Transcript Section 2 Determining Absolute Age Chapter 8

Chapter 8
The Rock Record
1
Chapter 8
Section 1 Determining Relative Age
Objectives
• State the principle of uniformitarianism.
• Explain how the law of superposition can be
used to determine the relative age of rocks.
• Compare three types of unconformities.
• Apply the law of crosscutting relationships to
determine the relative age of rocks.
2
James Hutton
• Scottish physician and
gentleman farmer is
often called the “founder
of modern geology.”
• Drew conclusions based
on observations.
• “The present is the key
to the past.”
1728-1797
3
Charles Lyell
also called the “founder of modern geology.”
1797 - 1875
• Rebelled against the
prevailing theories of
geology of the time.
• He thought the theories
were biased, based on the
interpretation of Genesis.
• He thought the action of
the rain, sea, volcanoes
and earthquakes explained
the geological history of
more ancient times.
4
Principle of Uniformitarianism
• The physical, chemical and biological
processes observed on a daily basis, have
also acted on the Earth over very long
periods of time.
• Observations of current geological
processes could be used to interpret the
rock record of very old geologic events.
5
Principle of Uniformitarianism
•
Rock form in flat, horizontal layers at the
surface of the earth (due to gravity)
6
Earth’s Age
• Hutton’s ideas raised serious questions
about Earth’s age
• Before Hutton most scientists thought
Earth was only 6,000 years old
• Hutton’s ideas about uniformitarianism
encouraged other scientists to learn
more about Earth’s history.
7
Geology Today
• Basic Assumption:
– The Earth works in an orderly fashion in
which natural phenomenon will recur
given the same set of conditions.
8
Chapter 8
Section 1 Determining Relative Age
Spot Check
• What evidence did Hutton propose to show
that Earth is very old?
• Hutton reasoned that the extremely slowworking forces that changed the land on his
farm has also slowly changed the rocks that
make up Earth’s crust.
9
Relative Age
• The age of an object in relation to the
ages of other objects
• Layers of rock, called strata, show the
sequence of events that took place in
the past.
10
Law of Superposition
• The law that a sedimentary rock layer is
older than the layers above it and
younger than the layers below it if the
layers are not disturbed
• As the sediments accumulate, they
harden into layers called beds.
• The boundary between two beds is
called a bedding plane.
11
Law of Superposition
• Rocks that are
deeper below the
earth’s surface are
older than those
close to the surface
12
Law of Superposition
13
Law of Superposition
The diagram below illustrates the law of Superposition.
14
Principle of Original
Horizontality
• Scientist know that sedimentary rock
generally forms in horizontal layers.
• States that sedimentary rocks left
undisturbed will remain in horizontal
layers.
• Tectonic forces can displace rock layers
from their original position
15
Principle of original horizontality
16
Principle of original horizontality
• Three clues to the principle
• #1 Graded Bedding- heavy particles
settle to the bottom of a lake or river
faster than smaller particles
17
18
Graded Bedding
19
Principle of original horizontality
Three clues to the principle
• #2 Cross-Beds As sand slides down
the slope of a large sand dune, the
sand forms slanting layers like those
shown in Figure 4.
20
Cross-Beds
21
22
Principle of original horizontality
Three clues to the principle
• # 3 Ripple Marks waves move back
and forth on a beach, ripple marks
commonly form
23
24
25
Unconformities
• A break in the geologic record created when
rock layers are eroded or when sediment is
not deposited for a long period of time
• Movements of Earth’s crust can lift up rock
layers that were buried and expose them to
erosion.
26
Unconformities
The diagram below illustrates the three types of
unconformities.
• A nonconformity
• An angular unconformity
• A disconformity
27
Relative Dating: Key principles
Unconformities
3 types
 Angular unconformity
Tilted/folded sedimentary rocks overlain by
younger, more flat layers
 Disconformity
 Nonconformity
28
Recipe for an angular unconformity
Deposition
Folding/Uplift
Erosion
Subsidence/more erosion
29
Relative Dating: Key principles
Unconformities
3 types
 Angular unconformity
 Disconformity
Layers on either side of
disconformity are parallel
 Nonconformity
30
31
Relative Dating: Key principles
Unconformities
3 types
 Angular unconformity
 Disconformity
 Nonconformity
Interface between sedimentary layers and
metamorphic or igneous rock
32
33
The Great Unconformity of the Grand Canyon
34
Angular unconformity,
Grand Canyon
35
South rim of the Grand Canyon
250 million years old
550 million years old
1.7 billion years old
Nonconformity
36
Nonconformity in the Grand Canyon
Tapeats Sandstone
(~550 million years old)
Vishnu Schist
(~1700 million years old)
37
Law of Crosscutting
Relationships
• The principle that a fault or body of
rock is younger than any other body of
rock that it cuts through.
