Ch09 geologic time_fall2007 (1)

Geologic TimeGeologic Time
• The rock layers of the Grand Canyon are likeThe rock layers of the Grand Canyon are like
pages in a history book.pages in a history book.
• However, some pages are missing or incomplete.However, some pages are missing or incomplete.
• Geologist study rocks to unravel the complexitiesGeologist study rocks to unravel the complexities
of Earth’s history.of Earth’s history.
• Geologic events must be put into a timeGeologic events must be put into a time
perspective – Theperspective – The Geologic Time ScaleGeologic Time Scale..
• The science that deals with determining the ages ofThe science that deals with determining the ages of
rocks is calledrocks is called geochronologygeochronology..

Methods of Dating Rocks
1. Relative Dating
• Using fundamental principles of geology to determine
the relative ages of rocks.
• Determining which rocks are older and which are
younger with respect to each other.
1. Absolute Dating
• Quantifying the actual date of the rock in years via
radiometric dating.
• Analysis of the breakdown of radioactive elements in
the rocks over time.

Relative Dating: Principles of Geology
• Geologic events must be put into a time perspective –
the Geologic Time Scale.
• Relative age dating methods were used before
numerical or absolute methods of dating were
developed:
 Principle of Original Horizontality
 Principle of Lateral Continuity
 Law of Superposition
 Principle of Cross-Cutting Relationships
 Inclusions
 Unconformities (Disconformity, Non-Conformity, Angular
Unconformity)
 Principle of Fossil Succession
 Index Fossils
 Uniformitarianism
• Only indicates the order of events relative to each other
– not how long ago they occurred.

Relative Dating: Steno’s Laws
• Principle of Original Horizontality
 Developed by Nicolaus Steno in 1669.
 Layers of sediment are generally
deposited in a horizontal position.
 Rock layers that are flat have not
been disturbed.
 Therefore, a sequence of
sedimentary rock layers that is
steeply inclined from horizontal
must have been tilted after
deposition and lithification.

Were these deposited like this?

Law of Original Horizontality –
These sedimentary rock layers have been moved from original
horizontal position by crustal disturbances after deposition

• Principle of Superposition
 Also developed by Nicolaus
Steno in 1669.
 In an undisturbed succession of
sedimentary rock layers (or
layered igneous rocks), the
oldest layer is at the bottom and
the youngest layer is at the top.

Where would you find the youngest rocks?
The oldest?

Superposition is well illustrated by the
strata in the Grand Canyon

• Principle of Lateral Continuity
 Sediment extends laterally in all
direction until it thins and pinches
out or terminates against the
edges of the depositional basin.

Figure 1-6 (p. 5)
Illustration of original lateral continuity. Cross-section A shows a sandstone
stratum deposited within a low-lying area or sedimentary basin that received
sediment eroded from surrounding uplands. Cross-section B shows the same
area after erosion has exposed the sandstone on hillsides.

Relative Dating: Unconformities
• Principles of
Unconformities
 Developed by James
Hutton
• There are two basic types
of contacts between rock
units:
 Conformable
 Unconformable

Relative Dating: Conformable Strata
• Principles of Unconformities
• Conformable Strata
 A vertical sequence of rocks in which deposition was
more or less continuous.
• Conformable contacts between beds of
sedimentary rocks may be either:
• Abrupt or
• Gradational
• Most abrupt contacts are bedding planes
resulting from sudden minor changes in
depositional conditions.
• Gradational contacts represent more gradual
changes in depositional conditions.

Abrupt vs.
Gradational
Contacts

Relative Dating: Unconformable Strata
• Principles of Unconformities
• Unconformable contacts (or unconformities) are
surfaces which represent a gap in the geologic
record, because of either:
 Erosion or
 Nondeposition
• The time represented by this gap can vary widely,
ranging from millions of years to hundreds of
millions of years.
• The rock record is incomplete
 The interval of time not represented by strata is a
hiatus.
• Unconformities represent significant geologic
events.

 For 1 million yearsFor 1 million years
erosion occurrederosion occurred
 removing 2 MY ofremoving 2 MY of
rocksrocks
The Origin of an Unconformity
• In the process of forming an unconformity:In the process of forming an unconformity:
 Deposition began 12 million years ago (MYA),Deposition began 12 million years ago (MYA),
 continuing until 4 MYA.continuing until 4 MYA.
• The last columnThe last column
 is the actualis the actual
stratigraphic recordstratigraphic record
 with an unconformity.with an unconformity.
 and giving rise toand giving rise to
 a 3 million yeara 3 million year
hiatus.hiatus.

