Chapter 15: Mass Wasting: The Work of Gravity

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Okay, let's dive in.

Imagine this.

The ground beneath your feet just suddenly acts like liquid.

Or a whole mountainside decides, you know, to move.

That 2014 Oso landslide in Washington, it wasn't just some headline, it was a really brutal reminder of this fundamental force we call mass wasting.

It's basically gravity doing its thing, constantly reshaping our planet.

So today we're really digging into this unseen hand, exploring what makes a slope stable one minute and catastrophic the next, and what these powerful movements actually mean for us, you know, living here.

Think of this as your quick guide to understanding one of Earth's most powerful but often overlooked external processes.

And what's really compelling, I think, right away is realizing this isn't just about those huge dramatic disasters.

Mass wasting, it ranges from this incredibly slow creep, almost invisible, to a roaring avalanche.

And it's happening everywhere, all the time.

It's like the planet constantly adjusting itself, shaping everything from massive canyons down to the little hills you might walk on.

Okay, so let's lay the groundwork then.

What exactly is mass wasting, beyond just, you know, stuff falling down a hill?

And maybe more importantly, how is it different from other things that wear down the land, like a river carving a valley or wind erosion?

Yeah, that's a really key distinction.

So fundamentally, mass wasting is the downslope movement of rock.

And regolith, that's just the loose rock and soil covering bedrock directly pulled by gravity.

But here's the crucial part you mentioned.

Unlike erosion by streams or wind, it doesn't need a transporting medium like water or air to carry the material away.

Gravity is the transport system.

You can think of it as the step after weathering.

Weathering breaks the rock down.

Right, breaks it apart.

Yeah.

Then mass wasting comes along and moves that debris downslope.

Often it dumps it right into a stream or onto a glacier, which can then carry it further.

Got it.

So it's like a handover process.

Exactly.

And think about the Grand Canyon.

If you've ever stood there, you probably think, wow, the Colorado River did all this.

Yeah, that's the common thought.

But actually, mass wasting is hugely responsible for widening those canyon walls way beyond where the river itself can cut.

It's constantly feeding material down to the river, creating those vast iconic slopes.

It's a partner in crime, essentially.

A silent partner doing a lot of the heavy lifting for the landscape.

A very important silent partner, yes.

If you connect this to the bigger geological picture, mass wasting is absolutely essential for the whole cycle of erosion.

It's kind of the counterpoint to mountain building, which is driven by plate tectonics pushing things up.

Mass wasting tears them down.

So uplift versus erosion.

Precisely.

Without it, the world would look completely different.

Imagine valleys being much, much narrower, almost like slots, instead of the wider forms we see.

Which brings us to a key question.

Why should you, listening right now, really care about this?

Right, the relevance.

Well, these events, often just called landslides by the public, are major geological hazards.

We're talking billions in damages, dozens of deaths, just in the U .S.

each year.

And crucially, they often happen alongside other disasters.

Think earthquakes, wildfires, huge storms.

They can amplify the danger.

OK, so gravity is the constant force, always there pulling down.

But obviously, slopes aren't collapsing all the time.

So what pushes them over the edge?

What turns stable ground into, well, moving ground?

Geologists just call these triggers, right?

Exactly.

Triggers are the immediate cause.

And one of the biggest, maybe the most common, is water.

Ah, water.

Yeah.

Always seems to be involved somehow.

It really is.

When heavy rain or snowmelt saturates the ground, the water doesn't necessarily carry the material like a river does.

Instead, it gets between the soil or rock particles.

This reduces the friction, the internal cohesion holding everything together.

OK.

So it makes things slippery internally.

Precisely.

Think about building a sandcastle.

Damped sand holds its shape perfectly.

But completely waterlogged sand, it just flows, it oozes.

Plus, all that water adds significant weight.

A huge extra burden on the slope.

We saw that in Oso, didn't we?

You mentioned Boulder, Colorado, too.

Absolutely.

Oso in 2014 was heavily linked to saturation.

And in 2013, Boulder had something like 1 ,100 debris flows triggered by just five days of relentless rain on steep terrain.

Shows how critical water saturation is.

Wow, 1 ,100.

That's incredible.

OK, so water is one trigger.

What else?

