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Measuring Geologic Time
Types of Rocks
• Igneous rock: any rock formed by cooling and
crystallization of magma (e.g., granite) or by
consolidation of debris ejected from volcanoes
(e.g., ash)
• Sedimentary rock: any rock composed of
sediment. The sediment may be particles of
various sizes formed by the breakdown of other
rocks (e.g., sandstone or shale) or the remains
of animals (e.g., shelly limestone) or plants
(e.g., coal), or chemical compounds that are
deposited directly from solution (e.g., cave
limestone).
• Metamorphic rock: any rock altered by high
temperature or pressure and the chemical
activity of fluids (e.g., slate, marble).
Examples of Sedimentary Rocks
Chert - a form of microcrystalline quartz
Anthracite coal
Siltstone
Reconstructing Geologic Time
• Relative dating: Determining the age of an event
relative to other events. Putting events in a
chronological order.
• Absolute dating: Using geochemical and other
methods to assign actual ages to specific events.
Construction of the Relative Time Scale of Rocks
• Stratigraphy: study of the composition, origin,
geographic extent and age of layered rocks.
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Principle of Original Horizontality
Principle of Original Lateral Continuity
Principle of Superposition
Principle of Uniformitarianism
Principle of Cross-Cutting Relationships
Principle of Inclusions
Unconformity
Principle of Original Horizontality
• Sedimentary particles settle from fluids under the influence of
gravity
• Thus, sediments must have been deposited in layers that were
nearly horizontal and parallel to the surface on which they were
accumulating
The Principle of Original Lateral Continuity
• At the time of deposition,
soft sediments spread out
laterally in all directions
until they either thin out (due
to a lack of material) or run
into a barrier, such as the
edge of a geologic
depression.
The Principle of Superposition
• For any sequence of undisturbed strata, the oldest layers are on
the bottom, and successively higher layers are successively
younger
Dipping strata
Principle of Uniformitarianism
• The same processes and principles (e.g., erosion, transportation of
materials, etc.) that govern change on the earth today, have operated
in the past to produce similar changes
Principle of Cross-Cutting Relationships
• Any structure that cuts across another feature must be younger
than that feature.
Principle of Inclusions
• Fragments with larger rock masses are older than the
rock masses in which they are enclosed
Unconformity
• A surface that
separates older from
younger rock, with
some amount of time
missing between the
older and younger
units
Kinds of Unconformity
Angular unconformity – steeply
inclined older strata have been beveled
by erosion and covered by flat-lying
younger layers
Disconformity – parallel strata are
separated by an erosional surface
May be caused by the withdrawal and
subsequent advance of the sea
Nonconformity – surfaces where
stratified rocks rest on older intrusive
rocks
Question
• How do we recognize the same time period
in different temporally-ordered stacks of
rock that are sometimes separated by large
distances?
Correlation
• The determination of the
equivalence of bodies of
rock in different
localities
• Rock bodies may be
equivalent in the
lithology (composition,
texture, color), in their
age, in the fossils
Lithostratigraphic Correlation
• Correlation based on similarity of rock type and position.
• A formation may
have changed
somewhat in
appearance between 2
localities, but if it
always lies above or
below a distinctive
stratum of consistent
appearance, then the
correlation of the
formation is confirmed
Biostratigraphic Correlation
• Correlation of strata based on biological similarity
• If life forms have varied
through time (and fossils
assemblages from different
times are distinctive, and the
relative ages of assemblages
can be determined by
superposition), then the
occurrence of the same fossil
assemblage in rocks from
different regions indicates that
the rocks formed at the same
time, and a relative time-scale
based on fossil assemblages
can be constructed.
Magnetostratigraphic Correlation
• Correlation based on different orientation of
materials in sediments due to changes in the Earth’s
magnetic field over time
• Using these
aforementioned
methods, especially
the biostratigraphy,
scientists working
over the last 200
years have developed
a Relative
Geologic Time
Scale.
Biostratigraphy and the Geologic Time Scale
• The German paleontologists Friedrich Quenstedt and Albert Oppel
indicated that each increment of time in a stratigraphy could be
characterized by a particular assemblage of fossil organisms,
formally termed a biostratigraphic "zone"
• These zones could then be traced over large regions, and
eventually globally.
• Groups of zones were used to establish larger intervals of
stratigraphy, known as geologic "stages" and geologic "systems".
• The time corresponding to most of these intervals of rock became
known as geologic "ages" and "periods", respectively.
Construction of an
Absolute Time Scale
Lord Kelvin's Challenge
• Kelvin believed that the earth was initially molten and had
cooled to its present state
• He attempted to determine the rate of cooling: because we
know the mean surface temperature of the Earth and we
know the heat loss of the Earth, Kelvin reasoned that we
can calculate how long it took this molten body to cool
• Kelvin determined the Earth could be as young as 10 to 25
million years old.
