Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

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Welcome back to the Deep Dive.

Great to be here.

So today we're tackling something huge, literally, mountains.

Yeah, and not just postcard peaks, but these enormous interconnected chains, mountain belts,

the geological term is origins.

Origins, right.

Yeah.

They really snick across the continents on maps.

And if you've ever looked at one or maybe even hiked in one and thought, how on earth did this get here?

Yeah.

Then, well, you're in the right place for this deep dive.

Absolutely.

Because building mountains, this process we call orogeny, it's so much more than just uplift.

It involves incredible forces reshaping the crust.

Deformation is really the key concept.

Deformation.

OK.

So that means rocks bending, breaking.

Or even flowing, which sounds strange, but yeah, flowing under intense stress.

Glowing.

Wow.

So it's like the Earth's crust gets put through the ringer.

That's a good way to put it.

And the results are these geologic structures.

We see joints, faults, folds, even a kind of internal layering called foliation.

These are the cracks, crumples, and crags the source material mentions.

Cracks, crumples, and crags.

I like that.

And it's not just physical twisting, is it?

Orogeny can also trigger other things.

Definitely.

It often leads to metamorphism, changing existing rocks with heat and pressure and igneous activity.

So creating new volcanic or plutonic rocks, the whole geological package deal.

And this doesn't happen overnight, right?

The time scale is immense.

Oh, completely.

We're talking tens of millions of years for an orogeny.

And there's this constant tug of war between uplift, pushing the mountains up, and erosion, trying to tear them down.

Like the Matterhorn example, uplift creates the height, erosion carves the sharp peaks.

So it's a dynamic balance.

Exactly.

And even when mountains are eventually worn down flat, which takes ages, they leave behind what our sources call crustal stars.

Scars.

Meaning zones of highly deformed metamorphosed rock deep down, evidence that a mountain range used to be there.

So even flat looking areas can hide this dramatic history.

Okay.

So our mission for this deep dive is clear.

Understand deformation, uplift, mountain building, and how to read these structures in the context of plate tectonics.

Right.

And we know geology has its jargon, but we'll break it down.

Let's start with that deformation idea.

How do rocks actually change?

Well, the sources give a great visual contrast.

Picture a simple road cut, maybe showing nice, flat, undisturbed layers of sandstone or shale.

Pretty straightforward.

Okay, got it.

Horizontal beds.

Now contrast that with a cliff face and a mountain range.

Huge difference.

Those same rock types might be there, but they're often metamorphosed sandstone into quartzite, shale into slate, limestone into marble.

So changed by heat and pressure.

Exactly.

And crucially, they're not flat anymore.

They're likely folded, maybe broken by faults.

You can even see the texture change, like the grains in quartzite look flattened and slate gets that characteristic layering, the foliation.

So deformation involves several things happening to the rock.

Yes, the sources break it down into three components.

Displacement, which is just changing location, rotation, changing orientation, and distortion, which is changing shape.

And distortion is what geologists call strain.

Precisely.

Strain is the measure of that shape change.

It can be stretching, making things longer.

Like pulling Cathy?

Sort of, yeah.

Or shortening, squishing things.

Or shear strain, where different parts slide past each other, changing the angles inside the rock.

OK, so rocks can get stretched, squished, or sheared.

But do they tend to snap or do they bend?

That seems like a key difference.

It's fundamental.

It's the difference between brittle and plastic deformation.

Brittle is like dropping a plate.

It shatters.

It breaks.

Easy enough.

And plastic.

Plastic, or sometimes called ductile, is like modeling clay or warm chewing gum.

You can bend it, squeeze it, change its shape.

Without it actually breaking into pieces, it flows.

And this happens at the atomic level.

Yeah, basically.

In brittle deformation, the chemical bonds holding the minerals together actually break.

Crack.

In plastic deformation, bonds break, but then they quickly reform, allowing atoms and grains to shuffle around without the whole structure falling apart.

So what makes a rock decide whether to be brittle or plastic?

Seems important.

It really is.

Several factors are key.

First, temperature.

Heat makes things softer, right?

Like a candle.

Cold, it snaps.

Warm, it bends.

Exactly.

