Chapter 6: 1 What are the five processes necessary to produce a sedimentary rock

Question

Chapter 6:

1 What are the five processes necessary to produce a sedimentary rock?

2 Describe each of the following types of physical/mechanical weathering:

  (a) Frost wedging & where it is most common  

(b) Unloading (pressure release) & the formation of exfoliation domes

  (c) mineral growth & plant/root action as wedging agents  

(d) thermal expansion & contraction and why it occurs

  (e) Abrasion - and how and in what environments it occurs

3 Describe each of the following types of chemical weathering:   

(a) Dissolution (including drawing and explaining the polarity of a water molecule);  

(b) Acid dissolution & which acid is the most common dissolution agent?

  (c) Oxidation - and its relation to the color of rocks; and,

   (d) Hydrolysis - how weathering of silicates makes clays & muds

4 What factors affect how fast or how much chemical weathering can occur?

5 How does mechanical weathering influence chemical weathering?

6 What is spheroidal weathering? Where and how does it occur?

7 How is chemical weathering related to the position of minerals on the   Bowen's Reaction Series?

8 What is differential weathering? Explain differences in the way different beds weather   at the Grand Canyon.

9 What are the necessary ingredients in order for a soil to support life?   What is pore space and how does this affect a soil?

10 What factors control how and at what rate a soil forms?

11 Describe each of the horizons that form in a mature soil. (O, A, E, B & C)

12 Describe the difference between pedalfers, pedocals, laterites in both  where they form and their distribution of horizons.

Chapter 7:

13 Draw the "mountain" we drew in class that shows the CLASTIC rocks and their  distance away from the source region.

14 How does the size, shape & sorting of sedimentary clasts change relative  to distance from the source?

15 How are sedimentary rocks lithified? What is the difference in these processes?

16 What is the difference between clastic and chemical sedimentary rocks  (both in the way they form and their texture)?

17 Describe and compare the following chemical sedimentary rocks:  Limestone, Dolostone, Chert, Evaporites (Rock salt & gypsum)

Explanation / Answer

1. Five processes necessary to produce a sedimentary rock

1) Weathering,

2) erosion,

3) deposition

4) compaction,

5) cementation.

Methods of Sedimentary Rock . The classic route begins with the weathering and erosion of sedimentary, igneous, or metamorphic rocks. Through these processes, larger rock is broken up into smaller particles which are transported by moving water, ice, gravity or wind, and deposited at the bottom of a lake, a river delta, an ocean, or similar location where further movement is restricted or slowed down. The rock particles can range in size from boulders to clay particles less than .002 mm in diameter. If these rock particles are covered by additional particles, eventually the weight from above will start the process of lithification. Lithification is the compaction and cementation of particles which form rock. Compaction squeezes out the fluids and space that exist between the particles, and cementation results when the fluids that are squeezed out are replaced with minerals that crystallized from the fluids.

2.(A) Frost/ice wedging is most common mainly in environments where is a lot of moisture and where the temperature constantly fluctuates above and below the freezing point. The Apline and Periglacial areas are two great examples of this.

(B) Exfoliation is the process in which rocks weather by peeling off in sheets rather than eroding grain by grain.Exfoliation can happen in thin layers on individual boulders, or it can take place in thick slabs as it does here, at Enchanted Rock in Texas.

The great white granite domes and cliffs of the High Sierra, like Half Dome, owe their appearance to exfoliation. These rocks were emplaced as molten bodies, or plutons, deep underground, raising the Sierra Nevada range. The usual explanation is that erosion then unroofed the plutons and took away the pressure of the overlying rock. As a result, the solid rock acquired fine cracks through pressure-release jointing. Mechanical weathering opened up the joints further and loosened these slabs.

(C) Living organisms may contribute to mechanical weathering (as well as chemical weathering, see 'biological' weathering below). Lichens and mosses grow on essentially bare rock surfaces and create a more humid chemical microenvironment. The attachment of these organisms to the rock surface enhances physical as well as chemical breakdown of the surface microlayer of the rock. On a larger scale, seedlings sprouting in a crevice and plant roots exert physical pressure as well as providing a pathway for water and chemical infiltration.

chemical weathering changes the composition of rocks, often transforming them when water interacts with minerals to create various chemical reactions. Chemical weathering is a gradual and ongoing process as the mineralogy of the rock adjusts to the near surface environment. New or secondary minerals develop from the original minerals of the rock. In this the processes of oxidation and hydrolysis are most important. Chemical weathering is enhanced by such geological agents as the presence of water and oxygen, as well as by such biological agents as the acids produced by microbial and plant-root metabolism.

(D) Weathering by the thermal expansion and contraction occurs mostly in arid and hot .

because there must be a significant daily change in temperature, this occurs is desert (aolian) type environments where it is hot during the day and the temperature falls rapidly at night due to the lack of cloud cover.

