Rocks: Materials of the Solid Earth opens with a discussion of the rock cycle which presents a general
overview of the origins and processes involved in forming the three major rock groups—igneous rock,
sedimentary rock, and metamorphic rock. A discussion of the crystallization of magma is followed by an
examination of the classification, textures, and compositions of igneous rocks. After presenting the processes
of mechanical and chemical weathering, the classification of sedimentary rocks, as well as some of their
common features, is discussed. The chapter concludes with an investigation of the agents of metamorphism,
the textural and mineralogical changes that take place during metamorphism, and some common metamorphic
rocks.
Popular Misconceptions
•
Rocks are unchanging—and although most students acknowledge sediments as having originatedfrom solid earth materials, they view the Earth’s surface as a static environment.
•
Natural processes such as volcanoes, streams, and glaciers are viewed as episodic, unrelated events
rather than part of the rock cycle and the formation of rocks.
Learning Objectives
After reading, studying, and discussing the chapter, students should be able to understand:
•
Rock cycle.•
The geologic processes involved in the formation of each rock group.•
Crystallization of magma.•
Criteria used to classify igneous rocks.•
Names, textures, and environments of formation for the common igneous rocks.•
The processes of mechanical and chemical weathering.•
Sources of materials that accumulate as sediment.•
Criteria used to classify sedimentary rocks.•
Detrital and chemical sedimentary rocks.•
Names, textures, and environments of formation for the common sedimentary rocks.•
Common features of sedimentary rocks.•
Agents of metamorphism.•
Criteria used to classify metamorphic rocks.•
Names, textures, and environments of formation for the common metamorphic rocks.Chapter Outline___________________________________________________________________
I. Rock cycle
A. Shows relations among three rock types
B. Proposed by James Hutton in late 1700s
C. The cycle
1. Magma
2. (Crystallization)
3. Igneous rock
4. (Weathering)
5. (Transportation)
6. (Deposition)
7. Sediment
8. (Lithification)
9. Sedimentary rock
10. (Metamorphism)
11. Metamorphic rock
12. (Melting)
13. Magma
D. Full cycle does not always take place
1. "Shortcuts" or interruptions
a. e.g. Sedimentary rock melts
b. e.g. Igneous rock metamorphosed
c. e.g. Sedimentary rock weathers
d. e.g. Metamorphic rock weathers
II. Igneous Rocks
A. Form as magma cools and crystallizes
1. Rocks formed inside Earth are called plutonic, or intrusive
rocks
2. Rocks formed on the surface are called volcanic, or extrusive
rocks
B. Crystallization of magma
1. Ions arranged into orderly patterns
2. Crystal size is determined by rate of cooling
a. Slow–large crystals
b. Fast–microscopic crystals
c. Very fast–glass
C. Classification
1. Uses texture and mineral composition
a. Texture
1. Size and arrangement of crystals
2. Types
a. Fine-grained
b. Coarse-grained
c. Porphyritic (two sizes)
d. Glassy
b. Mineral composition–explained by Bowen's reaction series
1. Mineral crystallization
2. Crystal settling
D. Naming igneous rocks
1. Basaltic rocks
a. From first minerals to crystallize
b. Rich in iron and magnesium
c. Low in silica
d. Common rock–basalt
2. Granitic rocks
a. From last minerals to crystallize
b. Mainly feldspar and quartz
c. High silica
d. Common rock–granite
III. Weathering
A. Response to a changing environment
B. Two types
1. Mechanical weathering
a. Broken into smaller pieces
b. Processes
1. Frost wedging
2. Expansion from unloading
3. Biological activity
2. Chemical weathering
a. Alters internal structure of minerals
b. Water most important agent (oxygen will oxidize materials)
c. Acidic water destroys a mineral's structure
d. Weathering of granite produces
1. From feldspar
a. Clay (most abundant)
b. Potassium ions (dissolved)
c.
Silica (dissolved)
2. Quartz
a. Resistant
b. Becomes
grains
IV. Sedimentary rocks
A. Form from sediment–weathered products
B. About 75% of outcrops on continents
C. Used to construct much of Earth's history
1. Clues to past environments
2. Information about sediment transport
3. Fossils
D. Economic importance
1. Coal
2. Petroleum and natural gas
3. Sources of iron and aluminum
E. Classification
1. Two groups based on source of material
a. Detrital rocks
1. Material is solid particles
2. Classified by particle size
3. Common rocks
a. Shale (most abundant)
b. Sandstone
c.
Conglomerate
d. Siltstone
b. Chemical rocks
1. From material once in solution
a. Precipitates to form sediment directly
b. Through life processes (biochemical origin)
2. Common rocks
a. Limestone (most abundant)
b.
Travertine
c. Microcrystalline quartz
1. Chert
2. Flint
3. Jasper
4. Agate
d. Evaporites
1. Rock salt
2. Gypsum
e. Coal
1. Lignite
2. Bituminous
F. Lithification
1. Sediments transformed into solid rock
2. Processes
a. Compaction
b. Cementation by the materials
1.
Calcite
2. Silica
3. Iron oxide
G. Features
1. Strata, or beds (most characteristic) (bedding planes separate
strata)
2. Fossils
a. Traces or remains of prehistoric life
b. Most important inclusions
c. Help determine past environments
d. Time indicators
e. Matching rocks from different places
V. Metamorphic rocks
A. "Changed form" rocks
B. Can form from
1. Igneous rocks
2. Sedimentary rocks
3. Other metamorphic rocks
C. Degrees of metamorphism
1. Shows in rock's texture and mineralogy
2. Types
a. Low-grade (e.g. shale becomes slate)
b. High-grade–original features are obliterated
D. Metamorphic settings
1. Regional
metamorphism
a. Over extensive
areas
b. Greatest volume of rock produced
2. Contact metamorphism
a. Near a mass of magma
b. "Bakes" surrounding rock
E. Metamorphic agents
1. Heat
2. Pressure
a. From burial
b. From stress
3. Chemically active fluids
a. Water (most common)
b. Ion exchange among minerals
F. Textures
1. Foliated
a. Minerals in parallel alignment
b. Minerals perpendicular to force
2. Nonfoliated
a. Contain equidimensional crystals
b. Resembles coarse igneous rock
G. Classification
1. Based on texture
2. Two groups
a. Foliated rocks
1. Slate
a. Fine-grained
b. Splits easily
2. Schists
a. Strongly foliated
b. "Platy"
c. Types based on composition (e.g. mica schist)
3. Gneiss
a. Elongated, granular minerals
b. Strong segregation of silicates
c. "Banded" texture
b. Nonfoliated rocks
1. Marble
a. Parent rock is limestone
b. Building stone
c. Variety of colors
2. Quartzite
a. Parent rock - sandstone