Warmer rocks deep in the earth are much more likely to deform plastically.

Colder rocks near the surface tend to be brittle.

Okay, temperature.

What else?

Pressure.

Specifically, confining the pressure pushing in from all sides deep underground.

High pressure makes it harder for fractures to open up, so it encourages plastic behavior.

Ah, it holds the rock together even as it deforms.

You got it.

Then there's the deformation rate, how fast you apply the stress.

Like hitting something quick versus pushing slowly.

Perfect analogy.

A sudden stress, like a hammer blow, tends to cause brittle fracture.

A slow, steady stress applied over geologic time allows the rock time to deform plastically.

Think of a marble bench sagging under its own weight over decades.

Wow, okay.

Rate matters.

Anything else?

Composition.

Just like some materials are naturally softer than others, some rocks deform plastically more easily.

Rock salt, for instance, flows relatively easily compared to, say, granite, which is much stronger.

So temperature, pressure, rate, and what it's made of.

That makes sense.

And all this leads to the concept of the brittle plastic transition zone in the crust, typically around maybe 10 to 20 kilometers down in continents.

And that's why earthquakes happen mostly above that depth, because the rocks are brittle and they break.

That's the main reason, yes.

Earthquakes are fundamentally brittle failure events.

But interestingly, you can see both types of deformations sometimes in the same outcrop.

How does that work?

Well, maybe conditions changed over time.

Perhaps deformation started slow, plastic folds, and then sped up, causing brittle faults later on.

Okay, complex histories.

Now, we've talked about deformation as the effect.

What about the cause, the forces involved?

Right, forces and stress.

Force is just a push or pull or shear.

Think of tectonic plates colliding that's applying immense forces.

But the sources emphasize stress over force.

Why is that?

Good question.

Stress is force per unit area.

The aluminum can analogy helps.

You can apply a large force spread over the whole can, or a smaller force concentrated on one tiny point with your thumb.

Which one dents the can?

The concentrated force.

So is that how focused the force is?

Exactly.

That concentration is stress, and rocks respond to stress.

And there are different kinds of stress mirroring the strain types.

Compression is squeezing stress.

Tension is pulling apart stress.

Sheer stress involves sideways movement,

like cards sliding in a deck.

And then there's pressure, which is equal stress from all directions, like underwater.

Got it.

Stress causes strain.

Cause and effect.

That's a fundamental relationship.

Okay, let's dive into the results of brittle deformation first.

Joints and veins.

They sound less dramatic than faults.

They are, in a way.

Joints are basically just cracks in rocks where there hasn't been any significant sliding.

The rock just pulled apart slightly,

usually due to tension stress.

Why do they form?

What causes that tension?

Several things.

When igneous rocks cool, they contract and crack.

When overlying rock has eroded away, the pressure decreases, and the underlying rock expands vertically and might crack horizontally.

Even just bending a brittle layer can cause joints on the outer part of the bend.

Are all joints the same?

No.

There are systematic joints, these long, straight, parallel cracks, often forming sets.

Think Arches National Park, where erosion along these joints creates fins and eventually arches.

And then non -systematic joints are more irregular, shorter, random looking.

And veins are related.

Yes.

Veins form when groundwater flows through these open joints and precipitates minerals like quartz or calcite, filling the crack.

That's why you see those white stripes cutting through rocks sometimes.

And joints matter for more than just looks, right?

Engineering.

Oh, absolutely.

Joints control how water flows through rock, which is critical for dams or tunnels.

And they create planes of weakness, increasing the risk of rock falls on cliffs or road cuts.

Engineers have to map them carefully.

OK, moving on to the bigger breaks faults, where actual sliding happens.

Right.

Faults are fractures with measurable slip.

And as we know, movement on active faults causes earthquakes.

The San Andreas is a classic example, offsetting streams, fences.

Just like joints, they're planar features, described by Strike and Dip.

Usually, yes.

And we distinguish between active faults, recent movement, potential for more, and inactive ones.

Some reach the surface, their intersection is the fault trace or fault line.

Others are buried, called blind faults.

Why spend so much time studying faults, apart from earthquakes?

Well, earthquakes are a big one.

