or re-dissolve into the ocean), leaving a piercing trail of destruction in their wake.
Bob came up with another one: serpentine at a subduction zone: hydrothermal alteration of oceanic crust produces serpentine, but then the serpentine is unstable when it gets subducted. It dehydrates (gives off water), and (poof!) there’s no more serpentine minerals. However, this dehydration is super duper important geologically: the addition of that water to the hot rocks of the subduction zone lowers the melting temperature of the rocks, and helps generate magma: the magma that rises to feed volcanic arcs. If we didn’t have oceanic crust to look at, would we have imagined serpentine beneath our convergent boundaries, a humble transformer of the world above?
Readers, I put the same question to you: Which minerals cause big effects, but then disappear? Who are the prime movers who flee the scene of the crime? These are minerals that aren’t just ghostly; they’re downright phantasmic! I’ll be eager to read your suggestions, or hear your thoughts on the three I’ve noted here.
I took a look at some interesting blobby structures in the Swift Run Formation last week, and walked readers through my logic in tentatively concluding that they were ball & pillow structures (soft sediment deformation), though overprinted by a pervasive (Alleghanian) cleavage. As we move west in the Appalachian mountain belt, the rocks are less cleaved: the strain is instead taken up in large anticlines and synclines with a few thrust faults thrown in. Though arched and fractured, the rocks’ fabrics remain pretty close to what they were at the time of deposition.
Fortunately, in the Valley & Ridge province along New Route 55 in West Virginia, you can see some sweet examples of (undeformed) ball & pillow in the Hampshire Formation (Devonian; part of the Acadian clastic wedge). Here’s a view looking up at the bottoms of some of these sandy sags (quarter for scale):
I like that partial weathering-out of the features into the third dimension…
To refresh your memory, ball & pillow forms when a heavy load of sand gets dumped (underwater) on top of a soft, squishy deposit of mud. The sand sags downward in broad “balls” (if you have a point locus of sinking) or “pillows” (if the sags have a linear axis to them). In between, the low-viscosity mud squirts up in cuspate “flame” structures. Check out this fine example (quarter for scale), found by GMU structure students Joe M. and Justin O. on the northern side of the road:
…And here we have the same photo, with the sand, mud, flames, and ball & pillow all labeled for you. The white arrows represent the downward sagging of the sand; the red arrows represent the upward squirting of the mud.
In some places, the pillows have weathered out. Here’s one (quarter for scale) that is now upside down on the grassy knoll beneath its source outcrop:
Note the thin laminae of mud clinging to its exterior, like a coat of paint!
Here’s a second sandstone pillow that has weathered out of the cliff and popped down onto the grassy slopes below. Obviously, the surface being photographed is a cross-section through the saggy pillow:
The “upper left” of this sample would have been the lowpoint of the sag if it were in situ on the outcrop. You can see the smooth margin on the left side, and the rougher zone on the right where it detached from the overlying part of the sandy layer. Let’s now zoom in on this box to see something really cool:
If we go deep here, we can see that the laminations of sand within the pillow show small Z-folds (and S-folds on the other side, though they’re not as obvious in the photograph) that are “parasitic” on the main fold. Here they are, highlighted (white arrows) and traced out (black lines):
The axes of these small-scale structures verge on the axis of the main fold, meaning that their axial planes (blue traces) “tip over” with increasing deformation towards parallelism with the main fold’s axial plane. We’ve seen similar things before. The axial planes play the same game as the cleavage plane. This is the first time I have ever observed such a structure associated with soft-sediment deformation, however.
Has anyone else ever seen S- or Z-folds associated with soft sediment deformation? It makes total sense to me that they would be there based on simple physics, but this was the first time I had ever seen it myself. I’d be curious to get a sense of how common or rare this might be.
I have a student this semester, Diane, who is a flight attendant. Every weekend, she’s off to some cool European city with her husband, a pilot. Up until now, she’s been bringing me little gifts of beer from each trip. She drops 60 cents in Germany, and then that week I get to taste a new variety of ale or lager. Very, very cool!
