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Geologic Time
Marble demo
Some Index Fossils
Foam strata
Determining geological ages
• Relative ages – placing rocks and geologic
events in their proper sequence, oldest to
youngest.
• Absolute dates – define the actual numerical
age of a particular geologic event. For example,
large dinosaurs died out 65 mya. The Lavas
along Rt 22 and Rt 78 were deposited about 205
mya.
First principle of relative dating
• Law of superposition
• Developed by Nicolaus Steno in 1669
• In an undeformed sequence of
sedimentary or volcanic rocks the oldest
rocks are at the base; the youngest are at
the top
Superposition illustrated by
strata in the Grand Canyon
2nd principle of relative dating
• Principle of original
horizontality
• Layers of sediment are originally deposited
horizontally (flat strata have not been
disturbed by folding, faulting)
3rd principle of relative dating
• Principle of cross-cutting relationships
If a rock layer is cut by a fault
or igneous intrusion, the rock
that is cut must be older than
the layer that cuts it.
Igneous Dike
3rd principle of relative dating
• Principle of cross-cutting relationships (example 2)
Fault cross-cuts
limestone and shale
Unconformities
(loss of rock record)
• An unconformity is a break in the rock record
produced by erosion and/or nondeposition
• Types of unconformities
– Nonconformity – sedimentary rocks
deposited above metamorphic or igneous
rocks (basement) with time lost
– Angular unconformity – tilted rocks overlain
by flat-lying rocks
– Disconformity – strata on either side of the
unconformity are parallel (but time is lost)
Layered
sedimentary
rocks
(a)
8_9
Nonconformity
Metamorphic
rock
Igneous
intrusive rock
(b)
Younger
sedimentary
rocks
Angular
unconformity
Older, folded
sedimentary
rocks
(c)
Disconformity
Brachiopod
(290 million years old)
Trilobite
(490 million years old)
Horizontal younger sediments over tilted older sediments
Cambrian Tapeats sandstone over Precambrian Unkar Group
What type of unconformity is this?
Grand Canyon in Arizona
Formation of an angular unconformity
Development of a Nonconformity
An intrusion occurs
The overburden is eroded away
Pennsylvanian sandstone over
Precambrian granite is a
nonconformity
Sea level rises, new sediment is
deposited
Nonconformity in the Grand Canyon - Sediments deposited over Schist
Cross Cutting Relationships in strata
Zoroaster Granite across Vishnu Schist
Correlation of rock layers
• Matching strata of
similar ages in
different regions is
called correlation
Correlation of strata in
southwestern United States
Sections are incomplete
Match with fossils and lithology
Because of sea-level changes
Fossils are more reliable than sequences
However, falling sea level
of sediment facies is useful for worldwide
correlation. Why?
Sauk Sequence
WEST
Transgression
Middle
Cambrian
http://www.geo-tools.com/trilobites.htm
Lower
Cambrian
http://www.wmrs.edu/projects/trilobites/images/trilo7-2.jpg
Note how western BAS is older than eastern BAS
EAST
http://www.unh.edu/esci/wmsmith.htm
Correlation of rock layers with
fossils
Correlation often relies upon fossils
• Principle of fossil succession (Wm. Smith)
http://www.csun.ed
u/~psk17793/ES9
CP/ES9%20fossils
.htm
– fossil organisms succeed one another in a recognizable order - thus
any time period is defined by the type of fossils in it
• Index Fossils - useful for correlation
– Existed for a relatively brief time
– Were widespread and common
•Most fossils are just
impressions. A few may have
small amounts of some
original tissue
How impression fossils form (the most common type)
8_10
Shells
settle on
ocean
floor
Cast forms when mold
is filled in with mineral
water
Rock broken
to reveal
fossil cast
Shells
buried in
sediment
Mold, or cavity,
forms when original
shell material
is dissolved
Rock broken
to reveal external mold
of shell
Fossil Succession shown with Index Fossils
Determining the ages of
rocks using overlap of fossils
Dinosaurs and flowering
plant fossils occur in
these sediments,
therefore sediments “A”
are Cretaceous
B contains trilobites, Cambrian to Permian, and
Ginkgo leaves, Permian to Recent, therefore B is
Permian
Overlapping fossil ranges
are the data used in
Biostratigraphy –
These are NOT Index Fossils- WHY? correlation with fossils
Two Measured Sections
Note the use of
overlapping fossil ranges
in two distant outcrops,
even though the
sediment facies are
different.