But faults also mess up the crest's arrangement, putting totally different rock types next to each other.

Understanding that helps piece together the geological history, and can even help predict where resources might be trapped.

How are faults classified?

Seems like there could be many ways they move.

The main classification is based on the dip of the fault plane and the direction of slip relative to that dip.

First, we have dip -slip faults.

Meaning movement is up or down the slope of the fault.

Exactly, parallel to the dip line.

Geologists talk about the hanging wall block the rocks above the fault plane, and the footwall block the rocks below it.

OK.

Hanging wall and footwall.

So, in a normal fault, the hanging wall moves down relative to the footwall.

This happens when the crust is being stretched or extended.

Think crustal rifting.

Makes sense.

Pulling apart lets the block above slide down.

Right.

Now, a reverse fault is the opposite.

The hanging wall moves up relative to the footwall.

This accommodates crustal shortening, squeezing things together.

Think continental collision.

Compression pushes the upper block upwards.

Precisely.

And a thrust fault is just a special type of reverse fault that has a gentle dip, less than 30 degrees.

They can cause huge amounts of shortening.

So normal faults for stretching, reverse, and thrust faults for shortening.

What about sideways movement?

That's a strike -slip fault.

Here, the movement is horizontal, parallel to the strike of the fault.

The fault plane is usually steep or vertical.

And how do you describe the direction?

You imagine standing on one side of the fault and looking across.

If the other side moved to your left, it's left -lateral.

If it moved to your right, it's right -lateral.

Like the San Andreas, which is right -lateral.

Okay.

And can movement be diagonal?

Yep.

That's an oblique slip fault.

It has both a dip -slip, up -down, and a strike -slip sideways component.

So how do you spot these in the field?

What are the clues?

The most obvious clue is offset, or displacement.

You see a distinctive rock layer, maybe a red sandstone bed, and suddenly it stops at the fault, and you find the continuation of that same bed shifted up, down, or sideways on the other side.

Layers just don't match up.

Exactly.

Also, knowing the relative ages helps.

Thrust faults tend to put older rocks on top of younger rocks, which is unusual.

Normal faults usually put younger on older.

Anything else.

Sometimes you see drag faults, little bends in the layers right near the fault caused by the friction during slip.

And faults can offset landscape features, too – rivers, ridges, even human -made things like roads or fences if the fault is active.

Like the San Andreas offsetting streams.

Perfect example.

Also, fault zones are often weaker rocks, so they erode more easily, sometimes forming long straight valleys.

The Great Glen Fault in Scotland is a good example.

Or if the rocks on either side erode differently, you can get a steep slope or cliff called a fault line scarp.

What about the actual fault surface itself, if you can see it?

Yeah, if it's exposed, you might see several things.

A fault scarp is a small step on the ground surface directly caused by recent dipslip movement.

Like a little cliff formed by the earthquake.

Exactly.

On the fault surface itself, you might find fault breccia, basically angular chunks of rock shattered during the movement, or fault gouge, which is rock pulverized into a fine powder.

Grand up rock.

Yeah.

The surface might be polished smooth, that's a slick inside, and often you'll see fine grooves or scratches called slip lineations or striations etched onto the slick inside, which actually show the direction the rocks last moved.

Wow.

Lots of clues locked in the rock.

Yeah.

Do faults usually occur alone?

Often not.

They frequently occur in fault systems, arrays of related faults working together.

For example, thrust systems can involve multiple thrust faults that curve downwards and merge into a common horizontal fault called a detachment.

Like sheets sliding over each other.

Yes.

Carrying huge sheets of rock called thrust sheets for potentially hundreds of kilometers.

These sheets often get folded as they move, creating what we call fold thrust belts.

The Canadian Rockies are a spectacular example.

Imagine a bulldozer pushing a pile of snow.

It wrinkles up and slides forward on sheets.

Okay.

That's for compression.

What about stretching?

That leads to normal fault systems, again, often linked to a detachment fault at depth.

As the crust stretches, blocks slide down along curved normal faults.

Creating basins.

Exactly.

You get wedge -shaped basins called half -gravens bounded by one main fault.

