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FOSSILS, RELATIVE & ABSOLUTE
DATING
FOSSILSFOSSILS 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
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
PALEONTOLOGY PALEONTOLOGYthe 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)
Radiometric / radioactive dating (isotopes) Relative dating (gives age before, after, during)
Observation of rock layers
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)
Radiometric / radioactive dating (isotopes) Relative dating (gives age before, after, during)
Observation of rock layers
Ways to tell the age of a rockWays to tell the age of a rock
Absolute Dating: All you need is a tiny sample of material (mineral,
bone) no larger than a grain of rice. Gives us the true “age” of a fossil or rock Mainly organic tissue or igneous crystals Measure the amount of unstable isotopes that have
“decayed” to figure out age
Absolute Dating: All you need is a tiny sample of material (mineral,
bone) no larger than a grain of rice. Gives us the true “age” of a fossil or rock Mainly organic tissue or igneous crystals Measure the amount of unstable isotopes that have
“decayed” to figure out age
Ways to tell the age of a rockWays to tell the age of a rock
Relative Dating: Places events in geologic history in the
proper order. The basis for the geologic time scale Mainly Sedimentary Rocks Does not provide a true “age”
Relative Dating: Places events in geologic history in the
proper order. The basis for the geologic time scale Mainly Sedimentary Rocks Does not provide a true “age”
RELATIVE DATING & AGE RELATIVE DATING & AGE Relative Dating: putting
rocks and geological events in correct chronological order
Relative Age: how old something is in comparison to something else
HOW? Use of sedimentary rocks Use of fossils Study of strata
Relative Dating: putting rocks and geological events in correct chronological order
Relative Age: how old something is in comparison to something else
HOW? Use of sedimentary rocks Use of fossils Study of strata
INDEX FOSSILINDEX FOSSIL
Fossil that defines and identifies geologic periods; often in only one layer of rock
Easily recognizable
Short-lived (found only in a few layers of rock worldwide)
Wide distribution (geographic range)
Fossil that defines and identifies geologic periods; often in only one layer of rock
Easily recognizable
Short-lived (found only in a few layers of rock worldwide)
Wide distribution (geographic range)
Ex/ INDEX FOSSIL: AMMONITEEx/ INDEX FOSSIL: AMMONITE
Ammonite fossils are found worldwide, but they existed for only a very specific period of time
this means ammonites are found in very specific layers of rock
when an index fossil is found, the age of the rocks it is preserved can be determined
Ammonite fossils are found worldwide, but they existed for only a very specific period of time
this means ammonites are found in very specific layers of rock
when an index fossil is found, the age of the rocks it is preserved can be determined
LAW OF HORIZONTALITY LAW OF HORIZONTALITYSediments are originally deposited in
horizontal layersSediments are originally deposited in
horizontal layers
Folds or inclines: layers must have been deformed after they were deposited.
Folds or inclines: layers must have been deformed after they were deposited.
LAW OF SUPERPOSITION LAW OF SUPERPOSITIONFor undisturbed rocks, the oldest layer is on the bottom and the youngest is on top (Supai is oldest)
For undisturbed rocks, the oldest layer is on the bottom and the youngest is on top (Supai is oldest)
What kind of rocks are these fossils in?
Which layer is oldest?
Which layer is youngest?
How do you know?
LAW OF SUCCESSIONLAW OF SUCCESSION Fossils are found in a predictable sequence Fossils in rock B are older then fossils in rock A
Fossils are found in a predictable sequence Fossils in rock B are older then fossils in rock A
LAW OF INCLUSIONSLAW OF INCLUSIONS
If a rock body (Rock B) contained fragments of another rock body (Rock A),
then Rock B must be younger than the fragments of rock it contained
If a rock body (Rock B) contained fragments of another rock body (Rock A),
then Rock B must be younger than the fragments of rock it contained
GEOLOGIC TIME SCALEa 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
GEOLOGIC DATING: ABSOLUTE AGE DETERMINATION
Marie Curie
Ernest Rutherford
• Radioactivity was first discovered by Henri Becquerel in 1896 and Polish-French chemist Marie Curie discovered that radioactivity produced new elements (radioactive decay).
• Ernest Rutherford first formulated the law of radioactive decay and was the first person to determine the age of a rock using radioactive decay methods.
M&W4 Fig. 3.3; M&W5 Fig. 3.4
• The number of protons (the atomic number) is fixed for any element and is unique for each element but the number of neutrons in atoms of different elements can vary. Atoms of an element having different numbers of neutrons are referred to as the isotopes (of that element).
GEOLOGIC DATING: ABSOLUTE AGE DETERMINATION
Radioactive decay occurs when an isotope of one element is transformed into a different element by changes in the nucleus. There are three different decay mechanisms:
GEOLOGIC DATING: ABSOLUTE AGE DETERMINATION
M&W4 Fig. 17.18; M&W5 Fig. 17.18
“Parent” “Daughter”
How can we tell age based on the number of parent
isotopes?
How can we tell age based on the number of parent
isotopes?
Radioactive isotopes “decay” at a particular rate. We express this rate as the “HALF-LIFE”, which is the time it takes for HALF of the parent isotopes to decay.
M&W4 Fig. 17.24; M&W5 Fig. 17.24
C14 is an isotope of carbon that forms from Nitrogen in the atmosphere. Living things consume this radioactive carbon.
Once dead, no new carbon is absorbed, and C14 turns back into Nitrogen.
The Half-Life of C14 is 5,730 years.
This method works best for fossils younger than 50,000 years. Why?
(end)
GEOLOGIC DATING: ABSOLUTE AGE DETERMINATION
• For radioactivity dating we use igneous rocks and minerals. The clock starts when radioactive atoms that are present in the magma get incorporated in the crystalline structure of certain minerals in the rocks.
• The crystals containing the parent atoms form and so we then have a “container” with parents that can begin decaying to form daughters.
• We can then use measure the parent-daughter ratio. This is our “atomic clock” that records the time since the rock crystallized.
M&W4 Fig. 17.21; M&W5 Fig. 17.21
GEOLOGIC DATING: ABSOLUTE AGE DETERMINATION
• To the oldest materials ever dated by the radioactive method are found in the Jack Hills of western Australia and are tiny zircon grains contained in sandstones and conglomerates. The zircons are 4.4 billion years old.
Scanning electron microscope image of a Jack Hills zircon. Scale bar is 0.1 mm
The very remote “outback” of western Australia--the Jack Hills
