Objectives

1
Objectives
The Geologic Time Scale

• Describe the geologic time scale.

• Distinguish among the following geologic time
  scale divisions eon, era, period, and epoch.

Vocabulary

• geologic time scale
• eon
• era
• period
• epoch

2
The Geologic Time Scale
The Geologic Time Scale

• By studying the characteristics of rocks and the
  fossils within them, geologists can interpret the
  environments in which the rocks were deposited,
  reconstruct Earths history, and possibly predict
  events or conditions in the future.

3
The Rock Record
The Geologic Time Scale

• Geologists have divided the history of Earth into
  time units based upon the fossils contained
  within the rocks.

The geologic time scale is a record of Earths
history from its origin 4.6 billion years ago to
the present.

• This scale allows the correlation of geologic
  events, environmental changes, and the
  development of life-forms that are preserved in
  the rock record.

4
The Rock Record
The Geologic Time Scale
5
Geologic Time
The Geologic Time Scale

• The oldest division of time is at the bottom of
  the geologic time scale.

• The time scale is divided into units called eons,
  eras, periods, and epochs.

• An eon, measured in billions of years, is the
  longest time unit of the geologic time scale.

• An era, defined by the differences in life-forms
  found in rock and measured in hundreds of
  millions to billions of years, is the
  second-longest span of time on the geologic time
  scale.

6
Geologic Time
The Geologic Time Scale

• Precambrian Time, which makes up approximately 90
  percent of geologic time, is divided into the
  Archean and Proterozoic Eons.

• The end of the Proterozoic, the more recent of
  the two, is marked by the first appearance of
  organisms with hard parts.

7
Geologic Time
The Geologic Time Scale

• Plants and Animals Evolve

• During the Paleozoic Era, the oceans became full
  of a wide diversity of plants and animals land
  plants appeared and were followed by land
  animals.
• The Mesozoic Era is known for the emergence and
  evolution of dinosaurs, reef-building corals,
  large predatory reptiles, and flowering plants
  and trees.
• During the Cenozoic Era, mammals increased both
  in number and diversity, human ancestors
  developed, and grasses and flowering plants
  expanded on land.

8
Geologic Time
The Geologic Time Scale

• Periods of Geologic Time

• Periods, usually measured in terms of tens of
  millions of years to hundreds of millions of
  years, are defined by the life-forms that were
  abundant or became extinct during the time in
  which specific rocks were deposited.

• The Cenozoic is divided into three periods the
  Paleogene, Neogene, and Quaternary.
• The boundaries between the periods of the
  Cenozoic are not marked by extinction events.

9
Geologic Time
The Geologic Time Scale

• Epochs of Geologic Time

• Epochs are smaller divisions of geologic time and
  are usually measured in millions of years to tens
  of millions of years.

• The Cenozoic Periods have been further divided
  into epochs including the Paleocene and the
  Oligocene.
• Different groups of organisms have been used to
  distinguish the various epochs.
• Regardless of how a geologic period was defined,
  each unit contains specific characteristics that
  set it apart from the rest of geologic history.

10
Section Assessment
The Geologic Time Scale

• 1. Match the following terms with their
  definitions.
• ___ eon
• ___ period
• ___ era
• ___ epoch

A. time periods defined by the life forms that
were present usually measured in terms of tens
of millions to hundreds of millions of
years B. smaller divisions of time usually
measured in millions to tens of millions of
years C. the longest period of time measured in
billions of years D. second longest period of
time measured in hundreds of millions to
billions of years
11
Section Assessment
The Geologic Time Scale

• 2. How does the geologic time scale correspond
  with the rock record?

12
Section Assessment
The Geologic Time Scale

• 3. Identify whether the following statements are
  true or false.

______ Precambrian Time represents 90 percent of
geologic time. ______ Human ancestors developed
during the Paleozoic Era. ______ The boundaries
between the periods of the Cenozoic are marked by
mass extinctions. ______ Organisms in Precambrian
Time had soft bodies with no shells or skeletons.
13
End of Section 1
14
Objectives
Relative-Age Dating of Rocks

• Apply the principles for determining relative age
  to interpret rock sequences.

• Describe an unconformity and how it is formed
  within the rock record.

Vocabulary

• uniformitarianism
• original horizontality
• superposition

• cross-cutting relationships
• unconformity
• correlation

15
Relative-Age Dating of Rocks
Relative-Age Dating of Rocks

• The principle of uniformitarianism states that
  the forces that continually change the surface
  features of Earth today have been occurring since
  Earth formed.

