Plate Tectonics (building things up) - driven by heat generated inside the earth trying to escape to the surface

The Atmosphere & Gravity (tearing things down) - the atmosphere is driven by solar energy

In this unit, we will concentrate on the forces tearing down things.  We start with weathering!

Weathering: The disintegration and decomposition of rock at or near the surface of the earth (breaking up rocks). 

• Mechanical Weathering (using direct force)
  • Frost Wedging (also known as frost action)
    • This is the most important mechanical weathering process of them all!
    • When water freezes it expands 9% in volume due to the unique way water molecules line up when crystallizing (forming a hexagon)
    • It is most active when the temperature frequently crosses 32 degrees (late fall to early spring in Wisconsin, also in mountains)
    • examples are potholes, cracks in sidewalks, rocks found on mountain roads etc.
  

• Internal Stress
  • Unloading - removal of overlying layers make the rocks below expand.  This is what happens when you get up from the couch ... the padding expands and returns to the original shape.
  • Exfoliation - chemical changes produce volume changes.  This tends to make rock peel away in layers in a process geologist call sheeting.
  • Together these two effects can make rock peel away like an onion as in Half Dome (Yosemite National Park)

  
  Half Dome

  

  This is an example of an Exfoliation Dome (Yosemite).  A granite chamber formed deep underground.  As overlying layers were removed, the rock weakened in layers which follow the domes present shape.  This dome is constantly being "peeled" off like an onion (sheeting).  Note the cool dude enjoying all this beauty.
   

  • Thermal Expansion - sudden heating of rock (forest fires).  The rock cracks due to internal stress (like dropping an ice cube in water).
     
  • Crystallization of Salts -  dissolved salts expand as they dry and crystallize in small cracks.
     
• Life - Root wedging, burrowing animals, man.  Also bacteria and decaying plants produce acids which chemically destroy rock.
   
• External Forces (Abrasion, shock, external stress)
  • Glaciers
  • Waves - abrasion by sand motion & "punch" of waves against rock (up to 6000 pounds/square foot)
  • Sandstorms - Arches in Utah
       

    

  Ask me about a great field trip offered by Dr. George Stone and Dr. Bob Helwig at MATC.

   

  • Faults (Tectonic Activity - plates pushing together, pulling apart, or rubbing across each other)
  • Streams - the force of running water & the sediments it "drags" with it does a VERY efficient job of weathering rock.  One example are the "pits" created by swirling sand as water runs over a granite block in Johnson Shut-In State Park, MO.
  • Earthquakes - shock wave weakens adjacent rocks (after shocks)
  • Pressure of moving magma as it rises and tries to escape to the surface
  • Collisions - falling rock hitting other rock (rock slides) & meteor impacts
         

    Rock Slide in Yosemite (July 10, 1996)

    Meteor Crater AZ

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• Chemical Weathering (dissolving rock with chemicals)
  • Water is the key!  Therefore, chemical weathering is most active in warm, moist climates.
     
    • Carbonic Acid - Carbon dioxide (CO₂ ) dissolved in water forms H₂CO₃ (the same stuff you find in most soda).  This produces a weak acid "rain" which attacks rock as it percolates through the ground.
       
      • To a lesser extent, sulfur emissions from the burning of fossil fuels also produce acids (sulfurous & sulfuric acids) which can effect areas far from the source of their emission.
         
      • This attacks and dissolves limestone and marble (Calcite CaCO3) creating caverns and destroying monuments - such as Cleopatra's Needle (when it was moved to NY)
         
      • This acid attacks the potassium feldspar which is a common mineral in granite.  The result is a fine clay mineral (Al₂Si₂O₅(OH)₄).  This gets transported easily in water and deposited as sediment.  The resulting rock forms shale.
         
      • The other by-product of this chemical weathering is bicarbonate  ... a dissolved form of carbon which will become important in the next unit when we discuss the carbon cycle.
         
      • The quartz in the granite is less resistant to chemical weathering and is freed as sand.  It, too, gets washed to the sea forming beaches, dunes, and perhaps sandstone.
         
    • Organic chemical weathering - acid can also be formed by organisms such as bacteria, lichens (see yellow patches in the image below), mosses and decaying plants.  This can be the first stage of breaking up a rock because these tiny organisms create the first tiny cracks in a rock.  Once formed, frost wedging can take over and make the cracks bigger.

      
       

      Here are some other ways rocks may be chemically weathered (just in case you were interested)! Please just skim over the list below. (until you get to the horizontal line)
       

    • Oxidation - reaction with atmospheric oxygen to form rust.  Mars looks red because most of the oxygen in its early atmosphere reacted with the surface rocks.
    • Hydration - Hydration is the combination of a solid mineral or element with water. When the water molecules are chemically bonded to the mineral, the size of the chemical structure is increased, thereby making a softer, more stressed, and more easily decomposed mineral.  Sub-surface hydration probably produced England's granite Tors.
    • Solution - water dissolves rock salt and to a lesser degree, other minerals.
    • Hydrolysis - Hydrolysis is the process of minerals reacting with water to form hydroxides, which usually are more soluble than the original mineral. Hydrolysis is one of the most important weathering processes causing soil profile changes.
    • Reduction - Reduction is the chemical process in which electrons are gained. In soils, reduction usually takes place when oxygen is scarce, as in stagnant water conditions. Reduction in minerals may result in electrically unstable compounds, more soluble ones, or more internally stressed ones, which eventually decompose more rapidly.
       

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  • Rates of chemical weathering vary greatly from place to place, depending on:
    • Types of chemical agents and minerals which are available
    • Climate (warm, moist vs. cold, arid)
    • Surface Area (the smaller the better)
      • Because edges and corners of a rock have a higher surface area/volume ratio. Chemical weathering is most efficient there.  This means a rock with sharp edges and corners soon will weather to a round flat surface.  This is known as spheroidal weathering.  Examples are most rocks you find on a beach or the Elephant Rocks in MO.

      
      
      Elephant Rocks State Park, MO  (Check out the cool dude in the blue shorts)
      

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      This is a mesa (Spanish for table) ... formed because a resistant layer of sandstone acts like a "cap" protecting the weaker rock below.

      
      This is Bryce Canyon (Utah)... very pretty!  This rock is very easily weathered and eroded (as much as several feet per year) ... so get there before it's gone!

Some General Ideas about Weathering

• All rock types are subject to mechanical weathering once they are exposed to the earth's surface but the degree can vary depending on the hardness of the rock.
   
• There is a much greater variation in the amount of chemical weather of rocks.  Some rocks are highly resistant to chemical action - quartz for example.  Other rock types (basalt) are highly susceptible to chemical action.
   
• Both actions (mechanical and chemical) will work together to break up a rock.  For example, once lichen create small pores in a rock (chemical), frost action can enlarge the crack.  Once the rock is broken into smaller pieces, the increase in surface area allows a dramatic increase in chemical attack.

©Jim Mihal 2004, 2006 - all rights reserved