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Chapter 8
Geologic Time
PowerPoint Presentation
Stan Hatfield . Southwestern Illinois College
Ken Pinzke . Southwestern Illinois College
Charles Henderson . University of Calgary
Copyright (c) 2005 Pearson Education Canada, Inc.
8-1
Geologic Time
Relative age dates – placing rocks and
events in their proper sequence of
formation
Numerical dates – specifying the actual
number of years that have passed since an
event occurred (known as absolute age
dating)
Geologic time scale – Earth’s history is
exceedingly long
Copyright (c) 2005 Pearson Education Canada Inc.
8-2
Relative Dating – Key Principles
Law of Superposition
• Developed by the physician Nicolaus
Steno in 1669
• In an undeformed sequence of
sedimentary rocks (or layered igneous
rocks), the oldest rocks are on the bottom
Copyright (c) 2005 Pearson Education Canada Inc.
8-3
Relative Dating – Key Principles
Applying the Law of Superposition to Grand Canyon formations.
Copyright (c) 2005 Pearson Education Canada Inc.
8-4
Relative Dating – Key Principles
Principle of Original Horizontality
• Layers of sediment are generally deposited
in a horizontal position
• Rock layers that are flat have not been
disturbed
Principle of Cross-Cutting Relationships
• Younger features cut across older features
Copyright (c) 2005 Pearson Education Canada Inc.
8-5
Relative Dating – Key Principles
Cross-cutting relationships represent one principle used in relative dating.
Copyright (c) 2005 Pearson Education Canada Inc.
8-6
Relative Dating – Key Principles
Inclusions
• An inclusion is a piece of rock that is enclosed
within another rock
• Rock containing the inclusion is younger
Unconformities
• An unconformity is a break in the rock record
produced by erosion and/or nondeposition of
rock units
• When there is no break the rocks are
considered conformable
Copyright (c) 2005 Pearson Education Canada Inc.
8-7
Relative Dating – Key Principles
Unconformities
• Types of Unconformities
– Angular unconformity – tilted rocks are
overlain by flat-lying rocks
– Disconformity – strata on either side of the
unconformity are parallel
– Nonconformity – metamorphic or igneous
rocks in contact with sedimentary strata
Copyright (c) 2005 Pearson Education Canada Inc.
8-8
Relative Dating – Key Principles
This Grand Canyon cross-section illustrates the three basic types of unconformities.
Copyright (c) 2005 Pearson Education Canada Inc.
8-9
Relative Dating – Key Principles
Formation of an angular unconformity.
Copyright (c) 2005 Pearson Education Canada Inc.
8-10
Correlation of Rock Layers
Matching of rocks of similar ages in
different regions is known as correlation
Correlation often relies upon fossils
• William Smith (late 1700s and early 1800s)
noted that sedimentary strata in widely
separated areas could be identified and
correlated by their distinctive fossil content
Copyright (c) 2005 Pearson Education Canada Inc.
8-11
Correlation of Rock Layers
Correlation often relies upon fossils
• Principle of Fossil Succession – fossil organisms
succeed one another in a definite and
determinable order that documents the evolution
of life; therefore any time period can be
recognized by its fossil content
• Index fossils – represent best fossils for
correlation; they are widespread geographically
and are limited to a short time span (i.e., they
evolved rapidly)
Copyright (c) 2005 Pearson Education Canada Inc.
8-12
Correlation of Rock Layers
Overlapping ranges of fossils held date rocks more exactly than using a single fossil.
Copyright (c) 2005 Pearson Education Canada Inc.
8-13
Dating with Radioactivity
Reviewing Basic Atomic Structure
• Nucleus
– Protons – positively charged particles with
mass
– Neutrons – neutral particles with mass
– Electrons – negatively charged particles that
orbit the nucleus
Copyright (c) 2005 Pearson Education Canada Inc.
8-14
Dating with Radioactivity
Reviewing Basic Atomic Structure
• Atomic number
– An element’s identifying number
– Equal to the number of protons in the atom’s
nucleus
• Mass number
– Sum of the number of protons and neutrons in
an atom’s nucleus
Copyright (c) 2005 Pearson Education Canada Inc.
8-15
Dating with Radioactivity
Reviewing Basic Atomic Structure
• Isotope
– Variant of the same parent atom
– Differs in the number of neutrons
– Results in a different mass number than the
parent atom
Copyright (c) 2005 Pearson Education Canada Inc.
8-16
Dating with Radioactivity
Radioactivity
• Spontaneous changes (decay) in the
structure of atomic nuclei
Types of radioactive decay
• Alpha particle emission
– Emission of 2 protons and 2 neutrons (an
alpha particle)
– Mass number is reduced by 4 and the atomic
number is lowered by 2
Copyright (c) 2005 Pearson Education Canada Inc.