Types of Unconformities
• Angular Unconformity – Tilted rocks
are overlain by flat-lying rocks.
 Indicates that during the pause in
deposition, a period of deformation and
erosion occurred.
• Nonconformity – Metamorphic or
igneous rocks in contact with
sedimentary strata.
 Indicates a period of uplift and erosion of
the rocks previously overlying the igneous/
metamorphic rocks prior to deposition of
the younger sedimentary rocks.
• Disconformity – Strata on either side
of the unconformity are parallel.
• Paraconformity – An unconformity
at which strata are parallel and the
contact is a simple bedding plane.

Formation
of an
Angular
Unconformity

Formation
of an
Angular
Unconformity
Insert Animation #1:
Angular Unconformities
and Nonconformities

Angular unconformity at Siccar Point, eastern Scotland. (A) It was
here that James Hutton first realized the historical significance of
an unconformity. The drawings (B) indicate the sequence of events
documented in this famous exposure.

Formation
of a
Nonconformity
with Inclusions

Formation
of an
Nonconformity
with Inclusions
Angular Unconformities
and Nonconformities

Formation
of an
Disconformity
Angular Unconformities,
Nonconformities, and
Disconformities

Unconformities in the
Grand Canyon

Relative Dating: Lyell’s Principles
• Principle of Cross-Cutting
Relationships
 Younger features cut
across older features.
• Principle of Inclusions
 An inclusion is a
fragment of rock that is
enclosed within another
rock.
 The rock containing the
inclusion is younger.

Principle of Cross-Cutting
Relationships – Faults
Where a fault cuts across a sequence of sedimentary
rock, the fault is younger than the rocks it cuts.
The sedimentary rocks are older than the fault which
cuts them, because they had to be there first, before
they could be faulted.

Principle of Cross-Cutting
Relationships – Intrusions
Where an igneous intrusion cuts across a sequence of
sedimentary rock, the sedimentary rocks are older
than the igneous rock which intrudes them.
The intrusion is younger than the rocks it cuts.

What Cross-Cutting Relationships
Are Illustrated in this Figure?

The Principle of Inclusions –
Sedimentary Rocks
Fragments of eroded rock overlie the unconformity.
These are gravel clasts or inclusions.
The pieces of gravel are older than the bed in which
they are found.

The Principle of Inclusions –
Igneous Rocks
A xenolith is a fragment of the surrounding rock
which has broken off during an intrusion and
fallen into the magma.
The xenolith is older than the igneous rock which
contains it.

Comparison of inclusions in a sedimentary rock (A)
with inclusions in an igneous rock (B).
Which rock unit is older in each figure?
Which are gravel clasts and which are xenoliths?

Figure 2-12 (p. 21)
(A) Granite inclusions in sandstone indicate that granite is the older unit.
(B) Inclusions of sandstone in granite indicate that sandstone is the older unit.

Relative Geologic Age Dating
Relative Geologic Dating #2

Interpreting a Sequence of Events

Interpreting a Sequence of Events
Determine the order in which the geologic events
occurred?

• An example of how the sequence of geologic events can be determined from
cross-cutting relationships and superposition.
• From first to last, the sequence indicated in the cross-section:
1. First deposition of D,
2. Then faulting to produce fault B,
3. Then intrusion of igneous rock mass C,
4. Then erosion forming the unconformity,
5. Followed by deposition of E.
• Strata labeled D are oldest, and strata labeled E are youngest

Interpret the Geologic History of the
Grand Canyon

The Principle of Uniformitarianism
• Principle of Uniformitarianism:
 Developed in the late 18th
century by James Hutton the
“Father of Modern Geology”
 States that physical laws which
operate today have operated in
the past.
 Argued that uniform natural
laws govern geologic
processes (cycles) that
operated over the immensity
of geologic time.
““The Present is the Key to the Past”The Present is the Key to the Past”

• Principle of Fossil Succession:
 Developed by William Smith (late 1700's)
 Fossilized organisms of each age of the Earth’s
history are unique and occur in a consistent vertical
order in sedimentary rocks all over the world.
 Therefore, any time period can be recognized by its
fossil content.
 Examples: Age of Trilobites, Age of Fishes, Age of
Coal Swamps, Age of Reptiles, Age of Mammals
• Geologists interpret fossil succession to be the
result of evolution – the natural appearance
and disappearance of species through time.
Principle of Fossil Succession

The Geologic
Time Scale –
Development
of Plants and
Animals Over
Geologic Time

• Index Fossil – A geographically
widespread fossil that is limited to a
short span of geologic time (specific
time indicator).
Principle of Fossil Succession

Dating Rocks using Overlapping
Fossil Ranges

Geologic Time Scale
• The geologic time scale has been
determined bit-by-bit over the years
through relative dating, correlation,
examination of fossils, and radiometric
dating.
• Boundaries on the time scale are drawn
where important changes occur in the fossil
record, such as changes in plant and animal
species or extinction events.

