Geologic Time
John Wesley Powell Headed an Expedition through the Grand
Canyon that Resulted in his Recognition of the Long &
Complicated Geologic History of the Earth
Determining geological ages
!  Relative age dates – placing rocks and
events in their proper sequence of
formation
!  Numerical dates – specifying the actual
number of years that have passed since
an event occurred (known as absolute age
dating)
Principles (Methods) of relative
dating
!  Law of superposition
•  Developed by Nicolaus Steno in 1669
•  In an undeformed sequence of
sedimentary rocks (or layered igneous
rocks), the oldest rocks are on the bottom
Superposition is well illustrated by
the strata in the Grand Canyon
Principles of relative dating
!   Principle of original horizontality
•  Layers of sediment are generally deposited in a
horizontal position
•  Rock layers that are flat have not been disturbed
!   Principle of original horizontality
•  Rock layers that are NOT flat have been
disturbed
Principles of relative dating
!  Principle of cross-cutting relationships
•  Younger features cut across older feature
Older
Rocks
Younger
(CrossCutting)
Rock
Cross-cutting relationships aid relative age dating
Principles of relative dating
!  Inclusions
• An inclusion is a piece of rock that is
enclosed within another rock
• Rock containing the inclusion is
younger
Inclusions of Older (in
this case, igneous) Rock
in the Overlying
Sedimentary Layers
Indicate that the Igneous
Rock was Formed &
Weathered Before the
Sedimentary Layers
Were Deposited. Hence
the Sedimentary Layers
are Younger
Principles of relative dating
!  Unconformity
•  An unconformity is a break in the rock
record produced by erosion and/or
nondeposition of rock units
•  The unconformity represents “missing
deposits for a given interval of time”,
although the amount of time may be
uncertain
Principles of relative dating
!  Unconformity
•  Types of unconformities
–  Angular unconformity – tilted rocks are
overlain by flat-lying rocks
–  Disconformity – strata on either side of the
unconformity are parallel
–  Nonconformity – metamorphic or igneous
rocks in contact with sedimentary strata
Creation of an Angular
Unconformity Requires
Deposition, Folding and
Probable Erosion of
Older Sedimentary
Rocks Before
Deposition of Younger,
Flat-Laying Sediments
Angular Unconformity - Rocks Beneath the Unconformity
Surface Are Vertical, Rocks Overlying the Surface Are Gently
Dipping to the Left
Disconformity (Missing Deposits) Between 2 FlatLying Formations in the Guadalupe Mountains
Grayburg Formation
San Andres Formation
Nonconformity Between Metamorphic Rocks of the
PreCambrian Vishnu Schist & Paleozoic
Sedimentary Rocks in the Grand Canyon
Paleozoic Sedimentary
Rocks
Vishnu Schist
All Three Types of Unconformities Occur in the Grand Canyon
Using Relative Dating
Methods in Hypothetical
Situations
F
E
C
J
I
H
G
E
B
D
A
C
D
B
A
Answer to Block Diagram
1.  Deposit Layers A-F
2.  Fault A Cuts Layers A-F
3.  Deposit Layers G-J
4.  Fault B Cuts Layers A-J
5.  Batholith Intrudes Deposits & Fault B
6.  Dike B Cuts Batholith, Fault B and Sediment Layers
Through E
7.  Sill Forms Between Layers E & G
8.  Dike A Cuts Sill & Sediments A-J
Relative Age Dating Even Works on Other
Planets/Moons
Younger Lava Flow
Superimposed
Above Older Flow
Older Lava Flow
Cross-Cutting
Relationships Can Also
Be Used on Other
Planets or Moons.
Which Impact Crater
is Younger in this
Photo from our Moon?
Correlation of rock layers
!  Matching of rocks of similar ages in
different vertical sections is known as
correlation
!  Locally, correlation may be accomplished
by “walking out” the same bed (deposit)
from one vertical section to another
In this Example from the North Wall of Slaughter
Canyon, Beds at Vertical Sections A and B can be
Literally Walked Laterally and Demonstrated to be
of the Same Age (Correlative)
A
B
Correlation of rock layers
!  Where rocks are separated by vast
distances or by “covered interval”,
correlation becomes more difficult
!  In such cases, correlation often relies
upon fossils
•  William Smith (late 1700s) noted that
sedimentary strata in widely separated
area could be identified and correlated by
their distinctive fossil content
Fossils: Evidence of past life
!  Fossil = traces or remains of prehistoric
life now preserved in rock
!  Fossils are generally found in sediment or
sedimentary rocks (rarely in
metamorphic rocks and never in igneous
rocks)
!  Paleontology = study of fossils
Fossils: Evidence of past life
!  Geologically fossils are important
because they
•  Aid in interpretation of the geologic past
•  Serve as important time indicators
•  Allow for correlation of rocks from
different places
Fossils: Evidence of past life
!  Types of fossils
•  The remains of relatively recent
organisms – teeth, bones, etc.
