The Rock
Record
I. The ages of past
events can be indicated
in 2 ways:
Relative Dating
Absolute Dating
A. Relative Dating
Dating-- places
events in a sequence, but
does not give an actual
age
There are 3 laws associated
with relative dating
1. Law of SuperpositionSuperpositionin sequences of undisturbed
sedimentary rock the oldest rock is
on the bottom and the youngest rock
is on the top
What happened here?
2. Law of crosscross-cutting
relationships-relationships
igneous intrusions are
younger than the rock in
which they are found.
3. Law of included fragmentsfragmentspieces of rocks found within
another rock must be older than
the rock in which they were
found
4. UnconformitiesUnconformitiesPlaces in rock strata where
layers are missing.
Can you find the unconformities in the picture?
B. Absolute DatingDatingprovides the age of an artifact
1. Counting tree ringsringsa. Each ring = 1 year
b. The width of the ring depends on
the temp & rainfall that year.
How old is
this tree?
2. Observing VarvesVarves- sediments
that show a yearly cycle
a.
b.
Ice from glaciers melts in spring
and carries sediments that deposit
in lakes
You get a layer of new and old soil
to represent 1 year.
3. Radioactive element decaydecay- the
process of energy emission from
an atom
a. Radioactive isotopeisotope- atom that gives
off radiation from its nucleus, or
“decays” at a fixed, measurable rate
b.Half
b. Half--life
life-- the time it
takes for half of the
atoms of a radioactive
isotope to decay to a
stable end product
c. Decay rates are assumed to be
unaffected by time or changes
in temp & pressure.
4. Common radioactive isotopes
a. Uranium
Uranium--238 (U(U-238)
1. has a half life of 4.5 billion
years
2. decays to LeadLead-206 (Pb(Pb-206)
3. Can only be used on some
igneous rocks
4. Most reliable if rocks are less
than 10 million years old
b. Radiocarbon (C(C-14)
1. HalfHalf-life=5,700 years
2. Decays to CC-12
3. Can ONLY be used to find
the age of things that were
once living
c. PotassiumPotassium-40 (K(K-40)
1. HalfHalf-life = 1.3 billion years
2. Decays to ArgonArgon-40 (Ar(Ar-40)
3. Is more common and can
be used on all 3 types of
rocks.
Example: Ur-238 decay
Years (billions)
0
4.5
9
13.5
18
22.5
UrUr-238 (g)
1000
500
250
125
62.5
31.25
PbPb-206 (g)
0
500
750
875
937.5
968.75
II. Geologic Time
A. The Eras
1. Precambrian
2. Paleozoic
3. Mesozoic
4. Cenozoic
1.
1.Precambrian
Precambrian EraEraa. Makes up 87% of Earth’s
history
b. First life appeared
2. Paleozoic Era
a. Rise of:
1. marine invertebrates
2. fish
3. amphibians
4. reptiles
3. Mesozoic
a. The era of the dinosaur
b. Has 3 periods
1. Triassic: 1st dinosaurs
2. Jurassic: height of the dinosaur
3. Cretaceous: extinction
4. Cenozoic Era
a. The Era of mammals
b. Video
c. At the end we see the planetary
spread of Homo sapiens
II. Fossils as Evidence for
Evolution
A.Evolution
A.Evolution-change over time (Darwin’s
Origin of Species,
Species, 1859)
B.Evolution
B. Evolution is driven by
natural selectionselection- organisms
who survive to reproduce are
those who inherited the most
beneficial traits for surviving in a
particular environment
Darwin’s Story
HMS BEAGLE
The Galapagos Finches
The Evolution of Man
Scopes Monkey Trial:
John Scopes prosecuted by William
Jennings Bryan in 1925
C. Finding Rock correlations
1. When you find correlations,
you match rock layers from 1
area to another
2. Look for similar appearance,
color, composition & fossils
3. Index fossilsfossilscommon, easily recognized
fossils that are typical of a
particular time in history
4. Key bedbedsimilar to an index fossil, but
covers a large area
The End!!!!!!!!