Geology 200

Study Guide

Unit 4:
Weathering, Soil, and Sedimentary Rocks

Overview

In Unit 4, we discuss the origin and classification of sedimentary rocks and how geologists analyze and interpret sedimentary rocks to learn about their environments of deposition. To understand the origin of sedimentary rocks, you must first learn about weathering and erosion, processes that create the material that accumulates to form sediments and sedimentary rocks. The rock cycle diagram at the beginning of Chapter 5 of the textbook shows how weathering fits into the rock cycle; weathering is the subject of Section 1 of this unit.

Section 2 deals with regoliths and soils, two of the products of weathering processes, while Section 3 addresses sedimentation and lithification, or rock-forming processes. In Section 4, we discuss the types of sedimentary rocks, and Section 5 and 6 examine sedimentary structures and their interpretation.

Objectives

After completing this unit, you should be able to

1. explain the difference between weathering and erosion.
2. describe the three processes of mechanical weathering.
3. explain why minerals weather.
4. describe the three main types of chemical weathering.
5. explain chemical weathering in terms of Bowen’s reaction series.
6. describe the effects of weathering on granites, basalts, sandstones, and shales.
7. list and explain the three most important factors in determining the rate of weathering.
8. explain why soil formed from weathering of a basalt can be more fertile than soil formed from the weathering of granite.
9. define the term laterite.
10. describe how sedimentary rocks are classified.
11. explain how sedimentary rocks can be distinguished from igneous and metamorphic rocks.
12. define the following terms:
    1. rounding
    2. sorting
    3. lithification
    4. pore space
    5. compaction
    6. cementation
13. explain the difference between breccias and a conglomerate.
14. distinguish among shale, siltstone, and mudstone.
15. describe how chemical sediments differ from clastic sediments, and give examples of each.
16. describe two ways in which extensive layers of dolomite can form.
17. describe (briefly) how chert, evaporates, and coal form.
18. draw sketches to illustrate bedding, cross bedding, graded bedding, mud cracks, and ripple marks.
19. explain how graded bedding might be used to identify the tops and bottoms of sedimentary rock layers in an area where sedimentary rock is no longer horizontal.
20. explain how sedimentary rock can be studied to determine the type of the original source rock, and the direction of movement and distance from the source.
21. give the main criteria for distinguishing and naming a formation.
22. draw a diagram to illustrate the common sedimentary environments of deposition.
23. recognize the major types of sedimentary rocks and explain the “genetic” significance of texture and composition.

Section 1: Weathering

The reading associated with this section explains what weathering is, and describes the different mechanisms of mechanical and chemical weathering. In their definition of weathering, Plummer, Carlson, and Hammersley refer to destructive processes operating at or near the Earth’s surface (2010). Judson, Kauffman, and Leet define weathering as “the response of materials that were in equilibrium within the Earth’s crust to new conditions at or near contact with air, water, and living matter” (1987, p. 7, after Reich, 1950).

As the textbook indicates, the destructive processes that work on rock can be mechanical or chemical. Plummer, Carlson, and Hammersley identify the main agents of mechanical weathering as frost wedging and frost heaving, abrasion, and pressure release. Although they discuss chemical weathering processes, they don’t use the technical terms, so a few of these terms are outlined here. Please keep these definitions in mind as you read the assigned textbook material.

• Oxidation is the process in which atmospheric oxygen combines with a mineral to form an oxide. This process is particularly important for minerals containing a large amount of iron, such as olivine, pyroxene, and amphibole. The iron in these minerals combines with oxygen to form hematite (Fe₂O₃) or (FeO(OH)).
• Hydrolysis is the chemical union of water with a mineral. A good example of hydrolysis is the chemical weathering of feldspar. Carbon dioxide from the atmosphere and soil combines with rainwater to form carbonic acid. If this acid comes into contact with a K feldspar, the hydrogen ion displaces the potassium ion of the feldspar and disrupts the crystal structure. The weathering of feldspar by weak acidic rainwater is described in the textbook. Please work through the chemical equation given in Table 5.1.
• Dissolution is a process whereby a rock material passes directly into solution. Some rocks can be completely dissolved and flushed away by water. Rock salt and gypsum are particularly susceptible to this process, and are rarely found as large outcrops in humid climates. Limestones and dolomites are easily dissolved by waters that contain carbon dioxide.

