Transcript Ch 4 ppt

Grand Canyon
• When looking down into the Grand
Canyon, we are really looking all the way
back to the early history of Earth
Chapter 4: Geologic Time—
Concepts and Principles
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
• Relative Dating – putting rock layers and events in
order relative to when they occurred.
• Absolute dating which results in specific
numerical dates for rock units or events
– 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 -- today
• The discovery of
radioactivity near the end
of the 1800s allowed
absolute ages to be
accurately applied to the
relative geologic time
scale
– The most recent
geologic time scale
model is a dual scale
– a relative scale and
an absolute scale
Fig. 4-1, p. 62
Changes in the Concept of
Geologic Time
• Attempts to give an age to the earth
– James Usher (1581-1665) in Ireland
– calculated the age of Earth based on genealogies in
Genesis
– Announced that Earth was created on
October 22, 4004 B.C.
• A century later it was still considered heresy to say
Earth was more than about 6000 years old.
Changes in the Concept of
Geologic Time
– Georges Louis de Buffon (1707-1788)
• calculated how long Earth took to cool gradually
from molten iron balls
– Earth about 75,000 years
Others calculated the rate of sediment deposition
Also the rate of salt build-up in the oceans from the continental rivers
Ages in millions to billions of years
In 1953, the dating of meteorites was accomplished. Age ~ 4.5 by
Relative-Dating Principles
• Six fundamental geologic principles are
used today in relative dating
1. 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
• (note: Steno lived contemporaneously with Usher)
– This method is used for determining the
relative age of rock layers (strata) and the
fossils they contain
Illustration of the principles of
superposition
Relative-Dating Principles
2. 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
Principle of Lateral Continuity
Nicholas 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 event must be younger
than the rocks it intrudes or cuts across
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.
Principle of inclusions
That which is included is _________
(older? Younger?)
Principle of inclusions
6. Principle of
fossil succession
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
• (also relatively modern, and built on Usher’s Biblical
age of the Earth)
Uniformitarianism
• Principle of uniformitarianism
– Present-day processes have operated throughout geologic
time. This includes the physical, chemical and biological
processes
– 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
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
• Old Earth: 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
Sequence of Events
Key to Rock Types
Unconformities: 3 Types
1
2
3
Using Radioactive Decay
to obtain numerical age
• 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
• 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
Dating of sedimentary rocks RARE: dating the mineral glauconite,
because it forms in certain marine environments as a reaction
with clay during the formation of the sedimentary rock
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
• 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 235
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
• 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