Overview of Geologic Structures Part 2: Faults and Folds

We just learned about the different types of rock deformation, so now let's get a closer look at some more specific structures. First we have faults, which can be normal, reverse, and strike-slip or reverse faults. We should also take a look at folds, which can be synform or antiform, and syncline or anticline. What do all these terms mean?! I'll show you!

Script by Jared Matteucci

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[lucyoneill]

praying professor dave videos come in clutch for my geo final tmr

[sx]

The only heartwarming YouTuber out there with real critical thinking. Congratulations on your 2 million subscribers man!

[philipliao]

Likely won't be taking any class related to this topic, but always love to learn

[roypatton]

Ahhhh! It's so good to hear Geology again. I miss it.

[qutopyo]

I'm not even close to needing to learn about this, but it can't hurt

[Askify]

My Transcript notes of the video
00:00
we just learned about the different types of rock deformation but there are two very important structures pertaining to this concept that must be discussed



00:15

in detail those being faults and folds faulting is the main type of brittle deformation in rocks faults are classified based on the type of deformation that creates them normal faults are caused by extension



00:30

reverse faults by compression and strike slip or transverse faults by shearing normal and reverse faults are called dip slip faults because fault motion or slip is parallel to the false dip this is in contrast with strike slip



00:49

faults where slip is perpendicular to the dip in geology dip refers to the direction that the fault plane is inclined to conceptualize this imagine that you pour a cup of water onto an inclined



01:04

surface the direction that the water flows is the dip which has two components an azimuth representing a horizontal direction and an angle representing the steepness of the plane



01:18

in any dip slip fault there is a hanging wall and a foot wall if you imagine drilling a tunnel longitudinally through the fault when standing in this tunnel you would be standing on the foot wall and the hanging wall would be above you



01:33

in reverse faults the hanging wall moves upward and in normal faults it moves downward reverse faults primarily form in areas where the tectonic plates are converging like along subduction zones and



01:47

continent continent collisions there are also areas where the plates are converging at an angle these transpressional faults experience both reverse and strike slip faulting a fault with both dip slip and strike



02:03

slip motion is called an oblique fault an example of this is the san andreas fault here the pacific and north american plates are converging at about a 10 degree angle



02:16

though predominantly a strike-slip fault it is also slightly compressional as is evidenced by the coastal mountain ranges of california normal faults are most common where the plates are moving apart and the crust is



02:31

being stretched this is currently occurring in east africa the basin and range province of north america and at the center of every ocean basin in the world as the crust thins during extension its



02:44

weight above the underlying mantle is reduced and spread out over a larger surface area this causes the crust to bob up in a process called isostatic adjustment or more specifically isostatic rebound



03:01

it's like walking off a boat when your weight is removed the boat bobs upward and rises in the water get back in and the boat bobs down in response to your weight back to the geological case the crust is



03:15

analogous to the boat and the mantle is analogous to the water with the crust bobbing up and down in response to the thickening and thinning of the crust the same thing occurs in areas that are experiencing compression the



03:29

compressional forces thicken and shorten the crust causing subsidence because the now heavier crust will sink into the mantle this is what leads to the development of four land basins or large areas of



03:43

depressed land located adjacent to large mountain ranges the weight of the mountain pushes the crust down into the mantle not only beneath it but also adjacent to it and it is these adjacent lowlands that sink



03:58

below sea level during isostatic adjustment which in this case is called subsidence basin subsidence is what makes the formation of ultra thick sedimentary deposits possible



04:12

for example as much as 10 000 feet of sediment accumulated in the catskill four land basin over a period of only about 20 million years during the upper devonian period this sediment was shed from the now



04:26

eroded acadian mountains that existed along the eastern seaboard subsiding forland basins like this have fostered deposition of some of the largest sedimentary formations in the world



04:39

in some areas compressional forces cause rocks to become folded like a car that gets crumpled during an accident folding occurs when a rock is being compressed and the temperature and pressure are just high enough for plastic deformation



04:53

to occur folds are classified based on their geometry and stratigraphy or arrangement of strata folds that are concave up are called sin forms and folds that are concave down



05:08

are called anti-forms when layered rocks are folded into a sin form the youngest rocks will usually be at the hinge of the fold which is similar to the local maximum or minimum of a parabolic function

To get the full transcript of the video with PDF screenshot notes - Try Askify chrome extension

[thjacobi]

Great video Professor Dave, normally it takes a full semester to explain this. Would love to see geophysics covered too!

[gooseart]

I can't believe it. Dave Grohl is such a talented musician and still has the time and intelligence to teach us about everything on YouTube!

[HinkosaAdugna]

Excellent lecture! As I'm one geology student I'm gain good explanation!

[acommunistdwarf]

This is a very exciting topic to learn about, thanks for making this so accessible Dave. I find it amusing how many of the terms, rock formations and types of minerals and rocks I've learned by playing Dwarf Fortress, I'm following this together with the walkthrough of the periodic table to compare how the game distributes some of the metal ores.

[robertwilsoniii]

Now I get why Stanford is made out of sandstone. The first professor at Stanford was a geologist, and he knew both that sandstone reflected the regions historical record in the stone itself but that it was also the stone most resistent to errosion for long term longevity of the university structure itself.

Stanford really is a cut above the rest when you dig deep.

[stevon]

Congratulations on 2mil man! I’ve always loved your normal style teaching videos but the debunking videos are so damn funny.

[Hammer]

As a fantasy worldbuilder, things like this help me immensely, knowing the reasoning behind structures helps in making realistic environments, or having an idea about the science gives you a baseline to play with when making them fantastic. Thanks so much!

[ashleygilliam]

Not only was this a great summary of faults and deformation but you pronounced Appa ”latch” ian correctly! Yay! Will always appreciate any videos about geology, geomorphology, geoarchaeology or archaeology. 🙂 love your content and appreciate all the hard work you do!

[TescoMealdeal]

Congratulations on hitting 2 million subscribers!!

[abenezerwondewosen]

does the earth's crust have that fault?

[ispylugaw]

Im not a geology student but this thing is interesting

[vintinoo]

Another great Vid! Didn’t even know I had an interest in this topic until I started watching your geology stuff. Keep up the educational stuff!

[muzikhed]

Nicely explained, very interesting, Cheers.

[Killbayne]

"when a rock has cleavage" is a line i didnt expect