Earth - Chapter 8

Report
Geologic Time
Marble demo
Some Index Fossils
Coin Toss Sheet
Color Copies of Expected Values for X2 in homework
Time Scale.doc
Determining geological ages
• Relative age dates – placing rocks and
geologic events in their proper sequence
• Numerical dates – define the actual age of
a particular geologic event (termed
absolute age dating)
Principles of relative
dating
Developed by Nicolaus Steno
in 1669
Niels Steensen (Nicolas Steno)
Steno recognized the organic origin of fossils and sketched a theory of geological
strata, which he used in an attempt to reconstruct Tuscany's geological development
• 1. Law of superposition
• In an undeformed sequence of
sedimentary or volcanic rocks,
oldest rocks at base; youngest at
top
Superposition illustrated by
strata in the Grand Canyon
Steno’s 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)
Steno’s 3rd principle of relative dating
• Principle of cross-cutting relationships
Chunks of this “country rock”
have broken off and are visible
in the intrusion
3rd principle of relative dating
• Principle of cross-cutting relationships
This fault shows the offset
of the two sides. See the key
beds? Notice this side is lower
Unconformities
(loss of rock record)
• An unconformity is a break in the rock record
produced by erosion and/or non-deposition
• 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)
Development of a Nonconformity
Pennsylvanian sandstone over
Precambrian granite is a
nonconformity
Nonconformity in the Grand Canyon - Sediments deposited over Schist
Formation of an angular unconformity
Horizontal younger sediments over tilted older sediments
Cambrian Tapeats sandstone over Precambrian Unkar Group
What type of unconformity is this?
Grand Canyon in Arizona
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
Matching lithology is risky, discussion
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
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
Determining the ages of
rocks using overlap of fossils
Overlap time span is shorter
than that of any one fossil.
Fossils allow correlation in spite of unconformities
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
Divisions based on fossils
Eon, Era, Period, Epoch
Homework
Learn
Timescale.doc
less Epochs
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
• 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 Ga)
• PreCambrian spans about 88% of Earth’s history
Geologic time scale
• Structure of the geologic time scale
• Era – subdivision of an eon
• Eras of the Phanerozoic (visible life”) eon
– Cenozoic (“recent life”) begins ~ 65 mya
– Mesozoic (“middle life”) begins ~ 248 mya
– Paleozoic (“ancient life”) begins ~ 540 mya
• Eras are subdivided into periods
• Periods are subdivided into epochs
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
show Earth not as old as Moon, Meteorites
• Included in some sediment from NW
Australia, 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
•Claims of older zircons 4.4 by.
Radiometric Age Determinations
of the Earth
• However, the age of the Earth is
thought to be about 4.5 - 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”
• Protons + neutrons called 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
Daughter
nucleus
pp
pp
p
p
Proton
Neutron
p
(a) Alpha decay
pp
p
p
p
p
Atomic mass decreases
by 4; atomic number
decreases by 2
p
p
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 not
changed much;
atomic number
decreases by 1
electron combines with a proton
to form a neutron
Using radioactivity in dating
• Parent – an unstable radioactive
isotope
• Daughter product – stable isotopes
resulting from decay of parent
• 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
Uranium to Lead used for granites; Potassium to Argon used for basalts
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. Measure parent/daughter ratio of
target isotopes - mass spectrometer
4. Substance heated – Ions – move in
Electrical Field, curved in Magnetic
Mineral
crystal
Dating a crystal
1 Mineral crystal
formed in igneous
rock
Parent
atoms
2
Daughter
atoms
Usual Case
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
Rock looks as if it just formed: it looks young
Age found dates from metamorphic event
Metamorphism Case
3 Deep burial and
metamorphism
during mountain
building causes
daughter atoms
to escape from
crystal
4 After mountain
building ends,
accumulation of
daughter atoms
in crystal
resumes
Easily recognized,
useful in studying
metamorphism
Dating sediments without
fossils: Superposition, Cross-cutting
Wasatch Fm. younger than 66 my
Mancos Shale and Mesa Verde Fm.
older than 66 my
Rule of Cross-cutting
Radiometric Dating with Igneous Rocks
Or Bracket between fossiliferous layers
Morrison Fm
older than 160
my
(superposition)
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
Dating with carbon-14 (Carbon Dating)
• Half-life only 5730 years
• Used to date very young rocks
• Carbon-14 is produced in the upper atmosphere
• Useful tool for geologists who study very recent
Earth history
Atoms split into
smaller particles,
among them neutrons
Carbon-14
Cosmic rays
bombard
atmospheric atoms
Neutrons strike
nitrogen atoms
Nitrogen atom gains a neutron and loses a
Proton; becomes carbon-14
C-14 mixes with atmospheric oxygen
to produce CO2
C-14 absorbed
by living
organisms
CO2 taken up
by plants, water
C-14 intake ceases when organism
dies; C-14 concentration decreases
CO2 dissolved
in water
Years of age
500
Tree Rings both modern and past 2000 years
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
Southern lakes track glaciation
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)
Lake deposits, fossil plants C14. Fossil tree pollen track climate.
http://bcornet.tripod.com/Cornet94/Cornet94.htm
http://www.bio.uu.nl/~palaeo/people/Hanneke/index.html
Hanneke Bos
End of Geologic
Time Lecture

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