• Only the rate, intensity, and scale with which
  the forces occur have changed.
• The resulting sediments and rocks all record an
  environment and fossils within the rocks preserve
  evidence of the life-forms that lived during the
  time of deposition.

16
Principles for Determining Relative Age
Relative-Age Dating of Rocks

• The concept of relative-age dating places the
  ages of rocks and the events that formed them in
  order, but without exact dates.

• This is done by comparing one event or rock layer
  to another.

17
Principles for Determining Relative Age
Relative-Age Dating of Rocks

• Geologic Principles

• The principle of original horizontality states
  that sedimentary rocks are deposited in
  horizontal or nearly horizontal layers.

• The principle of superposition states that in an
  undisturbed rock sequence, the oldest rocks are
  at the bottom and each successive layer is
  younger than the layer beneath.

18
Principles for Determining Relative Age
Relative-Age Dating of Rocks

• Geologic Principles

• The principle of cross-cutting relationships
  states that an intrusion or a fault is younger
  than the rock it cuts across.

19
Principles for Determining Relative Age
Relative-Age Dating of Rocks

• Inclusions

• Relative age also can be determined where an
  overlying rock layer contains particles of rock
  material from the layer beneath it.
• These particles, called inclusions, indicate that
  the rocks in the lower layer are older than those
  on top.

20
Other Means of Determining Relative Age
Relative-Age Dating of Rocks

• The fact that Earth is constantly changing makes
  it difficult to find an undisturbed sequence of
  rock layers.

An unconformity is a gap in the rock record
usually caused by an erosional surface becoming
buried by the deposition of younger rocks.
21
Other Means of Determining Relative Age
Relative-Age Dating of Rocks

• The unconformity is called a disconformity when
  horizontal sedimentary rocks overlie horizontal
  sedimentary rocks.

22
Other Means of Determining Relative Age
Relative-Age Dating of Rocks

• A different type of unconformity exists when
  sedimentary rocks overlie nonsedimentary rocks.

• The contact point between the nonsedimentary and
  sedimentary rock is called a nonconformity.

23
Other Means of Determining Relative Age
Relative-Age Dating of Rocks

• An angular unconformity is created when
  horizontal sedimentary rocks are uplifted and
  tilted, are exposed to erosional processes, and
  deposition resumes.

24
Other Means of Determining Relative Age
Relative-Age Dating of Rocks

• Correlation of Rock Strata

• Correlation is the matching of outcrops of one
  geographic region to another.

• Geologists examine rocks for distinctive fossils
  and unique rock or mineral features to help
  correlate the rock layers.
• Correlation allows geologists to accurately
  locate that same rock layer in another location.

25
Section Assessment
Relative-Age Dating of Rocks

• 1. Match the following terms with their
  definitions.
• ___ original horizontality
• ___ superposition
• ___ unconformity
• ___ correlation

A. principle which states that sedimentary rocks
are deposited in horizontal layers B. a gap in
the rock record C. principle which states that
oldest rocks are at the bottom and that each
successive layer is younger D. matching of
outcrops from one geographic region to another
26
Section Assessment
Relative-Age Dating of Rocks

• 2. What is the principle of uniformitarianism?

27
Section Assessment
Relative-Age Dating of Rocks

• 3. Identify whether the following statements are
  true or false.

______ Relative-age dating allows geologists to
determine the age of rock formations. ______ A
limestone layer is older than a shale layer that
is above it. ______ Rock layers are often found
undisturbed if you dig deep enough. ______ The
grains in a rock layer can be from a younger
layer of rock.
28
End of Section 2
29
Objectives
Absolute-Age Dating of Rocks

• Explain the several different methods used by
  scientists to determine absolute age.

• Describe how objects are dated by the use of
  certain radioactive elements.
• Explain how annual tree rings and glacial varves
  are used to date geologic events.

Vocabulary

• radioactive decay
• radiometric dating
• half-life

• dendrochronology
• varve
• key bed

30
Absolute-Age Dating of Rocks
Absolute-Age Dating of Rocks

• Absolute-age dating enables scientists to
  determine the actual age of a rock, fossil, or
  other object using the decay rate of radioactive
  isotopes.

• Radioactive substances emit nuclear particles at
  a constant rate regardless of any physical or
  environmental changes.
• The original radioactive element is referred to
  as the parent, and the new element is referred
  to as the daughter.
• As the numbers of protons and neutrons change
  with each nuclear emission, the element is
  converted to a different element.

31
Absolute-Age Dating of Rocks
Absolute-Age Dating of Rocks

• Radioactive decay is the emission of radioactive
  particles and the resulting change into other
  elements over time.

