Transcript Geologic Time - Florida Atlantic University

Geologic Time
4,560,000,000 Years - Estimated
Age of the Earth
GLY 2010 – Summer 2014
Lecture 12
1
Principle of Superposition
• Layers on the bottom were
deposited first, and are the
oldest (A older than B, B
older than C, etc.) - think of
paint layers on a wall
• In any unaltered sequence of
rocks, the oldest is at the
bottom, the youngest at the
top
2
Principle of Original Horizontality
Upper layers of the Grand Canyon
3
Tilted Beds
• Beds may be tilted a little
4
Tilted Beds
• Tilted considerably, here about 45° to the
horizontal
5
Tilted Beds
• Tilted vertically
6
Nicolaus Steno
• Formulated both
the Principle of
Superposition and
Principle of
Original
Horizontality in
1669
7
Principle of Uniformitarianism James Hutton
• Geologic processes
happening today operated
in a similar fashion in the
past, so provide guidance
in studying the earth’s
history
8
John Playfair, 1748-1819
In 1802, John Playfair
published "Illustrations of
the Huttonian Theory of the
Earth”
• Explained Hutton’s ideas
more clearly than Hutton
himself had done
•
9
Sir Charles Lyell
• Published
“Principles of
Geology”, which
appeared in three
volumes between
1830 and 1833
10
Relative Dating
• Relative Age is the answers to a question
like, “Which is younger?”
• Relative ages allow us to compare
different geologic formations, and
determine which is the oldest, next oldest,
etc.
11
Principle of
Cross-cutting
Relationships
– Dike
• A feature, such as a dike or fault, that
cuts formations is younger than the
formations it cuts
12
Principle
of Crosscutting
Relationships
– Fault
• The fault is younger than the beds it offsets
13
Principle of Inclusions
• Fragments of other rocks contained
within the body of a rock are older than
the rock
14
Xenolith
•Xenoliths are older than the granite
15
Sedimentary Conglomerate
• Rock fragments in this conglomerate
are older than the conglomerate itself
16
Principle of Faunal Succession William Smith (1769-1839)
• The principle states that over
time, the organisms on earth
have changed in a definite
order that is reflected in the
fossil record
• Within a succession of
fossil-bearing sedimentary
rocks, fossils preserved
within lower layers are older
- the organisms lived and
died before - than those in
overlying layers
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Faunal Succession
• Smith also recognized a key feature: The
order of change in types of fossils from
oldest to youngest in a particular area was
consistent and could be predicted on the
basis of preliminary study
18
Relative Dating in Action
19
First Geologic
Map
• Smith used the Principle
of Faunal Succession to
produce the first
Geologic Map, which is
shown in greatly reduced
format here
• He used colors to
represent rocks of
different ages
20
Index Fossils
• Organisms with
specific
characteristics:
 Short lived
(geologically)
 Widespread
occurrence
 Readily recognized
21
Unconformity
• Gaps in the rock record,
unconformities mark boundaries
between rocks of different ages
• Unconformities may result from
non-deposition (a hiatus), or from
deposition followed by erosion
22
Unconformity in Volcanic Ash
• Outcrop photo of
volcanic ash layers in
Japan
• There is an erosionial
unconformity that
separates earlier folding
in the lower half from
folding (above) after
later ash flows were
deposited.
23
Nonconformity
• Boundary between unlayered igneous or
metamorphic rocks, and overlying sequential
sedimentary rocks
• Lower rocks show evidence of erosion before
the deposition of the sedimentary rocks
24
Angular Unconformity
• Grand Canyon, Arizona
• Unconformity visible at left center of picture
25
Disconformity
• Parallel layers of sedimentary rocks
• Fossils in the rock layers reveal that,
although parallel, the sequence is not
continuous
26
Unconformity Animation
27
Contact Metamorphic Age
Relationship
• Red: Igneous intrusion
• Blue: Contact
metamorphosed country
rock
• Yellow: Country rock
• Country rock was
present when intrusion
occurred, so is older
than the intrusive rock28
Geologic Correlation
• Correlation seeks to establish age
relationships between distant sequences of
rock - often through the use of fossil
assemblages, or index fossils
• A key bed, a distinctive stratum that appears
at several localities, may also be used
29
Correlation
Example
• Grand
Canyon,
Zion,
and
Bryce
Canyon
National
Parks
30
Absolute Age
• Determination of the absolute age is
usually done using radiometric dating
• Absolute ages are expressed in years, or
millions of years, before present
• The use of “billion” is discouraged
because, in British usage, billion means
a million million, or an American
“trillion”
31
Radiometric Dating
• A parent isotope that is radioactive
decays to yield a daughter isotope at
a known rate
 Example:
 14C  14N
 Radioactive decay follows an
exponential decay law
32
Illustration of Radiometric Decay
33
Half-life, t½
• The time
necessary
for half of
the original
atoms of the
parent
isotope to
decay to the
daughter
isotope is
the half life
34
Parent and Daughter Isotopes
• In the previous example,

14C
 14N
• 14C is the parent, and 14N is the daughter
• The half-life, t½, is 5730 years
35
Isotope Systems
Parent
Daughter
Half-life
Dating Range
Materials
Dated
Rb87
Sr87
47 billion
10 million to 4.6
billion
IgneousMetamorphic
U238
Pb206
4.5 billion
10 million to 4.6
billion
IgneousMetamorphic
U235
Pb208
713 million
10 million to 4.6
billion
IgneousMetamorphic
K40
Ar40
1.3 billion
100,000 to 4.6
billion
IgneousMetamorphic
C14
N14
5730
100 to 100,000
Carbon
bearing
material
36
Decay
Chains
• Diagram
illustrates the
decay of 238U to
206Pb
37
Resetting of Radiometric Clocks
• Metamorphic fluids often produce ion
exchange - can partially or totally reset
radiometric clocks
• Because Argon (Ar) is a gas, it may be
lost during various events, especially
reheating during metamorphism
38
Dendrochronology
• Dating by counting tree-rings, which are added
annually
• Comparison of a series of tree rings from one
tree with tree-ring libraries for a particular area
allows exact dating of a sample
39
Technique of Dendrochronology
40
Bristlecone Pine
Dendrochronology
Colossal Bristlecone Ghost -Bristlecones have the ability
to remain standing for centuries after death
41
Varve Chronology
• Lakes often contain recognizable annual
deposits, especially if they freeze in winter
• The lower layer is coarser material deposited
by rapid melting of ice in the warmer months
and is thick and light in color
• Upper layer deposits are fine-grained, thin
and dark-colored, deposited from suspended
particles in quiet water in the winter months
42
Taconite Inlet Project, Ellesmere
Island, Canada
• Inter-disciplinary Taconite Inlet Lakes
Project was initiated on northern
Ellesmere Island to examine the
processes controlling varve sediment
formation in an Arctic lake
43
Location of Ellesmere Island
• The study location, Lake C2, is shown
• Glaciers are indicated by stippled shading
44
Varves in Core Sample, Lake C2
•Lake C2, Ellesmere Island, Canada
45
Implications of Varve Data
• Varve studies can provide accurately
dated samples of rock, which can be used
for isotopic studies
• Isotope studies can provide a great deal
of information about climate change,
especially about the rate of past changes
in climate
46
Geologic Time Scale
• Eons Largest divisions of time, beginning with
the Hadean (4600 to 3800 million years ago)
 Eras - Largest subdivision, defined by dominant life
forms
• Periods - Divisions of eras, based on smaller scale
changes
» Epochs - Divisions of periods in the Cenozoic era,
based on detailed, smaller scale changes
47
Geologic
Time
Scale
48