Judson, Kauffman, and Leet presented the following generalizations about products of chemical weathering (1987, pp. 75-76):

1. Any mineral that contains aluminium will yield clay.
2. Any iron-bearing mineral will oxidize to iron oxides or take water into its chemical makeup.
3. Chemical weathering commonly yields a collection of dissolved constituents of potassium, sodium, silicon, magnesium, and calcium, which go off in solution, as well as a collection of residual solids. The solids go on to make clastic (or fragmental) sedimentary rocks; the dissolved constituents go on to make chemical precipitates.
4. Because reactions take much time, erosion commonly removes debris before the reactions are complete. In fact, the debris may even be incorporated into sedimentary rocks before weathering processes are complete.

The rate at which mineral weathering takes place in igneous rocks can be related to Bowen’s reaction series (see Figure 4.1).

Olivine, the igneous mineral that crystallizes at the highest pressure and temperatures, is the most unstable at Earth’s surface. Olivine therefore weathers very rapidly. In contrast, quartz crystallizes last in the Bowen’s reaction series (at low temperatures and pressures), and as you might predict, quartz weathers most slowly at Earth’s surface.

The kinds of minerals present in a rock strongly influence its rate of weathering. Basalts, which are composed of high temperature and high pressure minerals (e.g., olivine, pyroxene, plagioclase), are generally highly porous and contain no quartz. They weather rapidly to clay and iron oxide. Granites are composed of low temperature minerals, and although the feldspars and micas in granite break down to clay, the quartz remains. The result is quartz and clay residue. Sandstones are composed mostly of quartz with small amounts of rock fragments, feldspars, and clay minerals. The grains are commonly held together by quart, calcite, or iron oxide cements. A sandstone cemented by quartz can be very resistant to weathering. Limestones are composed mostly of calcite; they weather by dissolution. In arid areas, limestones can form resistant cliffs, but in humid areas, solution activity creates networks or caverns and caves in limestone. Shales are the least resistant to weathering because of their fine grain size, their softness, and their ability to absorb large amounts of water.

In an area where different rock types are present, differential weathering can form irregular topography, with the weathering-resistant rock bodies protruding above the other rocks. The hoodoos in Drumheller, Alberta, for example, are formed by differential weathering. The caps on the hoodoos are zones of carbonate-cemented sandstone, which are resistant to erosion. They protect the less resistant underlying sediments from erosion. The areas between the hoodoos were probably uncemented sediments.

The surface area exposed to weathering elements is another important factor in the rate of weathering. The structure and texture of the rock control the surface area exposed to the atmosphere. Rock with closely spaced joints may break down quickly, even if it consists entirely of quartz. A highly porous rock may weather more rapidly than a denser rock of the same composition.

Climate is another important factor in weathering. In hot, moist climates, chemical weathering is intense, because heat and moisture promote chemical reactions. In cold climates, chemical weathering is very slow. In such climates, mechanical weathering may be more significant.

Rocks on steep slopes weather more rapidly than do rocks on gentle slopes, as the product of weathering is moved away quickly from steep slopes to expose fresh surfaces, which in turn are weathered and eroded.

Reading Assignment

Plummer, C. C., Carlson, D. H., & Hammersley, L. Physical Geology (15th ed.).

• Introduction to Chapter 5: Weathering and Soil (pp. 105-107).
• “Weathering, Erosion, and Transportation” (p. 106-107).
• “How Weathering Changes Rocks” (pp. 107).
• “Effects of Weathering” (p. 107-108).
• “Mechanical Weathering” (pp. 108-110).
• “Chemical Weathering” (pp. 110-117).

Study Questions

1. What is weathering?
2. Explain the difference between weathering and erosion, and between erosion and transportation.
3. Can a rock be eroded without being weathered?
4. Why are rocks rounded by weathering?
5. Describe three processes of mechanical weathering.
6. Why do minerals weather?
7. Explain (briefly) three types of reactions involved in chemical weathering.
8. How do hematite and limonite form as weathering products?
9. List three kinds of natural acids that can accelerate weathering, and identify the source of each acid. Which is the most significant weathering agent?
10. Explain how solution features, such as caves in limestones, form.
11. Describe the chemical weathering of feldspar.
12. Refer to Figure 4.1 in this Study Guide. Which member of each of the following pairs of minerals would weather more rapidly?
    1. augite or hornblende
    2. calcic feldspar or potassic feldspar
13. According to Bowen’s reaction series, which rock would you expect to break down first: gabbro or granite? Explain.
14. What causes differential weathering?
15. Describe the effects of weathering on each of the following: granites, basalts, sandstones, shales, and limestones.
16. List and explain the three most important factors in controlling the rate of weathering.

