Historical Geology
INTRODUCTION
MATTER AND ITS COMPOSITION
Bonding and Compounds
MINERALS—THE BUILDING BLOCKS OF ROCKS
How Many Minerals Are There?
Rock-Forming Minerals and the Rock Cycle
IGNEOUS ROCKS
Texture and Composition
Classifying Igneous Rocks
SEDIMENTARY ROCKS
Sediment Transport, Deposition, and Lithification
Classification of Sedimentary Rocks
METAMORPHIC ROCKS
What Causes Metamorphism?
Metamorphic Rock Classification
PLATE TECTONICS AND THE ROCK CYCLE
SUMMARY
The following content objectives are presented in Chapter 2:
¨ Chemical elements are composed of atoms, all of the same kind, whereas compounds form when different atoms bond together. Most minerals are compounds, which are characterized as naturally occurring, inorganic, crystalline solids.
¨ Of the 3,500 or so minerals known, only a few, perhaps two dozen, are common in rocks, but many others are found in small quantities in rocks and some are important natural resources.
¨ Cooling and crystallization of magma or lava and the consolidation of pyroclastic materials account for the origin of igneous rocks.
¨ Geologists use mineral content (composition) and textures to classify plutonic rocks (intrusive igneous rocks) and volcanic rocks (extrusive igneous rocks).
¨ Physical and chemical weathering yields sediment that is transported, deposited, and then lithified to form detrital and chemical sedimentary rocks.
¨ Texture and composition are the criteria geologists use to classify sedimentary rocks.
¨ Any type or rock may be altered by heat, pressure, fluids, or any combination of these, to form metamorphic rocks.
¨ One feature used to classify metamorphic rocks is foliation—that is, a platy or layered aspect,but some lack this feature and are said to be nonfoliated.
¨ The fact that Earth materials are continually recycled and that the three families of rocks are interrelated is summarized in the rock cycle.
To exhibit mastery of this chapter, students should be able to demonstrate
comprehension of the following:
¨ the nature of minerals, including the classification of substances as mineral or not mineral
¨ the basic atomic structure, and the formation of elements and compounds
¨ the types of minerals and their roles in the various types of rocks
¨ the rock cycle, including how each type of rock can yield the other rock types
¨ the formation and classification of igneous rocks
¨ sediment production through weathering, environments of deposition, and lithification
¨ the formation and classification of metamorphic rocks
¨ the effects of plate tectonics on the rock cycle
1. Minerals are naturally occurring, inorganic, crystalline solids with distinctive physical properties and specified chemical compositions. Rocks are made of one or more minerals.
Figure
2.1 Minerals and Rock
2. Elements are composed of atoms, which have a nucleus of protons and neutrons around which electrons occupy electron shells.
Figure 2.2 Shell Model for Atoms
3. The number of protons in a nucleus determines the atomic number of an element; the atomic mass number is the number of protons plus neutrons in the nucleus. The number of neutrons in the nucleus of an element may vary, forming different isotopes of the element.
Figure 2.3 Isotopes of Carbon
4. Atoms join together or bond by transferring electrons from one atom to another (ionic bond) or by sharing electrons (covalent bond). Most minerals are compounds of two or more different elements bonded together.
Figure 2.4 Ionic and Covalent Bonding
5. Atoms of minerals are arranged in a specific three dimensional framework. Although some minerals have a specific chemical composition, other minerals have a compositional range because of substitution.
Figure
2.5 A Variety of Mineral Crystal Shapes
6. By far the most common minerals are the silicates (composed of silicon and oxygen), but carbonate minerals (containing the CO32- ion) are prevalent in some rocks.
Figure 2.6 Some of the Common Silicate Minerals
Table 2.1 Some of the Mineral Groups Recognized by Geologists
Some minerals qualify as collectible and valuable. Your students can investigate the various types of gemstones that are considered precious through this PBS website (http://www.pbs.org/wgbh/nova/diamond/gemprimer.html ). The formulas of gemstones can be accessed at http://pubs.usgs.gov/gip/gemstones/formulas.html , and from these you may want to consider having your students classify gemstones into their corresponding mineral groups. Physical properties of minerals are listed at http://www.gemstone.org/gem-by-gem/gem-by-gem.html , while this USGS website discusses environments of formation: http://pubs.usgs.gov/gip/gemstones/environment.html.
