Transcript Dating the Earth

The Map That Changed the World
• William Smith, 1815
• Geologic map of
England, Wales, part of
Scotland
The Map That Changed the World
Relative Geologic Time Scale
• The relative geologic
time scale has a
sequence of
– eons
– eras
– periods
– epochs
– but no numbers
indicating how long
ago each of these
times occurred
Geologic Time Scale
• Large divisions based on…?
• Paleozoic Era –
• Mesozoic Era –
• Cenozoic Era –
Geologic Time Scale
• Large divisions based on characteristics of
fossils
• Paleozoic Era – early life dominated by
invertebrate animals
• Mesozoic Era – middle life
• Cenozoic Era – recent life
How was the timescale created?
How was the timescale created?
• Mapping in 1800s using the principles of
– Superposition
– Original Horizontality
– Original Lateral Continuity
– Cross-cutting relationships
– Also Fossil Correlation
Relative-Dating Principles
• Superposition
– Oldest on bottom, youngest on top
• Original Horizontality
– Sediment originally deposited in flat
parallel layers
Chattanooga Shale, TN
Relative-Dating Principles
• Lateral continuity
– sediment extends laterally in all directions until
it thins and pinches out or terminates against
the edges of the depositional basin
• Cross-cutting relationships
– an igneous intrusion or a fault must be
younger than the rocks it intrudes or displaces
Cross-cutting
Relationships
• A dark-colored
dike has intruded
into older light
colored granite:
the dike is
younger than the
granite
North shore of Lake Superior, Ontario Canada
Cross-cutting Relationships
• A small fault
displaces
tilted beds:
the fault is
younger
than the
beds
Templin Highway, Castaic, California
Unconformities
• What is an unconformity?
Unconformities
• What is an unconformity?
– A surface of erosion or non-deposition
– Recognizable surface in the rock record
Example of an Unconformity
• Tilted sandstone and
siltstone below,
conglomerate above
www.geology.sdsu.edu/visualgeology/geology101/erosion6.htm
Back to Steno
www.gly.uga.edu/railsback/1121Steno.jpg
Why are layers tilted?
• Deformation of rocks
– Occurs after they are deposited
– Important factor in relative dating
• Folding
– Anticlines, synclines
– Rock bends, but does not break
• Faulting
– Normal, reverse, transform
– Rock breaks
Folding
www.hill.anorak.org.uk/dhtml/glgchap5.html
Faulting
www.stmarys.ca/academic/science/geology/structural/faults.html
Relative Dating
• Principles
• Unconformities
• Deformation
Relative Dating – Cross-section
http://facweb.bhc.edu/academics/science/harwoodr/Geol101/labs/dating/
Relative Dating – Cross-section
http://facweb.bhc.edu/academics/science/harwoodr/Geol101/labs/dating/
Key:
E
Erosion
G
L
C
H
Tilting
Erosion
M
D
J
A
Erosion
N
K
B
Tilting
Erosion
F
Erosion
Relative Dating – Cross-section
http://facweb.bhc.edu/academics/science/harwoodr/Geol101/labs/Dating2/Index.htm
Relative Dating – Cross-section
Key:
H
F
D
G
I
Erosion
C
Erosion
A
B
M
L
K
Erosion
J
http://facweb.bhc.edu/academics/science/harwoodr/Geol101/labs/Dating2/Index.htm
Grand Canyon: history revealed
Grand Canyon
• More than 1 billion years of history are preserved
in the rock layers of the Grand Canyon
• Reading this rock book shows:
– periods of mountain building
– advancing and retreating shallow seas
– evolution of faunas
• Determine these things by:
– applying the principles of relative dating to the rocks
– and recognizing that present-day processes have
operated throughout Earth history - Uniformitarianism
Absolute Dating
• Radiometric dating is the most common
method of obtaining absolute ages
– calculated from the natural rates of decay of
various natural radioactive elements present in
trace amounts in some rocks
• Other methods?
– Tree ring counting
– Varves
– Ice cores
Geologic Time Scale
• The discovery of radioactivity
near the end of the 1800s
allowed absolute ages to be
accurately applied to the
relative geologic time scale
• The geologic time scale is a
dual scale
– a relative scale
– and an absolute scale
Changes in the Concept of
Geologic Time
• The concept and measurement of geologic
time has changed through human history
• James Ussher (1581-1665) in Ireland
– calculated the age of Earth based on recorded
history and genealogies in Genesis
• he announced that Earth was created on October
22, 4004 B.C.
