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PHYSICAL GEOLOGY FIRST SEMESTER LEARNING OBJECTIVES, Ch. 1-11
(Newcomb)
CHAPTER 1 – INTRODUCING GEOLOGY AND AN OVERVIEW OF IMPORTANT CONCEPTS
Overview
Geology uses the scientific method to explain natural aspects of the Earth - for example, how mountains form or why oil
resources are concentrated in some rocks and not in others. This chapter briefly explains how and why Earth's surface, and its
interior, are constantly changing. It relates this constant change to the major geological topics of interaction of the atmosphere,
water and rock, the modern theory of plate tectonics, and geologic time. These concepts form a framework for the rest of the
book. Understanding the "big picture" presented here will aid you in comprehending the chapters that follow.
Learning Objectives
1. Geology is the scientific study of the earth. Physical geology is that division of geology concerned with earth materials,
changes in the surface and interior of the earth, and the dynamic forces that cause those changes.
2. Earthquakes, like Northridge, reflect the sudden release of energy along faults that respond to plate motions.
3. Not all geologic hazards are immediately apparent. For example, most of the deaths associated with the 1985 eruption of the
Nevado del Ruiz volcano in Columbia were caused by a mudflow.
4. Western economic systems depend on abundant supplies of cheap energy.
5. Most geological resources are nonrenewable and their extraction poses potential ecologic damage.
6. Knowledge of geology can enhance appreciation of one's surroundings, such as the scenery produced as mountains are eroded.
7. Earth processes are driven by two heat engines: one that is internal powered by heat from the earth's core; one that is external
powered by the sun.
8. The earth's interior comprises three concentric zones: crust (thin, oceanic crust and thicker, continental crust), mantle (solid
and thickest zone), and core. Lithosphere is the crust and upper mantle that is broken into plates. Asthenosphere is the soft,
"lubricating" layer beneath lithosphere upon which the plates move. Tectonic forces cause vertical and horizontal deformation
from forces within the earth.
9. Plate tectonics is a theory that views the earth's lithosphere as broken into plates that are in motion. At mid-oceanic-ridges,
plate boundaries are diverging because magma rises from the asthenosphere, pushes the ridge crests apart, and solidifies in the
fissures created. Ridges spread at a rate of 1-18 centimeters per year. Transform boundaries occur where plates slide past each
other, such as the San Andreas fault. Converging boundaries reflect subduction, where oceanic plates descend into the mantle
creating either extrusive or intrusive igneous rocks from melting at depth. Metamorphic rocks may be formed from hightemperature and pressure at subduction zones, if melting does not occur.
10. Rocks formed within the earth are pushed to the surface by tectonic forces. They are unstable and reach equilibrium as new
materials formed by the affects of water, solar heating and other surficial processes. Removal of this material, called sediment,
by agents of erosion transports it to a site of deposition where it may become sedimentary rock, when lithified.
11. Geology follows the scientific method (see Box 1.3).
12. Geology involves deep time, vastly greater than human lifetimes or even human contemplation. The earth is about 4.5 billion
years old. Most geological processes are slow and take place over many million years. Fast, to a geologist, is an event or process
completed in a million years or less. Plate motions are relatively fast. Complex life forms have existed on the earth for the past
545 million years. Humans have only been on earth for about 3 million years. Geology follows the scientific method (see Box
1.3).
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CHAPTER 2 - ATOMS, ELEMENTS, AND MINERALS
Overview
This chapter is the first of six on the material of which Earth is made. The following chapters are mostly about rocks. Nearly all
rocks are made of minerals. Therefore, to be ready to learn about rocks, you must first understand what minerals are as well as
the characteristics of some of the most common minerals.
In this chapter, you are introduced to some basic principles of chemistry (this is for those of you who have not had a chemistry
course). This will help you understand material covered in the chapters on rocks, weathering, and the composition of Earth's
crust and its interior. You will discover that each mineral is composed of specific chemical elements, the atoms of which are in a
remarkably orderly arrangement. A mineral's chemistry and the architecture of its internal structure determine the physical
properties used to distinguish it from other minerals. You should learn how to readily determine physical properties and use them
to identify common minerals. (Appendix A of your text is a further guide to identifying minerals).
Learning Objectives
1. Rocks are naturally formed, consolidated material composed of grains or one or more minerals. Minerals are crystalline
(orderly three-dimensional-arrange of atoms).
2. Elements are substances that cannot be broken down by ordinary chemical methods. Atoms are the smallest particles of
elements. They are constructed of protons, neutrons (forming the nucleus) and electrons. Atomic mass number, atomic number
and atomic weight control the “character" of an element, particularly its isotopes.
3. Chemical activity is related to ions and their bonding.
4. Eight elements comprise 98% of the weight of the crust. Oxygen accounts for half the weight of the crust. Silicon is the second
most abundant element in the crust and silicate minerals, combinations of oxygen and silicon, are the most common in the crust.
5. Crystalline substances have a three-dimensional, regularly repeating, orderly pattern of their anions. The silica tetrahedron is
the basic “building block" of most common (silicate) minerals. Silicate structure reflects the arrangement of silica tetrahedra and
the numbers of shared oxygens. These structures include: isolated silicate structure (no shared oxygens), chain-silicates (two
shared oxygens), sheet silicates (three shared oxygens), and framework silicates (four shared oxygens).
6. Minerals are naturally occurring solids that are crystalline (which is to say that it has a periodically repeating arrangement of
atoms) and have a specific chemical composition. Specific chemical composition reflects the orderly internal arrangement of
atoms. Zoning further reflects the orderly arrangement.
7. A small number of rock-forming minerals comprise most of the crust. Five mineral groups (feldspar, quartz, pyroxene,
amphibole, and mica) account for greater than 90% of the earth's crust. Feldspars are the most common crustal mineral, while
olivine is the most abundant mineral in the earth as a whole. Nonsilicates are either native elements or are classified by their
negative ion. These include ore minerals or commercial value.
