Transcript Relative dating

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
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
► 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
Fig. 4-1, p. 62
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
Usher (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.
► 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
Illustration of the principles of original
horizontality
Illustration of the principles of
superposition
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
accepted
geologic column was widely
 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
► 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
►
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)
► 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
► Term uniformitarianism was coined by William Whewell in 1832
Unconformity at Siccar Point
► Hutton
explained that
 the tilted, lower rocks resulted from severe
upheavals that formed mountains
 The mountains were then worn away and covered by
younger flat-lying rocks
 the erosional surface represents a gap in the rock
record
Uniformitarianism
erosion
► Hutton
viewed Earth
history as cyclical
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
► For
the geologic processes envisioned by other
geologists at that time, this age was too young!
 What was the flaw in Kelvin’s calculation?
► 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
►
The nucleus of an atom is composed of
 protons – particles with a positive electrical charge
 neutrons – electrically neutral particles
 electrons – the negatively charged particles – encircling the
nucleus
►
atomic number
 Equal to the number of protons
 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 -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
Can vary from less than 1/billionth of a second to 49
billion years
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 halflife 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,
 25% means it is 2 half-lives
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
 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
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.
Dating the whole rock
yields a date of 700
million years = time of
crystallization.
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
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
 the number of
fission tracks
 the amount of
uranium
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 neutrona type of
cosmic ray strikes a nitrogen 14
atomit may be absorbed by the
nucleus and eject a proton changing
it to carbon 14
►
The
14C
formation rate
 is fairly constant
 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 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
► 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