physical geography: “landslides”

So I’m deeply interested in what we call “natural” “disasters.” I could spend a whole blog post explaining why I quoted these words (in fact, that’s going to be an entire thesis of mine! Stay tuned!). But let me just explain a few things about “landslides.”

Landslides is a very general term, but has a much deeper meaning in the world of geomorphology (tldr, basically how the earth’s landforms work). We have a few different types of ways that land material fails, but which are all umbrella-termed “landslides.” All of these processes are different in their mechanisms, processes and impacts. They all share basic characteristics that when stress exceeds the strength of the regolith or bedrock underneath, something has go to give.

  • Debris Flows
  • Rockfall
  • Rockslides
  • Slumps
  • Earth Flows
  • Sapping/Undercutting

Debris Flows

Think of a giant mess of mud, rock, vegetation traveling at a great distance at a high speed– or don’t because that’s kinda terrifying, right? Now that kinda sounds like some landslide-avalanche hybrid. Honestly, that’s not a bad way to think about it. Debris flows happen on steep rocky terrain. They happen because a channel was incised too much, so it left deposits that eventually turned depression. Debris flows can turn into debris flow fans– incredible. These are essentially debris flows coming out in different directions but from the same source.

Rockfall

These are triggered by earthquakes, snow melt, even mountain goats. These mechanisms dislodge rocks, which when falling, dislodge more rocks. The damage is determined by the source of stress and the size of the material dislodged. An example in the US is McGee Pass.

Rockslides

This occurs when bedrock fails on a surface and forms a tongue-deposit. Some examples on regolith-mantled slope include Mt. Cook and Mt. Rainier (Emmon’s Glacier). But on a planar slope with bedding and jointing at an angle, the Himalayas also experience this.

Slumps

Imagine a mass amount of land just slumping downwards– but like in a hundred years. Stronger rock masses maintain steeper slopes, and weaker bedrock masses tend to be strongly jointed and bedded and have weathered voids. Slump’s stresses and transport and proportional to the slope.

Earthflows

These are slow process that can take a century or so long.  They typically happen on marsh lands or ponds. They leave a very rumpled topography. Mudflows are much faster versions of this.

Sapping/Undercutting

In an odd sense, this one is my favourite. Imagine you have a cake with that thick matte frosting on the top and then this softer, moist cake below it. There’s only a few people in front of you in line for this beautiful cake. Everything is fine, you’re patient. But then the guy right before you decides he doesn’t want any frosting and only cuts out the cake under the matte frosting. You quiver as you watch the heavy, strong frosting fall because what previously supported it is now gone. Going back to geomorphology, you now have a hill. The bottom part is of a weaker rock material and is eroded away faster than the overlying material. Just like your cake, once the supporting bottom material is gone, the top falls over.

 

All of my posts are going to be physical geography/geomorphology based until after my exam! But I’ll try to relate the concepts with mapping!

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