8. Physical properties are used to identify minerals. These include color, streak, luster, hardness, external crystal form, cleavage,
fracture, specific gravity, special properties (smell, taste, striations, magnetism), and other properties (double refraction, effects
of polarized light, x-ray defraction). Chemical tests can be used to identify minerals.
CHAPTER 3 - IGNEOUS ROCKS, INTRUSIVE ACTIVITY, AND THE ORIGIN OF IGNEOUS ROCKS
Overview
Chapters 3 and 4 are about igneous rocks and igneous processes. (Either chapter may be read first). Chapter 4 focuses on
volcanoes and igneous activity that takes place at Earth's surface. Chapter 3 describes igneous processes that take place
underground. However, you will learn early in this chapter how volcanic as well as intrusive rocks are classified based on their
grain size and mineral content.
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We begin the chapter by introducing the rock cycle. This is a conceptual device that shows the interrelationship between igneous,
sedimentary, and metamorphic rocks. We then begin focusing on igneous rocks. After the section on igneous rocks classification,
we describe structural relationships between bodies of intrusive rock and other rocks in the earth's crust. This is followed by a
discussion of how magmas form and are altered. We conclude by discussing various hypotheses that relate igneous activity to
plate tectonic activity.
Learning Objectives
1. The rock cycle relates the three major rock types, igneous, sedimentary and metamorphic, and the processes by which they are
formed. These processes reflect a lack of equilibrium caused by external forces (weathering and erosion) or internal forces
(tectonism). The major rock types are also related to one another at convergent plate boundaries.
2. Intrusive rocks crystallize from magmas emplaced into country rock. They possess mineralogies identical to volcanic rocks,
but coarse-grained (= slow cooling) textures. Intrusions exhibit both "baked" and chill zone contacts, and they may contain
xenoliths.
3. Names of plutonic rocks are the counterparts of extrusive rocks, sharing their mineralogy, but distinguished by their coarsegrained textures. Mineralogically equivalent rocks include granite-rhyolite, diorite-andesite, gabbro-basalt. The gabbro-basalt
pair is dominated by ferromagnesian minerals and plagioclase feldspar. The granite-rhyolite pair is dominated by feldspars and
quartz. The diorite-andesite pair is composed of feldspars and significant ferromagnesian minerals (30%-50%).
4. Classification systems are arbitrary and there is considerable variation in the composition of granite and rhyolite.
5. Silica content varies significantly among rock types and influences the minerals comprising various rock types. Mafic rocks
contain 50% or less silica by weight. They are silica-deficient and have high magnesium, iron, and calcium content. Silicic
(Felsic) rocks are silica-rich (greater than 65%), and have significant content of aluminum, sodium, and potassium. Intermediate
rocks fall between mafic and silicic (felsic), and Ultramafic rocks, of which peridotite is the most abundant, are composed of
pyroxene and olivine and have less than 45% silica. They have no fine-grained counterparts.
6. Intrusive bodies are defined by size, shape and relationship to country rock. Volcanic necks are the solidified throats of
volcanoes, dikes are discordant, tabular intrusions, while sills, are concordant, tabular intrusions. Plutons crystallize at great
depth, and most are granite. Batholiths are large and discordant, while stocks are small and discordant. Detached bodies of
magma that moved to shallow depths are called diapirs.
7. Granite comprises the bulk of continents. Basalt and to a lesser extent gabbro underlay the oceans, while andesite forms most
volcanoes along continental margins. Ultramafic rocks are thought to form the mantle.
8. Magmas are melted by a combination of the effects of the geothermal gradient, mantle plumes, water under pressure, pressure
release, and mixed mineralogies.
9. Bowen's Reaction Series (Fig. 3.18) explains the variation in rock composition that can be produced from a single magma.
Crystallization proceeds simultaneously along two branches: a discontinuous branch for ferromagnesian minerals that remain
reactive with the magma, and a continuous branch for plagioclase feldspars that exhibit zoning from changes in calcium and
sodium content. These minerals are formed by silicon-oxygen tetrahedral that control their silica content. Any magma left after
the discontinuous and continuous branches are complete is enriched in silica, and the last minerals to form are potassium
feldspar, muscovite and quartz. Differentiation, crystal-settling, partial melting, assimilation, and magma mixing also account for
compositional differences in magmas.
10. Basaltic magmas are produced at diverging plate boundaries from partial melting of the asthenosphere and build oceanic
crust. Mantle plumes produce intraplate volcanism that is basaltic under oceanic crust, and rhyolitic under continental crust.
Converging plate boundaries produce andesite by partial melting, and magmatic underplating that promotes melting of the lower
continental crust for granite production.
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CHAPTER 4 - VOLCANISM AND EXTRUSIVE ROCKS
Overview
Chapters 3 and 4 cover igneous activity. Either may be read before the other. Chapter 3 emphasizes intrusive activity, but it also
covers igneous rock classification and the origin of magmas, which are applicable both to volcanic and intrusive phenomena.
Chapter 4 concentrates on volcanoes and related extrusive activity.
Volcanic eruptions, while awesome natural spectacles, also provide important information on the workings of the Earth's interior.
Volcanic eruptions vary in nature and in degree of explosive violence. A strong correlation exists between the chemical
composition of magma (or lava) and the violence of an eruption. The size and shape of volcanoes and lava flows and their
pattern of distribution on the Earth's surface also correspond to the composition of their lavas.
Understanding volcanism provides a background for theories relating to mountain building, the development and evolution of
continental and oceanic crust, and how the crust is deformed. Our observations of volcanic activity fit nicely into plate-tectonic
theory as described in chapter 3.
