#Fault (geology)

[artemistorm]

Silly Game Time: What was the most recent internet search you're comfortable with sharing?

Slickensides! Slickensides are rock races that are polished smooth by rubbing against another rock at a fault line (source of earthquakes). We found one on our field trip (and I looked them up online to see what other look like).

This polished surface was so smooth and hard that it felt like porcelain. It's just crazy to think that this was caused by rocks rubbing against rocks.

[tilbageidanmark]

The Al Naslaa Rock Formation

[rabbitcruiser]

Forest (No. 87)

Earthquake Fault, Mammoth Lakes

[quasarlasar]

SAN ANDREAS WATCHES SAN ANDREAS

Well I haven't done a geology comic since *checks archives*...2018??!

Okay then. Here's a new one after 6 years.

This joke is 9 years out of date, but I had the pencil sketch scan sitting on my computer for years so after I had a dream featuring San Andreas I figured I would finally ink and color it.

In this comic the San Andreas Fault (the guy with the hat) is watching the 2015 movie San Andreas, which was a disaster movie starring Dwayne Johnson. In it, the San Andreas Fault creates a steadily escalating series of earthquakes, culminating in a magnitude 9.6 quake.

This is...impossible for the San Andreas Fault to produce. It can only get up to magnitude 7.8, maybe 8.25 if the entire fault ruptured at once? But the San Andreas is not known to do that. It has a midsection that creeps instead of causing giant earthquakes, and ruptures to the north and the south have trouble breaking through this section.

But you know what kind of fault can produce a magnitude 9+ earthquake? Subduction zones. The giant earthquake and tsunami in San Andreas is way more appropriate for the Cascadia Subduction Zone than for the San Andreas Fault. So I imagine Cascadia (the guy with the volcanoes on his back) feels insulted that the humans gave this ability to San Andreas, who is comparatively weak and scrawny.

I should note San Andreas and Cascadia are roughly the same length, but San Andreas' body is mostly hidden by his couch here. It mostly exists as a zone of fractured rock in the ground, but he's capable of rearing the front portion out of the ground to watch TV.

Cascadia is depicted of course as much more powerfully built than San Andreas since he has a greater rupture area and overall is just a more powerful force of nature than San Andreas is.

If San Andreas wasn't a continental transform fault, and was an oceanic one, Cascadia would probably eat subduct him.

[thatsbelievable]

[geologyin-blog]

Transform plate boundaries are tectonic regions where two lithospheric plates slide horizontally past each other along strike-slip faults.

Transform plate boundaries are one of the three primary types of tectonic plate interactions, alongside divergent and convergent boundaries. Transform boundaries are characterized by horizontal motion between two plates, resulting in a transform fault.

[hit-with-rock]

I will actually do all the geology write ups about my trips last summer eventually but here’s a reverse fault underground .

I’m a bad geologist for not putting a scale in but the fault zone is maybe a foot?

Probably wouldn’t be a great time if it reactivated while we were standing next to it but we’d learn something new!!

[borealopelta]

this is where i'm blogging from btw

[geolensgist]

Faults in Capitol Reef National Park, UT

The first two photos are of the same near vertically-oriented fault. This is a major fault that stretches for miles across the northern portion of Capitol Reef and outside the park, towards Torrey, UT. The third photo shows two smaller parallel (synthetic) faults in the Entrada Sandstone where the top (hanging wall) layers were dropped down, indicating extension.

June 2023

[iamthepulta]

Lessons for future writers. Do not, I repeat, do not make fictional stratigraphic columns for your fictional universe. Do not do this.

[sepdet]

Calico Fault, Mojave Desert

One of the best geological anticlines (folds up) and synclines (U or V shaped) in the world, often used as the textbook example of both.

(My photo) I started camping in the adjacent campground during 2021 because SEXAY ROCKS to self-isolate on the drive to visit my parents in Utah.

But I'm not a geologist, just a rock groupie, so I'm happy to find a great short video Explaining The Thing.

Edit: I swear that's my own photo at top; if you look closely the vegetation's different. l I guess we both headed for the same corner of the parking lot for the best view!

[drhoz]

The Great ACT-NSW-NZ Trip, 2023-2024 - St. Arnaud

After getting across Cook Strait without being shipwrecked (the weather was actually quite pleasant compared to some of the unholy gales that come through the gap, with the wind merely howling), we started our explorations of Te Waipounamu, the Island of Greenstone Waters. Pounamu is such a beautiful and useful stone that the Māori named the entire island after it.

Europeans called it South Island, or archaically New Munster. It covers 150,437 square kilometres, making it the world's 12th-largest island. We stopped at the Omaka Aviation Museum, which was worth it, but our first night was spent at St. Arnaud, formerly Rotoiti, a tiny alpine village.

It's certainly surrounded by mountains, and shows some really nice alpine geomorphology - hanging valleys left where subsiduary glaciers got cut off by the larger glaciers in the main valley, scree slopes where the greywacke of the mountains is disintigrating, and alpine lakes like Lake Rotoiti itself, formed when the glaciers retreated at the end of the last Ice Age and left behind huge piles of pebbles, gravel, and boulders to dam the meltwater.

On the other hand St. Arnaud has also been built right on top of a considerably larger geological feature - the Alpine Fault. This tectonic boundary between the Australian and Pacific Plates runs for over 600km, and is one of the fastest moving faultlines in the world, moving, on average, almost 40mm a year. Geological formations that originally straddled the fault are now 480km apart. Unfortunately most of that movement happens during huge earthquakes every few hundred years - the last big one on the Alpine Fault happens around 1717, rupturing 400km of the fault at once.

