Relative Dating
How do we determine a rocks age by the surrounding rocks?
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Geologic History
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Relative Dating
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Uniformitarianism - the idea that forces working on our
planet today worked on our planet in the past in the
same manner
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“The present is the key to the past”
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Relative Dating
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Relative Dating - determination of the age of a rock or
event in relation to the ages of other rocks or events
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Relative Dating
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Principle of Superposition - basis for relative dating
and the idea that the bottom layer is the oldest and
each overlying layer gets progressively younger
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Youngest
Oldest
Principle of Superposition
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Relative Dating
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Original Horizontality - idea that sedimentary and
igneous rocks are deposited in parallel layers to Earth’s
surface
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Original Horizontality
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Relative Dating
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Extrusions - molten rock flows onto the surface
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Relative Dating
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Intrusions - when molten rock squeezes into preexisting
rock layers
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Younger than the rocks that they crosscut
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Exception to the principle of superposition
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Igneous Intrusions
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Relative Dating
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Contact Metamorphism - temperature induced change
of preexisting rocks along an intrusion
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Relative Dating
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Faults - a crack in the bedrock where movement has
occurred
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Younger than the rocks that they crosscut
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Faults
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Faults
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Relative Dating
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Folds - when thrusting rock layers cause preexisting
rock layers to overturn
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Exception to the principle of superposition
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Folds
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Relative Dating
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Relative Dating
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Correlation - the process of showing that rocks or
geologic events from different places are the same or
similar age
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Correlation is the most effective method when using
relative dating
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Correlation
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Relative Dating
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What to look for when correlating rocks:
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Similarities in Rocks
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Rock Sequence
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Mineral Composition
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Color
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Fossils
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Relative Dating
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Fossils - remains or evidence of former living things
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Examples: bones, shells, footprints, and organic
compounds (DNA)
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Relative Dating
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Index Fossil - fossil used to define and identify
geologic periods
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Best method for correlating rocks
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Relative Dating
Dinosaur Fossils
251 - 65 mya
Trilobite Fossils
544 - 251 mya
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Correlation
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Relative Dating
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To be considered a good index fossil it needs to meet
two criteria:
1. The organism existed over a large geographic area
• Large horizontal distribution
2. The organism existed over a short time
• Small vertical distribution
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Relative Dating
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Geologic Time Markers - deposits spread over large
areas that represent a specific date
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Examples: volcanic ash deposits and meteorite
impacts
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Relative Dating
KT Boundary
KT Asteroid - 65 mya
Meteorite Impact
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Relative Dating
Krakatau - 1883
Volcanic Ash Deposit
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KT Boundary
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Absolute Dating
How do we use radioactive decay in dating the
absolute age of a rock, fossil, or event?
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Absolute Dating
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Absolute Dating - using radioactive decay to determine
the exact age of a rock, fossil, or event
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Radioactive Decay - the disintegration of an isotope
over time
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Step 1: Geologists drill for core samples.
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Step 2: Geologists crush the samples into thin sections and a fine powder.
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Step 3: Geologists analysis the samples for composition and inconsistencies.
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Step 4: Geochronologists use spectroscopes to measure the ratio of stable to unstable products.
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Periodic Table
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Absolute Dating
• Isotopes
- variations of an element that have the same
atomic number but differing atomic masses
• Example:
• Stable
carbon has a mass of 12 units called
Carbon-12
• Isotopic
carbon has a mass of 14 units called
Carbon-14
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Absolute Dating
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Half-Life - the time required for half of a radioactive
product to decay to a stable product
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In a given sample of a radioactive isotope half of the
atoms will decay to a stable product, but the
remaining half is still radioactive
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Absolute Dating
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Each element has its own half-life that range from
fractions of a second to billions of years
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Absolute Dating
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The half-life of an isotope is not effected by any
environmental factors such as temperature, pressure,
or chemical reactions
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Absolute Dating
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Uranium-238 - one of the most important isotopes
when dating rocks or events millions of years ago
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Mass: 238 units
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Decay: Uranium-238 ! Lead-206
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Half-Life: 4,500,000,000 years
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Absolute Dating
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Carbon-14 - one of the most important isotopes when
dating organic remains within tens of thousands of
years
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Mass: 14 units
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Decay: Carbon-14 ! Nitrogen-14
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Half-Life: 5,700 years
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Age of the Earth
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Early Evolution
How did everything come to evolve on the Earth?
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Early Evolution
4.6 Billion Years Ago
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Radioactive decay shows that
Earth formed
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Early Evolution
4.5 Billion Years Ago
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During the early formation Earth heated up due to
radioactive decay of isotopes within the Earth’s interior
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Oldest Zircon Crystals - 4.4 billion years old
Western Australia
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Early Evolution
4.4 Billion Years Ago
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During early Earth’s melting, materials separated into
zones according to their densities
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Fe and Ni settled into the core
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Silicates formed the earliest crust
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Gaseous compounds made up the atmosphere
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Early Evolution
4.2 Billion Years Ago
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Solid crust formed and plate
tectonics started
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Gases trapped inside the Earth
seeped out in a process called
outgassing and created a
completely different second
atmosphere
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Oldest Rocks - 4.28 billion years old
Hudson Bay in Northern Quebec
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Early Evolution
3.9 Billion Years Ago
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After the crust had cooled enough, water vapor in the
atmosphere began to precipitate and form water on
Earth
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Early Evolution
3.8 Billion Years Ago
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Weathering, erosion, and deposition began and the first
sedimentary rock was formed
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The Potential for Life
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Early Evolution
3.5 Billion Years Ago
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Life forms that used CO2 and
released free oxygen began to
evolve
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This allowed for oxygen to
start collecting in our
atmosphere
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Early Evolution
3.5 - 2.8 Billion Years Ago
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Oxygen in the atmosphere
reacted with iron in the soil to
produce rust
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Resembled the surface color
of current day Mars
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Early Evolution
2.8 Billion Years Ago
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Most of the iron compounds
that could have reacted with
the oxygen had done so, thus
oxygen in the atmosphere
increased
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Early Evolution
2.8 - Present Billion Years Ago
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Life slowly evolved from single-celled bacteria to
multicellular to hard parts on life forms
Single-Celled
Multi-Celled
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Shelled
Burgess Shale
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Earth Science Reference Tables
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