Or if you have blocks dropping down between two faults dipping towards each other, that's a graven.

The uplifted blocks left in between are called horse.

Horses and gravens.

Like in the Basin and Range province.

Precisely.

That whole landscape is shaped by these normal fault systems, and these gravens and half -gravens tend to fill up with sediment eroded from the uplifted horse.

One more term here.

Yeah.

Shear zones.

How are they different from faults?

Shear zones are basically faults that formed deeper in the crust where it was hot enough for plastic deformation.

Instead of a sharp break with breccia, you get a band of rock where the minerals have been stretched, smeared out, and recrystallized into a fine -grained foliated rock called myelinite.

The boundaries are more gradual.

So a ductile fault zone, essentially.

That's a good way to think of it.

Okay, that covers brittle structures.

Now let's move into the plastic side.

Folds and foliations.

Folds are basically bends.

Right.

When layered rocks or other planar features get compressed or sheared under ductile conditions, they can bend into curves or folds.

Think of wrinkling a rug by pushing one end.

How do geologists describe the shape of a fold?

We look at key parts.

The hinge is the line where the fold has the tightest curvature.

The limbs are the less curved sides of the fold.

And the axial plane is an imaginary surface that connects the hinges of successive folded layers.

Are there different basic types of folds?

Yes.

The main ones are anticlines, which are arch -like folds where the limbs dip away from the hinge.

If you erode the top off an anticline, the oldest rocks are exposed in the middle.

Arch up, oldest in the core.

Got it.

And synclines are the opposite trough -like folds where the limbs dip towards the hinge.

When eroded, the youngest rocks are in the middle.

Trough down, youngest in the core.

Makes sense.

What about a monocline?

A monocline is like a single bend or step in otherwise horizontal layers.

Imagine draping a carpet over a stair step.

They often form over faults in the deeper basement rock.

Okay.

Anticline, syncline, monocline.

What about how the fold is oriented in 3D?

Good point.

We describe the hinge line if it's horizontal, it's a non -plunging fold.

If the hinge line itself is tilted, it's a plunging fold.

And then there's circular patterns.

Domes and basins.

Right.

A dome is like an upside down bowl.

The layers dip away from the center in all directions.

After erosion, the oldest rocks are in the middle, forming a bullseye pattern.

A basin is the opposite, an upright bowl.

Layers dip towards the center, and the youngest rocks are in the middle, also forming a bullseye.

How do these look on a geologic map?

Non -plunging anticlines and synclines make parallel stripes of rock layers.

Plunging folds make characteristic V or U -shaped patterns on the map.

Domes and basins make those circular or elliptical bullseye patterns.

And these folds influence the landscape too.

Absolutely.

Because different rock layers erode at different rates, areas with folded rocks often develop parallel ridges, formed by resistant layers, and valleys formed by weaker layers.

How do these folds actually form?

What are the mechanisms?

Two main ways.

Flexural slip folds happen when layers bend and slip occurs between the layers, like bending a deck of cards.

The layers mostly keep their original thickness.

Okay, slipping between layers.

The other type is passive flow folds.

This happens when the rock is very weak and flows like thick honey.

Different parts flow at different rates, causing folds.

Here, the layer thickness can change, often thinning on the limbs and thickening at the hinge.

And what causes the stress leading to folding?

Several things can do it.

Simple endon compression can buckle layers.

Shear stress can smear layers into folds.

Movement on a deep fault can warp overlying layers into a monocline, as we mentioned.

And sometimes, layers are forced to bend as they move up and over bends, called ramps, in an underlying thrust fold.

Lots of ways to make a fold.

Now, what about foliation again?

We mentioned it with metamorphic rocks.

Right.

Tectonic foliation is that layering or planar alignment of mineral grains that develops because of deformation.

Usually under metamorphic conditions.

Is it different from original sedimentary layering?

Yes.

Think of sandstone with roundish grains versus quartzite, where those grains look flattened and aligned.

Or shale with randomly oriented clay flicks versus slate, where the clay minerals have recrystallized and aligned perpendicular to the compression direction.

That alignment is the foliation, like slaty cleavage.

And it tells you about the strain?

It absolutely does.

