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