Learning Objectives
1. There is a strong correlation between magmatic (lava) chemistry and the violence of an eruption. Size and shape of volcanoes
and associated features also reflect the composition of lavas. Lava is magma that has reached the earth's surface.
2. Volcanic activity can have a beneficial or catastrophic effect on humans. Weathered lavas produce fertile soils, lava fields may
provide geothermal energy, eruptions may produce global cooling by reducing solar radiation.
3. Catastrophic eruptions have killed thousands in places such as Pompeii and Krakatoa, and threaten the Cascade region.
Fatalities have increased in recent centuries because of increased population. Pyroclastic flows represent the greatest threat
causing buildings to collapse or hitting people with fragments. Famines may follow eruptions from destruction of crops and
animals.
4. Viscosity of lava is its resistance to flow. Viscosity reflects gas content and its ability to escape the molten rock. Temperature
at extrusion, silica content, and amount of dissolved gas also control viscosity. Felsic lavas are very viscous, and are associated
with the most violent eruptions. Mafic lavas have low viscosity and produce flows. Observation of active volcanism provides
insight into past events.
5. Explosive volcanic eruptions produce great quantities of pyroclastic material that may be released in dangerous flows. Water
vapor is the most common gas released by volcanoes. Flows form from either outward exploding froth of gas and magma or
gravitational collapse of a vertical column of gas and pyroclastic debris.
6. Igneous rocks are composed primarily of silicate minerals. Felsic rocks form from high viscosity magmas that are silica-rich,
light and high in-potassium, sodium and aluminum; rhyolite is the most common example. Mafic rocks form from low viscosity
magmas that are silica-poor, dark, and exhibit high abundance of magnesium, iron and calcium; basalt is the most commonexample. Intermediate rocks are, of course, intermediate; andesite is the most common example.
7. Texture (size, shape and arrangement of grains). Grain size is the most important textural characteristic and it is controlled by
cooling history and viscosity (Table 4.1). Fast cooling = fine-grained texture, and distinguishes extrusive rocks. Obsidian is
volcanic glass that is not composed of minerals and reflects extremely rapid cooling of very viscous lavas. Porphyritic textures
exhibit phenocrysts from the intrusive, slow-cooling magmatic stage, and matrix from the extrusive, rapid-cooling eruptive stage.
Extrusive rocks are typically vesicular because decreased pressure releases gas from solution within the magma. Explosive
eruptions produce significant pyroclastic material in the form of dust, ash, cinders, bombs and blocks (increasing size) that can
form rocks tuff and volcanic breccia.
8. Volcanoes have a characteristic geomorphology including the cone, vent, and crater. Flank eruptions and caldera formation
may occur. The three major types of volcanoes - shield, cinder, composite - also reflect composition of the lava (Table 4.2).
9. Shield volcanoes have low flank slopes that reflect low viscosity, quiet eruptions of basaltic lavas. Aa and pahoehoe are
typical expressions of the basaltic composition of flows forming these cones.
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10. Cinder cones have very high flank slopes that reflect pyroclastic debris formed because of the high gas content in magmas of
any composition. Composite cones are constructed of alternating pyroclastic layers and lava. Most are composed of andesite and
reflect the circum-Pacific belt. Their eruptions can be very violent. Volcanic domes may form from felsic lavas that are very
viscous and are preceded by violent eruptions (e.g. Mt. St. Helens).
11. Not all volcanic eruptions result in cones. Plateau basalts form from very low viscosity lava floods and may exhibit columnar
jointing. Submarine eruptions produce pillow basalts, particularly along mid-oceanic ridges.
CHAPTER 5 - WEATHERING AND SOIL
Overview
In this chapter, you will study several visible signs of weathering in the world around you, including the cliffs and slopes of the
Grand Canyon and the rounded edges of boulders. As you study these features, keep in mind that weathering processes made the
planet suitable for human habitation. From the weathering of rock eventually came the development of soil, upon which the
world's food supply depends.
How does rock weather? You learned in chapters 3 and 4 that the minerals making up igneous rocks crystallize at relatively high
temperatures and sometimes at high pressures as magma and lava cool. Although these minerals are stable when they form, most
of them are not stable during prolonged exposure at Earth's surface. In this chapter, you see how minerals and rocks change when
they are subjected to the physical and chemical conditions existing at Earth's surface. Rocks undergo mechanical weathering
(physical disintegration) and chemical weathering (decomposition) as they are attacked by air and water. Your knowledge of the
chemical composition and atomic structure of minerals will help you understand the reactions that occur during chemical
weathering.
Weathering processes create sediments (primarily mud and sand) and soil. Sedimentary rocks, which form from sediments, are
discussed in chapter 6. In a general sense, weathering prepares rocks for erosion and is a fundamental part of the rock cycle,
transforming rocks into the raw material that eventually becomes sedimentary rocks.
Learning Objectives
1. Weathering creates sediment and soil by either mechanical or chemical processes. Erosion is the pick-up and removal of
weathering products (transportation).
2. Mechanical weathering causes physical disintegration without compositional change. Frost wedging, frost heaving, and
pressure release cause most mechanical weathering. Sheeting and exfoliation domes develop as a result of pressure release. Plant
growth, burrowing organisms and salt crystal development are additional mechanical weathering processes. Differential
weathering is also a common result of mechanical weathering
3. Chemical weathering causes rock decomposition and new mineral formation that reflects mineral instability because
conditions of formation of the original minerals are significantly different from those of the earth's surface. Oxygen and slightly
acidic rainwater (carbonic acid) are the agents of chemical weathering. The feldspars and ferromagnesian minerals weather to
clay minerals, while quartz does not weather chemically. Calcite dissolves adding calcium and bicarbonate ions to ground water,
while some silica is produced by chemical weathering of feldspars. This dissolved load may eventually be carried to the ocean.