You mentioned steep slopes.

Yes, the steepness itself.

Every pile of loose material, like sand or soil, has a natural stable slope limit.

It's called the angle of repose.

It's the steepest angle it can maintain without sliding down.

Usually somewhere between 25 and 40 degrees, depends on the material.

If you make the slope steeper than that angle naturally, like a river undercutting its bank.

Or unnaturally.

Or unnaturally, yes.

When we cut into a hillside for a road or level a pad for a house, you're essentially exceeding that natural limit.

You're pushing your luck with gravity.

Basically daring it to happen.

Pretty much.

And then there's vegetation, or rather, the removal of vegetation.

Plants.

How do they factor in?

Well, plants aren't just scenery.

Their root systems act like a natural net, binding soil and loose rock together.

They provide crucial support.

Like natural rebar and concrete.

That's a great analogy.

So if you remove that vegetation maybe through a wildfire or logging or even changing land use.

Like the example in France.

Exactly.

The Menton example.

Farmers replaced deep -rooted olive trees with shallow -rooted flowers, carnations, and boom, a deadly landslide followed because that deep, binding network was gone.

Wow.

Wildfires are a huge issue too, especially out west.

They strip the vegetation, leave behind dry, loose soil, sometimes even bake the ground, making it repel water initially.

Setting it up for disaster when the rains come.

Precisely.

That creates the perfect conditions for really dangerous, fast -moving debris flows with the first heavy downpour.

And then there are the really dramatic triggers.

Like earthquakes.

Oh, absolutely.

An earthquake delivers an instant, powerful shock.

The shaking can dislodge enormous amounts of rock and soil almost instantaneously.

Like North Ridge in 94.

Yeah.

The North Ridge quake triggered over 11 ,000 landslides.

And think about the 2008 Sichuan earthquake in China, magnitude 7 .9.

It caused hundreds of massive landslides.

Some were so big they actually dammed rivers, creating new lakes, which then posed a secondary threat the dam could burst, causing catastrophic downstream floods.

A double disaster.

That's terrifying.

Just layers of hazard.

It is.

And what's really interesting, or maybe sobering, is that the trigger isn't always the only cause.

Often it's just the final push after a long period of gradual weakening.

So the slope is already unstable, just waiting.

Exactly.

And sometimes, sometimes there's no obvious external trigger at all.

Like the 1999 Sacred Falls landslide in Hawaii, 10 hikers were killed.

It seems the slope materials just gradually weakened over time until they failed.

Without any warning.

Apparently so.

Which raises a really difficult question.

How do you predict something like that if there's no clear trigger event?

It highlights the hidden risks in some landscapes.

That is a crucial point.

And with earthquakes, there's another phenomenon too, right?

Liquifaction?

Yes, liquefaction.

That's when intense shaking hits water -saturated ground,

usually loose sands or silts.

The material temporarily loses all its strength, all its structure, and basically behaves like a liquid.

Like quicksand, almost.

Very much like quicksand.

This was a huge problem in Anchorage during the 1964 Good Friday earthquake.

Buildings tilted, foundations failed, because the ground underneath them literally turned to fluid.

Okay, so we have triggers, materials, underlying conditions.

It's complex.

How did geologists even begin to categorize all these different ways the ground can move?

That's where classification comes in.

To make sense of it all, we look at three main things.

First, the type of material that's moving.

Second, the type of motion.

And third, the speed or velocity of the movement.

Okay, let's break that down.

Type of material seems simple enough.

Relatively.

Is it solid bedrock moving?

We generally call it rock.

Is it loose stuff, soil, unconsolidated fragments?

We use terms like debris, mud, or earth, depending on the mix and particle size.

Fine stuff often gets called mud.

A mix is debris.

Soil -rich is often earth.

Got it.

Rock, debris, mud, earth.

Now the type of motion, that sounds key.

It really is.

It tells us how the material is moving.

We generally group motion into three broad categories.

Fall, slide, and flow.

Fall, slide, flow.

A fall is pretty much what it sounds like.

Pieces detached from a steep slope or cliff and just fall freely through the air, maybe bouncing on the way down.

Like rocks falling onto a road from a cliff.

Exactly.

That process builds up those slopes of broken rock you see at the base of cliffs.