It shows the rock has deformed plastically.

Slaty cleavage, for example, typically forms perpendicular to the direction of shortening.

Other foliations, like scistosity or Gneissach layering, often align parallel or at a low angle to shear planes, indicating shearing was involved.

Okay, we've got deformation, brittle structures, ductile structures.

Now, the really big picture.

What causes mountain building?

Why do origins form where they do?

The answer is fundamentally plate tectonics.

Mountain building is primarily linked to convergent plate boundaries, continental collisions, and also continental rifting.

The long, linear nature of mountain belts directly reflects the shape of these plate boundaries.

Let's take those one by one.

Subduction zones first.

Okay.

At subduction zones, we know we get volcanic arcs forming on the overriding plate.

But the compression there also squeezes the overriding plate, causing crustal shortening and often creating large fold -thrust belts behind the arc.

The Andes are a prime example of this kind of compressional subduction -related origin.

So compression and volcanoes working together.

What about when continents collide?

That's where you get the really major mountain ranges.

After the ocean basin between two continents closes completely, the continents themselves collide.

Since they're both buoyant, neither wants to subduct fully.

Like the India -Asia collision forming the Himalayas.

Exactly.

Or the collision that formed the Alps.

Or the ancient collision that formed the Appalachians.

These collisions involve intense compression.

You get massive fold -thrust belts pushed out onto the edges of the continents.

And in the middle of the collision zone.

That's where things get really intense.

High -grade metamorphism, complex folding, including passive flow folds, strong foliations.

The boundary where the two continents are stitched together is called the suture.

The whole process leads to significant crustal thickening.

The crust gets shorter horizontally, but much thicker vertically.

Often through stacking on thrust faults.

You also mentioned accretion.

Right.

Sometimes smaller bits of crust island arcs microcontinents collide with and get stuck onto the edge of a larger continent at a convergent margin.

This process, accretion, also builds up mountain belts over time, piece by piece.

The western part of North America, the Cordillera, is largely built from accreted terrains.

Okay, so subduction and collision create mountains through compression.

What about rifting?

That's pulling apart.

It seems counterintuitive, but rifting can create mountains.

As the crust stretches and thins, the brittle upper part breaks along normal faults.

Blocks drop down, forming basins, grab -ins and half -grab -ins.

And the blocks left up form mountains.

Exactly.

These are called fault block mountains.

They are typically narrow, elongated ranges bounded by normal faults.

The basin and range province is full of them.

The East African rift is another place where this is happening today.

Does thinning the lithosphere do anything else?

Yes, it allows the hot asinosphere below to rise closer to the surface.

This can cause melting, decompression melting, and lead to volcanism within the rift zone.

And the heat itself can cause some uplift.

So mountain building involves specific rock -forming processes too?

Absolutely.

Igneous activity is common.

Convergent margins and collisions produce lots of magma, leading to volcanic arcs and Rifting leads to decompression melting, often producing basalt, but sometimes more evolved magmas too.

And sediments.

Mountains are huge sources of sediment.

Erosion strips material off and it gets deposited in nearby basins.

The sheer weight of the mountain range can actually depress the crust nearby, creating space for these sedimentary basins to form and fill up.

Rift valleys also accumulate thick sequences of sediment.

And metamorphism is widespread.

Very much so.

You get contact metamorphism around the igneous intrusions, and the intense pressure and temperature deep within the thickened crust of collision zones cause regional metamorphism over vast areas, often accompanied by the development of tectonic foliation.

Is mountain building still happening?

Can we measure it?

Oh, definitely.

Earthquakes and volcanic eruptions are obvious signs of active tectonics.

Geologists also look for clues like uplifted ancient shorelines or rivers cutting down rapidly into the landscape.

And technology helps.

Massively.

GPS, the Global Positioning System, allows us to measure tiny movements of the crust, both vertical uplift and horizontal shortening with millimeter precision.

We can literally watch mountain ranges like the Andes growing and compressing year by year.

That's incredible.

Let's talk about the topography itself.

Why are mountains high?

It boils down to uplift the vertical rise at the surface.

Leonardo da Vinci figured this out, seeing seashells high in the Alps.