4. Soil is the layer of unconsolidated weathered material on top of bedrock. Clay minerals make important contributions in
holding water and nutrients on their surfaces because they are negatively charged.
5. Soil horizons develop as it matures. The O horizon is the top layer and consists of plant litter and other organic material. The
A horizon is the next layer and is characterized by leaching downward. The B horizon is the zone of accumulation for material
leached from the A horizon. The C horizon is transitional from soil to un-weathered bedrock. Most soils are residual, but
transported soils can be deposited by ice, wind, and running water. Soils thicken with time.
6. The character of the soil depends on the parent material. Soils forming on granite are sandy, while those forming on basalt are
never sandy. Soil types reflect climate. Soils containing large amounts of aluminum and iron oxides are found in wet climates.
Soils formed in arid climates are thinner and contain higher concentrations of calcite. Hardpans form in either wet or dry
climates and have thick B horizons. Laterites form in tropical regions, have thick A horizons and may be mined for aluminum.
The Soil Conservation utilizes a detailed classification of soils provided in Table 5.3.
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CHAPTER 6 - SEDIMENT AND SEDIMENTARY ROCKS
Overview
The rock cycle is a theoretical model of the constant recycling of rocks as they form, are destroyed, and then reform. We began
our discussion of the rock cycle with igneous rock (chapters 3 and 4), and we now discuss sedimentary rocks. Metamorphic
rocks, the third major rock type, are the subject of the next chapter.
You saw in chapter 5 how weathering produces sediment. In this chapter, we explain more about sediment origin, as well as the
erosion, transportation, sorting, deposition, and eventual lithification of sediments to form sedimentary rock. Because they have
such diverse origins, sedimentary rocks are difficult to classify. We divide them into clastic, chemical, and organic sedimentary
rocks, but this classification is not entirely satisfactory. Furthermore, despite their great variety, only three sedimentary rocks are
very common - shale, sandstone, and limestone.
Sedimentary rocks contain numerous clues to their origin and the environment in which they were deposited. Geologists
determine this information from the shape and sequence of rock layers and from the sediment grains and the sedimentary
structures such as a fossils, cross-beds, ripple marks, and mud cracks that are preserved in the rock.
Sedimentary rocks are important because they are widespread and because many of them, such as coal and limestone, are
economically important. About three-fourths of the surface of the continents is blanketed with a relatively thin skin of
sedimentary rocks. Concentrated in sedimentary rocks are important natural resources such as crude oil, natural gas, ground
water, salt, gypsum, uranium, and iron ore.
Learning Objectives
1. Sedimentary rocks have highly diverse origins and are difficult to classify, but only three are very common: shale, sandstone
and limestone. Seventy five percent of continental surfaces are covered by sedimentary rocks.
2. Sediment is unconsolidated particles of either preexisting rocks or chemical precipitates. It is classified by size: gravel> 2
mm< sand> 1/16 mm<"mud," without regard to composition, although most grains of “mud" size are clay minerals.
3. Rounding (grinding away sharp edges) and sorting (separation by size) occur during transportation, usually by streams. Size
decreases downstream in a river. Deposition occurs when agents of transportation lose their energy. Preservation of sediments
requires their burial and is favored in subsiding basins.
4. Lithification converts loose sediment to sedimentary rock, usually by compaction (reduces pore space) and cementation (fills
remaining pore space). These rocks have a clastic texture.
5. Not all sedimentary rocks form from sediment. Some form through crystallization of minerals from solution (example,
calcite). These rocks have a crystalline texture, but that texture can also result from recrystallization that has destroyed an
originally clastic texture.
6. A section on types of sedimentary rocks expands the discussion of clastic versus crystalline textures, and includes organic
rocks as well. Clastic sedimentary rocks are classified by grain size and composition. Breccia (angular) and conglomerate
(rounded) (the term "till" is deleted in 9th edition) have a gravel fraction. Sandstones contain sand-size grains, distinguished as
quartz sandstone (>90% quartz), arkose (>25% feldspar), and graywacke (>15% matrix = silt and clay). Graywackes result from
deposition by turbidity currents. Lithified silt forms siltstone, while combinations of silt and clay form shale. Predominately claysize particles form claystone and mudstone.
7. Limestone is composed mostly of calcite through the action of organisms or as an inorganic precipitate. Varieties include
coquina (cemented shells), bioclastic limestone (coarse-grained fossils), chalk (very fine-grained bioclastic limestone), oolitic
limestone (small spheres of calcite), tufa and travertine (crystalline precipitates) and recrystallized limestone (original texture
lost). Dolomite is a mineral CaMg(CO3)2 and a rock (sometimes called dolostone) that occurs as a replacement of limestone and
destroys its original texture.
8. Chert is a fine-grained, sedimentary rock composed almost entirely of silica. It
can be a replacement, inorganic precipitate, or bioclastic, and may be recrystallized. Evaporites are sedimentary rocks formed
from evaporation of seawater. They have crystalline textures and include rock gypsum and rock salt. Coal forms from
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consolidation of plant material, originally as peat. Compaction transforms peat to coal and several varieties are recognized.
Organic material preserved in marine muds changes to oil and natural gas through increased heat and pressure provided by
burial.
9. Sedimentary structures form before lithification. Horizontal bedding planes are the most common feature of sedimentary rocks
and reflect original horizontality and superposition. Cross bedding is inclined and most common in sandstones as a reflection of
wind or water currents. Graded bedding exhibits a vertical change in grain size and characterizes turbidity current deposition.
Mud cracks require air-drying of very fine-grained sediments. Ripple marks are either symmetric (waves) or asymmetric
(currents) and form in any clastic rock. Fossils are traces of plants or animals buried by sediment and preserved as unaltered
original material, replacements, molds or carbon films. Fossils may occur in any sedimentary rock type, but are most common in
limestones.
