Sample Test Questions:
  Weathering and Parent Material

 

Question 1:
   A rock which was dark in color would contain which class of minerals and 
	would weather at what rate?...

a.  quartz, slow
b.  aluminosilicates,  fast
c.  ferromagnesian, fast



Question 2:
   Match the following parent materials and their characteristics:


Till			Deposit along side of glacier
Aeolian			unsorted, angular stones
Outwash			highly productive, silt soils along E. Miss.
Moraine			often alternating layers of silt/clay
Loess			sorted by meltwater
Esker			matrix of rounded stones, cobbles in a linear ridge
Kame Terrace		deep, sand deposits at river mouths
Delta			deposited by wind
Lacustrine		ridge at glacier terminus

answer 



Question 3:
   Arrange the following classes of primary and secondary minerals in the
order they would weather, from fastest to slowest.   (ferromagnesian, 
primary aluminosilicate, secondary aluminosilicate, carbonates, oxides,
evaporites, quartz).

Answer.

 

 


Question 4:
   Write a general equation describing the soil weathering process.    Explain how soil leaching and LeChatelier's principle affect the rate of soil weathering.

Answer




Question 5:

The following chemical weathering equation represents which type of reaction?...

        Fe2SiO4   +   2H2O   +   0.5O2      =    H4SIO4   +    2Fe2O3

a. Hydration
b. Oxidation
c. Carbonation
d. Acidification
e. Hydrolysis

 


Question 6:
   Granite is a course grained rock containing a mixture of quartz and light-colored minerals.  Would the initial physical weathering proceed rapidly or slowly?   Would the subsequent chemical weathering proceed rapidly or slowly?   Describe the texture of the resulting soil..

     physical      chemical      texture
a.   rapidly         slowly      sandy-clay
b.   slowly         slowly       clayey
c.   slowly         rapidly      sandy
d.   rapidly        rapidly      silty
e.   rapidly        slowly       clayey

 

 

 

 

 

 

 

 

 

 

 

 



Answer to Question 1:

a.  quartz, slow
b.  aluminosilicate, fast
c.  ferromagnesian, fast

Quartz (colorless) and aluminosilicate minerals such as feldspars (light colored), both weather more slowly than ferromagnesian minerals, which are dark-colored due to the presence of reduced iron (Fe2+).  In a magma, the ferromagnesians (mafic minerals) precipitate out first at the high temperatures. Thus, in a low temperature soil, conditions are furthest away from where they formed (where they are "stable").   Aluminosilicates and quartz precipitate out last, and are consequently more stable in soils, and thus weather more slowly.  In addition, ferromagnesian minerals contain more ionic bonds than the first two choices.

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Answer to Question 2:

Till			unsorted, angular stones
Aeolian			deposited by wind
Outwash			sorted by meltwater
Moraine			ridge at glacier terminus
Loess			highly productive, silt soils along E. Miss.
Esker			matrix of rounded stones, cobbles in a linear ridge
Kame Terrace		Deposit along side of glacier
Delta			deep, sand deposits at river mouths
Lacustrine		often alternating layers of silt/clay


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Answer to Question 3:
   fastest   
evaporites     -   highly soluble
carbonates     -   moderately soluble
ferromagnesian -   primary, iron oxidizes, ionic bonding
primary aluminosilicate (feldspars) -  formed at high temp.
secondary aluminosilicate (clays)  -  formed at low temp.
quartz        -  formed at high temp., but covalent bonding
oxides        -  formed in soils, very insoluble
   slowest


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Answer to Question 4:

General Weathering Equation:        1o Þ 2o + solute

Primary minerals (geologic in origin) weather in soils to produce more stable secondary soil minerals, plus solutes (often soluble ions).   LeChatelier's principle states that if a reaction at equilibrium is perturbed, it will adjust to regain equilibrium.  Briefly, this means that if you add reactants or remove products, the reaction will go to the right.   Since leaching removes products (solutes), this would cause weathering, and consequently horizon development, to proceed faster.   This is why well-drained soils at the top of a hillslope often have thicker, more developed horizons than those at the bottom.  Conversely, in poorly-drained soils, solutes accumulate, slowing down weathering.

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Answer to Question 5:

a. Hydration
b. Oxidation
c. Carbonation 
d. Acidification
e. Hydrolysis

The presence of oxygen on the left hand side of the equation is a 
give-away for an oxidation reaction.   Also you should check the change
in the valence of Fe.  Since Si is always +4 and O is always -2, then Fe 
must be Fe2+ on the left.  On the right, iron oxide is obviously +3, to 
balance the six negative charges on the three oxygens.   The charge has
increased, hence an oxidation reaction. (note that the higher the positive 
charge, the more oxidized a chemical is.  
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Answer to Question 6: 
     physical      chemical     texture
a.   rapidly        slowly      sandy-clay
b.   slowly         slowly      clayey
c.   slowly         rapidly     sandy
d.   rapidly        rapidly     silty
e.   rapidly        slowly      clayey 

The coarse grains of granite would undergo physical weathering rapidly,
since they have little surface area to bind them.  Once unconsolidated,
chemical weathering would proceed slowly, since there is little surface
for weathering to occur on, which is mediated by water contact.  The quartz
would give rise to sand, and the light-colored (primary aluminosilicate) 
minerals would give rise to secondary aluminosilicate clays.  Hence the
resulting texture would be a mix of the two (sandy clay).

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