Download No Slide Title

Survey
yes no Was this document useful for you?
   Thank you for your participation!

* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

Document related concepts
no text concepts found
Transcript
Chapter 4
Geologic Time—
Concepts and Principles
Grand Canyon
• When looking down into the Grand Canyon,
we are really looking all the way back to the
early history of Earth
Grand Canyon
• More than 1 billion years of history are
preserved,
• like pages of a book,
– in the rock layers of the Grand Canyon
• Reading this rock book we learn
– that the area underwent episodes of
– mountain building
– advancing and retreating shallow seas
• We know these things by
– applying the principles of relative dating to the rocks
– and recognizing that present-day processes
– have operated throughout Earth history
What is time?
• We are obsessed with time, using
– clocks
– calendars
– appointment books
• Mostly we don’t have enough of it.
• Our common time units are
–
–
–
–
–
–
seconds
hours
days
weeks
months
years
• Ancient history involves
– hundreds of years
– thousands of years
• But geologic time involves
– millions of years
– even billions of years
Concept of Geologic Time
• Geologists use two different frames of reference
– when discussing geologic time
– Relative dating involves placing geologic events
• in a sequential order as determined
• from their position in the geologic record
– It does not tell us how long ago
• a particular event occurred
• only that one event preceded another
• For hundreds of years geologists
– have been using relative dating
– to establish a relative geologic time scale
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
Concept of Geologic Time
• The second frame of reference for geologic time
– is absolute dating
– Absolute dating results in specific dates
• for rock units or events
• expressed in years before the present
– It tells us how long ago a particular event occurred
• giving us numerical information about time
• Radiometric dating is the most common method
– of obtaining absolute ages
– Such dates are calculated
• from the natural rates of decay
• of various natural radioactive elements
• present in trace amounts in some rocks
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
• Early Christian theologians
– conceived of time as linear rather than circular
• James Ussher (1581-1665) in Ireland
– calculated the age of Earth based
– on recorded history and geneologies 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
– using melted iron balls of various diameters
• Extrapolating their cooling rate
– to an Earth-sized ball,
– he estimated Earth was 75,000 years old
Changes in the Concept of
Geologic Time
• Others used different techniques
• Using 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.
• Using 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
Relative-Dating Principles
• Six fundamental geologic principles are used in
relative dating
• 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
Relative-Dating Principles
• Principle of original horizontality
– Nicolas Steno
– Sediment is deposited
• in essentially horizontal layers
–
–
–
–
Therefore, a sequence of sedimentary rock layers
that is steeply inclined from horizontal
must have been tilted
after deposition and lithification
Principle of Horizontality
• Illustration of the principles of superposition
– and original horizontality
• Horizontality: These sediments
were originally
– deposited horizontally
– in a marine environment
– This outcrop is Chattanooga Shale, Tennessee
Principle of Superposition
• Illustration of the principles of superposition
– and original horizontality
• Superposition: The youngest
– rocks are at the top
– of the outcrop
– and the oldest rocks are at the bottom
Relative-Dating Principles
• Principle of lateral continuity
–
–
–
–
–
Nicolas Steno
Sediment extends laterally in all direction
until it thins and pinches out
or terminates against the edges
of the depositional basin
• 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
Relative-Dating Principles
• Principle of inclusions
– discussed later in the term
• Principle of fossil succession
– discussed later in the term
Cross-cutting
Relationships
• North shore of Lake
Superior, Ontario
Canada
• A dark-colored dike
has intruded into
older light colored
granite.
• The dike is younger
than the granite.
Cross-cutting Relationships
• Templin
Highway,
Castaic,
California
• A small fault
displaces
tilted beds.
• The fault is
younger than
the beds.
Neptunism
• 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
Neptunism
• Werner’s geologic column was widely accepted
– Alluvial rocks
• unconsolidated sediments, youngest
– Secondary rocks
• rocks such as sandstones, limestones, coal, basalt
– Transition rocks
• chemical and detrital rocks, some fossiliferous rocks
– Primitive rocks
• oldest including igneous and metamorphic
Catastrophism
• 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
– The last one was the biblical flood
Neptunism and Catastrophism
Were Eventually abandoned
• These hypotheses were abandoned because
– they were not supported by field evidence
• Basalt was shown to be of igneous origin
• Volcanic rocks interbedded with sedimentary
– and 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
• Principle of 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
• 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
