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FOSSILS &
RELATIVE DATING
GEOLOGIC TIME SCALE
a series of time
intervals that divides
Earth’s history
• Each layer of rock
represents specific interval
of time
• Index fossils help
determine specific period
• Time periods divided by
specific events like mass
extinctions
ROCKS TELL A STORY
 Rocks can tell where they were made and when
 Sedimentary rocks can have fossils in them
 Rocks can tell when mass extinctions happened
LAW OF SUPERPOSITION
For undisturbed rocks, the oldest layer is on
the bottom and the youngest is on top
Supai is oldest – WHY?
PALEONTOLOGY
the study of fossils  remains of ancient life
Body fossils vs. trace fossils
 Body = remain of organism, like bones;
 Trace = evidence of organism, like footprints
Scientific dating
 Absolute dating (gives age in years)
uses radiometric / radioactive dating (isotopes)
 Relative dating (gives age before, after, during)
uses observation of rock layers
FOSSILS
Traces and preserved remains of ancient life
found within rock layers
Fossils show:





Biodiversity
How species have changed over time
Correlation between rock layers from around the world
Relative ages to particular strata
Evidence for the geological time scale
SCIENTIFIC DATING
Absolute Dating: numerical
dating to give rocks an
actual date or date range,
in number of years
Relative Dating: compare
how old something is in
comparison to something
else;
used to put rocks and
geological events in correct
chronological order
HOW?
 Use sedimentary rocks
 Use fossils
 Study strata
INDEX FOSSIL
Fossil that defines and identifies geologic
periods; often in only one layer of rock
 Easily recognizable
 Exists over a short
geologic time range
(found only in a few
layers of rock)
 Wide distribution
(geographic range
is world-wide)
Ex/ INDEX FOSSIL: AMMONITE
Ammonite fossils
 are found worldwide,
 existed for only a very
specific period of time
This means ammonites are
found in very specific layers
of rock;
Once we know the ammonites,
then we can determine the age
of any fossil found next an
ammonite fossil.
Quiz yourself!
What kind of
rocks are these
fossils in?
Which layer is
oldest?
Which layer is
youngest?
How do you
know?
• Radiometric dating uses decay of unstable isotopes.
– Isotopes are atoms of an element that differ in their
number of neutrons.
– A half-life is the amount of time it takes for half of the
isotope to decay.