7. The rock cycle demonstrates that the three families of rocks—igneous, sedimentary, and metamorphic—are interrelated.
Figure 2.7 The
Rock Cycle
8. The two broad groups of igneous rocks, intrusive (or plutonic) and extrusive (or volcanic) are classified by composition and texture. However, for a few extrusive rocks, texture is the main consideration.
Figure 2.8 Block Diagram Showing Plutons or Intrusive Bodies of Igneous Rock
Figure 2.9 Igneous Rock Textures
Figure 2.10 Classification of Igneous Rocks
Figure 2.11 The Common Igneous Rocks
Figure 2.12 The Igneous Rocks in this Chart are Classified Mainly by their
Textures
9. Lithification involving compaction and
cementation is the process whereby
sediment is transformed into sedimentary rock.
Figure 2.13 Lithification and Classification of Detrital Sedimentary Rocks
10. Sedimentary rocks are also grouped into two broad categories: detrital (made up of solid particles of preexisting rocks) and chemical/biochemical (composed of minerals derived by inorganic chemical processes or the activities of organisms).
Figure 2.14 Detrital Sedimentary Rocks
Figure 2.15 Chemical Sedimentary Rocks
Table 2.2 Classification of Chemical and Biochemical Sedimentary Rocks
Figure 2.16 Evaporites, Chert, and Coal
11. Metamorphic rocks result from compositional and/or textural transformation of preexisting rocks by heat, pressure, and fluid activity. Most metamorphism is regional, occurring deep within the crust over large areas. However, some metamorphism (contact metamorphism) occurs when rocks adjacent to an igneous body are altered.
12. Metamorphic rocks result from compositional and/or textural transformation of
other rocks by heat, pressure, fluid activity, or a combination of these agents. Most metamorphism is regional, occurring deep within the crust over large areas, but some, called contact metamorphism, takes place adjacent to hot igneous rocks. Dynamic metamorphism occurs in zones adjacent to faults where high levels of differential pressure develop.
13. Metamorphism imparts a foliated texture to many rocks (parallel alignment of minerals), but some rocks have a mosaic of equidimensional minerals and are nonfoliated.
Figure 2.17 Foliated Texture
14. The several varieties of metamorphic rocks are
classified largely by their textures,
but
composition is a consideration for some of these rocks.
Table 2.3 Classification of Common Metamorphic Rocks
Figure 2.18 Foliated Metamorphic Rocks
Figure 2.19 Nonfoliated Metamorphic Rocks
15. Plate tectonics, which is driven by Earth’s internal heat coupled with surface
processes such as weathering, erosion, and deposition, accounts for the recycling of
Earth materials in the rock cycle.
Rocks and minerals provide many of the natural resources for the physical structures we build. “If you don’t grow it, you mine it.” Have students investigate the grand variety of raw materials supplied by the mined rocks and minerals in the building industry. The USGS mineral website (http://www.usgs.gov/themes/mineral.html) discusses various commodities. Some mined natural resources are discussed within a collection of fact sheets (http://water.usgs.gov/wid/index-resources.html).
The Mineral Information Institute (http://www.mii.org/commonminerals.php) also lists the common minerals and their uses.
Students can investigate the materials within their own dwellings, and identify them as far as the mineral or rock type, and the location from which items are mined. Some possibilities include aluminum, gypsum (for sheetrock/wallboard), clay, and sand.
The more obvious “rock” building materials—such as that used as flooring material, countertops, or fireplace hearths—are usually classified by building contractors as “marble” or “granite.” However, there are a variety of rocks that are used in construction which are not marble or granite! Students can investigate these various rock countertops and floor tiles, and correctly classify them using the information in the chapter.
1. Is the element mercury a mineral by our definition? Why or why not? Does glacial ice classify as a mineral?
2. If a laboratory gemstone has identical composition and physical properties as a substance found in nature, is it a mineral? Does coal classify as a mineral? Ask students to think of examples where a substance may classify as a rock, but it is not made of minerals.
Common Rock-Forming Minerals
Students can learn more about the common rock-forming minerals. Although there are over 3,500 different minerals, only a few are common within rocks. About.com Geology lists the common minerals that are abundant and widespread, the accessory minerals that are widespread but rarely abundant, and interesting but uncommon minerals. (http://www.mii.org/commonminerals.php) Each mineral has a clickable link that includes a photograph of the mineral, as well as a general description. Ask your students to locate the formulas of the common rock-forming minerals, and then classify them into the mineral groups recognized by geologists (Table 2.1).