32
Use of Radioactive Isotopes
Absolute-Age Dating of Rocks

• In a process called radiometric dating,
  scientists attempt to determine the ratio of
  parent nuclei to daughter nuclei within a given
  sample of a rock or fossil to determine its
  absolute age.

• Because it often takes a long time for the entire
  amount of an isotope to decay, geologists use the
  half-life of an isotope.

33
Use of Radioactive Isotopes
Absolute-Age Dating of Rocks

• Half-life is the length of time it takes for
  one-half of the original amount of an isotope to
  decay.

34
Use of Radioactive Isotopes
Absolute-Age Dating of Rocks

• Carbon-14

• Carbon-14 (C-14) is a radioactive isotope that is
  commonly used to determine the absolute age of
  an object, especially one that is of organic
  origin.

• C-14 is accurate for dating objects up to 75
  000 years old.
• For the dating of a particularly old rock
  sample, a radioactive isotope with a longer
  half-life must be used.

35
Other Ways to Determine Age
Absolute-Age Dating of Rocks

• Naturally occurring materials, such as trees,
  lake-bottom sediment, and volcanic ash can also
  be used to help geologists determine the age of
  an object or event.

36
Other Ways to Determine Age
Absolute-Age Dating of Rocks

• Tree Rings

• The age of a tree can be determined by counting
  the number of annual tree rings in a cross
  section of the tree.
• The widths of tree rings are directly related to
  the climatic conditions during growth periods.
• Dendrochronology is the science of comparing
  annual growth rings in trees to date events and
  changes in past environments.

37
Other Ways to Determine Age
Absolute-Age Dating of Rocks

• Seasonal Climatic Changes

• About 11 000 years ago, continental glaciers
  covered the northern part of the United States.
• Varves are bands of alternating light- and
  dark-colored sediments of sand, clay, and silt
  found in lakes that resulted from summer and
  winter runoff from glaciers.

• Varves from different lakes can be compared to
  determine the ages of glacial lake sediments from
  about 15 000 to 12 000 years ago.

38
Other Ways to Determine Age
Absolute-Age Dating of Rocks

• Distinctive Sediment Layers

• When a widespread sediment layer is formed by an
  instantaneous or short-lived event, geologists
  may be able to determine the time of the event
  through radiometric dating.
• A key bed is a layer which has been dated and
  acts as a time marker, which can be used to
  correlate rock layers across large areas.

39
Section Assessment
Absolute-Age Dating of Rocks

• 1. Match the following terms with their
  definitions.
• ___ half-life
• ___ dendrochronology
• ___ varve
• ___ key bed

A. alternating light- and dark-colored
sedimentary deposits in glacial lakes B. the time
period until the ratio of parent-to-daughter
atoms is equal C. a widespread layer that has
been accurately dated D. the science of comparing
annual growth rings in trees to date events and
environmental changes
40
Section Assessment
Absolute-Age Dating of Rocks

• 2. How old is an object of organic origin if it
  has 25 percent of carbon-14 remaining? Why?

41
Section Assessment
Absolute-Age Dating of Rocks

• 3. Why would rubidium-87 with a half-life of
  48.6 billion years probably not be useful in
  dating an object that is 100 000 years old?

42
End of Section 3
43
Objectives
Remains of Organisms in the Rock Record

• Define fossil.

• Explain several methods by which fossils can be
  preserved.
• Describe the characteristics of an index fossil.
• Discuss how fossils can be used to interpret
  Earths past physical and environmental history.

Vocabulary

• fossil
• evolution
• original preservation
• altered hard part

• permineralization
• index fossil
• mold
• cast

44
Remains of Organisms in the Rock Record
Remains of Organisms in the Rock Record

• Fossils are the evidence or remains of
  once-living plants or animals.

• The fossil record provides evidence of evolution.
  
• Evolution is an adaptive change in the DNA of
  populations as a result of mutation and/or
  environmental change.

• Fossils preserved in the rock record also provide
  information about past environmental conditions
  and can be used to correlate rock layers from one
  area to another.

45
Types of Fossils
Remains of Organisms in the Rock Record

• Fossils with original preservation are the soft
  and hard parts of plant and animal remains that
  have not undergone any kind of change since the
  organisms deaths.

• Such fossils are uncommon because their
  preservation requires extraordinary circumstances
  such as freezing, drying out, or oxygen-free
  environments.

46
Types of Fossils
Remains of Organisms in the Rock Record

• Altered Hard Parts

• Altered hard parts are fossils in which all the
  organic material has been removed and the hard
  parts of a plant or animal have been changed
  either by mineral replacement or by
  recrystallization.

• Permineralization is the process by which pore
  spaces in a fossil are filled in with mineral
  substances.

