Download Geologic Time

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Geologic Time
Earth 9th edition, Chapter 9
Geology 100
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Key Concepts
Geologic time and the “rock record.”
Putting events in order – Relative dating laws and principles.
Using principles of relative dating – Interpretation and correlation of rock units.
Evidence of past life and environments – Fossils.
Determining ages of events – Radioactive dating.
The geologic time scale.
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Determining geological ages
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
Also known as absolute age dating
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Principles of relative dating
Law of superposition
Developed Nicolaus Steno in 1669
In an undeformed sequence of sedimentary rocks (or layered extrusive igneous rocks),
the oldest rocks are on the bottom
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Superposition illustrated in the Grand Canyon
Principles of relative dating
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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Cross-cutting
Relative Geologic Dating
Cross-cutting
Cross-cutting
Inclusions
Inclusions
Inclusions
Unconformity
Principles of relative dating
Types of unconformities
Angular unconformity
Tilted rocks are overlain by flat-lying rocks
Disconformity
Strata on either side of unconformity are parallel
Nonconformity
Metamorphic or igneous rocks are in contact with sedimentary rocks
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Formation of an angular unconformity
Unconformity Types
Any unconformities in this picture?
Unconformities present in the Grand Canyon
Starting at the bottom: nonconformity
Starting at the bottom: angular unconformity
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Toward the top: disconformities
From the top: Redwall limestone
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 1700’s) 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
Principle of fossil succession
Fossil organisms succeed one another in a definite and determinable order
Any time period can be recognized by its fossil content
Index fossils
Widespread geographically
Limited to short span of geologic time
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Determining the ages of rocks using fossils
Relative dating
Using radioactivity in dating
Reviewing basic atomic structure
Nucleus
Protons
zPositively-charged particles with mass of 1 a.m.u.
Neutrons
zNeutral particles with mass of 1 a.m.u.
Electrons
Negatively-charged particles that orbit the nucleus
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Using radioactivity in dating
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 the atom’s nucleus
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Using radioactivity in dating
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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Using radioactivity in dating
Radioactivity
Spontaneous changes (decay) in the structure of atomic nuclei
Types of radioactive decay
Alpha 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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Using radioactivity in dating
Types of radioactive decay
Beta emission
An electron (beta particle) is ejected from the nucleus
Mass number remains unchanged and the atomic number increases by 1
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Types of radioactive decay*
Using radioactivity in dating
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Parent
An unstable radioactive isotope
Daughter product
The isotopes resulting 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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A radioactive decay curve
Radioactive Decay
Uranium – 238 decay series
Using radioactivity in dating
Principle of radioactive dating
The percentage of radioactive atoms that decay during one half-life is always the same
(50%)
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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Using radioactivity in dating
Sources of error
A closed system is required
To avoid potential problems, only fresh, unweathered rock samples should be used
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Using radioactivity in dating
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, archeologists and Quaternary geologists
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Using radioactivity in dating
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
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
Names of the eons
Phanerozoic
“Visible life”
Most recent eon (we’re still there…)
Began about 540 million years ago
Proterozoic
2.5 Ga Æ 540 Ma
Archean
Oldest rocks (up to ~4 Ga)
Hadean
Oldest eon (no rock record)
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Geologic time scale
Era – subdivision of an eon
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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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Major divisions of geologic time
Simplified divisions of geologic time
Complex divisions of geologic time
Geologic time scale
Precambrian time
Nearly 4 billion years up to the start of the Cambrian period
Not divided into smaller time units
Events of Precambrian history are not known in great enough detail
zFirst 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
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Dating sedimentary strata using radiometric dating
End of Chapter 9
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