In the diagram, the presence of three species of index fossils is shown for two
measured sections. The limestone marked A on the left is the same age as
a. sandstone 1
b. mudstone 2
c. shale 3
d. sandstone 4
Correlation with index fossils in spite of unconformities
Highway cut A
Correlation
Highway cut B
Especially useful in well cores
conodonts
diatoms
pollen
are microfossils
foraminifera
radiolaria
http://www.ucl.ac.uk/GeolSci/micropal/
Magnetostratigraphy dated with
fossils
or with
absolute
(radiometric)
dating
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, Era, Period, Epoch
Geologic Timescale
Eons
Phanerozoic
PreCambrian
Divisions based on fossils
Eon, Era, Period, Epoch
Eras
Origin of Period Names
Geologic time scale
• Structure of the geologic time scale
• Names of the eons
– Phanerozoic (“visible life”) – the most recent
eon, began about 545 million years ago
– PreCambrian (Cryptozoic)
• PreCambrian subdivisions:
• Proterozoic – begins 2.5 billion years ago
• Archean – begins 3.8 bya
• Hadean – the oldest eon begins 4.6 bya
Read from bottom to top – Oldest to Youngest
Geologic time scale
• Precambrian time
• Nearly 4 billion years prior to the
Cambrian period (.545 billion= 545 million)
• Long time units because the events of
Precambrian history are not know in
detail – few fossils, most rock modified
• Immense space of time (Earth is ~ 4.6 by)
• PreCambrian (4.6 -0.545)/4.6 ~ 88%
Geologic time scale - Eras
• Structure of the geologic time scale
• Era – subdivision of an eon
• Eras of the Phanerozoic eon
– Cenozoic (“recent life”) begins ~ 65 mya
– Mesozoic (“middle life”) begins ~ 248 mya
– Paleozoic (“ancient life”) begins ~ 545 mya
• Eras are subdivided into periods
• Periods are subdivided into epochs
• Eon>Era>Period>Epoch
Using radioactivity in dating
• Importance of radiometric dating
• Allows us to calibrate geologic
timescale
• Determines geologic history
• Confirms idea that geologic time is
immense
Radiometric Age Determinations
of the Earth
• The oldest known minerals ever found on
Earth include some from NW Australia. The
containing rock (a conglomerate) is about 3.0
billion years old. The rock contains detrital
grains of the mineral zircon that are 3.96
billion years old. The dates are based on
datable Uranium in the Zircons.
•Similar dates are known from Yellow Knife
Lake, NWT, Canada
Radiometric Age Determinations
of the Earth
• The age of the Earth is thought to be
about 4.54 (roughly 4.6) billion years
• Based on the dates obtained from
meteorites and samples collected on
the moon, assumed to have formed at
the same time.
Recall Isotopes
• The number of protons in an atom's nucleus is
called its atomic number –defines “element”
• Mass protons + neutrons called atomic mass or,
more loosely, atomic weight
• The number of neutrons can vary
• Atoms of the same element with different numbers
of neutrons are called isotopes. Some are
radioactive
Radioactive
parent nucleus
pp
p
p
p
p
Decay process
8_19
pp
pp
p
p
p
(a) Alpha decay
p
p
p
p
Atomic mass decreases
by 4; atomic number
decreases by 2
p
p
Proton
Neutron
pp
Daughter
nucleus
pp
p
p
p
p
Alpha particle
Emission of 2 protons and 2
neutrons (alpha particle)
pp
p
p
p p p
Atomic mass not changed
much; atomic number
increases by 1 because
Neutron becomes proton
Beta particle
(b) Beta decay
An electron (beta particle) is
ejected from the nucleus
pp
p
p
p
p
p
pp
p
p
p
p
p
Beta particle
(c) Electron capture
pp
p
p
p
p
Atomic mass unchanged;
atomic number
decreases by 1
electron combines with a proton
to form a neutron
Using radioactivity in dating
• Parent – an unstable radioactive
isotope. Parent atom fits in crystal.
