There's something terrifyingly strange about a landslide, of any size. The solid ground moves under your feet, and geographical features are displaced. But the larger they get, the weirder landslides become. When they get big enough, they start acting like fluids. Here's why really big landslides are completely bizarre.

Top image: Landslide warning sign in Victoria, Australia buried in sand and surrounded by rubble. Credit: Mika McKinnon

Small landslides can still be deadly — but really large landslides are almost impossible to stabilize. Once a landslide hits around a half a million cubic meters in volume, not only is it large enough to bury an entire town and its inhabitants, but it also fluidizes, becoming unnervingly mobile and difficult to predict.

You can use physics to predict how far a smaller landslide will move across the landscape: it's basically a block sliding down a ramp.

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Once you approximate the mass of the landslide's "failure area" and the angle of the friction between the landslide and the runout path, a simple calculation lets you predict the fahrböschung angle — the angle between the very top of the crown, where the landslide starts, and the tip of its toe, where the mass will come to a rest.

The runout distance of a small landslide can be predicted by simple sliding-block physics. Image credit: Mika McKinnon

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A damaged landslide warning sign on the Canary Islands. Image credit: vosta

But when it comes to predicting the course of a landslide that's over a half a million cubic meters in volume, good luck! Instead of sliding like somewhat-tidy intact blocks scooting down a slope, the landslides start acting like a fluid flowing out along the landscape. These fluid-like flows consistently run farther than the predicted fahrböschung angles, forcing disaster researchers to develop more complicated models in an effort to make useful predictions. But it's incredibly tough to build useful models of these landslides unless we understand what causes them to misbehave. And that's a whole problem in itself.

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Since gentleman-geologist Albert Heim first noticing these unnerving massive landslides in the European Alps in 1881, many people have proposed (and sometimes discredited) mechanisms to account for the unusually long runout of large landslides. But none of these theories has been widely accepted. The assorted theories fit within four broad categories: 1) ideas that reduce the internal friction of a landslide; 2) theories that reduce the basal friction between the landslide and the ground it is running out over; 3) location-specific traits, where the landscape itself enhances landform runout; or 4) a physical consequence of the process whereby a large landslide bulks up, to be even more massive as it flows.

A debris flow on Vancouver Island jumbles sediment, rock, wood, water, and even a truck in a complex, difficult-to-model mess. Image credit: Dru!

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How to Reduce The Internal Friction of a Landslide

Landslides are messy, heterogenous unsorted chaos of rock, mud, silt, vegetation, water — and anything else unlucky enough to be entrained in the flow. All of this stuff jostles together, creating internal friction, and reducing the fluidity of the landslide. Something may be reducing that internal friction, allowing the landslide to become more fluid-like and cover a greater distance. But what could cause the friction inside a huge landslide to be reduced?

Here are a few possible culprits:

Theoretically, under high shear conditions like a landslide, the internal friction of the material could spontaneously reduce — but that hasn't been observed in experiments which makes it somewhat questionable as a major mechanism behind increased landslide runout. But as strange as it sounds, that lack of lab observation doesn't necessarily invalidate the theory: the physics of landslides doesn't tractably scale down to lab-sized experiments, and idealized landslides of sand, glass beads, or ping-pong balls don't necessarily reflect the complex interactions of real geological materials.

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Vibration within the landslide could reduce internal friction, if only we could come up a continuous source of energy or a mechanism that could perpetuate the shaking once it got started. Even if we could figure out a vibration source to rattle a landslide into fluidity, one problem remains: why wouldn't it impact smaller landslides?

The landslide could also be incorporating saturated material into the flow, lubricating the flow by literally having a higher water content. For dry landslides, rock dust ground by internal collision of rock fragments during the landslide's flow could act as a lubricant, or with enough of it, could even produce buoyancy within the flow.

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Powdered rock gouge at the Hope Slide in British Columbia. Image credit: Mika McKinnon

Unlike the other theories for reducing internal friction, this one doesn't have any outstanding counter-arguments, leaving some form of lubrication as a plausible mechanism that could drop internal friction and consequently increase fluidity and runout distance for large landslides.