• a century later it was considered heresy to say
Earth was more than about 6000 years old
Changes in the Concept of
Geologic Time
• During the 1700s and 1800s Earth’s age
was estimated scientifically
– Georges Louis de Buffon (1707-1788)
calculated how long Earth took to cool
gradually from a molten beginning
• used melted iron balls of various diameters
• he estimated Earth was 75,000 years old
Changes in the Concept of
Geologic Time
– Others used rates of deposition of various
sediments and thickness of sedimentary rock
in the crust
• gave estimates of <1 million
• to more than 2 billion years
– Or the amount of salt carried by rivers to the
ocean and the salinity of seawater
• John Joly in 1899 obtained a minimum age of 90
million years
History of Historical Geology
• Neptunism
– proposed in 1787 by Abraham Werner (1749-1817)
– all rocks, including granite and basalt, were precipitated
in an orderly sequence from a primeval, worldwide ocean
– Werner was an excellent mineralogist, but is best
remembered for his incorrect interpretation of Earth
history
History of Historical Geology
• Catastrophism
– proposed by Georges Cuvier (1769-1832)
– dominated European geologic thinking
– the physical and biological history of Earth resulted
from a series of sudden widespread catastrophes
which accounted for significant and rapid changes in
Earth and exterminated existing life in the affected
area
– six major catastrophes occurred, corresponding to the
six days of biblical creation, he last one was the
biblical flood
History of Historical Geology
• Neptunism and Catastrophism were
eventually abandoned
–
–
–
–
they were not supported by field evidence
basalt was shown to be of igneous origin
volcanic rocks interbedded with sedimentary
primitive rocks showed that igneous activity had
occurred throughout geologic time
– more than 6 catastrophes were needed to explain field
observations
• The principle of uniformitarianism became
the guiding philosophy of geology
Uniformitarianism
• Developed by James Hutton, advocated by Charles Lyell
(1797-1875)
• Present-day processes have operated throughout
geologic time
• Term uniformitarianism was coined by William Whewell in
1832
• Hutton applied the principle of uniformitarianism when
interpreting rocks at Siccar Point Scotland
• We now call what he observed an unconformity
– but he properly interpreted its formation
Unconformity at Siccar Point
Uniformitarianism
• Hutton viewed Earth
history as cyclical
erosion
deposition
uplift
• He also understood that geologic
processes operate over a vast amount of
time
• Modern view of uniformitarianism
– geologists assume that the principles or laws
of nature are constant
– but the rates and intensities of change have
varied through time
Crisis in Geology
• Lord Kelvin (1824-1907)
– knew about high temperatures inside of deep
mines and reasoned that Earth is losing heat
from its interior
• Assuming Earth was once molten, he used
• the melting temperature of rocks
• the size of Earth
• and the rate of heat loss
– to calculate the age of Earth as between 400
and 20 million years
Crisis in Geology
• This age was too young for the geologic
processes envisioned by other geologists at that
time
– leading to a crisis in geology
• Kelvin did not know about radioactivity as a heat
source within the Earth
Absolute-Dating Methods
• The discovery of radioactivity destroyed Kelvin’s
argument for the age of Earth
• Radioactivity is the spontaneous decay of an atom’s
nucleus to a more stable form
• The heat from radioactivity helps explain why the
Earth is still warm inside
• Radioactivity provides geologists with a powerful tool
to measure absolute ages of rocks and past geologic
events
Absolute-Dating Methods
• Understanding absolute dating requires knowledge of
atoms and isotopes: we have it!
• Atomic mass number
= number of protons + number of neutrons
• Isotopes: different numbers of neutrons
• Different isotopes have different atomic mass numbers
but behave the same chemically
• Most isotopes are stable
– but some are unstable
• Geologists use decay rates of unstable isotopes to
determine absolute ages of rocks
Radioactive Decay
• Radioactive decay is the process whereby an
unstable atomic nucleus spontaneously changes
into an atomic nucleus of a different element
• Three types of radioactive decay:
– alpha decay, two protons and two neutrons (alpha
particle) are emitted from the nucleus
Radioactive Decay
– beta decay, a neutron emits a fast moving electron
(beta particle) and becomes a proton
– electron capture decay, a proton captures an electron
and converts to a neutron
Radioactive Decay
• Some isotopes undergo only one decay step
before they become stable
– rubidium 87 decays to strontium 87 by a single beta
emission
– potassium 40 decays to argon 40 by a single electron
capture
Radioactive Decay
• Other isotopes undergo several decay
steps
– uranium 235 decays to lead 207 by 7 alpha
steps and 6 beta steps
– uranium 238 decays to lead 206 by 8 alpha
steps and 6 beta steps
Uranium 238 decay
Half-Lives
• Half-life of a radioactive isotope is the time it
takes for one half of the atoms of the original
unstable parent isotope to decay to atoms of a
new more stable daughter isotope
• The half-life of a specific radioactive isotope is
constant and can be precisely measured
Half-Lives
• The length of half-lives for different isotopes of
different elements can vary from
– less than 1/billionth of a second
– to 49 billion years
• Radioactive decay
– is geometric not linear
– a curved graph
Geometric Radioactive Decay
• In radioactive
decay, during
each equal
time unit, one
half-life, the
proportion of
parent atoms
decreases by
1/2
Determining Age
• By measuring the parent/daughter ratio and
knowing the half-life of the parent which has
been determined in the laboratory geologists can
calculate the age of a sample containing the
radioactive element
• The parent/daughter ratio is usually determined
by a mass spectrometer
– an instrument that measures the proportions of atoms
with different masses
Determining Age
• For example:
– If a rock has a parent/daughter ratio of
1:3  a parent proportion of 25%
– and the half-live is 57 million years, how
old is the rock?
