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• The Map That Changed the World– William Smith, 1815– Geologic map of
England, Wales, part of Scotland
Dating the Earth
The Map That Changed the World
• 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
Relative 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
Geologic Time Scale
How was the timescale created?
• Mapping in 1800s using the principles of
– Superposition
– Original Horizontality
– Original Lateral Continuity
– Cross-cutting relationships
– Also Fossil Correlation
How was the timescale created?
• Superposition– Oldest on bottom, youngest on top
Relative-Dating Principles
Chattanooga Shale, TN
• Original Horizontality– Sediment originally deposited in flat
parallel layers
• 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
Relative-Dating Principles
• A dark-colored dike has intruded into older light colored granite: the dike is younger than the granite
Cross-cutting Relationships
North shore of Lake Superior, Ontario Canada
• A small fault displaces tilted beds: the fault is younger than the beds
Cross-cutting Relationships
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:EErosionGLCHTiltingErosionMDJAErosionNKBTiltingErosionFErosion
Relative Dating – Cross-section
http://facweb.bhc.edu/academics/science/harwoodr/Geol101/labs/Dating2/Index.htm
Relative Dating – Cross-section
http://facweb.bhc.edu/academics/science/harwoodr/Geol101/labs/Dating2/Index.htm
Key:HFDGIErosionCErosionABMLKErosionJ
Grand Canyon: history revealed
• 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
Grand Canyon
• 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
Absolute Dating
• 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
Geologic Time Scale
• 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
Changes in the Concept of Geologic Time
• 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
History of Historical Geology
• 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
Uniformitarianism
Unconformity at Siccar Point
• Hutton viewed Earth history as cyclical
Uniformitarianism
• 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
erosion
deposition uplift
erosion
• 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
Crisis in Geology
• 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
Absolute-Dating Methods
• 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
Radioactive Decay
– electron capture decay, a proton captures an electron and converts to a neutron
• 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
Radioactive Decay
Uranium 238 decay
• 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
Half-Lives
• In radioactive decay, during each equal time unit, one half-life, the proportion of parent atoms decreases by 1/2
Geometric Radioactive Decay
• 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?
Determining Age
– 25% means it is 2 half-lives old.
– the rock is 57 x 2 =114 million years old.
• 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
What Materials Can Be Dated?
• Crystallization of magma separates parent atoms from previously formed daughters
• This resets the radiometric clock to zero• Then the parents gradually decay
Igneous Crystallization
• 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
Sources of Uncertainty
a. A mineral has just crystallized from magma.
Dating Metamorphism
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.
• 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 Daughters Half-Life (years)
Long-Lived Radioactive Isotope Pairs Used in Dating
Uranium 238 Lead 206 4.5 billionUranium 234 Lead 207 704 millionThorium 232 Lead 208 14 billionRubidium 87 Strontium 87 48.8 billionPotassium 40 Argon 40 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
• 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
Radiocarbon Dating Method
• 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
Carbon 14
• 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
Tree-Ring Dating Method
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
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