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A landslide on Mars undercuts the theory that large landslides gain mobility by floating on a cushion of air. Image credit: NASA/JPL-Caltech/University of Arizona

How to Reduce Basal Friction Between a Landslide and its Runout Path

A landslide doesn't flow smoothly over a landscape. It chews up terrain — breaking trees, scouring loose sediment, and entraining material by incorporating it into the original failure mass. All of this costs energy, which is lost to friction between the landslide and the runout path. If something reduced this basal friction, the landslide could run out more smoothly, travelling farther before depositing. The physical processes for how to decrease the friction between a chaotic landslide and rough, real-life terrain are borderline-fanciful with one exception.

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Some of the earliest theories for reducing basal friction were concepts that something could cushion the landslide. After dreaming of ways that a landslide could trap air or pressurized water vapor and create a "cushion" to reduce friction, researchers spotted long-runout landslides on Mars and the moon. Since whatever mechanism allows large landslides to be so outrageously mobile still works with little or no atmosphere, that put an end to the "air cushion" theories.

But there's another possibility — internal sorting while the landslide road out across the landslide could produce a thin layer of "ball bearings" to lubricate the flow. Unfortunately, the effect would be inversely proportional to size as larger volumes would crush ball bearings, so it doesn't work as a theory for why larger landslides run out disproportionally farther than smaller landslides.

On a more plausible note, just as rock dust and water could lubricate within a flow, they could also lubricate between the flow and the landscape. The landslide could squeeze out a layer of water through undrained loading, or the heat of friction flash-heating ice and snow to melt it into water. In dry regions, the landslide could grind rock into a thin powdery layer akin to the gouge observed in active faults, or for a truly epic landslide of gargantuan proportions, could even melt the rock into frictionite, a smooth geological glass.

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A Rockslide near Point Reyes National Seashore. Image credit: AP Photo/National Parks Service

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How Local Landscapes Could Increase Runout Distance

Just like everything else involving real estate, when it comes to really large landslides, location matters.

Part of the difficulty in understanding extremely large landslides comes from the fact that they just don't happen that often. But by looking at scaled-down laboratory experiments and smaller landslides, we're starting to see some patterns that show how the specific details of a location — especially the geological materials and the shape of the landscape — will impact an individual landslide's behaviour.

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Not all geological materials are created equal. Continuing on the theme of reducing basal friction, certain types of material are smoother to flow over than others. A landslide running out over limestone or evaporite rocks like gypsum would likely be more mobile, while glaciers would are downright slippery surface for a landslide.

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Both the slippery glacial ice and the canyon containing flow probably enhanced the runout of this landslide in Alaska. Image credit: NASA

Not all landforms are created equal, either. Some shapes like valleys channelize the flow, so the landslide's kinetic energy is confined to one direction, allowing the landslide to expend its energy in running out farther instead of spreading out as it flows. While this certainly has an impact on landslides — researchers have found clear correlation with confined landslides running farther than similar-sized unconfined landslides — it doesn't explain long-runout landslides in other terrains.

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Landslide at Cascade Head on the Oregon coast. Image credit: Alex Derr

The Farther A Landslide Flows, The Bigger It Gets

The larger a landslide is, the farther it will flow out. But landslides also grow larger as they flow.

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When the source area destabilizes and breaks free to flow as a landslide, the failed mass breaks into fragments. This fragmentation can cause the volume of a landslide to bulk up from 18% to 35% larger than the original volume.

The Mount Meager landslide deposit in British Columbia spreads out over a far larger area than you'd guess from the volume of the failed rupture area. Image credit: Mika McKinnon

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Since larger landslides run out farther, that bulking increases the runout distance. During fragmentation, intact rock elastically deforms and then finally breaks. This may release energy, imparting it to the flow and increasing runout distance.

Landslides can also entrain additional material as they flow, scooping up more material and incorporating it into the flow to grow even larger.

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A Landslide along the coast near Nan Diamant, Haiti. Image credit: USGS

While we don't have a firm answer on what makes large landslides so catastrophically mobile, we have good reason to keep trying to find out. If we can understand why these landslides are so fluid and run out so far, we will hopefully be better able to predict their runout distance before they they happen.

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If we can do that, then once we identify an unstable slope that's too big to mitigate, we can at least make sure not to plop a hospital or school in the danger zone where the landslide could smush it.

This article substantially draws from Chapter 2 of my thesis, Landslide runout: statistical analysis of physical characteristics and model parameters. For more on this topic, I recommend The mobility of long-runout landslides and The Hypermobility of Rock Avalanches.