The amount of rock uplifted to form a major range is just colossal.

But even Everest is tiny compared to the Earth's size.

Relatively speaking, yes.

The source uses the analogy of a billiard ball.

Earth shrunk down would feel smoother than one.

And remember, the peak elevation isn't the whole story.

The base of the range might already be quite high.

So what controls how high the lithosphere floats?

That concept of isostasy.

Exactly.

Isostasy is like buoyancy for the Earth's crust.

The lithosphere floats on the denser, more fluid asthenosphere.

How high it floats depends on its thickness and its density.

Thicker crust floats higher.

Less dense crust floats higher.

Like wood blocks in water.

Perfect analogy.

So how do mountains achieve that extra thickness or lower density to float high?

Collision seems like a big one thickening the crust.

Definitely.

Continental collision can double crustal thickness, say from 40 kilometers to 70 or 80 kilometers, like under the Himalayas or Tibet.

This creates a deep, buoyant crustal route extending down into the mantle, which supports the high elevation above.

That's the classic airy model of isostasy.

What else causes uplift?

Adding igneous rock adds volume and mass volcanoes pile lava on top, and intrusions add magma within and below the crust.

Underplating is another way basalt magma rising from the mantle gets trapped at the base of the crust, essentially plastering onto it from below and thickening it.

And removing something dense from below.

Delamination.

Yes, that's a fascinating idea.

If the dense lowest part in the lithosphere, the lithospheric mantle, detaches and sinks into the deeper mantle, like dropping ballast from a ship, the remaining lighter crust and upper mantle will bob up isostatically.

This might be involved in uplifting plateaus like Tibet.

What about in rifts?

How does stretching cause uplift?

Two things happen in rifts.

First, as you thin the dense lithosphere, you replace it with hotter, less dense asthenosphere rising from below.

Second, the remaining lithosphere heats up and expands, also becoming less dense.

Both effects contribute to isostatic uplift of the rift flanks, creating those high rift shoulders like in East Africa.

But what goes up must come down, right?

Erosion is always working against uplift.

Always.

As soon as there's topography, gravity drives erosion.

Landslides, rivers, carving canyons, glaciers, grinding away rock, the famous glacial buzzsaw effect.

So the actual height of a mountain depends on the balance between uplift rate and erosion rate.

Precisely.

While tectonics are active, uplift might win or they might roughly balance.

Once tectonics stop, erosion inevitably takes over and over millions of years wears the mountains down.

But the mountains fight back a bit.

Isostatic rebound.

Yeah, as erosion removes weight from the top, the crustal root below becomes overly buoyant and the whole range slowly rises, partly compensating for the erosion.

It prolongs the life of the mountains, but erosion wins in the end.

Is there a maximum height for mountains on earth?

Probably.

Our sources mention orogenic collapse.

Deep in the thickened crust of large mountain belts, maybe 15, 30 kilometers down, quartz -rich rocks get hot and weak.

They can't support the immense weight above.

So the mountains start to sag.

And spread outwards.

Yeah, like soft cheese or slow motion geological pancakes.

The upper, brittle part of the crust responds to this spreading by breaking along normal faults.

It's the mountain range essentially collapsing under its own weight.

And all this erosion and uplift eventually exposes rocks from deep down.

That's right.

It's called unroofing, or exhumation.

Rocks that were metamorphosed or intruded maybe 10, 20, 30 kilometers deep can be brought to the surface over millions of years by this combination of erosion from the top and uplift from below.

Okay, that covers the dynamic mountain belts.

What about these stable parts of continents?

The cretins?

Right.

Cretins are the old, strong, stable hearts of continents.

They haven't seen major mountain building for a billion years or more.

They're generally cool and rigid, often with deep, old lithospheric mantle roots beneath them.

North America has a huge cretin at its core.

And cretins have different parts.

Shields and platforms.

Yeah.

Shields are where the ancient Precambrian basement rocks heavily metamorphosed and deformed from those really old orogenies are exposed right at the surface.

They're usually broad, low -relief areas like the Canadian Shield.

And platforms.

Cretonic platforms are areas where those same ancient basement rocks are covered by a relatively thin blanket of much younger, mostly flat -lying sedimentary rocks, usually Paleozoic or Mesozoic in age.