 Answer Key

Section 2: Regoliths and Soils

Regoliths and soils are important products of weathering. The textbook defines a regolith as “any loose, unconsolidated material that covers most of the Earth’s land surface” (2010, p. 124), but you will find the following definition most useful. Regolith is the layer of soft, disaggregated rock material formed in place by the decomposition and disintegration of bedrock. Soil is the uppermost layer of the regolith, the layer that contains varying amounts of decomposed organic matter. Highly leached regoliths, called laterites, form in tropical regions, where temperatures are high and rainfall is abundant.

Reading Assignment

Plummer, C. C., Carlson, D. H., & Hammersley, L. Physical Geology (15th ed.).

• “Soil” (pp. 117-125).

Study Questions

17. Explain the difference between soil and regolith.
18. What two types of minerals are left after complete chemical weathering of a rock has occurred?
19. Soil formed from the weathering of a basalt can be more fertile than soil formed from a granite. Why?
20. What is a laterite?

 Answer Key

Section 3: Origin of Sedimentary Rocks

Chapter 5 of the textbook describes how weathering and erosion transforms rock into loose, solid particles. Sediment is the name given to the loose, solid particles that originate from weathering and erosion of pre-existing rock (detrital sediments) or from chemical precipitation from solution (nondetrital or chemical sediments). In this section we will learn how unconsolidated particles are transformed into sedimentary rock.

Sedimentation is the general process by which rock-forming material is deposited. The textbook mentions sorting, a process in which sediments settle (from the water carrying them) at different rates depending on their size. The text, however, does not mention that water bodies carry their sediments in different ways, depending on the grain size of the sediment. In a stream, for example, the coarsest particles move along the stream bed by rolling and sliding. Medium-sized particles move within the flowing water, falling at times down to the stream bed, and then bounce back into the stream flow. The finest material is suspended in the flowing water. Material dissolved during weathering and erosion moves in the flowing water in solution.

You should note that while detrital material is deposited (in a manner that depends on the energy of the agent of transport), dissolved or nondetrital material is precipitated. Precipitation is a “chemical process by which dissolved material is converted into a solid and separated from the liquid solvent” (Judson et al., 1987, p. 92). Precipitation may be caused by chemical interaction, evaporation-concentration, or both.

Some rocks that form by chemical processes have a nonclastic texture—one in which grains interlock. When crystals form by precipitation from fluids, they tend to be small, but they may increase in size as a result of pressure during burial. Nonclastic textures may be fine grained (finely crystalline), medium-grained, or coarse-grained.

While the textbook discusses lithification, it does not mention diagenesis. Diagenesis is a series of changes—caused by physical, chemical, and biological agents—that a sediment undergoes after it is deposited. Diagenesis includes lithification, but is not limited to it: lithification is the general term for processes that convert unconsolidated sediment into rock; diagenesis, however, also includes processes that occur after the rock has been lithified. After lithification, for example, secondary porosity may occur through the dissolution of detrital grains or cement, or detrital minerals may be altered and transformed to other minerals.

Reading Assignment

Plummer, C. C., Carlson, D. H., & Hammersley, L. Physical Geology (15th ed.).

• Introduction to Chapter 6: Sediment and Sedimentary Rocks (pp. 127-128).
• “Sediment” (pp. 128-132).

Study Questions

21. What important resources are concentrated in sedimentary rocks?
22. Define sediment.
23. Explain the difference between deposition and precipitation.
24. What is the texture of a sedimentary rock?
25. Define rounding and sorting.
26. Describe a nonclastic texture. How do rocks with a nonclastic texture form?
27. Define lithification, pore space, compaction, and cementation.
28. What is diagenesis?

 Answer Key

Section 4: Types of Sedimentary Rocks

We examine the three types of sedimentary rocks (detrital, chemical, and organic) in this section. Sedimentary rocks may also be classified as

• detrital rocks;
• carbonate rocks (any rocks composed of carbonate minerals); and
• other (chert, evaporites, and coal).

Detrital sediments are classified by their grain size into gravel, sand, and mud. The mud category contains silt and clay-sized material (see Figure 4.2). As sedimentary rocks, lithified gravel and sand would be termed conglomerate and sandstone, respectively. Lithified mud would be termed shale, siltstone, or mudstone, depending on grain sizes. Note that use of the term mud as a classification category is somewhat unusual. More commonly, mud is an informal term describing a substance that includes sand, silt, and clay fractions in a wet environment.

Figure 4.2. Grain size scale for clastic sediments and rocks.

Rocks composed exclusively of gravel can be either sedimentary breccias or conglomerates, as described in the textbook. For rocks containing mixtures of grain sizes, terms such as sandy conglomerate, muddy conglomerate, and conglomeratic sandstone are used.