This weekend, it was off to Rome, and she brought me back a piece of paper. This piece of paper had something written on it, though, something she thought I would appreciate because of my environmental concerns:
Pretty cool, eh? Despite his being a politician, I have a lot of respect for Gore for bringing the issue of climate change into the national consciousness. Unfortunately, because he’s a Democrat and a climate activist, the Republican response has been anti-Gore, and thus anti-climate science and anti-climate action. Hopefully Gore’s consciousness-raising outweighs the politicization of the issue he has wrought by advocating on its behalf.
Either way, the autograph is a great gift. Thanks, Diane!
(Did I mention she brought me a bottle of beer, too?)
On my structure field trip just over a week ago, we found the contact between the Mesoproterozoic-aged Blue Ridge basement complex and the overlying Neoproterozoic Catoctin flood basalts (now metamorphosed to greenstone). This nonconformity can be found just west of the Appalachian Trail at the Little Stony Man parking area in Shenandoah National Park. Here’s four photos, with my left index finger for scale, in raw and annotated versions:
It’s not as glaringly obvious as some other unconformities profiled here, but it’s an important horizon in understanding the geologic history of the mid-Atlantic region.
In places, small inclusions of the basement complex may be found inside the base of the Catoctin Formation, a nice example of the principle of relative dating by inclusions. The basement rock must be older than the Catoctin if pieces of the basement have been broken off and enveloped in the Catoctin:
You’ll notice that the Swift Run Formation isn’t present at this location, though stratigraphically, it belongs between the basement and the Catoctin. The Swift Run is patchy and discontinuous, probably reflecting low-lying areas on the paleo-landscape, which paleo-hills poked up above the sediment-laden paleo-valleys, and were last to be smothered beneath the advancing flood basalts.
It’s a great pleasure to be able to find and “put your finger on” such a significant surface, such a gap in the geologic record. Given that the basement complex formed during the Grenvillian Orogeny (1.1-1.0 Ga), and the Catoctin erupted sometime before 565 Ma, there’s probably more than 400 million years of time that passed between the formation of the rock below my finger and the rock above it. Unconformity surfaces like this are geologic contacts which are emblematic of time passing, but going unrecorded in the geologic record. They are high-contrast reminders of how incomplete the geologic record is at any single location on the planet. They remind us to be humble in our interpretations. They remind us to strive for a multi-referenced correlation between different locations’ outcrops in order to get closer to the full story of our planet’s checkered past.
For the twenty-fourth edition of the Accretionary Wedge, I selected “heroes” as the theme. For those of you new to the geoblogosphere, the Accretionary Wedge is a ~monthly geoblog “carnival,” wherein various and sundry geobloggers write posts on a common theme.
Broadly speaking, submissions to this edition fell into three categories: (1) professional heroes, (2) personal heroes, and (3) individuals who were both professional and personal heroes.
Neil of Microecos was the first one to submit a tribute to three women paleontologists: Mary Anning, Annie Alexander, and Tilly Edinger. The three profiles he offers show conclusively that not all fossilists are old white men.
Ian Stimson of Hypo-theses also chose a woman as his hero: the woman who discovered in the inner core, Inge Lehmann. She made on of the most fundamental interpretations of the structure of our planet, using Ian’s favorite medium, seismic waveforms.
Above the core is the mantle, and plate tectonic theory suggested in the 1960s that the mantle might move around, dragging around lithospheric plates above. Chris Rowan of Highly Allochthonous reminds us that Arthur Holmes suggested something very similar in 1928. Chris explores Holmes’ ideas, style, and influence on geologic thinking at a critical time.