Detrital sedimentary rocks are classified primarily by the size of the included particle, not composition. Students should discuss the common meaning of the words “sand” and “clay.” “Sand” and “clay” are used as categories of particle size, although we commonly use “sand” to convey “quartz composition.” (If the green beach sands of Hawaii, which are composed partly of the mineral olivine, were lithified, olivine sandstone would result. Conversely, a sedimentary rock composed of sand-sized feldspar grains is commonly referred to as an “arkose.”). “Clay” is another term that can be used as both a size category, as well as a group of minerals.
Discuss the various paths that rocks can take within the rock cycle, and how one rock type can be transformed into all the other rock types.
1. Can a sedimentary rock form metamorphic rock and igneous rock? Discuss a pathway by which an existing sedimentary rock can become a new sedimentary rock.
2. How does the processing of Earth materials through the various transitions of the rock cycle affect the aspects of the rocks that allow us to date them? In which types of rocks do we find fossils? Are these fossils preserved if the rock containing the fossils is transformed to other rock types within the rock cycle?
1. How can you identify a nonfoliated metamorphic rock, such as marble, from the original rock? What characteristics might nonfoliated metamorphic rocks have that make their identification easier? Would nonfoliated metamorphic rocks be easier to identify in the field, rather than in the laboratory as an assortment of samples?
2. Can we identify specific environments associated with plate tectonic boundaries in which various rock types would form? Are igneous rocks associated with convergent, divergent, and transform plate boundaries? In which environments are metamorphic, and sedimentary, rocks produced?
|
atom |
detrital sedimentary rock |
magma |
|
atomic mass number |
dynamic metamorphism |
metamorphic rock |
|
atomic number |
Element |
mineral |
|
bonding |
evaporite |
pyroclastic material |
|
carbonate mineral |
extrusive igneous volcanic rock |
regional metamorphism |
|
carbonate rock |
geologic record |
rock |
|
chemical
sedimentary rock |
igneous rock |
rock cycle |
|
compound |
intrusive igneous plutonic rock |
rock-forming mineral |
|
contact
metamorphism |
Lava |
sedimentary rock |
crystalline solid lithification silicate
1. Minerals: The Materials of Earth, Earth Revealed #12, Annenberg/CPB
2. Intrusive Igneous Rocks, Earth Revealed #14, Annenberg/CPB
3. Metamorphic Rocks, Earth Revealed #18, Annenberg/CPB
4. Rocks and Minerals, Cambridge Educational Products
5. Standard Deviants School Geology Minerals, Cerebellum Corporation
6. Standard Deviants School Geology, Sedimentary
& Metamorphic Rocks, Cerebellum Collection
7. Core Geology, Ambrose Video
8. Igneous Rocks, Ambrose Video
9. Introduction to Rocks and Minerals, Ambrose Video
10. Metamorphic Rocks, Ambrose Video
11. Sedimentary Rocks, Ambrose Video
1. Hands-On Mineral Identification, Tasa Graphics Arts, Inc.
2. The Study of Minerals, Tasa Graphics Arts, Inc.
3. Tasa Photo CD-ROM, Rock Cycle I, Tasa Graphics Arts, Inc.
4. The Wonders of Rocks and Minerals, Tasa Graphics Arts, Inc.
5. Study of Minerals, RockWare, Inc.
6. Explore Silicate Minerals, Geological Society of America
1. Rocks and Topography,
slide set, Educational Images, Ltd.
2. Rock Specimens and Crystals, slide set, Educational Images, Ltd.
3. Introductory Earth Science Rocks and Minerals, Science Stuff
4. Natural Crystal Collection, Science Stuff
5. Ores of Common Metals, rock collection, Science Stuff
6. Advanced Rock & Minerals Collection, Science Stuff
|
1. b |
5. b |
9. a |
|
2. c |
6. c |
10. d |
|
3. e |
7. d |
|
|
4. a |
8. a |
|
11. Plutonic rocks cool from magma below the Earth’s surface. These rocks
cool slowly and have larger, better-formed crystals. Volcanic rocks that from
in a lava flow at the Earth’s surface are extrusive. These rocks cool more
rapidly and have smaller or nonexistent crystals. Plutonic rocks include
granite and gabbro. Some volcanic rocks are rhyolite, basalt, and obsidian.
12. Atoms are the building blocks of
elements, or the smallest particles that retain the properties of an element.
Elements are composed of only one kind of matter, and have only one kind of
atom. When two or more atoms combine chemically, they form molecules. Molecules
are the smallest units of compounds that still retain the properties of the
compound.