– 25% means it is 2 halflives old.
– the rock is 57 x 2 =114
million years old.
What Materials Can Be Dated?
• Most radiometric dates are obtained from
igneous rocks
• As magma cools and crystallizes, radioactive
parent atoms separate from previously formed
daughter atoms
– they fit differently into the crystal structure of certain
minerals
• Geologists can use the crystals containing the
parents atoms to date the time of crystallization
Igneous Crystallization
• Crystallization of magma separates parent atoms
from previously formed daughters
• This resets the radiometric clock to zero
• Then the parents gradually decay
Sources of Uncertainty
• Closed system is needed for an accurate date
– neither parent nor daughter atoms can have been
added or removed from the sample since
crystallization
• If leakage of daughters has occurred
– it partially resets the radiometric clock and the age will
be too young
• If parents escape, the date will be too old
• Most reliable dates use multiple methods
Sources of Uncertainty
• Dating techniques are always improving
– Presently measurement error is typically <0.5% of the
age, and even better than 0.1%
– A date of 540 million might have an error of ±2.7
million years or as low as ±0.54 million
Dating Metamorphism
a. A mineral has just crystallized
from magma.
b. As time passes, parent atoms
decay to daughters.
c. Metamorphism drives the
daughters out of the
mineral (to other parts of
the rock) as it
recrystallizes.
d. Dating the mineral today
yields a date of 350 million
years = time of
metamorphism, provided the
system remains closed
during that time.
•Dating the whole rock yields a
date of 700 million years =
time of crystallization.
Long-Lived Radioactive
Isotope Pairs Used in Dating
• The isotopes used in radiometric dating need to be
sufficiently long-lived so the amount of parent
material left is measurable
– Such isotopes include:
Parents
Uranium 238
Uranium 234
Thorium 232
Rubidium 87
Potassium 40
Daughters
Lead 206
Lead 207
Lead 208
Strontium 87
Argon 40
Half-Life (years)
4.5 billion
704 million
14 billion
48.8 billion
1.3 billion
Most of
these are
useful for
dating older
rocks
Mass Spectrometer
www.mines.unr.edu/isotope/gallery.html
How do we know the Earth can’t
be older than about 6-7 b.y.?
• Moderate half-life isotopes (1 b.y.)
• If Earth was > 6-7 b.y. old, there wouldn’t
be many parents left
Radiocarbon Dating Method
• Carbon is found in all life
• It has 3 isotopes
– carbon 12 and 13 are stable but carbon 14 is not
– carbon 14 has a half-life of 5730 years
– carbon 14 dating uses the carbon 14/carbon 12 ratio of
material that was once living
• The short half-life of carbon 14 makes it suitable
for dating material < 70,000 years old
• It is not useful for most rocks, but is useful for
archaeology and young geologic materials
Carbon 14
• Carbon 14 is constantly forming
in the upper atmosphere
– when a high-energy neutron, a type
of cosmic ray , strikes a nitrogen 14
atom it may be absorbed by the
nucleus and eject a proton changing
it to carbon 14
• The 14C formation rate
– is fairly constant
– and has been calibrated against tree
rings
Carbon 14
• The carbon 14 becomes part of
the natural carbon cycle and
becomes incorporated into
organisms
• While the organism lives it
continues to take in carbon 14
– when it dies the carbon 14 begins
to decay without being replenished
• Thus, carbon 14 dating
measures the time of death
Tree-Ring Dating Method
• The age of a tree can be determined by counting the
annual growth rings in lower part of the stem (trunk)
• The width of the rings are related to climate and can be
correlated from tree to tree
– a procedure called cross-dating
• The tree-ring time scale now extends back 14,000 years!
Tree-Ring Dating Method
• In cross-dating, tree-ring patterns are used from
different trees, with overlapping life spans
Summary
• Uniformitarianism holds that
– the laws of nature have been constant through
time
– and that the same processes operating today
have operated in the past
– although not necessarily at the same rates
Summary
• The principles of superposition
– original horizontality,
– lateral continuity
– and cross-cutting relationships
– are basic for determining relative geologic
ages and for interpreting Earth history
• Radioactivity was discovered during the
late 19th century
– and lead to radiometric dating
– which allowed geologists to determine
absolute ages for geologic events
Summary
• Half-life is the length of time it takes for
one-half of the radioactive parent
isotope to decay to a stable daughter
isotope of a different element
• The most accurate radiometric dates
are obtained from long-lived radioactive
isotope/daughter pairs
– in igneous rocks
Summary
• The most reliable radiometric ages are
obtained using two different pairs in the
same rock
• Carbon 14 dating can be used only for
organic matter such as
– wood, bones, and shells
– and is effective back to about 70,000 years