physical geography: chemical weathering

Weathering is this crazy concept that makes things change. We already went over some basics of physical weathering, now let’s review chemical weathering, which involves compositional changes or dissolution. Thus, the pH is the crucial factor here. Simplified, pH is the concentration of H+ ions. Most of what is going to be weathered chemically are metal cations, Silicates, Carbonates, etc. Chemical weathering primarily happens in the following three processes:

  • Acid Attacks
  • Oxidation
  • Chelation

Acid Attacks

Acid attacks involve water and acidity to act upon the soon-to-be weathered material (eg rock). Foremost, acidity is mostly generated via biota. Plants and animals release a lot of CO2 into the ground. This happens because plants breakdown animals as they decompose, which releases a lot of CO2. The climate connection here is that with greater tropical climates and flora/fauna, the greater the soil’s acidity. There are a few types of acid attacks. Solution is essentially dissolving (think salt into water, sugar into coffee, etc). This is mostly important for carbonate rocks, as their solubilities change with pH. Hydrolysis is the replacement of H+ ions with other metal cations. Cations have a positive charge, making them easy to substitute H+ ions. However, this greatly weakens rocks structure. It turns silicates (granite, basalts, etc) into clays. As a byproduct, it also releases OH. Carbonation mixes water with carbon dioxide to make carbonic acid. Fundamentally, that is the release of metal cations and HCO3- (bicarbonate). It converts silicates to clays and also dissolves the carbonate. This process is important in the formation of caves.

Oxidation

This occurs via dissolved oxygen in water. This reaction with oxygen causes the cation to give up an electron to oxygen. In example, Fe+2 –> Fe+3 as it bonds with oxygen. The product of this is that oxide minerals are formed. A common oxidation is rusting. For rocks, clays are also produces with oxides.

Oxidation potential is measured in Eh. Eh = log{oxidants/reductants}. Simplified, this is a measure of the prevalence of O2.

Chelation

Organic chemistry allows a large and complex range of decomposing reactions to take place. These chelation agents are organic compounds that do the job. They form ring-like structures around the metal ion in order to make it soluble. These agents come from leaf litter and other organic compounds.This make the metal ions soluble. The metal ions are carried away from the upper soil.

physical geography: physical weathering

Physical weathering is the mechanical breakdown of a material (broadly rock in this example) that fractures the material for natural pulverisation. Physical weathering makes a good catalyst for chemical weathering as well. There are a number of ways that material can be weathered in this way.

  • Ice Wedging
  • Ice Lens Growth
  • Expansion & Contraction
  • Exfoliation
  • Biological Activity

Ice Wedging

Imagine you have a rock that has a few cracks in it, but it’s still solidly your rock! But you left your rock out in the cold, cold winter. Rain dropped onto your rock and found itself deep in the cracks. As the night progressed and the temperature dropped, the ice froze. Ice takes up more volume that water, so it pushed outwards against the cracks’ walls. Although this function isn’t initially catastrophic to the rock, it further weakens the material and increases its susceptibility to other weathering events.

Ice Lens Growth

Let’s say underneath your lawn there’s ice. This ice is surrounded by low pressure water because the ice pieces and rock are repulsive of each other. This low water film attracts more water, which grow the ice shards. This outwards pushing fractures your lawn. You should probably consider moving if this process continues!

Expansion & Contraction

This is a daily process, but it’s also intensified with large fires and snow storms. This is a volume change in the material. This process is dependant on wetting , drying and temperature.

Exfoliation

So you have a rock that’s buried deep below the regolith, bedrock, etc. On all sides of the rock, there is a strong pressure that densifies this by pushing it inwards strongly. With a long series of erosion, the regolith and bedrock have disappeared, but now that rock is beginning to surface. However, the intense central push inward is no longer being applied to the top, so it expands upwards. This, in turn, fractures rock.

Biological Activity

Things like animal burrows and plant roots cause hillslope hollowing. This loosens the regolith mantle.

Next will be chemical weathering.

 

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!