This means that the rock's expansion during the day is significant and as the temperature falls more rapidly than the rock is able to conduct heat the outer layer cools and contracts whilst the inner rock is still expanded. The outer layer will thus crack and shatter coming away like the layers of an onion, hence the common term onion-skin weathering.In hot and humid conditions there is no significant temperature fall at night.

3 (A) Dissolution is the process by which a solute forms a solution in a solvent. The solute, in the case of solids, has its crystalline structure disintegrated as separate ions, atoms, and molecules form. For liquids and gases, the molecules must be adaptable with those of the solvent for a solution to form. The outcome of the process of dissolution (the amount dissolved at equilibrium, i.e., the solubility) is governed by the thermodynamic energies involved, such as the heat of solution and entropy of solution, but the dissolution itself (a kinetic process) is not. Overall the free energy must be negative for net dissolution to occur. In turn, those energies are controlled by the way in which different chemical bond types interact with those in the solvent.

(B) ARD can also produce sulfuric acid at a slower rate, so that the acid neutralizing capacity (ANC) of the aquifer can neutralize the produced acid. In such cases, the total dissolved solids (TDS) concentration of the water can be increased from the dissolution of minerals from the acid-neutralization reaction with the minerals.

Sulfuric acid is used as a defence by certain marine species, for example, the phaeophyte alga Desmarestia munda (order Desmarestiales) concentrates sulfuric acid in cell vacuoles.

Sulfuric acid is formed naturally by the oxidation of sulfide minerals, such as iron sulfide. The resulting water can be highly acidic and is called acid mine drainage (AMD) or acid rock drainage (ARD). This acidic water is capable of dissolving metals present in sulfide ores, which results in brightly colored, toxic streams. The oxidation of pyrite (iron sulfide) by molecular oxygen produces iron(II), or Fe2+
:

2 FeS
2 (s) + 7 O
2 + 2 H
2O ? 2 Fe2+
(aq) + 4 SO2?
4 (aq) + 4 H+

The Fe2+
can be further oxidized to Fe3+
:

4 Fe2+
+ O
2 + 4 H+
? 4 Fe3+
+ 2 H
2O

The Fe3+
produced can be precipitated as the hydroxide or hydrous oxide:

Fe3+
(aq) + 3 H
2O ? Fe(OH)
3 (s) + 3 H+

The iron(III) ion ("ferric iron") can also oxidize pyrite:

FeS
2 (s) + 14 Fe3+
+ 8 H
2O ? 15 Fe2+
(aq) + 2 SO2?
4 (aq) + 16 H+

When iron(III) oxidation of pyrite occurs, the process can become rapid. pH values below zero have been measured in ARD produced by this process.

(C)

Ferrous and ferric iron are components in many minerals, especially within sandstones. Fe2+ is in clay, carbonates, sulfides, and is even within feldspars in small amounts. Fe3+ is in oxides, hydrous, anhydrous, and in glauconites.[6] Commonly, the presence of iron is determined to be within a rock due to certain colorations from oxidation. Oxidation is the loss of electrons from an element. Oxidation can occur from bacteria or by chemical oxidation. This often happens when ferrous ions come into contact with water (due to dissolved oxygen within surface waters) and a water-mineral reaction occurs. The formula for the oxidation/reduction of iron is:

Fe2+ ? Fe3+ + e-

The formula works for oxidation to the right or reduction to the left.

Fe2+ is the ferrous form of iron. This form of iron gives up electrons easily and is a mild reducing agent. These compounds are more soluble because they are more mobile. Fe3+ is the ferric form of iron. This form of iron is very stable structurally because it's valence electron shell is half filled.

(D) Clay minerals and oxide minerals (including quartz) are the most common byproducts of chemical weathering. Thus clay minerals and quartz are the most abundant contributors to clastic sediment and soil. We here discuss the structure, properties, occurrence, and identification of clay minerals, but first we need to discuss the phyllosilicates in general.

4)

Climate, which is usually measured in terms of temperature and moisture, can drastically affect the rate of weathering. High amounts of water and higher temperatures generally cause chemical reactions to run faster. Thus warm humid climates generally have more highly weathered rock, and rates of weathering are higher than in cold dry climates. Example: limestones in a dry desert climate are very resistant to weathering, but limestones in a tropical climate weather very rapidly. A high temperature and high amounts of water also controls vegetation which indirectly affects rate. Seasonality of precipitation affects rate to a degree.

Rock Primary Minerals Residual Minerals* Leached Ions Granite Feldspars Clay Minerals Na+, K+ Micas Clay Minerals K+ Quartz Quartz --- Fe-Mg Minerals Clay Minerals + Hematite + Goethite Mg+2 Basalt Feldspars Clay Minerals Na+, Ca+2 Fe-Mg Minerals Clay Minerals Mg+2 Magnetite Hematite, Goethite --- Limestone Calcite None Ca+2, CO3-2

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