10. Formations are bodies of rock recognized as a convenient means to map, describe and interpret the geology of a region. The
first name is a geographic location where it is well exposed, and the second name is its rock type. The bounding surfaces are
called contacts.
11. Source area of the sedimentary rock is determined by the composition of its grains (for example feldspar, quartz and mica
indicate a granitic source). Sedimentary deposits thin away from their source, and sedimentary structures may help determine
direction of current flow.
12. Continental environments include glacial environments, alluvial fans, river channels, that usually have a gravel component.
Flood plains and lakes usually develop shales. Dunes have high angle cross-bedding. Shallow marine environments include
deltas (usually with thick siltstone and shale, cut by sandstone channels), beaches and barrier islands (well sorted, quartz
sandstone), dunes (high angle cross bedding), lagoons (shales), shallow marine shelves (widespread sandstone, siltstone and
shale), and reefs (massive limestone cores). Deep marine environments receive deposition from turbidity currents.
13. The distribution of sedimentary rocks may be controlled by plate tectonics. Convergent boundaries accumulate thick clastic
deposits in sedimentary basins. Turbidity currents dominate forearc basins, while sediments derived from rising mountains fill
backarc basins. These deposits may now be found in mountains marking those plate boundaries. Transform boundaries may have
organic rich deposits, while diverging boundaries form rift valleys with gravels, lake deposits and evaporites.
CHAPTER 7 - METAMORPHISM, METAMORPHIC ROCKS, AND HYDROTHERMAL ROCKS
Overview
This chapter on metamorphic rocks, the third major category of rocks in the rock cycle, completes our description of earth
materials (rocks and minerals). The information on igneous and sedimentary processes in previous chapters should help you
understand metamorphic rocks, which form from pre-existing rocks.
After reading the chapter on weathering, you know how rocks are altered when exposed at Earth's surface. Metamorphism (a
word from Latin and Greek that means literally "changing of form") also involves alterations, but the changes are due to deep
burial, tectonic forces, and/or high temperature rather than surface conditions.
As you study this chapter, try to keep clearly in mind how the chemical composition of a rock and the temperature, pressure, and
water present each contribute to the metamorphic process and the resultant metamorphic rock.
We also discuss the hydrothermally deposited rocks and minerals, which are usually found in association with both igneous and
metamorphic rocks. Hydrothermal ore deposits, while not volumetrically significant, are of great importance to the world's
supply of metals.
Because nearly all metamorphic rocks form deep within the earth's crust, they provide geologists with many clues about
conditions at depth. Therefore, understanding metamorphism will help you when we consider geologic processes involving
Earth's internal forces. Metamorphic rocks are a feature of the oldest exposed rocks of the continents and of major mountain
belts. They are especially important in providing evidence of what happens during subduction and plate convergence.
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Learning Objectives
1. Metamorphism changes the texture or mineralogy (or both) of its parent rock usually in response to high temperature and
pressure within the earth's interior and under conditions that produce ductile (plastic) strain.
2. The chemical composition of the parent rock controls that of the metamorphic rock, although mineralogy may change.
Minerals are stable within a particular temperature range, but that range varies with pressure and presence of other substances.
The upper limits of metamorphism may overlap partial melting.
3. New metamorphic minerals crystallize under high confining pressure and tend to be denser than their low-pressure
counterparts. Differential stress (compression and/or shearing) produces foliated textures described as slaty, schistose, or
gneissic. Water triggers metamorphism and promotes new mineral formation. Metamorphism is a slow process taking place over
millions of years.
4. The classification of metamorphic rocks is based on texture (foliated versus nonfoliated) and mineralogy (chemical
composition is controlled by parent rock).
5. Metamorphism is either contact (high temperatures but low confining pressure) or regional (high-temperatures and high
confining pressure). Metamorphic rocks produced in a contact aureole ("baked zone" of Chapter 3) include hornfels (shale parent
rock), marble (limestone parent rock), and quartzite (quartz sandstone parent rock). All of these rocks have nonfoliated textures.
6. Regional metamorphism usually produces foliated rocks such as greenschist (basalt parent rock) and amphibole schist (basalt
parent rock), although marble and quartzite also form, if their appropriate parent is present. Progressive metamorphism of shale
can produce slate, phyllite, schist or gneiss as temperature and pressure increase. Partial melting produces migmatites.
Retrogressive metamorphism reflects the effects of water movement that allows recrystallization under conditions below the
peak of metamorphism.
7. Plate tectonics, particularly subduction, explain differential stress and temperature variations, which increase toward the
continents because of rising magma from melting at depth.
8. Water plays an important role in metamorphism. Metasomatism involves hot water transporting ions from outside the rock,
and form significant ore deposits. Hydrothermal rocks are formed by crystallization from hot water, most commonly quartz veins
and disseminated ore deposits.
9. The water involved in metamorphism originates as either ground water or water trapped in descending oceanic crust in a
subduction zone.
CHAPTER 8 - TIME AND GEOLOGY
Overview
The immensity of geologic time is hard for humans to perceive. It is unusual for someone to live a hundred years, but a person
would have to live 10,000 times that long to observe a geologic process that takes a million years. In this chapter, we try to help
you develop a sense of the vast amounts of time over which geologic processes have been at work.
Geologists working in the field or with maps or illustrations in a laboratory are concerned with relative time - unraveling the
sequence in which geologic events occurred. For instance, a geologist looking at a photo of the Grand Canyon can determine that
the tilted sedimentary rocks at the bottom of the canyon are older than the horizontal sedimentary rocks above them, and that the
lower layers of the horizontal sedimentary rocks are older than those above them. But this tells us nothing about how long ago
any of the rocks formed. To determine how many years ago rocks formed, we need the specialized techniques of radioactive
isotope dating. Through isotopic dating we have been able to determine that the rocks in the lowermost part of the Grand Canyon
are well over a billion years old.
