Unit 2: ABSOLUTE/RELATIVE DATING
How do scientists know how old rocks and fossils are?
Relative Age: The age of a rock or fossils __________________to another rock
or fossil.
Absolute Age: The ______________ of _____________ since a rock or fossil
was formed.
Relative Dating
• Relative dating allows scientists to __________ the relative age of a fossil.
• Remember! Relative dating does __________ tell the exact age of a fossil!
• It only tells us if a rock or fossil is ______________ or _____________
than other rocks and fossils.
Law of Superposition- States that each sedimentary rock layer is
____________ than the one above it and ____________than the one
below it.
Intrusions - ____________ will burn its way through layers of
sedimentary rock, cool and harden forming intrusions. Intrusions are
always _______________ than the rock they cut through
Folding and Uplifting - Sometimes rock layers are folded or uplifted by
______________ plate movement. This will cause the horizontal
layers to become ____________ or ________________upside down.
Faulting - Rock layers ______________ from extreme pressure or
tension that occurs during tectonic plate movement. This causes rock
layers to ______________ up or down.
Unconformity - Occurs when rock layers are _______________ away
and then _________________ under more rock layers. This results
in a _____________ in the fossil record because the fossils that
would have existed in these eroded layers are _______________.
Absolute Dating
• Allows scientists to date the _____________ age of a rock or fossil
• Some of the atoms in rocks are _________________(unstable) and will
break down over time to a more stable form. We call this
_________________
________________.
Half-Life - Scientists measure the decay in half-lives. Half-life is the
time it takes for ____________ of a sample of radioactive atoms to
_____________. Different atoms will decay at a _______________
rates, so scientists __________________ the number of decayed
atoms to radioactive atoms left in a sample of rock or fossil.
Carbon 14 - Carbon is found in all _______________ ___________.
Carbon-14 Dating is used to date fossils of actual remains that are less
than _______________ years old.
Uranium 235 - Uranium is an element found in some ______________
rocks. Uranium 235 can be used to date rocks ________________ to
_______________ of years old. This element has been used to
estimate the age of the Earth at __________________ years old.
Quick Check
1. Which statement tells the relative age.
3.____ cause rock layers to shift up or down due
a. Agnes is younger than Mary.
to extreme pressure and tension from tectonic
b. Mary is 7 years old.
plate movment.
c. John is 4 years old.
a. folds
2. According to the Law of Superposition…
b. intrusions
a. Older rock layers are found near the
c. unconformities
surface.
d. faults
b. Younger rock layers are always below
older rock layers.
c. Older rock layers are often below
younger rock layers.
4. ____ occurs when rock layers are eroded
away then buried under more rock layers.
d. All rock layers in an area are the same
age.
5.
a. folds
b. intrusions
c. unconformities
d. faults