• During the process of recrystallization, the
  exterior of the shell or a bone remains the same,
  but the shell microstructures are destroyed.

47
Types of Fossils
Remains of Organisms in the Rock Record

• Altered Hard Parts

48
Types of Fossils
Remains of Organisms in the Rock Record

• Index Fossils

• Index fossils are remains of plants or animals
  that can be used by geologists to correlate rock
  layers over large geographic areas or to date a
  particular rock layer.

• An index fossil is easily recognized, abundant,
  and widely distributed geographically and must
  also have lived during a short period of time.

49
Types of Fossils
Remains of Organisms in the Rock Record

• Molds and Casts

• A mold is formed when the original shell parts of
  an organism within a sedimentary rock are
  weathered and eroded.

• A hollowed-out impression, or mold, of the shells
  is left in their place.
• A cast of an organism is created if the cavity
  later becomes filled with minerals or sediment.

50
Types of Fossils
Remains of Organisms in the Rock Record

• Molds and Casts

51
Types of Fossils
Remains of Organisms in the Rock Record

• Indirect Evidence of Past Life

• Trace fossils are indirect evidence of plant and
  animal life.
• Trace fossils can provide information about how
  an organism lived, how it moved, or how it
  obtained food.

52
Why study fossils?
Remains of Organisms in the Rock Record

• The study of fossils allows scientists to
  interpret and describe Earths history.

• Fossils may help scientists find patterns and
  cycles that can be used to predict future
  phenomena, such as climatic changes.
• The study of fossils allows geologists to locate
  energy resources.

53
Section Assessment
Remains of Organisms in the Rock Record

• 1. Match the following terms with their
  definitions.
• ___ original preservation
• ___ altered hard part
• ___ permineralization
• ___ cast

A. fossils in which all organic material has been
removed and bones or shells have been
structurally changed B. process by which pore
spaces are filled in with mineral
substances C. fossils in which soft and hard
parts of an organism have not undergone any kind
of change D. formed when a mold becomes filled
with minerals or sediments
54
Section Assessment
Remains of Organisms in the Rock Record

• 2. What conditions are necessary to allow
  original preservation?

55
Section Assessment
Remains of Organisms in the Rock Record

• 3. How might petroleum geologists use fossils?

56
End of Section 3
57
Section 21.1 Main Ideas
Section 21.1 Study Guide

• Geologists have separated Earths history into
  divisions based upon the fossil record.

• The divisions of the geologic time scale, in
  descending order and decreasing length of time
  spans, are eons, eras, periods, and epochs.

58
Section 21.2 Main Ideas
Section 21.2 Study Guide

• The principles of uniformitarianism, original
  horizontality, superposition, and cross-cutting
  relationships are used to interpret Earths rock
  record and, thus, to describe the planets
  history.

• Unconformities caused by weathering and erosion
  or by periods of nondeposition mark missing
  layers in the rock record.

59
Section 21.3 Main Ideas
Section 21.3 Study Guide

• Absolute-age dating measures the actual age of an
  object such as a mineral, rock, or fossil.

• Radioactive decay is the emission of particles
  from a radioactive atom. The decay rate can be
  used to determine the age of a rock or fossil.
  The time it takes a radioactive element to decay
  to 50 percent of its original mass is known as
  its half-life.
• Tree rings and varves can also determine the
  dates of events and changes in the environment.
  Volcanic ash and meteorite-impact debris create
  key beds that mark the time of the event.

60
Section 21.4 Main Ideas
Section 21.4 Study Guide

• The remains and evidence of plants and animals
  that once lived on Earth are called fossils.

• Fossils preserved in the rock record provide
  information about past environmental conditions,
  evolutionary changes in life-forms, and help
  geologists to correlate rock layers from one area
  to another.

61
Short Answer
Chapter Assessment

• 6. What does the principle of superposition state?

62
Short Answer
Chapter Assessment

• 7. What does the principle of cross-cutting
  relationships state?

63
True or False
Chapter Assessment

• 8. Identify whether the following statements are
  true or false.
• ______ Fallout from a major volcanic eruption
  can form a key bed.
• ______ Homo sapiens evolved in the past .01
  M.Y.B.P.
• ______ Varves are direct evidence of past
  life-forms.
• ______ Dendrochronology is limited to the age
  of the oldest living tree.
• ______ The first fish appeared in the Paleozoic
  Era.
• ______ All organisms contain carbon.

64
Chapter 21 Images
Image Bank
65
Chapter 21 Images
Image Bank
66
Chapter 21 Images
Image Bank
67
End of Custom ShowsThis slide is intentionally
blank.