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Lecture Four: Geologic Time, Principle of Cross-Cutting Relationships -…
Lecture Four: Geologic Time
Catastrophism
produced all rocks and major geologic features (Alps)
Uniformitarianism
idea that the processes that went on in the past are the same as they are today
James Hutton (1736-1797)
noted relatively flat-lying sedimentary rocks overlaying steeply dipping sedimentary rocks (Siccar Point) --> major gap in time
first clue: Hadrian's Wall: had little to no deterioration
time
Relative Time
comparative fashion between events
Absolute Time
specific number
Tree ring dating
Cross dating
diff tree systems - extend back 100,000 years
Varves
glacial lake sediments (layers to date)- <50,000 years
Radiometric dating!
certain isotopes are not stable and will break down over time by radioactive processes to form new, stable elements
Mechanisms of decay:
Alpha Emission
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exact dating is based upon the idea that each radioactive decay is based upon an exponential decay curve. The number of radioactive atoms declines at a precise, fixed rate over time, with that rate being given by the half-life of that isotope
Date requires:
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Uranium series dating is good for 10 million years to billions of years
individual crystals can record multiple stages of growth with different ages
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Principles:
Principle of Original Horizontality
all rocks that are deposited or formed at surface will do so in horizontal layers/beds
fossils
FAUNAL SUCC
some found only in limited time periods, but widely distributed (index/guide fossils)
Unconformity
breaks in the sedimentary rock record where an erosional surface is present before deposition resumes
Disconformity
Horizontal sedimentary layers overlaying horizontal sedimentary layers
Angular unconformity
Horizontal sedimentary layers overlaying angled/dipping sedimentary layers
Nonconformity
Horizontal sedimentary layers overlaying plutonic igneous rocks or metamorphic rocks
Modern Geological Time Scale
Eon
Era
Paleozoic
explosion of complex life
Mesozoic
evolution of dinosaurs
Cenozoic
evolution of large mammals
"Time of Life"
Period
Epoch
major global unconformities and extinction events became boundaries in time
Hadean
before known rocks, when Earth was molten
Archean
poorly defined by a specific characteristic
Proterozoic
before advanced life
eras (not widely used)
Important Dates
Age of Earth- 4.5 billion
Proterozic-Phanerozoic boundary- 542.0 million
Paleozoic-Mesozoic boundary 251.0 million
Mesozoic-Cenozoic boundary 65.5 million
Pliocene-Pleistocene boundary- 1.8 million
Principle of Cross-Cutting Relationships
Principle of Superposition
each undisturbed layer of sedimentary rocks is younger than the layer beneath it, older than layer overlying it
whatever cuts across a rock layer has to be younger than that rock layer
Principle of Inclusions
whatever is included within something else has to be older than the surrounding rock
Principle Of Lateral Continuity
how sedimentary layers will form continuous sheets that go for long distances and thin out to nothing