Chapter 12

Earth Materials and the Cycle of Rock Change

OVERVIEW

This is the first chapter of Part Three which deals with the systems and cycles of the solid Earth. This chapter discusses the basic materials of the solid Earth – rock and minerals – and some principals of their formation. These are linked to the cycle of rock change which describes how different rock types develop as Earth materials are cycled and recycled through geologic time.

The elements oxygen and silicon account for about seventy-five percent of the Earth’s crust, while the metallic elements iron, aluminum, and the base elements account for most of the rest.

The elements of the crust are combined in inorganic chemical compounds called minerals.

These minerals are mixed together in various proportions to form many kinds of rock.

The rocks of the Earth’s crust are grouped into three major classes: igneous, sedimentary, and metamorphic rocks.

Igneous rocks form when molten material from the Earth’s interior cools and solidifies in the crust.

Magma cooled slowly below the surface forms coarse-textured intrusive igneous rocks. Lava cooled rapidly at the surface forms fine-textured extrusive igneous rocks.

Igneous rocks consist mainly of silicate minerals containing silicon, oxygen, and metallic elements. The kind of metallic elements present determines the mineral density.

Less dense felsic minerals dominate the igneous rocks of the upper crust, while more dense mafic and ultramafic minerals dominate those of the lower crust.

Silicate minerals undergo chemical changes called mineral alteration when exposed to air and water at the Earth’s surface.

Most clay minerals are produced by mineral alteration.

Weathering, or the breakdown of rocks into smaller particles known collectively as sediment, occurs through both mineral alteration and physical disintegration.

Layers of mineral sediment and organic matter accumulate in oceans and low-lying land areas to be compacted and hardened into sedimentary rocks. Different types of sediment produce different kinds of sedimentary rock.

Igneous and sedimentary rocks can be altered by heat and pressure to form metamorphic rocks.

The cycle of rock change describes the circulation of rock material between the Earth’s interior and the crust. This is a very slow process powered by the heat of radioactive decay deep within the Earth.

KEY TERMS

mineral
rock
igneous rocks
sedimentary rocks
metamorphic rocks
quartz / felsic
mafic
extrusive igneous rock
intrusive igneous rock
mineral alteration
cycle of rock change / lava
magma
andesite
basalt
granite
pluton

STUDY QUESTIONS

  1. What is the Earth’s crust and what are its major chemical constituents?
  2. What is the difference between a mineral and a rock?
  3. How does the rate at which molten material cools affect the character of the resulting igneous rock?
  4. Define silicate mineral. Distinguish between the felsic, mafic, and ultramafic types and give an example of each.
  5. Name two igneous rocks that have the same mineral composition but a different texture.
  6. List and describe the major chemical processes by which silicate minerals are altered when they are exposed to air and water at the Earth’s surface.
  7. How are most clay minerals formed?
  8. What are the three main classes of sedimentary rocks?
  9. What is the process of chemical precipitation and how can this contribute to the formation of sedimentary rocks?
  10. Hydrocarbon compounds are found in some sedimentary rocks. How are they formed?
  11. What are metamorphic rocks? Name three metamorphic rocks and describe how they are formed.
  12. Sketch a diagram of the cycle of rock change. Show the main flow pathways, the kinds of rocks formed within the cycle, and the power sources for the cycle.
  13. Temperature change with depth below the Earth’s surface is different beneath the oceans than beneath the continents. Sketch a graph to show how it differs. Explain why this difference exists.

CHAPTER QUIZ

Multiple Choice Questions

1.The three most abundant elements in the Earth’s crust are

a)oxygen, silicon, and iron

b)silicon, iron, and aluminum

c)oxygen, iron, and aluminum

d)aluminum, oxygen, and silicon

2.Which of the following is a silicate mineral which contains magnesium and iron, but not aluminum?

a)plagioclase feldspar

b)quartz

c)amphibole

d)olivine

3.Which of the following is an intrusive igneous rock?

a)diorite

b)andesite

c)rhyolite

d)basalt

4.The process of mineral alteration in which water combines chemically with a primary mineral to form a new, stable secondary mineral is called:

a)oxidation

b)hydrolysis

c)solution

d)carbonation

5.Which of the following is a chemically precipitated sedimentary rock?

a)coal

b)dolomite

c)conglomerate

d)shale

True/False Questions

1.Solar radiation is the primary power source driving the cycle of rock. (T/F)

2.The Earth’s crust is about 100 kilometers thick. (T/F)

3.Intrusive igneous rocks tend to be finer textured than extrusive igneous rocks. (T/F)

4. Rhyolite has the same mineral composition as basalt. (T/F)

5. None of the hydrocarbon compounds qualify physically as rock. (T/F)

Short Answer Questions

1.Describe how the mineral composition of the Earth’s crust varies with depth, and explain this variation.

2.What are carbonate rocks, and how are they formed?

3.What are the major processes by which metamorphic rocks are formed?

Short Essay Questions (1 - 2 paragraphs)

1.Clastic sedimentary rocks form from accumulations of mineral and rock particles. Discuss how these particles are derived and how they can accumulate to eventually form sedimentary rock.

2.Identify the power sources that drive the cycle of rock change and discuss their relative importance within the cycle.

Internet Resources

  1. The rock cycle: <
  2. Definitions of rock types: <
  3. Images and descriptions of minerals: <
  4. INTRO? <
  5. Rock classification: <