2.6 - Weathering Process Activity

Determining the process of weathering for a given rock under a set environment.    

ACTIVITY

Choose a rock type (see Lesson  1, sections 1.21.3, and 1.4) and an environment in which it exists. (Hint:  You may want to choose a type of rock that appears where you live.)

Use the Annual Temperatures World Map and the Annual Rainfall World Map to estimate temperature and rainfall for your chosen rock.

Then show the different weathering processes as they lead to rock degradation (refer to Figure 10, below).

 

Example: What happens to limestone in Brazil?

Limestone, a sedimentary rock, existing in northwest Brazil.  

Annual rainfall world map

Annual temperature world map

Figure 9. Calcite.  (Image courtesy of USGS)

  • Limestone is primarily composed of calcite, with some quartz.
  • Expansion due to temperature will be minimal, since the temperature in this region does not fluctuate.
  • Some abrasion by water may occur, due to the intensity of rainfall (physical weathering).
  • The high rainfall, or precipitation, combined with carbon dioxide forms carbonic acid and carbonation begins.

H2O + CO2 = H2CO3

  • The carbonation process dissolves calcite and releases calcium.

CaCO3 + H2CO3 = Ca2+ + 2HCO3-

  • The calcium is removed by water carrying it away (leaching).
  • The process is accelerated, since the temperature of the area is high and water is plentiful.
  • The quartz, if present in the limestone, will not break down because of its resistance to weathering.
  • With time, this disintegrated limestone will become the soil parent material.  Principles Lesson 3.6 - How Parent Material Affects Soil Profile Development

Figure 10.  The influence of the interaction of temperature and rainfall on processes of physical and chemical weathering. Notice that as annual rainfall and temperature increase, chemical weathering dominates over physical weathering. On the contrary, notice that as the temperature lowers, physical weathering begins to dominate over chemical weathering.  Image courtesy of UNL, 2005.

Now, run through a scenario where this limestone rock exists in Barrow, Alaska (Tundra Climate, see maps at links below). How will weathering proceed?

View Annual Average Rainfall Map

View Annual Average Temperature Map

Figure 11.  Annual precipitation and temperature in Barrow, Alaska.  Note:  The bars represent precipitation and the line represents temperature.

Image courtesy of  http://www.uwsp.edu/geo/faculty/ritter/ interactive_climate_map/climate_map.html

  • The area is characterized as subartic tundra, so physical weathering dominates.
  • Expansion due to temperature may be a factor where the surface of the rock is heated, then cooled down.
  • Physical weathering processes may be slow in disintegrating the limestone, because the degree of temperature fluctuation and frost action may not be adequate enough for rapid changes to occur.
  • Rock disintegration will take a significant period of time.
  • Once the limestone has disintegrated, soil formation will begin, but at a very slow rate. Soils developed in this cold climate are typically classified as Gelisols.

Now, run through a scenario where this limestone rock exists in Duluth, Minnesota (Humid Continental Climate, see maps at links below). How will weathering proceed?

View Annual Average Rainfall Map

View Annual Average Temperature Map

 

Figure 12.  Annual precipitation and temperature in Duluth, Minnesota.  Note:  The bars represent precipitation and the line represents temperature. Image courtesy of http://www.uwsp.edu/geo/faculty/ritter/interactive_climate_map/climate_map.html  

  • The area is characterized as humid continental so all three processes can be important in disintegrating the limestone. Note: The area receives an ample amount of moisture.
  • Frost action could also be a factor where freezing and thawing weakens the limestone.
  • During the summer months (June, July, August), the warm temperatures and ample moisture will enhance chemical weathering.
  • During the summer months, temperature fluctuation can also be a factor. Day time temperatures can heat and expand the limestone, and cool night temperatures can shrink and contract the limestone (physical weathering).
  • During the spring and fall, there is still enough moisture for physical weathering, but the moisture that seeps into the limestone fractures can freeze during the cool night temperatures (frost action - physical weathering).
  • Summer months also enhance the growth of lichens on the limestone. The lichens may possess acidic properties which dissolves the limestone and indirectly enhances chemical and physical weathering.
  • Large tree roots growing in the area can also exert pressure on limestone.
  • The combination of all 3 weathering processes subsequently disintegrates the limestone, and soil genesis begins.