WEATHERING AND EROSION

Introduction
Mechanical Weathering
Chemical Weathering
The Effects of Weathering
Soils
Erosion
Other Links That You Can Go To
Back To Intro of Geoscience
 
 
 
 
 
 
 
 
 
 
 
 
 

INTRODUCTION

           Weathering and erosion is the breakdown of rock due to exposure to the atmosphere and due to the differences in conditions from where rock had formed and surfaced. An example is granite. It is formed deep and in it is hornblende, biotite, quartz, and feldspar. The feldspar starts to weather and then it breaks down into kaolinite. As it breaks down it is moved by wind, water, and gravity. As it moves, it wears down other material (s). There are 2 types of weathering and erosion - mechanical and chemical.

MECHANICAL WEATHERING

           Mechanical is physical disintegrating. Smaller pieces are formed, cliffs break off into pebbles and the pebbles into sand. Only the size changes, not the chemical composition. Frost, wetting and drying, plants and animals, and roots are part of mechanical erosion. When water freezes, it has an unusual property. Most materials expand when heated and contract when cooled. This is true of water as well, except when it is cooled from 4*C to 0*C. At these temperatures, water expands. Water expands most when it solidifies into ice. WHen water becomes ice, it takes up 9%-10% more space. This exerts incredible pressure on rocks and then splits apart rocks. When water gets into tiny pores, it is called ice wedging or frost action. Ice wedging  occurs in porous rocks and in rocks with cracks. This causes pot holes. Wetting and drying  is very effective at breaking up clay. It swells and shrinks and then falls apart. PLants, such as lichens, mosses, and tiny roots, wedge their way into pores and crevices, and cracks. The seeds grow and expand, causing cracking. Animals, such as earthworms and small mammals, act as transportation for particles that get broken down. Humans cause much erosion. Abrasion is where rocks rub against each other. This occurs mostly in streams where fragments bounce off each other and round out like a drum creating gems. This also occurs in the stream bed as well. The wind is a form of abrasion. This can be compared to sand blasting. Exfoliation is the peeling off of sheets of rock. This usually occurs in rocks that contain feldspar. Clay is formed where water gets deep into the feldspar. The clay then has a greater volume and peels off in layers when it expands. Pressure unloading  is when pressure is removed and the granite expands and leads to joints, or sheet joining. This can also occur when glaciers recede. All this is examples of mechanical erosion.

CHEMICAL WEATHERING

       Rainwater, oxygen, carbon dioxide, and plant decay acids are all examples of chemical erosion. These are called agents.
       Hydration and Hydrolysis is a chemical reaction of water and other substances. Hydration is when water combines with another substance. An example is the hydration of anhydrite turning into gypsum. The formula is CaSO4 (anhydrite) + 2H2O (water) + CaSO4 times 2H2O (gypsum). Water can also form hydrogen  ions (H+) and (OH-). When these ions replace the minerals, the reaction is called Hydrolysis.  Minerals such as feldspar, hornblende, and augite combine with water to form clay.
        Oxidation is where the atmosphere is 21% oxygen. THe O2 combines with another substance. This is oxidation. It reacts with iron bearing minerals such as magnetite, pyrite, and mafic silicates such as hornblende, augite, and biotite yields rust. When the  oxygen combines with the iron, chemical bonds are broken down, weakening the structure.  Other elements such as Al, and Si, do the same thing. The red in the hematite is from the oxygen combining with the iron. Water can also produce a blackish color in geothite. If the geothite is dehydrated, it then forms hematite. The color can also indicate with lies underneath. This process can be compared to rusty nails vs. new ones.
      Carbonic acid is created when CO2 dissolves in water. Carbonic acid attacks feldspar, augite, biotite, and dissolves out of elements such as K, Na, Mg, and Ca. The compounds that form when the carbonic acid reacts with these elements are called carbonates.  Clay can also form. Carbonic acid also dissolves calcite and this hollows out caverns. An example is the Luary caverns. Acids are formed from plants and animals and are then dissolved by rainwater and carried underground. There, they attack minerals. Industry also plays a part. Gases are released by the industry and they mix with the atmosphere and water vapor and this creates acid rain. This is a serious problem since this alters the Ph of the lakes.

