1. Rock Groups
Interlude A

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EARTH5 Figure A.1
A Rock Is Coherent
A rock is a coherent, naturally occurring solid that consists of an aggregate of minerals or, less commonly, a body of glass. Note on the photos the coherence of the rocks. This coherence is the reason they were used to build castles.
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Figure A.2d
A Rock Is Natural
Natural
The illustration shows rocks (formed by nature) and a roadway (not rock since it is not natural).
Manufactured
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4. A Rock Is an Aggregate of Minerals
Figure A.1b
A Rock Is an Aggregate of Minerals
The photo illustrates that rocks are aggregates of numerous minerals.
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5. Cement Holds Rocks Together
Figure A.1a
Cement Holds Rocks Together
Clastic rocks are held together by cement. (Be sure to explain what cement is and how it forms.)
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6. Interlocking Crystals Hold Rocks Together
Figure A.1b
Interlocking Crystals Hold Rocks Together
Crystalline rocks are held together by interlocking crystals.
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7. Rocks Are Classified By Their Origin
Figure A.3a
Rocks Are Classified By Their Origin
Point out that the modern classification scheme is a genetic scheme based upon origin (genesis) of rocks. Now would also be a good time to discuss previous classification schemes such as the Neptunist/Plutonist debate of the late 1700s.
Igneous rocks form by freezing (solidification) of molten rock.
Igneous rock forms from molten material.
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Figure A.3b
Sedimentary rocks form by the cementation of grains or the precipitation of minerals from a water solution.
Sedimentary rocks form by cementation or precipitation.
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Figure A.3c
Metamorphic rocks form when pre-existing rocks change character due to a change in temperature and/or pressure conditions. (Be sure to note that these changes occur in the solid state.)
Metamorphic rocks form from pre-existing rocks due to changes in temperature and/or pressure.
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10. Rock-Forming Environments
Figure A.4
Rock-Forming Environments
Be sure to explain the three different rock types and how/where they form due to different geologic condition represented on Figure A.4
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11. Physical Characteristics of Rocks
Figure A.5a,b
Physical Characteristics of Rocks
Grain size is variable.
Note that grain size and shape are very variable.
Grain shape is variable.
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Figure A.5b
Composition refers to the proportions of chemicals that make up a rock and affects minerals present in rocks.
Texture refers to the configuration of grains in a rock and how those grains connect.
Composition can be variable within the same outcrop.
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Figure A.6
Layering is defined by the difference in grain size.
Foliation is defined by the difference in mineral composition.
Layering can be created by bands of different compositions, grain sizes, or textures.
Foliation can be created by the parallel alignment of inequant grains.
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EARTH5 Figure A.8a
Studying Rocks
Rocks are studied at various scales.
Let students know that geologists study rocks at various scales to understand different aspects of those rocks.
Outcrops reveal relationships between rock and surrounding rocks.
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Figure A.7a
Rock hammer used for its intended purpose
Hand Sample—rock hammers are used to obtain hand samples (fist-sized pieces of rock) for study.
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Figure A.7b
Hand Sample
A hand sample can reveal the mineralogy and texture of a rock.
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Figure A.7c
Hand Lens
Geologists use a hand lens to see details within a hand sample.
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Figure A.8a,b
Making Thin Sections
Thin sections are very thin (~0.03 mm) slices of rock mounted on small glass plates.
Thin sections are studied with polarized light through a petrographic microscope that reveals characteristics of individual mineral grains.
Photomicrographs are photographs of these thin sections.
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19. Petrographic Microscope
Figure A.8c,d
Petrographic Microscope
Thin sections are very thin (~0.03 mm) slices of rock mounted on small glass plates.
Thin sections are studied with polarized light through a petrographic microscope that reveals characteristics of individual mineral grains.
Photomicrographs are photographs of these thin sections.
Thin section viewed in polarized light.
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Figure A.9
Electron Microprobe
For high-tech, more detailed studied of rocks, a scanning electron microscope (SEM) or an electron microprobe can be used. Discuss here how each of these works and what they reveal about rocks.
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21. X-Ray Defractometer (XRD)
EARTH5 Figure A.10b
X-Ray Defractometer (XRD)
For high-tech, more detailed studied of rocks, a mass spectrometer or an X-Ray defractometer can be used. Discuss here how each of these works and what they reveal about rocks.
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Think–Pair–Share
A geologist working in the field finds an outcrop of rock with layers in it.
Since all three classes of rock can contain layers, how can the geologist determine if the rock is igneous, sedimentary, or metamorphic?
Students should think about things like the minerals (or glass) of which the rock is composed, the texture (clastic, foliated, etc.) and also the surrounding bedrock, which might help understand the rock outcrop in question. Certain minerals are found in certain types of rocks (e.g., kyanite forms in metamorphic rocks). Clastic textures tend to be found in some sedimentary and some igneous rocks while interlocking crystals are found in all three rock types. Surrounding rock types can help a geologist get an idea of the environment of formation. Of course, if they are standing at an unconformity, this is more difficult. (of course, this observation depends on whether the students have learned about unconformities yet.)
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Think–Pair–Share
In the late 1700s, the Neptunists and Plutonists vigourously debated the origin of rocks.
Think about where you live.
What observations of rocks and geology can you make to support each of these views?
What observations of rocks and geology show why the Plutonists won the debate?
The answer to these questions will vary depending on location. The instructor needs to consider what rocks the students are able to observe.
This question can also be expanded by showing things like videos of lava flows cooling. Does this change the view of the validity of Neptunism? It can open a discussion of the necessity of studying a wide range of geologic environments to gain a full understanding of how the Earth works. If a geologist never observed an active lava flow or never saw the metamorphic aureole around an igneous intrusion, perhaps Werner’s Neptunist view would be more acceptable. As it is often said, “The best geologist is the one who has seen the most geology.”
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24. Interlude A Photo Credits
1 Photos 12 / Alamy
2-3 (both): Stephen Marshak
4 sciencephotos/Alamy
5 (left): Stephen Marshak;(center): Courtesy David W. Houseknecht, USGS
6 (left): sciencephotos/Alamy; (center): Courtesy of Kent Ratajeski, Dept. of Geology and Geophysics, U of Wisconsin, Madison
7-9 Stephen Marshak
13 (left): Tom Bean; (right): Stephen Marshak
14-17 (all): Stephen Marshak
19 (left): Stephen Marshak; (right): Scenics & Science / Alamy
20 Product photo courtesy of JEOL, USA
21 Courtesy of Joseph H. Reibenspies, Texas A & M University
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