Atoms
• An atom is the smallest unit of matter that still retains the properties
of an element
• In the nucleus of an atom are neutrons and protons
• A neutron is a subatomic particle that is electrically neutral (no electric
charge)
• A proton is a subatomic particle with a single positive electric charge (+)
• The nucleus of an atom is surrounded by rapidly moving electrons
• An electron is a subatomic particle with a single negative electric charge (-)
• The positive charge of a proton is equal to the negative charge of an
electron
• Electrons are very small subatomic particles - they have a mass that
is 2000 times less than that of a proton or a neutron
Model of an
atom of the
element Helium
Atomic Number and Mass Number
• Elements differ from one another in the number of
subatomic particles in their atoms
• All atoms of a particular element have the same unique
number of protons; this is called the element's atomic
number
• An atom's mass number is the sum of the numbers of
protons and neutrons in its nucleus
Isotopes and Radioactivity
• Some elements have variant forms called isotopes
•Isotopoes are atoms of an element which have the same number of
protons in the nucleus, the same atomic number, and the same
chemical properties , but which have different atomic masses because
they have different numbers of neutrons in their nuclei
• A radioactive isotope is one in which the nucleus decays
spontaneously, giving off particles and energy
• Radioactive decay involves the spontaneous decay of unstable
atomic nuclei (parent nuclei) until they reach a stable state, which we
call daughter nuclei.
Radioactive decay series of uranium-238 to lead-206
Alpha decay is a type of radioactive decay that yield alpha
particles - positively charged ions of helium (An alpha particle
contains two protons and two neutrons.)
Beta decay another type of radioactive decay that emits a beta
particle - an electron discharged from the nucleus when a neutron
splits into a proton and an electron)
A third kind of emission in radioactive decay is called gamma
radiation
Its consists of a form of invisible electromagnetic waves having
even shorter wavelengths than x-rays
Radioactive Decay
• The rate of decay of radioactive isotopes is uniform and is not
affected by changes in pressure, temperature, or the chemical
environment
• Each radioactive isotope has a particular mode of decay and a
unique decay rate
• As time passes, the quantity of the original parent nuclide
diminishes, and the number of the newly formed, or daughter, atoms
increases, thereby indicating how much time has elapsed since the
clock began timekeeping
•The determination of radioactive decay is accomplished using a
mass spectrometer
Mass Spectrometry
Radioactive Decay and Half-Life
• The decline in the number of atoms is rapid at early stages but
becomes progressively slower in the later stages
• Because of this feature of radioactivity, it is convenient to consider
the number of years needed for half of the original quantity of atoms
to decay
• This span of years is termed the half-life
Half-Life cont.
• Some nuclides have very long half-lives, measured in billions or even trillions of
years; others have extremely short half-lives, measured in tenths or hundredths of a
second.
• The decay rate and therefore the half-life are fixed characteristics of a nuclide; the
half-life of a given nuclide is a constant.
• At the end of the years constituting one half-life, 1/2 of the original quantity of
radioactive element still has not undergone decay; after another half-life, 1/2 of
what was left is halved, so that 1/4 of the original quantity remains; after a third
half-life, only 1/8 would remain, and so on
• Every radioactive
element has its own halflife
Dating Methods
Carbon-14 Dating Method
 Carbon 14 is being created
continuously in the earth’s
atmosphere
 14C originates in the upper
atmosphere when a neutron
strikes an atom of 14N; as a
result of the collision, the N
atom emits a proton and
becomes 14C
• The newly formed 14C
combines quickly with O2 to
form CO2
• The 14C that is tied up in CO2 can be utilized by plants during
photosynthesis; also, when plants are eaten by herbivores this 14C
can become incorporated into animal (tissues) food chain as well
Carbon-14 Dating Method cont.
 Eventually 14C decays back to 14N by the emission of a beta particle;
 A plant removing CO2 from the atmosphere should receive a share of
14C proportional to that in the atmosphere; a state of equilibrium
 But the age of some ancient piece of organic material is not
determined by the ratio of parent (14C) to daughter (14N) nuclides
 The age is estimated from the ratio of 14C to all other carbon (namely
12C) in the sample
• When an animals dies, there can be no further replacement of carbon
from atmospheric CO2, and the amount of 14C already present in the
once living organism begins to diminish in accordance with the rate of
14C decay
Summary of Radiometric Dating
• Radiometric dating provides numerical values for the age of an
appropriate rock, usually expressed in millions of years.
• Therefore, by dating a series of rocks in a vertical succession of
strata previously recognized with basic geologic principles (recall
Stratigraphic principles and relative time), it can provide a
numerical calibration for what would otherwise be only an ordering
of events -- i.e. relative dating obtained from biostratigraphy
(fossils), superpositional relationships, or other techniques.
• The integration of relative dating and radiometric dating has
resulted in a series of increasingly precise "absolute" (i.e. numeric)
geologic time scales