Geochronologic Units
• The Geologic Time Scale is divided into a number
of types of units of differing size.
• From the largest units to the smaller units, they
are:
»Eons
»Eras
»Periods
»Epochs
• These divisions are based on changes in life forms
recorded by fossils.
• These units are geochronologic units.
• Geochronologic units are time units.

Geologic Time Scale.
The age for the base of each division is in accordance with recommendations
of the International Commission on Stratigraphy for the year 2000.

Periods: Geochronologic Nomenclature
Development of Geochronologic Nomenclature for
Geolgic Periods:
• Cambrian System: Cambria (Roman name for Wales)
• Silurian and Ordovician Systems: Silures and Ordovices
were ancient Celtic Tribes
• Devonian System: Devonshire, England
• Carboniferous System: British coal measures
• Permian System: Perm Province, Russia
• Triassic System: Set of three formations in Germany
• Jurassic System: Jura Mountains, Franco-Swiss border
• Cretaceous System: Latin for Chalk (creta)
• Tertiary System: “Montes Tertiarii" of Italian Alps
• Quaternary System: Soft sediments of northern France

Eons
• Eons are the largest
division of geologic time.
• In order from oldest to
youngest, the eons are:
 Hadean Eon – Origin of the
Earth – oldest rocks on Earth
 Archean Eon – “Ancient or
Archaic” – first single-celled
organisms
 Proterozoic Eon – “Beginning
Life” – first multi-celled
organisms (2.5 billion to 542
million years ago)
 Phanerozoic Eon – “Visible
Life” (542 million years ago to
present)

The Precambrian
• The Hadean, Archean,
and Proterozoic Eons
are together referred
to as the Precambrian,
meaning “before the
Cambrian Period”.
• The Precambrian
covers ~88% of
geologic history.

Eras
• The Phanerozoic Eon is
divided into three Eras.
• Eras are divided into
geologic Periods.
• In order from oldest to
youngest, the three Eras
are:
 Paleozoic Era – “Ancient
Life” (such as trilobites)
 Mesozoic Era – “Middle
Life” (such as dinosaurs)
 Cenozoic Era – “Recent
Life” (such as mammals)

Periods of the Paleozoic Era
• Permian Period
• Carboniferous Period
 (Mississippian and
Pennsylvanian Periods
in North America)
• Devonian Period
• Silurian Period
• Ordovician Period
• Cambrian Period
(oldest)

Periods of the Mesozoic Era
• Cretaceous Period
• Jurassic Period
• Triassic Period
(oldest)

Periods of the Cenozoic Era
• Quaternary Period
(youngest – today)
• Tertiary Period
(oldest)

Epochs
Periods can be subdivided into epochs.
Epochs can be subdivided into ages.

Epochs of the Cenozoic Era
• Quaternary
Period
 Holocene Epoch
(youngest – today)
 Pleistocene Epoch
• Tertiary Period
 Pliocene Epoch
 Miocene Epoch
 Oligocene Epoch
 Eocene Epoch
 Paleocene Epoch
(oldest)

Principles of
Radiometric Age Dating

Review of Atoms
• Atom = smallest particle of matter that can
exist as a chemical element.
• The structure of the atom consists of:
 Nucleus composed of protons (positive charge)
and neutrons (neutral).
 Electrons (negative charge) orbit the nucleus.
 Various subatomic particles.

Ions
• Most atoms are neutral overall, with the
number of protons equaling the number of
electrons.
• If there is an unequal number of protons
and electrons, the atom has a charge
(positive or negative), and it is called an ion.

Atomic Number
• Atomic number of an atom = number of
protons in the nucleus of that atom.
Example: The atomic number of Uranium is
92. It has 92 protons.

Mass Number
• Mass number is the sum of the number of
protons plus neutrons.
Example: Uranium-235 has 92 protons and
143 neutrons.
• The mass number may vary for an element,
because of a differing number of neutrons.

Isotopes
• Elements with various numbers of neutrons
are called isotopes of that element.
Example: Uranium-235 and Uranium-238
• Some isotopes are unstable. They undergo
radioactive decay, releasing particles and
energy.
• Some elements have both radioactive and
non-radioactive isotopes.
Examples: carbon, potassium.

What Happens When Atoms Decay?
• Radioactive decay entails spontaneous
changes (decay) in the structure of unstable
atomic nuclei.
• Nuclei are unstable because the forces
binding the protons and neutrons together
are not strong enough causing the nuclei to
break apart or decay.
• Radioactive decay occurs by releasing
subatomic particles and energy.
• The radioactive parent element is unstable
and undergoes radioactive decay to form a
stable daughter element.