Over the last 12 million years a significant upwards element to the fault movement has been added, creating the Southern Alps. Most of what is now the South Island got pushed 20 kilometers up, whereupon New Zealand's weather promptly ground it 16 kilometers back down again. The assorted rubble forms the plains on the east and southern coast, or got swept north by prevailing currents on the west coast. Exposed basement rock on the South Island is mostly greywacke, or heavily metamorphised rocks such as schist from even deeper. That's where the greenstone originally formed.

Anyway, the next big quake will probably trash St. Arnaud completely, and cut every road across the mountains for months. Happily that didn't happen on this trip - @purrdence had enough problems with a cyclone cutting roads and trainlines last time.

The original forest around St. Arnaud is mostly Antarctic Beech (Nothofagus sp.) and forms the basis of a unique and seriously threatened ecosystem. I'll tell you all about that over the upcoming posts.

Here's some species I've covered before.

[responsivethoughts]

Roughly 10 million years ago, the formation of this stunning valley commenced as a result of tectonic forces. The separation of the Eurasian Plateau and the northern China bloc initiated the creation of the Pinglu Rift Valley along the boundary of these plates.

[quasarlasar]

IT'S MID-OCEAN RIDGE BASALTIN' TIME

After getting too many episodic-tremor-and-slip stomach rumbles, Cascadia goes absolutely feral and tries to eat/subduct the Juan de Fuca Ridge. The words in the last panel are courtesy of @iamthepulta. The line gets even sillier when you take into consideration that I imagine Cascadia to have an Austrian accent.

The Cascadia subduction zone is nowhere near doing this IRL, but it (or its precursor) did subduct part of the Farallon-Pacific Ridge in the past. I have drawn Cascadia grabbing Farallon-Pacific by the throat and dragging her into his trench, and in this comic he's doing it to the Juan de Fuca Ridge, so I guess he just uses chokeholds on ridges he wants to subduct. The Farallon-Pacific Ridge did not get completely subducted, with large portions of her ending up being strewn over the continent. And of course Cascadia’s southern portions got replaced by the San Andreas Fault in the process.

I imagine Cascadia’s true mouth is not the one he has on his face but the trench on his underside, and the big flaming maw on the right represents that in cross section. I have drawn the Juan de Fuca Plate as subducting at a much steeper angle than it actually is (it actually is almost horizontal) for dramatic effect.

Because my IRL friends and family did not get this comic at all, I’m going to answer their questions below.

“Why do the faults look like snakes??!”

Faults are drawn on maps as curved lines, so sticking a face on one end makes them look snakelike. It is also the case that faults extend vertically under the ground as roughly planar regions of damaged rock, so in a way they’re kind of like rock snakes that are very flattened from side to side. More like rock ribbon eels than most snakes lol.

“Why is Cascadia dressed like a sailor???”

In the setting with the natural disaster characters, the subduction zones are the officer class of Gaia’s military who conquer oceanic plates and attach their mountains to the continents they fight for (yes this means Gaia fights herself…but she also created the Siberian Traps on a whim one day so she’s not the most sane planet/goddess). So Cascadia is kinda like a naval officer, with his tsunamis, volcanoes etc being his “soldiers”. His pipe is specifically inspired by Douglas MacArthur. It just seemed to fit with his naval officer theme, and it seems appropriate for a guy who makes volcanoes to be puffing ashes all the time.

“Why is Cascadia super buff???”

His design is based on the general design of the thrust fault characters. Thrust faults in maps are typically drawn as lines with teeth pointing in the direction of the dip of the fault:

So the thrust fault avatars kind of look like snakes with back spikes. Subduction zones are drawn similarly in maps, but they’re broader in extent and create more powerful earthquakes (they’re called “mega-thrusts”) so Cascadia looks like a bigger and really buff thrust fault avatar.

Depending on his orientation, the back spines are revealed to actually be his volcanic arc as shown here:

“Who is the other snake guy???”

That is the Juan de Fuca Ridge. I have drawn him/her/them like how mid-ocean ridges are drawn on maps, as rift segments offset jaggedly from each other by transform faults (which I haven’t drawn faces on to not clutter up the design too much). He/she/they wears a beret because they’re like an artist whose canvas is new ocean floors.

“What are those little snake/brown things?”

The little snakes are smaller faults that are in the ocean crust of the Juan de Fuca plate. The brown guys are seamounts. They’re all running as fast as they can to avoid being conveyor-belted into hell.

“Why is subduction portrayed as being eaten by Cascadia?”

There is a popular metaphor of subduction zones as factories being fed oceanic lithosphere, processing it into new substances and extracting volatiles from it, and then expelling the disintegrated waste products into the core-mantle boundary. Since Cascadia is alive in this setting and is himself the factory, this implies he is eating oceanic plates and melting/digesting/processing them in his stomach and intestines before pooping the remnants out into the core-mantle boundary lol.

(Though oddly enough I also depict him as having a literal factory for making new rocks inside of him that he runs by projecting an avatar inside himself. It’s weird.)

“How is a subduction zone even a character??? That’s just kind of a place/region!”

Personified countries are a whole genre so why not plate boundaries lol?

[ktz-tl]

September 22, 2022.

[bettergeology]

Geology of the Alvord Desert

The Alvord Desert, a huge playa in southeast Oregon, is the closest you can get in the Pacific Northwest to Death Valley - or any proper desert. From a geologic perspective, the two places are different but share similarities in their geologic structure. Tall mountains on one side, shorter mountains on all other sides, all bounded by geologic faults. There's a lot more than that though that makes the Alvord Desert truly special and unique - learn more on BetterGeology!