Transcript Geologic time

Understanding Geologic Time
The map that changed
the world by William
Smith (1815) links:
fossils
rock patterns
3Dapproach
Grand Canyon: history revealed
Grand Canyon
• Preserves more than 1 billion years of
history
• This rock book shows
– mountain building
– advancing and retreating seas
– evolution of faunas
• Determine these things by:
– applying the principles of relative dating to the
rocks
– Uniformitarianism
Concepts of Geologic Time
•
Two frames of reference
1) Relative dating – describes sequential order
2) Absolute dating – timing of events in years
before present
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, just the order
of occurrence
Absolute Dating
- specific dates for rock units or events
• expressed in years before the present
• gives us numerical information about events
Absolute Dating
• Radiometric dating is the most common
method of obtaining absolute ages
– calculated from the rates of decay of various natural
radioactive elements present in trace amounts in
some rocks
•Other methods
– tree ring counting
– varves (layers year sediment accumulations)
– ice (count layers of ice for annual scale)
Geologic Time Scale
• Radioactivity (late 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
• James Ussher (1581-1665) in Ireland
– calculated the age of Earth based on
recorded history and genealogies in
Genesis
– announced that Earth was created on
October 22, 4004 B.C.
– widely accepted
http://star.arm.ac.uk/his
tory/USSHER.GIF
Changes in the Concept of
Geologic Time
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
– considered an "old Earth!"
http://www.nceas.ucsb.edu/~alro
y/lefa/Buffon.jpg
Changes in the Concept of
Geologic Time
– 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
– 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
Relative-Dating
• Six fundamental geologic principles
1) Superposition
2) Original horizontality
3) Lateral continuity
4) Cross-cutting relationships
5) Inclusions
6) Fossil succession
Relative-Dating Principles
• Principle of superposition
– Nicolas Steno (1638-1686)
– in an undisturbed succession of
sedimentary rock layers, the oldest layer
is at the bottom and the youngest layer
is at the top
– this method is used for determining the
relative age of rock layers (strata) and
the fossils they contain
http://www.science.siu.edu/zoology/k
ing/304/biogrphy.htm
Relative-Dating Principles
• Principle of original horizontality
– Nicolas Steno
– sediment is deposited in essentially horizontal
layers
– a sequence of sedimentary rock layers that is
steeply inclined from horizontal must have
been tilted after deposition and lithification
• Principle of lateral continuity
– Sediment extends laterally in all directions
until it thins and pinches out or terminates
against the edges of a basin (also Steno)
Relative-Dating Principles
• Horizontality
– sediments were originally deposited
horizontally in a marine environment
• Superposition
– old to young
Relative-Dating Principles
• Principle of cross-cutting
relationships
– James Hutton (1726-1797)
– an igneous intrusion or a fault must be
younger than the rocks it intrudes or
displaces
http://www.physicalgeography.net/fundamentals/10c.html
Cross-cutting
Relationships
• A dark-colored
dike has intruded
into older light
colored granite:
the dike is
younger than the
granite
Cross-cutting Relationships
• A small fault
displaces
tilted beds:
the fault is
younger
than the
beds.
Relative-Dating Principles
• Principle of inclusions
– discussed later in the term
• Principle of fossil succession
– discussed later in the term
History of Historical Geology
• Neptunism
– all rocks, including granite and basalt,
were precipitated in an orderly
sequence from a primeval, worldwide
ocean
– proposed in 1787 by Abraham Werner
(1749-1817)
– Werner was an excellent mineralogist,
but is best remembered for his
incorrect interpretation of Earth history
http://de.wikipedia.org/wiki/Abraham_Gottlob_Werner
History of Historical Geology
• Catastrophism
– proposed by Georges Cuvier (17691832)
– dominated European geologic
thinking
– the history of Earth resulted from a
series of sudden widespread
catastrophes which exterminated
existing life in the affected area
– six major catastrophes occurred,
corresponding to the six days of
biblical creation, the last one was the
biblical flood
http://search.eb.com/dinosaurs/dinosaurs/ocuvier001p1.html
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
• Present-day processes have operated
throughout geologic time
• Developed by James Hutton
• Advocated by Charles Lyell
(1797-1875)
– term uniformitarianism was
coined by William Whewell in 1832
http://www.stephenjaygoul
d.org/people/charles_lyell.
html
http://cepa.newschool
.edu/het/profiles/whe
well.htm
Unconformity at Siccar Point
• Hutton applied the principle of uniformitarianism
when interpreting rocks
• We now call what he observed an unconformity
.
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
http://www.energyquest.ca.gov/scientists/kelvin.html
Crisis in Geology
• This age was too young for the geologic
processes envisioned by other geologists
at that time
• 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:
• Atomic mass number
= number of protons + number of neutrons
• Isotopes: different numbers of neutrons, same
number of protons
• 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
Age Dating with 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
– < 1/1000000000 of a second
up to 49 billion years
• Radioactive decay
– is geometric not linear
– a curved graph
Uniform Linear Change
• In this
example of
uniform linear
change, water
is dripping
into a glass at
a constant
rate
Geometric Radioactive Decay
During each
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,
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 ,
the remaining parent proportion is 25%
– 25% = 2 half lives
– If half life is 57 milliion
years then the rock is
57 million years 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 daughter atoms
– Parent and daughter fit differently into the crystal
structure of certain minerals
• Geologists can use the crystals containing the
parent 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
Sedimentary Rocks
• Generally, sedimentary rocks cannot be
radiometrically dated
– the date obtained would correspond to the time of
crystallization of the mineral, not the time that it
was deposited as a sedimentary particle
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.
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
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
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
Fission Track Dating
• Uranium in a crystal will damage the crystal
structure as it decays
• The damage can be seen as fission tracks under
a microscope after etching the mineral
• The age of the
sample is related to
• This method is
– the number of
fission tracks
useful for
samples between
– and the amount
of uranium
1.5 and 0.04
– with older
million years old
samples having
more tracks
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 < 50,000 years
old
Carbon 14
• Carbon 14 is constantly
forming in the upper
atmosphere
• 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