What Happens When Atoms Decay?
• Radioactive Decay
Example:
 Uranium, the parent
element, undergoes
radioactive decay,
releases subatomic
particles and energy,
and ultimately
decays to form the
stable daughter
element, lead.
Radioactive
Parent Isotope
Stable
Daughter
Isotope
Potassium-40 Argon-40
Rubidium-87 Strontium-87
Thorium-232 Lead-208
Uranium-235 Lead-207
Uranium-238 Lead-206
Carbon-14 Nitrogen-14

Subatomic Particles and Radiation
Released by Radioactive Decay
• Types of Radioactive Decay
 Alpha Emission:
• Emission of 2 protons and 2 neutrons (an
alpha particle = positively charge helium ion).
• Alpha Particles – Charge = +2 Mass = 4
Mass number is
reduced by 4 and
the atomic number
is lowered by 2.

 Beta Emission:
• An electron (beta particle) is ejected from the nucleus
when a neutron splits into a proton and electron.
• Beta Particles – Charge = -1 Mass = Negligible
• Mass number remains unchanged (because electrons
have practically no mass).
Atomic number increases by 1,
because a neutron is a
combination of a proton and
an electron; therefore, the
nucleus contains one or more
protons than before.

 Electron Capture:
• An electron is captured by the nucleus and combines
with a proton to form a neutron.
• Mass number remains unchanged and the atomic
number decreases by 1 (because the nucleus now
contains one less proton).

As Uranium-238
(U238
) to decays to
Lead-206 (Pb206
),
there are 13
intermediate
radioactive daughter
products formed
(including radon,
polonium, and other
isotopes of uranium),
along with and 8
alpha particles and 6
beta particles
released.

Radioactive Decay Rate
• Many radioactive elements can be used as geologic
clocks.
• Each radioactive element decays at its own
constant rate.
• The rate of decay is not affected by changes in
pressure, temperature, or other chemicals.
• Once this rate is measured, geologists can estimate
the length of time over which decay has been
occurring by measuring the amount of radioactive
parent element and the amount of stable daughter
elements.

Half-Life
• Each radioactive isotope has its own unique
half-life.
• A half-life is the time it takes for half of the
parent radioactive element to decay to a
daughter product.

Half Lives for Radioactive Elements
Radioactive
Parent Isotope
Stable
Daughter
Isotope
Currently
Accepted Half-
Life Values
Potassium-40 Argon-40 1.25 billion years
Rubidium-87 Strontium-87 48.8 billion years
Thorium-232 Lead-208 14.1 billion years
Uranium-235 Lead-207 704 million years
Uranium-238 Lead-206 4.47 billion years
Carbon-14 Nitrogen-14 5,730 years

Measuring Decay Rates
• The decay rates of the
various radioactive
isotopes are measured
directly using a mass
spectrometer.
• “Counts” the quantities of
parent and daughter
isotopes in a rock sample
by:
1. Measuring the mass of a
quantity of a radioactive
element.
2. Analyzing the mass again
after a particular period of
time.
3. The change in the number of
atoms over time gives the
decay rate.

Radioactive
Decay and
Half-Life
Radioactive Decay

Rocks That Can Be Dated
• Igneous rocks are best for age dating:
 Igneous rock crystallize minerals from magma.
 When the mineral crystals are formed, they “lock in”
the atoms of radioactive elements.
 The newly formed crystals may contain some
radioactive elements, such as Potassium-40 or Uranium
that can be dated.
 If the crystals remain undisturbed, fresh samples of
igneous rock are not likely to have lost any daughter
atoms.
 The dates they give tell when the magma cooled
(emplaced).

Bracketing Sedimentary Ages using
Igneous Rocks
Igneous rocks that have
provided absolute
radiogenic ages can
often be used to date
sedimentary layers.
(A)The shale is bracketed
by two lava flows.
(B) The shale lies above
the older flow and is
intruded by a younger
igneous body.

Bracketing Sedimentary Ages using
Igneous Rocks

Difficulties in Dating
• Radiometric dating is a complex procedure
that requires precise measurement
• Sources of Error
• A closed system is required: No parent or daughter
atoms gained or lost since system formation.
• No daughter atoms present when the system formed.
• To avoid potential problems, only fresh,
unweathered rock samples should be used.

Difficulties in Dating
• Not all rocks can be dated by radiometric
methods:
 Grains comprising detrital sedimentary rocks are
often weathered and may have lost some of their
daughter atoms.
 The age of the detrital grain in a sedimentary rock
gives the age of the original parent rock from which
it was derived.
 Metamorphic rocks have undergone trauma from
intense heat and pressure that may cause the loss of
some daughter atoms.
 The age of a particular mineral in a metamorphic
rock may not necessarily represent the time when
the rock formed – may represent age of
metamorphism due to recrystallization of the
minerals.

Ch09 geologic time_fall2007 (1)

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