Those are called talus slopes.

And falls can be incredibly powerful.

You mentioned Peru earlier.

The 1970 earthquake triggered a massive chunk of rock and ice off Nevado's Huascan.

It fell, pulverized on impact, and became a rock avalanche that buried towns, killing over 20 ,000 people.

Just unimaginable energy.

Absolutely.

And another example, Yosemite in 1996.

A huge rock fall hit the valley floor so hard it generated an air blast, like an atmospheric pressure wave, that knocked down over a thousand trees.

Wow.

Not even the impact itself, but the air pressure.

Just the air pressure wave.

It shows the immense forces involved.

Okay, so that's fall.

What about slide?

A slide is different.

Here, a block or mass of material moves downslope along a distinct surface or zone of weakness.

It's not free -falling.

It can be a rotational slide, where the movement is along a curved, concave surface, kind of like the shape of a spoon.

The top of the sliding block often tilts backward.

We also call these slumps.

Okay, like it scoops out.

Right.

Or it can be a translational slide, where the mass moves along a relatively flat, planar surface.

Maybe a fault line, or a join, or a tilted bedding plane in the rock.

There's less rotation here.

It just slides down the ramp, essentially.

Got it.

Rotational is curved.

Translational is flat.

And the third type,

flow.

Right, flow.

This is when the material moves downslope like a thick, viscous fluid.

It's often saturated with water, and it forms these characteristic lobe or tongue shapes as it moves.

Like a really thick, muddy river moving across the land.

That's a good way to picture it, yeah.

But not necessarily confined to a channel.

Okay, fall, slide, flow.

And the third classification factor was speed.

Velocity, yes.

And this varies hugely.

It can be incredibly fast, like those rock avalanches we mentioned, moving over 200 km per hour, faster than a speeding train.

Wow.

Or it can be almost imperceptibly slow,

measured in just millimeters or centimeters per year.

Like creep.

Exactly like creep.

So that range in velocity is critical for understanding the hazard.

Obviously fast is usually more immediately dangerous, but slow can cause immense long -term damage.

That fast movement, the rock avalanche is moving hundreds of kilometers per hour.

How is that even possible?

You'd think friction would slow down that much rock much faster.

That's a fantastic question, and it puzzled geologists for a long time.

How can these enormous masses of rock travel so far, so fast, sometimes even over relatively gentle slopes near the end?

Yeah, that seems to defy physics.

It does.

The leading hypothesis involves air.

The idea is that, as the massive amount of rock falls and breaks apart, it traps and compresses air underneath it.

This trapped, high -pressure air acts like a cushion, significantly reducing friction between the moving debris and the ground surface below.

So it's literally floating on a cushion of air, like a hovercraft.

Kind of like a giant, natural, chaotic hovercraft, yes.

This air -layer lubrication allows them to maintain momentum and travel incredible distances with terrifying speed.

Fascinating.

Okay, let's maybe look closer at some specific types, starting with the faster ones.

You mentioned slumps earlier.

Right, a slump.

That's the classic rotational slide.

A coherent block of material slides down along that curved, spoon -shaped surface.

You often see a steep, curved cliff at the top that's the scarp where it pulled away, and the slumped block itself might be tilted backward.

Where do these typically happen?

They're very common where slopes have been over -steepened.

Maybe a river cuts into the base of a valley wall, or waves erode the bottom of a coastal cliff.

Point Furman in California is a famous example of coastal slumping due to wave action.

Okay, and rockslides.

You said they were translational.

Yes, rockslide.

This involves blocks of actual bedrock breaking loose and sliding down a relatively flat or planar surface, like a tilted layer of rock, or joint plane.

And these are fast and dangerous.

Among the fastest and most destructive, yes.

Often triggered when water lubricates that failure surface, or by earthquake shaking.

The 1959 Madison Canyon slide in Montana was a massive rockslide triggered by an earthquake, dammed a river instantly.

You also mentioned the Gros Ventre slide earlier as a deep -dive example.

Can you recap that?

It sounds like a perfect storm scenario.

It really was.

Gros Ventre, near Kelly, Wyoming in 1925.

What geologists found was a setup just waiting to fail.

