Ch 8-Rock Record
• Objectives
– Principle of uniformitarianism
– Law of superposition and how it is used to determine
relative age of rock
– Compare types of unconiformities
– Apply law of crosscutting relationships to determine
relative age of rocks
– Summarize limitation of using rates of erosion and
deposition for determining absolute age
– Describe formation of varves
– Radioactive decay
– How entire organisms can be preserved as fossils
– Examples of fossilized traces of organisms
– Describe how index fossils can be used to determine
age of rocks
Sec 1-Determining Relative Age
• Uniformitarianism-principle that geologic
processes that occurred in past can be
explained by current geologic processes
– One of basic foundations of the science of
geology
– James Hutton
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Relative Age
• Age of an object in relation to ages of other objects
• Strata-layers of rock that shows sequence of events
that took place in the past
• Law of Superposition
– Sedimentary rock layer is older than the layers above it
and younger than layers below, if layers are not disturbed
– Beds-sedimentary rock layers
– Bedding plane-boundary between 2 beds
• Principle of Original Horizontality
– Sedimentary rocks left undisturbed remain in horizontal
layers
– Graded bedding, cross beds, ripple marks
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Unconformities
• Breakage in geologic record created when rock
layers are eroded or when sediment is not
deposited for long period of time
• Types
– Nonconformity=stratified rock rests upon unstratified
rock
– Angular=boundary between set of tilted layers and set
of horizontal layers
– Disconformity=boundary between horizontal layers of
old sedimentary rock and younger, overlying layers
deposited on eroded surface
Law of Crosscutting
• Faults and intrusions make it difficult to
determine rocks age
• Fault=break or crack in Earth’s crust along which
rocks shift their position
• Intrusion=mass of igneous rock that forms when
magma is injected into rock and cools then
solidifies
• Law of crosscutting
– Fault or igneous intrusions are always younger than
rock layers it cuts through
– Cuts through unconformity then the fault or intrusion
is younger than all rocks above and below
Sec 2-Determining Absolute Age
• Numeric age=absolute age
• Limitations using erosion and deposition
– Rate of erosion is only useful if geologic
feature was formed w/in past 10,000-20,000
yrs.
• Ex. Niagara Falls, What about Grand Canyon?
– Rate of Deposition calculates rate of sediment
deposition
• Collect data for long time-limestone, shale,
sandstone
• 30 cm of sedimentary rock=1,000 yrs
Varves
• What is a varve count?
– Summer time=coarse particles, winter=fine
particles. Fine settles on coarse at bt of lake
• Varve-banded layer of sand and silt that is
deposited annually in a lake
– Ice sheets or glaciers
– Used to determine absolute age
– Light colored=coarse, dark=fine
Radioactive Decay
• Use natural breakdown of isotopes to
measure absolute age of rocks
• Radiometric dating-method of determining
absolute age of object by comparing relative
percentages of radioactive isotope and stable
isotope
• Radioactive isotopes’ nuclei emit particle and
energy at constant rate
• Why is it used? How is it used?
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Half-Life
• Radioactive decay happens at relatively
constant rate-not changed by temperature,
pressure, or other environmental
conditions
• Half-life-time required for half of a sample
of radioactive isotope to break down by
radioactive decay to form a daughter
isotope
Carbon Dating
• Isotope Carbon 14 combines w/ oxygen to
form radioactive CO2
• Ratio of C 14 and C 12 in living organisms
is relatively common
• Organism dies, amount of 14 C decreases
steadily
• Half life of 14 C=5,730 yrs
• Used to date ages of wood, bones, shells,
and other organic remains that are less
than 70,000 yrs old
Sec 3-Fossil Record
• What are fossils?
– Remains of animals or plants that lived in
previous geologic time
– Provides clues to past
• Paleontology-study of fossils
– Most found in sedimentary rock. Why?
How fossils form
• Mummification
– Ancient civilizations extracted internal body organs and wrapped
body in prepared strips of cloth
– Found in dry places
• Amber
– Insects get trapped in sticky amber from tree. DNA can be
recovered
• Tar seeps
– Animals drink water that covers petroleum, get stuck, remains
preserved in tar
• Freezing
– Low temps of frozen soil and ice can protect and preserve
organisms
– Bacteria cannot survive
• Petrification
– Mineral solutions replace original organic material, results in
formation of mineral replica of original organism
– Silica, calcite, pyrite
Types of Fossils
• Imprints
– Carbonized imprints of leaves, stems, flowers, and fish in soft
mud or clay in sed rock
• Molds
– Shells leave empty cavity w/in hardened sediment. Shells decay
and leave empty space
• Casts
– Sand or mud fills mold, cast forms, replica of original organism
• Coprolites
– Fossilized dung or waste materials from ancient animals.
Dinosaurs
• Gastroliths
– Stones in digestive system of dinosaurs to help grind food,
survive as fossils
• Trace fossils-fossilized mark formed in sed rock by
movement of animal on or w/in soft sediment
Index Fossils
• Fossil used to establish age of rock layers.
Why?
– Distinct, abundant, widespread, existed for
short span
• Use them to date rock layers in separate
areas
• Use them to locate rock layers likely to
contain oil and natural gas deposits