•  Entire animals, flesh included
•  Given enough time, remains may be
petrified (literally “turned into stone”)
•  Molds and casts
•  Carbonization
Fossils of many
relatively recent
organisms are
unaltered
remains. Such
objects as bones,
teeth, & shells
are common
examples. In
this historic 1914
photo of the La
Brea tar pits in
Los Angeles,
bones of an IceAge mammal are
being excavated.
Petrified Wood (tree
truck) from Petrified
Forest National Park,
Arizona
Ammonite casts from
Europe (internal
molds of invertebrate
cephalopods)
Insect in amber –
does it contain
dinosaur DNA?
Fossil bee preserved
as thin carbon film
Fossils: Evidence of past life
!  Types of fossils
•  Others
–  Tracks
–  Burrows
–  Coprolites (fossil dung)
–  Gastroliths (polished stomach stones)
Dinosaur track
from Tuba City,
Arizona
Carbon films &
impressions of
Eocene fossil fish
from Wyoming
Fossils: Evidence of past life
!  Conditions favoring fossil preservation
•  Rapid burial
•  Possession of hard parts (skeleton, shell,
etc.)
The Cambrian Burgess Shale – An Example of
a Spectacular Fossil Locality
The Burgess Shale crops out in British Columbia,
Canada. It contains a magnificant fauna of soft
bodied marine organisms that lived adjacent to
a tropical reef. They were preserved in quite,
deep-water muds when turbidity currents
transported them down a steep slope. They
offer an incredible snapshot of Cambrian
marine life
Burgess Shale Faunal
Elements, as soft-bodied
organisms, their
preservation is very
unusual
8 inch long ?sea pen
Relative of modern velvet worm
Fossils & Correlation
•  Principle of fossil succession – fossil
organisms succeed one another in a definite
and determinable order, and therefore any
time period can be recognized by its fossil
content
•  Index fossil – geographically widespread
fossil that is limited to a short span of
geologic time
Dating rocks using overlapping
fossil ranges
Correlation
of strata at 3
locations on
the Colorado
Plateau
reveals the
total extent
of
sedimentary
rocks in the
region
Using radioactivity in dating
!  Reviewing basic atomic structure
•  Nucleus
–  Protons – positively charged particles with
mass
–  Neutrons – neutral particles with mass
•  Electrons – negatively charges particles that
orbit the nucleus
Using radioactivity in dating
!  Reviewing basic atomic structure
•  Atomic number
–  An element’s identifying number
–  Equal to the number of protons in the atom’s
nucleus
•  Mass number
–  Sum of the number of protons and neutrons
in an atom’s nucleus
Using radioactivity in dating
!  Reviewing basic atomic structure
•  Isotope
–  Variant of the same parent atom
–  Differs in the number of neutrons
–  Results in a different mass number than the
parent atom
Using radioactivity in dating
!  Radioactivity
•  Spontaneous changes (decay) in the
structure of atomic nuclei
!   Types of radioactive decay
•  Alpha emission
–  Emission of 2 protons and 2 neutrons (an
alpha particle)
–  Mass number is reduced by 4 and the atomic
number is lowered by 2
Using radioactivity in dating
!   Types of radioactive decay
•  Alpha emission
–  Emission of 2 protons and 2 neutrons (an alpha
particle)
–  Mass number is reduced by 4 and the atomic number
is lowered by 2
Using radioactivity in dating
!   Types of radioactive decay
•  Beta emission
–  An electron (beta particle) is ejected from the nucleus
–  Mass number remains unchanged and the atomic
number increases by 1
Using radioactivity in dating
!   Types of radioactive decay
•  Electron capture
–  An electron is captured by the nucleus
–  The electron combines with a proton to form a neutron
–  Mass number remains unchanged and the atomic
number decreases by 1
Using radioactivity in dating
!  Parent – an unstable radioactive isotope
!  Daughter product – the isotopes resulting
from the decay of a parent
!  Half-life – the time required for one-half
of the radioactive nuclei in a sample to
decay
A Radioactive
Decay Series from
Uranium (U-238) to
Lead (Pb-206). A
Number of
Unstable Isotopes
Are Formed As
Intermediate Steps
Using radioactivity in dating
!  Radiometric dating
•  Principle of radioactive dating
–  The percentage of radioactive atoms that
decay during one half-life is always the same
(50 percent)
–  However, the actual number of atoms that
decay continually decreases
–  Comparing the ratio of parent to daughter
yields the age of the sample
The
radioactivedecay curve
shows change
that is
exponential.