You had layers of sandstone tilted downhill, sitting on top of a weak, thin layer of clay.

Uh -oh.

Sandstone on wet clay sounds bad.

It is.

The Gros Ventre River had been slowly carving away at the base of the slope, removing support from the sandstone.

Then came heavy spring rains and snowmelt.

That water saturated the clay layer underneath, making it incredibly slick.

Basically, zero friction.

So the sandstone lost its footing.

Completely.

Eventually, it just couldn't hold on anymore.

A huge mass estimates are around 38 million cubic meters let go and crashed down into the valley.

It instantly created this huge dam about 70 meters high, across the river.

Blocking the river entirely.

Yes, forming Slide Lake behind it.

And then, inevitably, about two years later, that natural dam failed.

The lake overflowed catastrophically and caused devastating floods downstream in Kelly.

So the insight there is, sometimes it's not if, but when, the geology had preset the disaster.

Precisely.

It was an inevitable geological event just waiting for the right conditions, the right trigger.

A sobering lesson in reading the landscape.

Definitely.

Okay, moving on.

Debris flows.

You describe them as flowing tongues.

Yes, a debris flow.

Think of it as a dense, slurry -like mixture of mud, soil, rock fragments, sometimes big boulders and water all flowing downhill together.

If it's mostly fine -grained sand and silt, we might call it a mudflow.

Exactly.

Mudflow is often used for finer material.

Debris flows are especially common and hazardous in mountainous areas, particularly semi -arid ones.

They tend to follow existing canyons and stream channels, but they can spill out.

They can move incredibly fast and carry immense debris cars, trees, even houses.

And they build alluvial fans, right?

Those cone -shaped deposits at the mouth of canyons.

They do.

Which is ironic, because those fans look like attractive, flat places to build, but they are literally built by past debris flows, indicating a recurring hazard.

People often build there unaware of the potential danger lurking upslope.

That leads to Lahars, the volcanic version.

Right, Lahars.

A specific and terrifying type of debris flow associated with volcanoes.

The name comes from Indonesia.

They're composed of volcanic ash and debris mixed with water.

Where does the water come from on a volcano?

Several sources.

During an eruption, hot ash and gas can melt snow and ice on the volcano slopes very rapidly, or heavy rain can fall on loose ash deposits on the slopes even long after an eruption,

or crater lakes can be breached.

And these are particularly destructive.

Exceptionally so.

Mount St.

Helens in 1980 produced several Lahars that surged down valleys, destroying bridges, roads, over 200 homes.

But the 1985 Nevado del Riz eruption in Colombia?

That was truly catastrophic.

Lahars, generated by melting snow and ice, raced down river valleys.

They completely buried the town of Armero, which was built on older Lahar deposits.

Over 25 ,000 people died.

25 ,000.

Just underscores the immense power and reach of these volcanic hazards, even miles away from the volcano itself.

It really does.

It's a stark reminder of the connection between volcanic activity and mass wasting.

Okay, one more relatively rapid type you mentioned.

Earthflows.

How are they different from debris flows?

Earthflows are generally slower and less water saturated than debris flows.

You typically find them on hillsides, often in humid regions, especially during or after heavy rainfall.

They're usually rich in clay and silt.

They tend to form a distinct tongue or teardrop shape, with a cigarp at the head.

So slower, stickier, maybe.

Slower, stickier, more viscous is a good way to think of it.

Movement might be millimeters to maybe several meters per day.

They often happen in association with slumps.

You might see an earthflow forming at the toe, the bottom end of a larger slump.

Okay, so those are the faster, more dramatic events.

But you said a lot of mass wasting is actually slow, almost invisible.

That's right.

While the big, rapid events grab headlines, much more land surface is affected by slow, gradual movement.

It's less spectacular, but cumulatively, it moves a vast amount of material and causes significant property damage over time.

And the main type here is creep.

Creep is the big one, yes.

The super slow, gradual, downhill movement of soil and regolith.

You literally cannot see it happening in real time.

But you can see its effects.

Definitely.

You see tilted fences, utility poles leaning downhill, retaining walls bulging outwards, maybe even foundations cracking.

And those curved tree trunks you sometimes see on hillsides.

That's classic evidence of creep.