Detrital rocks can be further classified according to their grain composition. As many factors are involved in determining the grain composition of a detrital rock, such a classification is a concise way of providing a lot of information.

Most compositional classifications are based on the proportions of quartz, feldspar, and unstable rock fragments (detrital fragments of rocks that break down easily, e.g., fragments of granite, basalt, volcanic rocks, shales).

Rocks with a high proportion of feldspar are called arkoses. Rocks composed dominantly of quartz and siliceous rock fragments are called quartz arenites. (Note: Arenite is simply another term used for rocks with sand-sized particles.)

Carbonate rocks can be clastic in texture, composed, for example, from wave broken fragments of carbonate debris. These carbonate rocks can be described in a way similar to that used for detrital rocks. Figure 6.14 in the textbook illustrates the different kinds of clastic carbonate sediments in a reef setting. Such sediments can be referred to as bioclastic sediments, to distinguish them from noncarbonate clastic sediments. Other carbonate rocks are precipitated directly as rocks, and have crystalline textures. To a large extent, the study of carbonate rocks involves identifying the types of calcite particles (the type of shell, fragment of coral, or algae), and examining the textures of the cement that holds the bioclastic grains together.

Other sedimentary rocks, such as chert, evaporates, and coal, are described briefly in the textbook.

Reading Assignment

Plummer, C. C., Carlson, D. H., & Hammersley, L. Physical Geology (15th ed.).

• “Types of Sedimentary Rocks” (p. 132).
• “Detrital Rocks” (pp. 132-136).
• “Chemical Sedimentary Rocks” (pp. 136-142).
• “Organic Sedimentary Rocks” (p. 142).
• The Origin of Oil and Gas” (pp. 142-143).

Study Questions

29. Describe how sediments are classified.
30. Explain the difference between a breccia and a conglomerate.
31. Distinguish among shale, siltstone, and mudstone.
32. Define limestone.
33. Describe two ways in which extensive layers of dolomite can form.
34. Describe briefly how chert forms.
35. Name two types of evaporates.

 Answer Key

Section 5: Sedimentary Structures

Sedimentary structures are characteristic features of sedimentary rocks. Cross bedding, graded bed, mud cracks, ripple marks, and fossils, which are described in the following reading, are only a few of the many types of sedimentary structures.

Reading Assignment

Plummer, C. C., Carlson, D. H., & Hammersley, L. Physical Geology (15th ed.).

• “Sedimentary Structures” (pp. 143-146).
• “Fossils” (pp. 147-149).
• “Formations” (p. 149).

Study Questions

36. Draw sketches to illustrate bedding, cross-bedding, graded bed, mud cracks, and ripple marks.
37. What do mud cracks tell us about the environment of deposition of a sedimentary rock?
38. What are fossils? Why are coal and petroleum sometimes called fossil fuels?
39. How might graded bedding be used to identify the tops and bottoms of sedimentary rock layers in an area where sedimentary rock is no longer horizontal? What sedimentary structures besides graded bedding can be used to determine the tops and bottoms of tilted beds?
40. What is a formation?
41. What is the name for a boundary between two different rock types?

 Answer Key

Section 6: Interpretation of Sedimentary Rocks

Examining sedimentary rocks provides a great deal of information about the character of the sediment’s source area and the environment of its deposition. The reading for this section describes how the type of the source rock and the distance of the sedimentary rock from the source area can be determined from a sedimentary rock. The reading also describes the most important characteristic of deposits formed in the common sedimentary environments.

Reading Assignment

Plummer, C. C., Carlson, D. H., & Hammersley, L. Physical Geology (15th ed.).

• “Interpretation of Sedimentary Rocks” (pp. 150-155).

Study Questions

42. What is the probable source rock type of a sedimentary rock containing each of the materials listed below?
    1. feldspar, quartz, and biotite
    2. well-rounded quartz
43. Study Figure 6.34 in the textbook. Describe three ways in which a geologist could determine the distance of a sedimentary rock from the source area by studying the sedimentary rock.
44. How might one determine the direction of the source rock from a sedimentary rock?
45. List four characteristics of a sedimentary rock that are used to determine its environment of deposition.
46. Draw a diagram to illustrate the common sedimentary environments of deposition.
47. In which environment are thick sequences of silt and shale likely to form?
48. In which environment does the most poorly sorted sediment form?

 Answer Key

Unit 4 Self Test

You have now finished Unit 4, so please complete and submit the associated laboratory exercise and assignment. Instructions can be found in the Assignment Drop Boxes section of the course homepage.