• Daughter product – stable isotopes
resulting from decay of parent. Doesn’t
fit, so none present when crystal
formed.
• Half-life – time required for one-half of
the parent isotope in a sample to decay
into stable daughter product
A radioactive decay curve
1/2 = 50% parent: 1 half-life has passed
1/2x1/2 = 1/4 = 25% parent: 2 half-lives have passed
1/2x1/2x1/2 = 1/8 = 12.5% parent: 3-half-lives have passed
Marble Demo
Used up around Sparta, Byram – Zircons in pegmatites, rule of cross cutting
Used around here, East Africa, Biotite etc. in volcanics
U-238 decays to Pb-206
How do we actually “date” a rock?
1. Collect sample
2. Process for minerals by crushing,
sieve, separate magnetically and/or
with heavy liquids
3. Pass through extreme heat, electric
and magnetic fields.
4. Measure parent/daughter ratio of
target isotopes - mass spectrometer
Dating sediments without
fossils
Wasatch Fm. younger than 66 my
Mancos Shale and Mesa Verde Fm.
older than 66 my
Morrison Fm
older than
160 my
Radiometric Dating with Igneous Rocks
Or Bracket between fossiliferous layers
Even better: we get lucky. A layer we need to date is between two datable beds
So we have and upper and lower bound on the age of this limestone:
Basalt Lava flow 2
200 mya
We can bracket this
limestone’s age
between 209 and 200
mya
Lava flow 1
209 mya
Mineral
crystal
Dating a crystal
1 Mineral crystal
formed in igneous
rock
Parent
atoms
2
Daughter
atoms
Igneous rock
buried beneath
younger rocks;
daughter atoms
formed by
normal decay
(3) We calculate age
based on half-life
But IF:
8_22b
Heat
Resets the clock
3 Deep burial and
metamorphism
during mountain
building causes
daughter atoms
to escape from
crystal
Rock looks as if it just formed: it looks young
Age found dates from metamorphic event
4 After mountain
building ends,
accumulation of
daughter atoms
in crystal
resumes
Easily recognized,
useful in studying
metamorphism
Dating with carbon-14
(Carbon Dating)
• Half-life only 5730 years
• Used to date organisms
• Carbon-14 is produced in the upper
atmosphere
• Useful tool for geologists who study
very recent Earth history
Carbon-14
Atoms split into
smaller8_24
particles,
among them neutrons
Cosmic rays
bombard
atmospheric atoms
Neutrons strike
nitrogen atoms
Nitrogen atoms lose a
proton and becomes
carbon-14
C-14 mixes with atmospheric oxygen
to produce CO2
C-14 absorbed
by living
organisms
CO2 taken up
by plants
CO2 dissolved
in water
C-14 intake ceases when organism
dies; C-14 concentration decreases
Years of age
500
Tree Rings
8_27
Annual-ring similarities
show correlation
Current year
50
400
100 150
200
Tree
growth
rings
A
B
C
D
A
Sediment layers
with tree logs to
be collected for
dendrochronology
B
C
D
Buried tree
logs
Lakes south of the glacier track its advance - NH
8_28
Dating with Lake Varves
Very
little
or no
runoff
Heavy
runoff
into
lake
Ice
Summer
Turbid water
Summer layer
(coarse, thick, and
light-colored)
Winter Clear water
Winter layer
(fine, thin, and
dark-colored)
Modern Lakes, just count back from present. Fossil pollen track climate.
Examples of fossil pollen
http://bcornet.tripod.com/Cornet94/Cornet94.htm
http://www.bio.uu.nl/~palaeo/people/Hanneke/index.html
Hanneke Bos
End of Geologic
Time Lecture