• Hutton explained that
– the tilted, lower rocks
– resulted from severe upheavals that formed
mountains
– these were then worn away
– and covered by younger flat-lying rocks
– the erosional surface
– represents a gap in the rock record
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
– Today, 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
– and provided a clock to measure Earth’s age
• 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
Atoms
• Understanding absolute dating requires
– knowledge of atoms and isotopes
• All matter is made up of atoms
• The nucleus of an atom is composed of
– protons – particles with a positive electrical charge
– neutrons – electrically neutral particles
• with electrons – negatively charged particles –
encircling the nucleus
• The number of protons (= the atomic number)
– helps determine the atom’s chemical properties
– and the element to which it belongs
Isotopes
• Atomic mass number
= number of protons + number of neutrons
• The different forms of an element’s atoms
– with varying numbers of neutrons
– are called isotopes
• Different isotopes of the same element
– 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:
– In alpha decay, two protons and two neutrons
– (alpha particle) are emitted from the nucleus.
Radioactive Decay
– In beta decay, a neutron emits a fast moving
electron (beta particle) and becomes a proton.
– In 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.
– Examples:
• rubidium 87 decays to strontium 87 by a single beta
emission
• potassium 40 decays to argon 40 by a single electron
capture
• But other isotopes undergo several decay steps
– Examples:
• 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
• The 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,
– so has 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
– 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
• Because they fit, some radioactive parents
– are included in the crystal structure
– of certain minerals
What Materials Can Be Dated?
• The daughter atoms are different elements
–
–
–
–
with different sizes
and, therefore,
do not generally fit
into the same minerals as the parents
• 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.
Not Sedimentary Rocks
• Generally, sedimentary rocks cannot be
radiometrically dated
– because the date obtained
– would correspond to the time of crystallization of
the mineral,
– when it formed in an igneous or metamorphic rock,
– not the time that it was deposited as a sedimentary
particle
• Exception: dating the mineral glauconite,
– because it forms in certain marine environments as
a reaction with clay
– during the formation of the sedimentary rock
Sources of Uncertainty
• In glauconite, potassium 40 decays to argon 40
– because argon is a gas,
– it can easily escape from a mineral
• A closed system is needed for an accurate date
– that is, 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.
• The most reliable dates use multiple methods.
Sources of Uncertainty
• During metamorphism, some of the daughter
atoms may escape
– leading to a date that is too young.
– However, if all of the daughters are forced out
during metamorphism,
– then the date obtained would be the time of
metamorphism—a useful piece of information.
• 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
Dating the whole rock
yields a date of 700
million years = time of
crystallization.
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 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.
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
Daughters
Half-Life (years)
Uranium 238
Uranium 234
Thorium 232
Rubidium 87
Potassium 40
4.5 billion
704 million
14 billion
48.8 billion
1.3 billion
Lead 206
Lead 207
Lead 208
Strontium 87
Argon 40
Most of these
are useful for
dating older
rocks
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
– the number of
fission tracks
– and the amount of
uranium
– with older samples
having more tracks
• This method is
useful for samples
between 1.5 and
0.04 million years
old
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
but 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
• Early Christian theologians viewed time
– as linear and decided that Earth
– was very young (about 6000 years old)
• A variety of ages for Earth were estimated
– during the 18th and 19th centuries
– using scientific evidence,
– ages now known to be too young
• Neptunism and catastrophism were popular
– during the 17th, 18th and early 19th centuries
– because of their consistency with scripture,
– but were not supported by evidence
Summary
• James Hutton viewed Earth history
– as cyclical and very long
– His observations were instrumental
– in establishing the principle of uniformitarianism
• Charles Lyell articulated uniformitarianism
– in a way that soon made it
– the guiding doctrine of geology
• 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
• Geologists determine how many half-lives
– of a radioactive parent isotope
– have elapsed since the sample crystallized
• 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
Summary
• The most accurate radiometric dates
–
–
–
–
are obtained from
long-lived radioactive isotope/daughter pairs
in igneous rocks
Common pairs include:
•
•
•
•
•
uranium 238 – lead 206
uranium 235 – lead 207
thorium 232 – lead 208
rubidium87 – strontium 87
potassium 40 – argon 40
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