• A fault or an intrusion is always younger
than the layers it cuts through,
38
Crosscutting Relationships
The law of crosscutting relationships can be used to
determine the relative ages of rock layers .
39
Relative Dating: Key principles
Principle of cross-cutting relationships
40
41
Telling time geologically
 Earth’s history concealed in rocks
 Goal of geology: unraveling Earth history
 Principle time keeping devices:
Relative dating - putting rocks/events in proper order
Absolute dating - determining event’s actual time
42
Chapter 8
Section 3 The Fossil Record
Interpreting the Fossil Record
• Fossils: the trace or remains of an
organism that lived long ago, most
commonly preserved in sedimentary
rock
• Paleontology: the scientific study of
fossils
62
Chapter 8
Section 3 The Fossil Record
Interpreting the Fossil Record
• Fossils are an important source of
information for finding the relative and
absolute ages of rocks.
• Fossils also provide clues to past
geologic events, climates, and the
evolution of living things over time.
63
Chapter 8
Section 3 The Fossil Record
Interpreting the Fossil Record
• Almost all fossils are discovered in
sedimentary rock.
• The fossil record provides information
about the geologic history of Earth.
• Scientists can use this information to
learn about how environmental changes
have affected living organisms.
64
Stephen Marshak
65
66
Chapter 8
Section 3 The Fossil Record
Fossilization
• Only dead organisms that are buried
quickly or protected from decay can
become fossils.
• Generally only the hard parts of
organisms, such as wood, bones, shells,
and teeth, become fossils.
• In rare cases, an entire organism may
be preserved.
67
68
Chapter 8
Section 3 The Fossil Record
Fossilization
Mummification
• Mummified remains are often found in
very dry places, because most
bacteria which cause decay cannot
survive in these places.
69
Chapter 8
Section 3 The Fossil Record
Fossilization
Amber
• Hardened tree sap is
called amber. Insects
become trapped in the
sticky sap and are
preserved when the
sap hardens.
70
Chapter 8
Section 3 The Fossil Record
Fossilization
Tar Seeps
• When thick petroleum oozes to Earth’s
surface, the petroleum forms a tar seep.
• Tar seeps are commonly covered by water.
• Animals that come to drink the water can
become trapped in the sticky tar.
71
Mummification
72
Chapter 8
Section 3 The Fossil Record
Fossilization
Freezing
• The low temperatures of
frozen soil and ice can
protect and preserve
organisms.
73
Chapter 8
Section 3 The Fossil Record
Fossilization
Petrification
• Mineral solutions such as groundwater
replace the original organic materials
that were covered by layers of sediment
with new materials.
• Some common petrifying minerals are
silica, calcite, and pyrite.
74
Petrification
• Tree
75
Chapter 8
Section 3 The Fossil Record
Types of Fossils
• Trace fossil a fossilized mark that
formed in sedimentary rock by the
movement of an animal on or within
soft sediment
• A trace fossils in an important clue to
the animal’s appearance and activities.
76
Chapter 8
Section 3 The Fossil Record
Spot Check
• What is a trace fossil?
• A trace fossil is a fossilized evidence of
past animal movement, such as tracks,
footprints, borings, or burrows, that can
provide information about prehistoric
life.
77
Chapter 8
Section 3 The Fossil Record
Types of Fossils
Imprints
• Carbonized imprints of leaves, stems, flowers,
and fish made in soft mud or clay have been
found preserved in sedimentary rock.
• When original organic material partially
decays, it leaves behind a carbon-rich film. An
imprint displays the surface features of the
organism.
78
Stephen Marshak
79
D.05
80
81
Chapter 8
Section 3 The Fossil Record
Types of Fossils
Molds and Casts
• Shells often leave empty cavities called molds
within hardened sediment.
• When a shell is buried, its remains eventually
decay and leave an empty space.
• When sand or mud fills a mold and hardens,
a natural cast forms.
• A cast is a replica of the original organism.
82
Chapter 8
Section 3 The Fossil Record
Index Fossils
• A fossil that is used to establish the age
of rock layers
• It must be
–
–
–
–
It must be distinct
abundant
widespread
existed for only a short span of geologic time.
• Paleontologists can use index fossils to
determine the relative ages of the rock
layers in which the fossils are located.
83
Coprolites
84
Gastroliths
85
Chapter 8
Section 3 The Fossil Record
Index Fossils and Absolute Age
• Scientists can use index fossils to
estimate absolute ages of specific rock
layers.
• Scientists can also use index fossils to
date rock layers in separate area.
• Index fossils are used to help locate
rock layers that are likely to contain oil
and natural gas deposits.
86
Chapter 8
Section 3 The Fossil Record
Index Fossils
87