  THE EFFECTS OF WEATHERING

      Climate can affect weathering and erosion. The 2 factors include temperature and moisture. Warm climates favor chemical weathering while cold climates favor physical weathering.  The more moisture there is, the more weathering occurs. Chemical reactions occur at a faster rate when the temperatures increase. Water is needed for many of the acidic reactions that take place. The more hot and moist it is, the better it is for biological weathering. An example of this is Cleopatra's Needle in Central Park. (1880). It is granite obelisk and it stood unchanged for 3,000 years in Egypt's hot and dry climate.  100 years in NY, and the hieroglyphics are now almost unreadable and the surface is cracked, worn and discolored. Igneous and metamorphic rocks weather more rapidly in wet climates. Mechanical weathering often opens up cracks  and the water attacks minerals. First, particles break into smaller particles. That means a greater surface area. Once the rock is opened up, there is more chemical action. Limestones in the west often are on top of a ridge. They are resistant in dry climates since there are no dissolved acids. In the east. the limestone would be in a valley.
       The type of rock and the mineral composition can affect weathering. Rocks that are composed of minerals that react with acids, water or oxygen will weather more quickly than those that are composed of less active minerals. Limestone (calcite) will dissolve in mildly acidic rainwater, but granite (silicate), will not.  Mineral composition affects physical weathering. (abrasion) Harder rocks abrade less than softer rocks. Solid crystalline rocks will have fewer openings  and therefore fewer places where the water can penetrate. An example is quartz. It is dense and not susceptible to weathering  because there is so much sandstone.

SOILS

Soils are a result of weathering.  Soil is loose, weathered rock and organic materials. There are 3 basic kinds of soils.
       - Parent material - material that soil is formed from.
       - Residual soil - if the bedrock is the parent material.
       - Transported soils - when soil comes from other areas such as glacial action.
       There are 3 horizons (A,B,C) in soils. The A horizon is the topsoil and is dark in color because of the organic material. The organic material is formed from decayed plants and animals. The B horizon is a subsoil that contains clay and has soluble minerals that were washed down to it. The C horizon is made up of slightly weathered parent material and rock fragments.
       Soil profiles are when the soil is looked at in cross sections. When looking at soil profiles, there are mature and immature soils to look for. Mature soils have had enough time to develop distinct horizons. Immature soils have horizons that are lacking.
     There are different types of soils that grow in different climates.
          - tropical soil - this soil thrives in high temperatures and heavy rain. Most of this soil is infertile because the rains have washed away the nutrients.
          - grassland soils - this soil must have enough rain for heavy grass, but not enough for trees.
          - Forest soils - these soils grow in humid weather, cool seasons and forests of hardwood and evergreen. These soils have well developed horizons.
          - dessert soils - these soils are very dry, shallow, and they contain much calcium. They can be fertile when watered.
          - Arctic soils - the bottom layer is permafrost and is poorly drained.
     In the USA, there are 2 major types of soil. (Pedocals and Pedalfers) Pedocals grow in the West where the rainfall is less than 63 inches a year. This soil is rich in calcium. Pedalfers grow in the eastern USA, where the rain is over 63 inches a year. This soil is rich in Al and Fe, which are produced when water and oxygen react with common rock forming minerals and with soluble calcium compounds.

EROSION

          After the material is all weathered, it is usually moved by a process called erosion. Sediments are moved from one place to another. Waves that crash on the beach, streams, gravity, wind, and glaciers are all means of moving weathered material. There are many agents of erosion.
         Mass movements and mass wasting is the downhill movement under the direct influence of gravity. Gravity acts in components. Normal force perpendicular to the surface that the sediment is on, and the other force acts parallel to the surface, As the slope gets steeper, the more force there is pulling the parallel.
            The Angle of repose is the steepest angle  at which a particular remains stable. This depends on the size of the particles as well as the shape and density. Sand, gravel, and clay all have different angles of repose. Once the angle is exceeded, the sediments move downhill and mass wasting occurs.
        Talus slopes are called "fallen rocks zones" This is a clear example of gravity.
       Rock slides, or landslides, have a less steep slope which are often triggered by rains. As the rain decreases, the friction between the surfaces also decreases and chunks fall off.
       A slump is huge amounts that slide in a large piece along the plane of weakness. This is common where there are streams or ocean waves undercutting a bank. Mud flow is the rapid movement of water saturated mass of silt and clay.
       Gradual slopes include earth flow and creep. Earth flow is the shallow layer of earth, vegetation, and rock that gradually moves. This takes about several hours to move. Creep is a very, very, slow movement. You can notice fence posts leaning downhill after some years of being there. Water in the soil can lubricate and add to the movement.
        Erosion can also occur by the movement of water.
 
 

Other Links That You Can Go To
http://www2.nature.nps.gov/grd/usgsnps/misc/gweaero.html

http://vishnu.glg.nau.edu/people/jhw/GLG101/Weathering.html
Back To Intro of Geoscience