When the mantle partially melts, it makes basaltic magma. One spot where that liquid rock makes it to the surface is Hawaii, where Thomas Jaggar worked. Jess (a.k.a. ‘Tuff Cookie’) of the blog Magma Cum Laude says that the reason she finds Jaggar heroic is that he studied volcanoes in service to mankind. Jaggar valued the practical application of his ideas towards protecting people’s lives.
Chris M of Pools and Riffles wrote a post about one of his professional heroes, the pioneering fluvial geomorphologist Luna Leopold. Chris’s post dovetails nicely with a piece by Anne of Highly Allochthonous about Leopold’s collaborator Reds Wolman (see the “both” category below).
John Van Hoesen came out of the blogging deep-freeze with his thoughtful post about Louis Agassiz over at his rejuvenated blog Geological Musings in the Taconic Mountains. Agassiz, of course, is the reason that today we all think that the Pleistocene was a time of massive continental glaciation in the northern hemisphere. John’s understanding of his hometown is an Agassiz legacy.
Another John, this one the author of Karmasotra, opted for yet another John, John Wesley Powell, as his hero. Blogger John profiles Explorer John with a short biography and a sense of profound respect at how willingly Powell immersed himself in the unknown.
Finally, David Bressan of Cryology and Co. wrote an essay that discusses an important group of professionals: those who make us laugh. He discusses caricaturists and cartoonists, especially those who critiqued Victorian geologic thought.
The personal heroes:
Mel of Ripples In Sand came out of blogging semi-retirement to heap accolades on the geologically-aware individuals who forecast avalanche risk for skiers in the Rocky Mountain West.
I was the last person to submit an entry to this Wedge, and that was my tribute earlier today to Larry Wiseman, developmental biologist, artist, scholar, mentor, and connoisseur of life. His influence was instrumental in giving me my interests, enthusiasms, and priorities.
Larry was the one to put me onto the works of the author and philosopher (he hated being called a “nature writer”) Edward Abbey, and Abbey also happens to be the hero of Garry Hayes, a.k.a. Geotripper. Garry explores Abbey’s influential thinking and gorgeous prose, and relates it to issues like overpopulation and the climate crisis.
The “both” heroes:
Thinking along the same lines of Chris M (above), Anne Jefferson of Highly Allochthonous led the pack with a tribute to the fluvial geomorphologist Reds Wolman, who recently passed away. Reds taught Anne at Johns Hopkins when she was an undergraduate, and then she took up her doctoral study under an earlier Wolman PhD student, Gordon Grant, which meant that Reds became her “academic grandfather.” Anne’s tribute was penned before the Wedge was announced, but she followed up later with a supplementary tribute, including video of her hero.
‘Pascal,’ the author of Research at a Snail’s Pace, couldn’t choose a single hero, but some were clearly professional (Robert Bakker and Stephen Jay Gould) and others were clearly personal (parents and wife). See his many positive influences here.
Likewise, ‘Silver Fox’ of Looking for Detachment couldn’t pick just one, so she shared a collection of vignettes about why she finds Tanya Atwater and Tom Dibblee heroic, as well as her many advisors and mentors through life and geologic training.
Though the Sandglass‘s Michael Welland has both a professional and personal connection to his hero, thesis advisor Alan Smith. Michael relates the story of a day in the field, and how a thoughtful question from Alan revolutionized his thinking about field work.
Bill Normark was a marine geologist who produced work that was influential on the thinking of Brian Romans, author of Clastic Detritus. Brian describes how Bill’s work came off the printed page and into his life when Bill served on Brian’s PhD committee, and stimulated his interest in integrating modern sedimentation with the ancient sedimentological record. Apparently, he also made some tasty wine!
Some wine seems to be called for here: reviewing this stellar list of extraordinary individuals, I feel compelled to (virtually) pour each of you glass. Let’s raise this virtual wine aloft with a toast to these men and women who influenced our lives, our thinking, our science, and our goals.
May our heroes’ memories inspire us; may their legacies inspire others!
Long live the heroes; Cheers, everyone!
* < much clinking of glasses, shouts and huzzahs > *