This chapter explains how to apply several basic principles to decipher a sequence of events responsible for geologic features.
These principles can be applied to many aspects of geology - as, for example, in understanding geologic structures (chapter 15).
Understanding the complex history of mountain belts (chapter 20) also requires knowing the techniques for determining relative
ages of rocks.
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Determining age relationships between geographically widely separated rock units is necessary for understanding the geologic
history of a region, a continent, or the whole Earth. Substantiation of the plate tectonics theory depends on intercontinental
correlation of rock units and geologic events, piecing together evidence that the continents were once one great body.
Widespread use of fossils led to the development of the standard geologic time scale. Originally based on relative age
relationships, the subdivisions of the standard geologic time scale have now been assigned numerical ages in thousands, millions,
and billions of years through isotopic dating. Think of the geologic time scale as a sort of calendar to which events and rock units
can be referred. Its major subdivisions are referred to elsewhere in this book.
Learning Objectives
1. Uniformitarianism (actualism) implies that geologic processes operating today also operated in the past; “the present is the key
to the past." Since rates of deposition and other activities are slow, the expanse of geologic time was necessarily broadened by
application of uniformitarianism, but the term doesn't imply rates were uniform.
2. Absolute time provides a date in years or some other time unit to a rock, while relative time merely arranges events in a
sequence.
3. Geologists think of the geology of an area in terms of the sequence of events that form its history. Four basic principles are
applied to recognize the various steps in the geologic history of an area. Original horizontality implies that the rocks were
horizontal when first formed, and any change from horizontal took place after deposition. Superposition implies that rock
sequences get younger toward their tops. Lateral continuity states that sedimentary layers extend laterally until their edges pass
into another environment reflected by different sediment types. Crosscutting relationships imply that a disrupted rock unit is
older than the cause of its disruption. Figure 8.1-8.11 and Table 8.1 apply these principles to understanding the sequence of
events that developed Minor Canyon, a fictitious location similar to Grand Canyon.
4. Correlation establishes age relationships between rock units or events in separate areas. Physical continuity implies that a rock
unit can be traced from one area to another. Similarity of rock types allows correlation, particularly if the character and sequence
are distinctive. Correlation by fossils utilizes the observation of faunal succession: fossil species occur in a definite and
recognizable succession through time. Index fossils are short-lived and widespread, but most correlations are accomplished by
fossil assemblages.
5. The standard geologic time scale reflects relative time and is based on fossil assemblages. Eras are the largest divisions,
followed by periods, and then epochs. Fossils become common with the beginning of the Paleozoic Era, and rocks that precede
that era are called Precambrian. The Mesozoic Era succeeds the Paleozoic Era, followed by the Cenozoic Era. Those era
boundaries are times of mass extinction. The Cenozoic Era includes the Holocene Epoch in which we are now living. Most
geological investigations involve the use of relative time.
6. Unconformities are gaps in the geologic record developed as buried surfaces of erosion. A disconformity separates beds that
parallel one another. Fossils may indicate a break in the record. Angular unconformities separate tilted older strata from
horizontal younger strata. Fossils and crosscutting relationships may be used to determine the relative time of the folding and
tilting. Nonconformities separate older plutonic or metamorphic rocks from younger sedimentary rocks.
7. Absolute time provides ages, usually in years, to geological features and events. The earth is estimated to be 4.5-4.6 billion
years old. The oldest rocks on earth, 4.03 billion years old, are found in northwestern Canada. The oldest dated mineral is a
zircon from Australia, which is 4.4 billion years old.
8. Absolute dating is based on the decay of radioactive isotopes, particularly uranium. Decay is expressed in half-lifes, the time it
takes for one-half of a given amount of radioactive isotope to be reduced to its stable daughter product. Radioactivity involves
emission of alpha and beta particles and electron capture that may change the atomic number or atomic mass number of the
atom. In contrast, radiocarbon dating involves the formation and breakdown of C-14. Dates are determined by comparing the
amount of C-14 present to what would be expected in a living organism. The half-life is only about 5,730 years and the system
only provides dates on organic materials for the last 40,000 years with accuracy.
9. Dating is based on a comparison of the amount of isotope originally present compared to the amount present at the time of the
analysis and the half-life of the isotope involved. Usually the date provided by the analysis is the time that the rock or mineral
became a closed system. In igneous rocks, that would date mineral crystallization, while in metamorphic rocks, it would be time
of metamorphism. Absolute dates have been assigned to the geologic time scale by bracketing events whose relative time is
known. It has been used to subdivide the Precambrian, which comprises the bulk of geologic time. (N. B. the Precambrian-
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Cambrian boundary has been assigned a date of 551 billion years on the geologic time scale illustrated in Figure 8.26, but it is
stated to be 545 million years on p. 189 and p.194 in the 9th edition).
10. The age of the earth has been a controversial subject and several attempts were made to determine its antiquity prior to the
discovery of radioactivity (biblical, rate of cooling). The age of the earth has been established as 4.5 - 4.6 billion years old based
on isotopic dating of meteorites.
11. Geologic time is vast, mostly represented by the Precambrian, and human history represents an exceedingly small portion of
that time.
CHAPTER 9 - MASS WASTING
Overview
When material on a hillside has weathered (the process described in chapter 5), it is likely to move downslope because of the pull
of gravity. Soil or rock moving in bulk at Earth's surface is called mass wasting. Mass wasting is one of several surficial
processes. Other processes of erosion, transportation, deposition - involving streams, glaciers, wind, and ocean waves - are
discussed in following chapters.
Landsliding is the best-known type of mass wasting. Landslides destroy towns and kill people. While these disasters involve
relatively rapid movement of debris and rock, mass wasting can also be very slow. Creep is a type of mass wasting too slow to be
called a landslide.