Think of the interior plains of North America.

But even these stable platforms aren't perfectly flat.

They have basins and domes.

That's right.

Over vast areas, the cretonic platforms show gentle, large -scale warping.

Cretonic basins are huge, shallow depressions where the sedimentary layers dip inwards very gently and thicken towards the center.

The Illinois Basin is an example.

They formed by slow, long -term subsidence.

These domes are the opposite.

Yes.

Domes and related broad arches are areas that stayed high or were gently uplifted.

The sedimentary layers dip very gently outwards, away from the center, and erosion exposes the oldest rocks in the middle, like the Ozark Dome.

These broad, gentle vertical movements are called epirogyny, distinct from the intense deformation of orogeny.

Are there faults within cretins, too?

Yes.

Sometimes.

Even though the sedimentary layers are mostly undeformed, they can be cut by intracretonic faults.

Movement on these can cause overlying layers to bend into monoclanes.

Are these new faults?

Mostly not.

They're usually ancient faults in the Precambrian Basement, maybe formed during old rifting events that get reactivated much later.

Often, stresses from collisions happening way off at the edge of the continent are just enough to cause slip on these pre -existing weak zones within the cretin, but not enough to cause major folding or metamorphism there.

Fascinating how everything is connected.

To wrap it all up, the sources use the Appalachians as a case study.

Yeah.

It's a great example of just how complex the history of a mountain belt can be, involving multiple cycles of collision, rifting, and ocean basin opening and closing.

So what's a quick story?

Okay, super quick version.

Way back, around 1 .1 billion years ago, the Grenville Orogeny involved a major collision, forming the deep basement rocks of eastern North America.

Then, around 600 million years ago, rifting happened, an ocean opened up to the east, and North America's east coast became a passive margin collecting sediment.

Passive margin, like the east coast today.

Sort of, yes, but this was the precursor ocean.

Then things reversed.

Starting around 420 million years ago, several collisions happened.

The Taconic and Acadian Orogenies, where island arcs and continental bits slammed into North America from the east, turning it back into an active convergent margin.

Building mountains again.

Exactly.

And the grand finale was the Alleghenian Orogeny, around 270 million years ago, when Africa collided with North America as part of the formation of the supercontinent Tangaya.

This created a truly massive Himalayan -scale mountain range.

The folds we see in the Valley and Ridge province today are remnants of that collision.

Wow.

But then it rifted apart again.

Around 180 million years ago, Pangaea started breaking up, the Atlantic Ocean opened, and the east coast became a passive margin again, which it still is today.

That whole cycle, ocean opening -closing collision, has a name, right?

The Wilson Cycle.

It captures this recurring theme in Earth history.

So mountain building is incredibly complex, long -lasting, driven by plate tectonics involving deformation, uplift, erosion.

It really ties so much geology together.

It absolutely does.

These seemingly permanent mountains have such dynamic histories, recording everything from tiny mineral changes to massive continental collisions.

Understanding stress, strain, brittle, plastic, faults, folds, it all comes together here.

And seeing the contrast between these active belts and the quiet old cretins really highlights the different personalities of Earth's crust.

Definitely.

It's a reminder of the constant change and evolution our planet undergoes.

So a final thought for our listeners.

Next time you see mountains, or even just look at a geologic map of your area, think about the immense journey those rocks have been on, what clues might tell the story of their formation.

It's a fantastic way to connect with the deep history beneath our feet.

And with that, we've reached the end of this particular deep dive.

We set out to explore the whole process of mountain building, and I think we've managed to cover all the key ground laid out in our source material for today.

We looked at the basic concepts of orogeny and deformation,

the difference between brittle and plastic behavior,

the details of joints, faults, folds, and foliations.

We connected it all to plate tectonic subduction, collision, rifting, and looked at the resulting rocks and how we measure ongoing uplift.

We dove into topography, isostasy, erosion, and even orogenic collapse and exhumation.

We contrasted mountain belts with stable cratons, their basins and domes, and wrapped up with the complex life story of the Appalachians.

It's been quite a journey through the Earth's crust.