discovering earth’s history chapter 12, section 1
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Discovering Earth’s Discovering Earth’s HistoryHistory
Chapter 12, Section 1Chapter 12, Section 1
Rocks Record Earth Rocks Record Earth HistoryHistory
Rocks record geological events and Rocks record geological events and changing life forms of the pastchanging life forms of the past
Erosion has removed much of Earth’s rock Erosion has removed much of Earth’s rock record, but enough remains to study and record, but enough remains to study and interpret the pastinterpret the past
The geologic time scale revolutionized the The geologic time scale revolutionized the way people think about time and how they way people think about time and how they perceive our planetperceive our planet
We have learned that Earth is much older We have learned that Earth is much older than anyone had previously imagined and than anyone had previously imagined and that its surface and interior have been that its surface and interior have been changed by the same geological processes changed by the same geological processes that continue todaythat continue today
John Wesley Powell
A Brief History of A Brief History of GeologyGeology The primary goal of geologists is to interpret The primary goal of geologists is to interpret Earth’s historyEarth’s history
Uniformitarianism –Uniformitarianism – the concept that the concept that processes that shaped Earth in the past are processes that shaped Earth in the past are essentially the same as those operating todayessentially the same as those operating today
The forces and processes that we observe The forces and processes that we observe today have been at work for a very long timetoday have been at work for a very long time
The acceptance of uniformitarianism meant The acceptance of uniformitarianism meant the acceptance of a very long history for Earththe acceptance of a very long history for Earth
Remember, to us it may seem that a Remember, to us it may seem that a landscape is unchanging in our lifetimes, but landscape is unchanging in our lifetimes, but they are changing on a scale of hundreds, they are changing on a scale of hundreds, thousands, or even millions of yearsthousands, or even millions of years
Concept CheckConcept Check
How do the laws that govern How do the laws that govern geological processes change geological processes change through time?through time?
The same laws that operated in The same laws that operated in the past still operate today.the past still operate today.
Relative Dating—Key Relative Dating—Key PrinciplesPrinciples
To establish a relative time scale, a few To establish a relative time scale, a few basic rules or principles had to be appliedbasic rules or principles had to be applied
These principles were major breakthroughs, These principles were major breakthroughs, and their discovery and acceptance was an and their discovery and acceptance was an important scientific achievementimportant scientific achievement
Relative Dating –Relative Dating – process by which rocks process by which rocks are placed in their proper sequence or orderare placed in their proper sequence or order
Relative dating tells us the sequence in Relative dating tells us the sequence in which events occurred, not how long ago which events occurred, not how long ago they occurredthey occurred
Nicolaus Steno, a Danish anatomist, Nicolaus Steno, a Danish anatomist, geologist, and priest (1636-1686), is geologist, and priest (1636-1686), is credited with describing a set of geologic credited with describing a set of geologic observations that are the basis of relative observations that are the basis of relative datingdating
Relative DatingRelative Dating
Nicolaus Steno
Law of SuperpositionLaw of Superposition
Law of Superposition –Law of Superposition – states states that in any that in any undeformedundeformed sequence sequence of sedimentary rocks, each bed is of sedimentary rocks, each bed is older than the layers above and older than the layers above and younger than the layers belowyounger than the layers below
This rule also applies for other This rule also applies for other surface-deposited materials, such surface-deposited materials, such as lava flows and beds of ash as lava flows and beds of ash from volcanic eruptionsfrom volcanic eruptions
Law of SuperpositionLaw of Superposition
Principle of Original Principle of Original HorizontalityHorizontality
Principle of Original Horizontality Principle of Original Horizontality –– layers of sediment are generally layers of sediment are generally deposited in a horizontal or nearly deposited in a horizontal or nearly horizontal positionhorizontal position
If you see rock layers that are flat, it If you see rock layers that are flat, it means they haven’t been disturbed means they haven’t been disturbed and are still in their original positionand are still in their original position
If the rocks are tilted, they must have If the rocks are tilted, they must have been moved sometime after been moved sometime after depositiondeposition
Concept CheckConcept Check
To what rock type can the law of To what rock type can the law of superposition and the principle of superposition and the principle of original horizontality be best original horizontality be best applied?applied?
Undisturbed sedimentary rocksUndisturbed sedimentary rocks
Principle of Cross-Principle of Cross-Cutting RelationshipsCutting Relationships Principle of Cross-Cutting Principle of Cross-Cutting
Relationships –Relationships – a rock or fault is a rock or fault is younger than any rock or fault younger than any rock or fault through which it cutsthrough which it cuts
For an example, look at the For an example, look at the following figurefollowing figure
Principle of Cross-Cutting Principle of Cross-Cutting RelationshipsRelationships
InclusionsInclusions
Sometimes inclusions can help the Sometimes inclusions can help the relative dating processrelative dating process
Inclusions are pieces of one rock Inclusions are pieces of one rock unit that are contained within unit that are contained within anotheranother
The rock unit next to the one The rock unit next to the one containing the inclusions must containing the inclusions must have been there first in order to have been there first in order to provide the rock fragmentsprovide the rock fragments
Therefore, the rock containing Therefore, the rock containing inclusions is the younger of the twoinclusions is the younger of the two
InclusionsInclusions
UnconformitiesUnconformities Unconformity –Unconformity – a surface that represents a a surface that represents a
break in the rock record, caused by erosion or break in the rock record, caused by erosion or lack of deposition (angular unconformity, lack of deposition (angular unconformity, disconformity, and nonconformity)disconformity, and nonconformity)
An unconformity represents a long period during An unconformity represents a long period during which deposition stopped, erosion removed which deposition stopped, erosion removed previously formed rocks, and then deposition previously formed rocks, and then deposition resumedresumed
An angular unconformity indicates that during An angular unconformity indicates that during the pause in deposition, a period of deformation the pause in deposition, a period of deformation (folding or tilting) and erosion occurred(folding or tilting) and erosion occurred
Two sedimentary layers that are separated by an Two sedimentary layers that are separated by an erosional surface are called a disconformityerosional surface are called a disconformity
Nonconformities mean the erosional surface Nonconformities mean the erosional surface separates older metamorphic or intrusive separates older metamorphic or intrusive igneous rocks from younger sedimentary rocksigneous rocks from younger sedimentary rocks
Grand Canyon Cross-Grand Canyon Cross-SectionSection
Concept CheckConcept Check
What are the three basic types of What are the three basic types of unconformities?unconformities?
Angular unconformity, Angular unconformity, disconformity, and nonconformitydisconformity, and nonconformity
Correlation of Rock Correlation of Rock LayersLayers In order to develop a geologic time In order to develop a geologic time
scale that can be applied to the entire scale that can be applied to the entire Earth, rocks of similar age in different Earth, rocks of similar age in different regions must be matched upregions must be matched up
Correlation –Correlation – establishing the establishing the sequence of rocks of similar age in sequence of rocks of similar age in different areasdifferent areas
By correlating the rocks from one By correlating the rocks from one place to another, it is possible to place to another, it is possible to create a more complete view of the create a more complete view of the geologic history of a regiongeologic history of a region
Correlation in the Correlation in the Colorado PlateauColorado Plateau
AssignmentAssignment
Read Chapter 12, Section 1 (pg. 336-Read Chapter 12, Section 1 (pg. 336-342)342)
Do Chapter 12 Assessment #1-33 (pg. Do Chapter 12 Assessment #1-33 (pg. 359-360)359-360)
For Section 1: Do #’s 1, 2, 7, 9-10, 12-For Section 1: Do #’s 1, 2, 7, 9-10, 12-1515
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