In this chapter, we describe how different types of mass wasting shape the land and alter the environment and what factors
control the rapidity or slowness of the process. Understanding mass wasting and its possible hazards is particularly important in
hilly or mountainous regions.
Learning Objectives
1. Mass wasting is movement of bedrock, rock debris or soil downslope as a mass in response to gravity.
2. Mass wasting is classified on the basis of rate of movement, type of material, and nature movement. Rates vary from slow (< 1
cm/year) to rapid. Types of material include bedrock, unconsolidated debris, and soil. Movements include flow (movement as
viscous fluid), slide (movement as coherent mass along defined surface), and fall (free-fall).
3. Slides may be either translational (plane parallel to slope) or rotational (also called slump; movement along a curved surface).
4. Factors promoting mass wasting include: steep slopes, high local relief, thick debris above bedrock, planes of weakness
parallel to hillside, freeze and thaw, saturation of debris with water, long periods of drought with episodes of heavy precipitation,
and sparse vegetation (Table 9.2).
5. Gravity is the driving force for mass wasting. Shear force is parallel to slope. Shear strength is resistance to movement or
deformation. If shear force is greater than shear strength, mass wasting occurs.
6. Water is a critical factor in mass wasting. Small amounts of water actually inhibit mass wasting because surface tension
increases shear strength. As the water content increases, the rate of movement increases, for example a change from creep to
mudflow.
7. Creep is very slow, continuous downslope movement of soil or unconsolidated debris promoted by water in the soil and cycles
of freeze and thaw where shear forces are slightly greater than shear strength.
8. Debris flows are mass wasting taking place as a moving mass, including earthflows, mudflows and debris avalanches.
Earthflow is slow or rapid downslope movement of water saturated debris as a viscous fluid. Rotational sliding (movement along
a curved surface; previously called slumping) is commonly associated with earthflow. Solifluction is a variety of earthflow that
occurs above permafrost in colder climates. Mudflow is a flowing mixture of debris and water usually moving in a channel.
Debris avalanche is a very rapidly moving mass of debris, air and water.
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9. Rockfalls are blocks of bedrock that fall freely or bounce down a cliff. A rockslide is a mass of bedrock moving along an
inclined surface; a rock avalanche is a larger version of the same process. Debris slides and debris falls are the same process
involving movement of a coherent mass of debris rather than bedrock.
10. Landslides can be prevented by recognizing potential problems and proper engineering. Avoiding oversteepening of slopes
and undercutting slopes as well as providing adequate drainage for excess water are useful measures.
CHAPTER 10 - STREAMS AND FLOODS
Overview
Running water, aided by mass wasting, is the most important geologic agent in eroding, transporting, and depositing sediment.
Almost every landscape on earth shows the results of stream erosion or deposition. Although other agents - groundwater,
glaciers, wind, and waves - can be locally important in sculpturing the land, stream action and mass wasting are the dominant
processes of landscape development.
We begin by examining the relationship of running water to the other water in the Earth system. The first part of this chapter also
deals with the various ways that streams erode, transport, and deposit sediment. The second part describes landforms produced
by stream action, such as valleys, flood plains, deltas, and alluvial fans, and shows how each of these is related to changes in
stream characteristics. The chapter also includes a discussion of the causes and effects of flooding, and various measures used to
control flooding.
Learning Objectives
1. Running water (aided by mass wasting) is the most important geologic agent for erosion, transportation, and deposition of
sediment and landscape development on earth.
2. The longitudinal profile of a stream changes from steep to gentle as the stream flows from its headwaters (where valleys are
V-shaped) to its mouth (where valleys are surrounded by a flat flood plain). Stream channels usually contain the stream, but
unchanneled sheetwash can occur, commonly in deserts.
3. Streams drain drainage basins separated from each other by divides. Drainage patterns reflect rock type and structure.
Dendritic drainages form on horizontal, unfractured bedrock. Radial drainages form on high conical mountains. Rectangular
drainages form on fractured or jointed bedrock. Trellis drainages form in areas of tilted bedrock of varying resistance to erosion.
4. Stream erosion and deposition are controlled by velocity and discharge. Velocity is the distance water travels per unit of time.
Maximum velocity is near the middle of the water column and is displaced to the outside of its curves (Fig. 10.6). Figure 10.7
(Hjulstrom’s Diagram, but not labeled as such) illustrates that as velocity increases (for example during a flood), erosion and
transportation of larger grain sizes is accomplished. The point is also made that more velocity is required to erode silt and clay
than sand. Gradient (high vs low), channel shape (narrow vs wide), roughness (smooth vs rough), and discharge (increased
volume of water) influence velocity.
5. Stream erosion involves hydraulic action (ability to pick up and move sediments), solution, and abrasion (grinding of stream
bed by coarse sediment load, resulting in potholes).
6. Stream transportation of sand and gravel is accomplished as bed load (movement by traction that maintains contact with the
stream bed or saltation that involves bouncing along stream bed). Silt and clay are transported by suspension in the water.
Dissolved load comprises soluble ions. Suspension and solution comprise the bulk of a stream's load.
7. Stream deposition reflects a drop in velocity. Bars and braided streams are formed usually by gravel deposited as velocity falls
in streams with high discharge and bed load, and may contain placer deposits. Meandering in the lower reaches of a stream
produce points bars in the inside of meander loops and erode the outside of meander loops. Flood plains are formed by a
combination of point-bar deposits, fine-grained flood deposits, and channel-fill. Natural levees reflect drop in velocity and
deposition along stream channels during flooding.
8. Deltas and alluvial fans reflect drop in velocity as a stream enters a body of water or at the base of mountains respectively.
Sediment supply, waves, and tides control the shape of a delta. Bottomset, foreset and topset beds characterize deltas in
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freshwater lakes. Alluvial fans usually exhibit grading, with coarsest material deposited closest to the mountain front because of
a drop in stream velocity where the channel leaves a canyon.