13. When ions of two elements have similar sizes, and the same electrical charge,
they can substitute for each other in the crystal framework. (Both ions “fit”
within a given space, and maintain the overall electrical neutrality of the
compound.)With olivine, the iron and magnesium ions are about the same size,
and have the same electrical charge.
14. As sand or mud accumulates,
compaction occurs from the overlying sediments. This reduces the pore space and
the volume of the deposits. Compaction is using sufficient for the
lithification of mud. For sand participles, cementation is also necessary, and
this involves the precipitation of minerals within the pores. This binds the
sand sediments. Sand-size sediments produce sandstone upon lithification, while
mud-size sediments produce mudrocks. If the mudrock is composed of only silt-sized
particles, siltsone results. If the mudrock is composed of only clay-sized
particles, a claystone results. A shale is a claystone which is fissile.
15. Contact metamorphism occurs when heat and
chemical fluids from an igneous body alter adjacent rocks. The degree of
metamorphism decreases with increasing distance from the body of magma until
the surrounding rocks are unaffected. A type of rock that forms from contact
metamorphism is hornfels. Regional metamorphism takes place over large but
elongated areas as the result of tremendous pressure, elevated temperatures,
and fluid activity. This type of metamorphism is most obvious along convergent
plate boundaries where the rocks are intensely deformed during convergence and
subduction. It can also occur along divergent boundaries. Gneiss is found in
areas with regional metamorphism.
16. The definition of mineral states that the substance must be in the solid phase. Ice is the solid phase of H2O, while liquid water and water vapor are not.
17. Evaporites form by inorganic chemical precipitation of minerals from solutions that are concentrated by evaporation. Common evaporites include rock salt and rock gypsum.
18. Conglomerates and sedimentary breccias are detrital sedimentary rocks. Both are made of gravel-sized particles (particles greater than 2 mm), but a conglomerate has rounded clasts, while a sedimentary breccia has angular clasts.
19. The physical properties of pyrite, galena, and halite differ so that it is possible to distinguish them apart. Pyrite is generally brassy yellow in color with a metallic luster. Although galena also possesses a metallic luster, it is generally lead grey in color, and has a much lower hardness than pyrite. Pyrite can scratch galena, but galena cannot scratch pyrite. Halite is distinguishable from galena and pyrite because it has a nonmetallic luster, and it generally colorless or white. Halite can also be easily distinguished by its salty taste.
20. Silicate minerals include a combination of silicon and oxygen. The subgroups are ferromagnesian silicates, which contain iron, magnesium or both, and nonferromagnesian silicates, which lack these elements. Ferromagnesian minerals are darker and denser than nonferrmagnesian ones.
2. You
could use a salad analogy to distinguish between minerals and rocks. A salad is
composed of many different ingredients, such as lettuce, carrot slices, and
tomatoes. All of these ingredients produce a unique salad. A small variation in
the amount of carrots, lettuce, or tomatoes will still produce a typical
“garden salad,” but if we vary the composition to include boiled shrimp and
crabmeat, we would have to reclassify our salad as a “seafood salad.” With this
analogy, the individual salad vegetables and ingredients are the “minerals” and
the resulting composite salad is the “rock.” Changing the minerals will change
the rock type.
3. The
clasts in Figure 2.14a were transported the greater distance when compared with
the clasts in Figure 2.14b. The clasts in 2.14a are more rounded, and rounding results from wear and
abrasion of transport. Conversely, the angular particles of the breccia were
not transported a great distance before deposition. The clasts would be more
rounded if they were transported any distance from the source.
4. Sedimentary
Rocks: Sedimentary, metamorphic, and igneous rocks at the Earth’s surface can
be weathered and eroded into sediment. These sediments are thentransported and
deposited. Over time, compaction (rom the overburden) and cementation result in
lithification. Sedimentary rocks result.
Metamorphic Rocks: Sedimentary, metamorphic, and igneous rocks may be metamorphosed through contact with magma or lava (contact metamorphism), along a fault zone (dynamic metamorphism), or within large areas where mountain building is taking place. As long as heat, pressure, and/or fluid activity results in the solid state transformation of an existing rock, new metamorphic rocks result.
Igneous Rocks: Sedimentary, metamorphic, and igneous rocks may be heated to the melting point, form magma, and then cool to form new igneous rocks. The magma may cool below the Earth’s surface to form plutonic intrusive igneous rocks, or erupted to the Earth’s surface and cool from lava to form volcanic extrusive igneous rocks.
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