Transcript Dating with Radioactivity

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.
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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
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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
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Relative Dating – Key Principles
Applying the Law of Superposition to Grand Canyon formations.
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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
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Relative Dating – Key Principles
Cross-cutting relationships represent one principle used in relative dating.
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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
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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
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Relative Dating – Key Principles
This Grand Canyon cross-section illustrates the three basic types of unconformities.
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Relative Dating – Key Principles
Formation of an angular unconformity.
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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
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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)
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Correlation of Rock Layers
Overlapping ranges of fossils held date rocks more exactly than using a single fossil.
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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
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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
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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
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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
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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)
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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
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Dating with Radioactivity
Common types of radioactive decay.
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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
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Dating with Radioactivity
The radioactive-decay curve shows change that is exponential.
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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
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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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Dating with Radioactivity
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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
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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
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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
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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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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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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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Geologic Time Scale
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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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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
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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…
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End of
Chapter 8
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