9. Flooding is a natural process caused by heavy rains and snow melt. Recurrence intervals predict the average time separating
flood events, particularly 100-year floods. Flood erosion, high water and flood deposits are the undesirable results of flood
events. Urbanization enhances flooding by paved areas, storm sewers, and channel constrictions (bridges, docks, buildings).
Flash floods are short-lived events often caused by thunderstorms. Two catastrophic flash floods struck north-central Colorado in
1976 (Big Thompson River) and 1997 (Spring Creek, Cache la Poudre River). Flooding may be partially controlled by dams,
artificial levees, protective walls, and bypasses, but prohibiting building within 100-year flood plains should be encouraged.
10. The Great Flood of 1993 exceeded 100-year discharges for many rivers in the midwest and even the 500-year flood at
Hannibal, Missouri.
11. Erosional downcutting forms stream valleys and is limited by base level, either sea level or a local base level, such as a pond
or lake. Glaciation may lower base level promoting downcutting in stream valleys, or raise base level promoting deposition.
12. Ungraded streams use downcutting to smooth their gradients. Graded streams exhibit a balance between capacity and load
maintained by downcutting and deposition to smooth their gradients. Graded streams typically exhibit downcutting, lateral
erosion accompanied by meandering and valley widening, and headward erosion that lengthens its valley, and produce stream
piracy.
13. Stream terraces are either rock benches, or “stepped" sediment. They reflect a change from deposition to erosion caused by
either regional uplift (which lowers base level and promotes downcutting), or change from dry to wet climate (which increases
the erosional capability of the stream).
14. Incised meanders have no flood plain, as typically found in meandering streams, and reflect either lowered base level or
simultaneous downcutting and lateral erosion of graded streams.
15. Superposed streams occur when uplift allows a stream to erode through sediment burying mountain ranges (e.g. folded
Appalachians).
CHAPTER 11 - GROUND WATER
Overview
Surprisingly, water underground is about 60 times as plentiful as fresh water in lakes and rivers on the land surface (not
including water stored as ice in glaciers). Groundwater is a tremendously important resource. How it gets underground, where it
is stored, how it moves while underground, how we look for it, and, perhaps most important of all, why we need to protect it are
the main topics of this chapter.
Also important is how groundwater is related to surface rivers and springs. Groundwater can form distinctive geologic features,
such as caves, sinkholes, and petrified wood. It also can appear as hot springs and geysers. Hot groundwater can be used to
generate power.
Learning Objectives
1. Ground water represents percolation of about 15% of the precipitation falling to the earth, accounts for .61% of the world's
water, and is second only to glaciers as a source of freshwater.
2. Porosity is the percentage of a rock's volume comprised of openings and it measures the rock's ability to hold water. Most
rocks hold some water in either pores or joints, but porosity is highly variable. Permeability measures the capacity of a rock to
transmit fluids. Many porous rocks are permeable, but shale has high porosity and low permeability.
3. Water percolates into the earth as far as porosity exists and saturates the lower portions of the porous intervals (saturated
zone). The upper surface of the saturated zone is the water table. Above the water table, porosity is filled by air and water,
forming an unsaturated zone (= vadose zone). Water is drawn by capillary action from the saturated zone into the vadose zone.
Perched water tables result from local variations in permeability, such as shale lenses in sandstones.
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4. Ground water movement reflects Darcy's Law (velocity = permeability x hydraulic gradient; explained in Box 11.1). Water
table slope influences ground water velocity: steeper slopes have faster movement.
5. Aquifers are porous and permeable rock bodies through which ground water moves easily. Aquitards have low permeability
and/or porosity that prevent ground water movement. Unconfined aquifers have water tables and exhibit rapid ground water
movement. Confined aquifers lack water tables and exhibit slow ground water movement.
6. Wells must penetrate a saturated zone to produce ground water. Water tables fluctuate with season. If the water table falls
below the bottom of a well, it is dry. Drawdown from pumping wells produces local lowering of the water table. Recharge raises
the water table. A cone of depression may form around a well because of drawdown. In artesian wells, water rises above its
confined aquifer because the ground water is under pressure because of elevation differences.
7. Springs form where the water table intersects the surface, or structures bring water to the surface. The surface of gaining
streams is the water table, while losing streams lie above the water table.
8. Rain can leach surface contaminants and move them into ground water. Human activity produces potential pollution from
pesticides, herbicides, fertilizers, heavy metals and toxic compounds, bacteria, viruses and parasites from animal, plant and
human waste, acid mine drainage, and radioactive waste (both low level and high level). Gasoline may float on the water table.
Some pollutants are naturally occurring. Some filtration and purification can be expected through ground water flow, if it is slow.
Heavily pumped wells near coasts can be contaminated by saltwater intrusion.
9. Dropping water tables create problems with supply and subsidence, through compaction. Artificial recharge may offset these
problems.
10. Natural ground water is slightly acidic because of dissolved carbon dioxide from the atmosphere or soil gases. Its contact
with calcite in limestone causes solution forming caves, sinkholes and karst topography. Calcium and bicarbonate in solution can
be precipitated as calcite in the form of stalactites, stalagmites, columns and flowstone.
11. Ground water may also form petrified wood, concretions, geodes (bodies in Figure 11.26 are amygdules), cement
sedimentary rocks, and develop alkali soils.
12. Hot springs have ground water warmer than the human body. Heating of the water is by either proximity to a magma
chamber, or through the geothermal gradient. Geysers erupt periodically because constrictions in conduits to the surface allow
the temperature of the ground water to rise to vapor, which then condenses as the eruption proceeds. Hot ground water produces
deposits of sinter (silica) or travertine (calcite). Geothermal energy is derived from hot ground water through the production of
electricity from natural steam. It is also utilized for heating, paper manufacturing, ore processing, food preparation and other
non-electric uses.
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