Half of the
radioactive
parent remains
after one halflife. After a
second halflife, onequarter of the
parent
remains, etc.
Using radioactivity in dating
!  Radiometric dating
•  Useful radioactive isotopes for providing
radiometric ages
–  Rubidium-87
–  Thorium-232
–  Uranium-238 & Uranium 235
–  Potassium-40
Using radioactivity in dating
!  Radiometric dating
• Sources of error
– A closed system is required
– To avoid potential problems, only
fresh, unweathered rock samples
should be used
Using radioactivity in dating
!   Dating with carbon-14 (radiocarbon dating)
•  Half-life of only 5730 years
•  Used to date very recent events
•  Carbon-14 is produced in the upper atmosphere &
incorporated into all living things
•  Ratio of radioactive carbon-14 to stable carbon-12 remains
constant during life, but upon death, the carbon-14
gradually decays to nitrogen-14
•  Useful tool for anthropologists, archeologists, and
geologists who study very recent Earth history
Production (A) and Decay (B) of the Radioactive Isotope
of Carbon (C14)
Correlation of Tree Rings (Annual Growth
Bands) from Various Locations is Another
Method for Cross-Dating Recent Events
Using radioactivity in dating
!  Importance of radiometric dating
•  Radiometric dating is a complex
procedure that requires precise
measurement
•  Rocks from several localities have been
dated at more than 3 billion years
•  Confirms the idea that geologic time is
immense
Using radioactivity in dating
!  Problems with Direct Dating of
Sedimentary Rocks
•  Datable minerals are present in
sedimentary rocks (i.e. K-feldspars)
•  Dates obtained from these minerals reflect
the time of mineral formation, not the age
of the rock itself
•  Weathering of minerals in sedimentary
rocks can “reset the clock”
K-Feldspars (Orthoclase & Microcline)
KAlSi3O8 Making Porcelain & Glaze
microcline
microcline
Green or
Salmon
Pink Color
orthoclase
microcline
Hardness (6)
orthoclase
2 Good
Cleavages
Bentonite (Volcanic Ash) Layer Can be Used to Approximate the
Actual Dates of Deposition of These Algal Laminated Carbonates
Bentonite
Sedimentary Rocks are Best Dated by Relating Them
to Associated Igneous Formations
Geologic time scale
!  The geologic time scale – a “calendar” of
Earth history
•  Subdivides geologic history into units
•  Originally created using relative dates
!  Structure of the geologic time scale
•  Eon – the greatest expanse of time
Geologic time scale
!  Structure of the geologic time scale
•  Names of the eons
–  Phanerozoic (“visible life”) – the most recent
eon, began about 540 million years ago
–  Proterozoic
–  Archean
–  Hadean – the oldest eon
Geologic
Time Scale
Emphasizing
Eons (the
Greatest
Expanses of
Time)
Geologic time scale
!  Precambrian time
•  Nearly 4 billion years prior to the
Cambrian period
•  Not divided into smaller time units
because the events of Precambrian history
are not know in great enough detail
–  First abundant fossil evidence does not
appear until the beginning of the Cambrian
Geologic time scale
!  Structure of the geologic time scale
•  Era – subdivision of an eon
•  Eras of the Phanerozoic eon
–  Cenozoic (“recent life”)
–  Mesozoic (“middle life”)
–  Paleozoic (“ancient life”)
•  Eras are subdivided into periods
•  Periods are subdivided into epochs
Geologic Time
Scale
Numbers = Millions
of Years Before
Present -Note
Major Events in
Evolutionary
History of Earth
Geologic time scale – The K/T boundary &
extinction of the dinosaurs
!   Dinosaurs (& many other groups of organisms) became
extinct at @ the end of the Cretaceous 65 million years
ago. What happened??
•  Major asteroid impact in Yucatan
–  Dust & gases from impact may have initially
triggered global cooling
–  CO2 that remained in the atmosphere could
have subsequently triggered global warming
–  These rapid & drastic climate changes may
have wiped out many living organisms,
including the dinosaurs
Did this cause this????
Geologic time scale
!  Difficulties in dating the geologic time
scale
•  Not all rocks can be dated by radiometric
methods
–  Grains comprising detrital sedimentary rocks
are not the same age as the rock in which they
formed
–  The age of a particular mineral in a
metamorphic rock may not necessarily
represent the time when the rock formed
Geologic time scale
!  Difficulties in dating the geologic time
scale
•  Datable materials (such as volcanic ash
beds and igneous intrusions) are often
used to bracket various episodes in Earth
history and arrive at ages