The tree tries to grow vertically, but the ground beneath it is slowly moving downhill, so the base gets tilted and the trunk curves upwards.

Old gravestones leaning downhill in cemeteries.

That's often creep too.

What causes this super slow movement?

It's often driven by cycles of expansion and contraction of the surface material.

Think freeze -thaw cycles in colder climates.

Water freezes in the soil, expands, lifting particles perpendicular to the slope.

Then it thaws and gravity pulls those particles straight down, resulting in a tiny net movement downslope.

Like a microscopic ratchet mechanism.

Exactly.

Wetting and drying cycles can do something similar.

Clay minerals expand when wet, contract when dry.

Over many cycles, it adds up to slow, persistent downhill movement.

It's relentless.

And then there's solid flexion, sounds related to soil flow.

It literally means soil flow.

Solid flexion is the slow downslope movement of water saturated soil.

It happens in environments where water can't easily grain downwards.

Like if there's solid rock or dense clay underneath.

Yes, or perhaps most significantly, in areas with permafrost.

Okay, let's talk about permafrost.

That seems like a really unique environment for this.

It absolutely is.

Permafrost is ground that stays frozen for two or more consecutive years, common in the Arctic and high altitude regions.

Above the permafrost layer, there's usually an active layer at the surface that thaws during the summer.

Right, the top layer melts.

But the water from that thawed layer can't drain down because the permafrost below is frozen solid, impermeable.

So the active layer becomes completely saturated with water.

Even on very gentle slopes, just two or three degrees, this saturated layer will slowly flow downhill.

That's solid flexion in a permafrost environment.

It creates distinctive landscape features, like lobes and sheets of slowly moving soil.

And human activity makes a big difference here.

A huge difference.

Permafrost environments are incredibly sensitive.

If we disturb the surface, say, remove the insulating layer of vegetation, or build roads, or construct heated buildings directly on the ground.

We thaw the permafrost.

Exactly.

We melt the ice within the frozen ground.

Thawing permafrost loses its strength, becomes unstable.

It can lead to the ground sliding, slumping, subsiding unevenly.

It causes massive damage to buildings, roads, pipelines.

There's that example from Alaska.

Yes, a building near Fairbanks.

The heated side of the building caused the permafrost underneath to thaw significantly, leading to major subsidence and tilting on that side.

The unheated porch side, over still frozen ground, remains stable.

A really clear illustration.

So how do they build in these areas then?

Often, structures have to be built on piles or stilts.

This raises the building off the ground, allowing cold air to circulate underneath and keep the permafrost frozen and stable.

It's a critical engineering adaptation for living and working in these regions, showing how understanding these processes directly impacts how we interact with the environment.

Okay, so wrapping this up, what are the main takeaways here?

We've covered a lot of ground, literally.

We have.

I think the main thing is understanding that mass wasting gravity's work is this constant powerful force shaping our planet.

It ranges from the immediate terrifying devastation of things like landslides, rockslides, lahars, debris flows.

To the slow, almost invisible, but relentless reshaping by creep and soliflexion.

Exactly.

And we've seen how critical those triggers are, things like water saturation, making slopes too steep, removing ventation, or earthquake shaking.

They're the catalysts that turn potential energy into kinetic energy, often with damaging results.

Understanding the different types fall, slide, flow, and the materials involved helps us classify and hopefully anticipate these events.

Absolutely.

And connecting it all back, mass wasting isn't just some abstract geological concept.

It's fundamental to living on this dynamic earth.

Understanding these processes is absolutely vital for identifying potential hazards, for planning where and how we build our communities and infrastructure,

essentially for living more safely and sustainably on this constantly changing surface.

So a final thought for our listeners to maybe chew on.

Maybe consider this.

As our climate changes altering rainfall patterns, melting glaciers, and permafrost, and as human populations continue to grow, often expanding into more challenging terrains, how is our relationship with these powerful gravitational forces likely to evolve?

What new challenges might arise, and what responsibilities do we have collectively and individually to understand these risks and mitigate their impacts?

It's an ongoing conversation between humanity and the geology right beneath our feet.

A really important question to keep in mind.

Thanks for breaking down this powerful force with us today.

My pleasure.

It's a fascinating and crucial part of how our planet works.