geologic time. the rock record rocks record geological events and changing life forms...
TRANSCRIPT
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Geologic TimeGeologic Time
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The Rock RecordThe Rock Record
• Rocks record geological events and changing life forms
• UniformitarianismUniformitarianism: (James Hutton) the forces and processes that we observe today have been at work for all of earth’s history
• Earth is a lot older than originally thought!
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Relative DatingRelative Dating
• Tells us the sequence of events, not the actual time the events occurred
• Law of Superposition: Law of Superposition: in an undeformed sequence of sedimentary rocks, each bed is older than the one above it and younger than the one below it
• Original Horizontality: Original Horizontality: layers of sediment are deposited in a horizontal position
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Relative DatingRelative Dating
• Cross-Cutting:Cross-Cutting: when faults cut through rock layers, or magma intrudes, the intrusion or fault is younger than the rock affected
• Inclusions: Inclusions: Pieces of one rock are contained within another; the rock containing inclusions is younger
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Relative DatingRelative Dating
• Unconformities:Unconformities: a long period during which deposition stops, erosion removes previously formed rocks, then deposition resumes
• 3 Types: 3 Types: angular unconformity, discomformity, and nonconformity
• Correlation: Correlation: matching up rocks of similar age in different parts of the earth
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Relative DatingRelative Dating
• Angular unconformity: an unconformity where horizontally parallel strata of sedimentary rock are deposited on tilted and eroded layers
• Nonconformity exist when the sedimentary rock lies above and was deposited on the pre-existing and eroded metamorphic or igneous rock
• Disconformity: unconformity between parallel layers of sedimentary rocks which represents a period of erosion or non-deposition.
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A = Angular unconformity
B= Nonconformity
C=Disconformity
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Co
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Fossil RecordFossil Record
• Fossils are the remains or traces of prehistoric life
• Fossil type depends on conditions at death and how it was buried
• Fossils are typically found in sedimentary rock
• Unaltered RemainsUnaltered Remains: bones, teeth or shells of an animal remain unchanged
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Fossil RecordFossil Record
• Altered Remains:Altered Remains: most likely event
• Petrified: “turned into stone”
• Molds: the structure is buried and dissolved. Reflects only shape and surface markings
• Casts: If the hollow spaces are filled with mineral matter
• Trace fossils: indirect evidence; tracks
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Fossils and CorrelationFossils and Correlation
• Fossils succeed one another in a definite order; any time period can be recognized by the fossil content of the rock
• Index Fossils:Index Fossils: fossils used as time indicators
• Sometimes groups of fossils are used as the index
• Also used to interpret the environment
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Fossils and CorrelationFossils and Correlation
• Conditions favoring preservation:Conditions favoring preservation:– Rapid burialRapid burial
– Possession of hard body parts Possession of hard body parts (skeleton)(skeleton)
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Types of Fossilization
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Overlapping Ranges of Fossils
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Dating with Radioactivity
Basic Atomic Structures• Atomic number is the number of protons in
the atom’s nucleus
• Mass number is the number of protons plus the number of neutrons in an atom’s nucleus.
• Isotopes are atoms of the same element containing a different number of neutrons
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Dating with Radioactivity
Radioactivity Radioactivity is the spontaneous decay of
certain unstable atomic nuclei. Particles and energy are released from the
nucleus A more stable daughter is the result
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Common Types of Radioactive Decay
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Dating with Radioactivity
Half-Life A half-life is the amount of time necessary
for one-half of the nuclei in a sample to decay to a stable isotope.
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The Half-Life Decay Curve
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Dating with Radioactivity
Radiometric Dating
• Each radioactive isotope has been decaying at a constant rate since the formation of the rocks in which it occurs
• Radiometric dating is the procedure of calculating the absolute ages of rocks and minerals that contain radioactive isotopes
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Dating with Radioactivity
• As a radioactive isotope decays, atoms of the daughter product are formed and accumulate
• An accurate radiometric date can be obtained only if the mineral remained in a closed system since its formation.
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Radioactive Isotopes Frequently Used in Radiometric Dating
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Dating with Radioactivity
Dating with Carbon-14• Radiocarbon dating is the method for
determining age by comparing the amount of carbon-14 to the amount of carbon-12 in a sample
• When an organism dies, the amount of carbon-14 it contains gradually decreases as it decays. By comparing the ratio of carbon-14 to carbon-12 in a sample, radiocarbon dates can be determined
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The Geologic Time Scale
Structure of the Time Scale
• Based on their interpretations of the rock record, geologists have divided Earth’s 4.56-billion-year history into units that represent specific amounts of time.
• Taken together, these time spans make up the geologic time scale
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The Geologic Time Scale
Structure of the Time Scale
• Eons represent the greatest expanses of time.
• Eons are divided into eras.
• Each era is subdivided into periods.
• Finally, periods are divided into smaller units called epochs
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The Geologic Time Scale
Structure of the Time Scale
There are three eras within the Phanerozoic eon:
• Paleozoic, which means “ancient life,”
• Mesozoic, which means “middle life,”
• Cenozoic, which means “recent life.”
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The Geologic Time Scale
• Each period within an era is characterized by somewhat less profound changes in life forms as compared with the changes that occur during an era
• The periods of the Cenozoic era are divided into still smaller units called epochs, during which even less profound changes in life forms occur.
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The Geologic Time Scale
Precambrian Time
• During Precambrian time, there were fewer life forms. These life forms are more difficult to identify and the rocks have been disturbed often
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The Geologic Time Scale
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