8-17
Dating with Radioactivity
Types of radioactive decay
• Beta particle emission
– An electron (beta particle) is ejected from the
nucleus
– Mass number remains unchanged and the
atomic number increases by 1 (remember a
neutron is a combination of a proton and an
electron)
Copyright (c) 2005 Pearson Education Canada Inc.
8-18
Dating with Radioactivity
Types of radioactive decay
• Electron capture
– An electron is captured by the nucleus
– The electron combines with a proton to form a
neutron
– Mass number remains unchanged and the
atomic number decreases by 1
Copyright (c) 2005 Pearson Education Canada Inc.
8-19
Dating with Radioactivity
Common types of radioactive decay.
Copyright (c) 2005 Pearson Education Canada Inc.
8-20
Dating with Radioactivity
Parent – an unstable radioactive isotope
Daughter product – the isotopes that
result from the decay of a parent
Half-Life – the time required for one-half
of the radioactive nuclei in a sample to
decay
Copyright (c) 2005 Pearson Education Canada Inc.
8-21
Dating with Radioactivity
The radioactive-decay curve shows change that is exponential.
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8-22
Dating with Radioactivity
Radiometric Dating
• Principle of Radioactive Dating
– The percentage of radioactive atoms that
decay during one half-life is always the same
(50 percent)
– However, the actual number of atoms that
decay continually decreases
– Comparing the ratio of parent to daughter
yields the age of the sample
Copyright (c) 2005 Pearson Education Canada Inc.
8-23
Dating with Radioactivity
Radiometric dating
• Useful radioactive isotopes for providing
radiometric ages
– Rubidium-87
– Thorium-232
– Two isotopes of uranium (235 and 238)
– Potassium-40
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8-24
Dating with Radioactivity
Copyright (c) 2005 Pearson Education Canada Inc.
8-25
Dating with Radioactivity
Radiometric dating
• Sources of error
– A closed system is required
– If temperature is too high, daughter products
may be lost
– To avoid potential problems, only fresh,
unweathered rock samples should be used
Copyright (c) 2005 Pearson Education Canada Inc.
8-26
Dating with Radioactivity
Dating with carbon-14 (radiocarbon
dating)
• Half-life of only 5730 years
• Used to date very recent events
• Carbon-14 is produced in the upper
atmosphere
• Useful tool for anthropologists,
archaeologists, historians, and geologists
who study very recent Earth history
Copyright (c) 2005 Pearson Education Canada Inc.
8-27
Dating with Radioactivity
Importance of radiometric dating
• Radiometric dating is a complex procedure
that requires precise measurement
• Rocks from several localities have been
dated at more than 3 billion years
• Confirms the idea that geologic time is
immense
Copyright (c) 2005 Pearson Education Canada Inc.
8-28
Geologic Time Scale
The geologic time scale – a “calendar” of
Earth history
• Subdivides geologic history into units
• Originally created using relative dates
Structure of the geologic time scale
• Eon – the greatest expanse of time
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8-29
Geologic Time Scale
Structure of the geologic time scale
• Names of the eons
– Phanerozoic (“visible life”) – the most recent
eon, began just over 540 million years ago
– Proterozoic
– Archean
– Hadean – the oldest eon
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8-30
Geologic Time Scale
Structure of the geologic time scale
• Era – subdivision of an eon
• Eras of the Phanerozoic eon
– Cenozoic (“recent life”)
– Mesozoic (“middle life”)
– Paleozoic (“ancient life”)
• Eras are subdivided into periods
• Periods are subdivided into epochs
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8-31
Geologic Time Scale
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8-32
Geologic Time Scale
Precambrian time
• Nearly 4 billion years prior to the
Cambrian period
• Not divided into smaller time units because
the events of Precambrian history are not
known in great enough detail
– Also, first abundant fossil evidence does not
appear until the beginning of the Cambrian
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8-33
Geologic Time Scale
Difficulties in dating the geologic time
scale
• Not all rocks can be dated by radiometric
methods
– Grains comprising detrital sedimentary rocks
are not the same age as the rock in which they
formed
– The age of a particular mineral in a
metamorphic rock may not necessarily
represent the time when the rock formed
Copyright (c) 2005 Pearson Education Canada Inc.
8-34
Geologic Time Scale
Difficulties in dating the geologic time scale
• Datable materials (such as volcanic ash beds and
igneous intrusions) are often used to bracket
various episodes in Earth history and arrive at
ages
• Dates change as brackets become narrower and
methods refined; e.g., base of Triassic is now 252
Ma, base of Permian is now 299 Ma, base of
Cambrian is 543 Ma…
Copyright (c) 2005 Pearson Education Canada Inc.
8-35
End of
Chapter 8
Copyright (c) 2005 Pearson Education Canada Inc.
8-36