Lecture Outlines
Physical Geology, 14/e
Plummer, Carlson & Hammersley
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Time and Geology
Physical Geology 14/e, Chapter 8
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Age of earth
Prior to the 19th century, accepted age of Earth was based on
religious beliefs
•
•
Biblical: ~6,000 years for Western culture
Chinese/Hindu: Old beyond comprehension
James Hutton: “Father of Geology”
• realized geologic processes require vast amounts of time
Charles Lyell
• popularized Hutton’s concepts in book Principles of Geology
“The
present is the key to the past”
James Hutton
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Relative vs. numerical age
Relative age - the order of events or objects, from oldest to
youngest
• original horizontality
• superposition,
• lateral continuity,
• cross-cutting relationships,
• inclusions,
• unconformities,
• correlation of rock units and fossils
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Relative vs. numerical age
Numerical age - the age of events or objects, expressed as a
number or numbers
• determined using radiometric dating (determining how much
radioactive decay of a specific element has occurred since a rock
formed or an event occurred)
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Relative age determination
Contacts - surfaces separating
successive rock layers (beds)
Formations - bodies of rock of
considerable thickness with
recognizable characteristics
allowing them to be distinguished
from adjacent rock layers
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Relative age determination
Original horizontality - beds of
sediment deposited in water are initially
formed as horizontal or nearly horizontal
layers
Superposition - within an
undisturbed sequence of sedimentary or
volcanic rocks, layers get younger from
bottom to top
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Relative age determination
Lateral continuity - original
horizontal layer extends laterally until it
tapers or thins at its edges
Cross-cutting relationships - a
disrupted pattern is older than the cause of
the disruption
• Intrusions and faults are younger
than the rocks they cut through
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Relative age determination
Baked contacts - contacts between
igneous intrusions and surrounding
rocks, where surrounding rocks have
experienced contact metamorphism
Inclusions - fragments embedded in
host rock are older than the host rock
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unconformities
Unconformity: a surface (or contact) that represents a gap
in the geologic record
Disconformity - an unconformity in
which the contact representing missing
rock layers separates beds that are
parallel to each other
Angular unconformity - an
unconformity in which the contact
separates overlying younger layers from
eroded tilted or folder layers
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unconformities
Nonconformity - an
unconformity in which an
erosional surface on plutonic
or metamorphic rock has been
covered by younger
sedimentary or volcanic rock
• plutonic and metamorphic
rocks exposed by large
amounts of erosion
• typically represents a
large gap in the geologic
record
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correlation
The determination of the time-equivalency of rock units
Physical continuity—physically
tracing a continuous exposure of a rock unit
Similarity of rock types
•assumes similar sequences of rocks formed
at same time
•can be inaccurate if very common rock
types are involved
Correlation by fossils
•fossil species succeed one another through
the layers in a predictable order
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Geologic time scale
• Worldwide relative time
scale
• Subdivides geologic time
based on fossil
assemblages
• Divided into eons, eras,
periods, and epochs
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Geologic time scale
Precambrian
• represents 87% of geologic time
Paleozoic era - Old Life
• appearance of complex life
Mesozoic era - Middle Life
•
•
dinosaurs abundant
era ended by mass extinction
Cenozoic era - New Life
• mammals and birds abundant
• most recent ice ages occurred during the Pleistocene Epoch of the
Quaternary Period
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Numerical age dating
Numerical dating - puts absolute
values (e.g., millions of years) on
the ages of rocks and geologic time
periods
• uses radioactive decay of
unstable isotopes
• only possible since radioactivity
was discovered in 1896
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Numerical Age Dating
Half-life - time it takes for a
given amount of radioactive
isotope to be reduced by half
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Combining relative and Numerical ages
Radiometric dating gives numerical time brackets
for events with known relative ages
• individual layers may be dated directly
• radiometric dating of units above and below brackets age
of units in between
Geologic Time Scale
• divided into four Eons
o
o
o
o
Hadean
Archean
Proterozoic
Phanerozoic
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Age of earth
Numerical dating gives age for
Earth of ~ 4.56 billion years
• oldest age obtained for
meteorites, believed to have
been unchanged since the
formation of the solar system
• Earth and rest of solar system
very likely formed at this time
Geologic (deep) time is vast
• a long human lifetime represents
only about 0.000002% of
geologic time
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End of Chapter 8
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