Transcript Document

Geological Time - really, really, really long!
Motion pictures are generally projected at 32 frames
per second. Therefore, each frame (image) is on the
screen for only split second- let each frame represent
100 years.
Start movie at present and go back in time.
•The Declaration of Independence would show up 1/16
of a second into the movie.
•The Christian era (BC-AD boundary) would be 3/4 of a
second into the movie.
•The most recent Ice Age would be 7 seconds into it.
•The movie would run about 6 hours before we got to
the end of the Mesozoic era (extinction of the
dinosaurs).
•We'd have to watch the movie for about 2 days to see
the beginning of the Paleozoic era (macroscopic life).
•The whole movie (to the beginning of geologic time on
Earth) would be approximately 16 days long!
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Geologic Time
• Two ways to relate time in geology:
> Relative: Placing events in a
sequence based on their positions
in the geologic record.
> Chronologic : Placing a specific
number of years on an event or rock
sample.
Geologic Time Scale
• a combination of the two types of age
determinations
> a relative sequence of lithologic units
- established using logical principles
> measured against a framework of
chronologic dates.
Geologic Time and the "geologic column"
• Developed
Developed usingusing
logical logical
rules to establish
relative
rules to
establish
sequences
of events of events
relative
sequences
- superposition
- cross-cutting relationships
- original horizontality
- lateral continuity
• Added to as new information is obtained and
data is refined
refined
- Use of fossils for correlation and age
•
Numerical Dates attached to strata after the
- development of Radiometric techniques
determination
Still being refined as more information
becomes available
The Geologic Time Scale (1:2)
The Geologic Time Scale (2:2)
Relative Dating Methods
• determines the relative sequence of events.
> which came first, which came last.
> no numeric age assigned
• 6 Relative age principles:
> Superposition
> Lateral continuity
> Inclusions
> Original Horizontality,
> Cross-cutting Relationships
> Fossil succession.
Those in yellow are most useful
History of Historical Geology
• Niels Stensen (Nicolaus Steno)
- Fundamental Principles of Relative Time
> Principle of Superposition- see below
> Principle of Original Horizontality- see below
> Principle of Original Lateral Continuity- see below
Law of Superposition
• In undisturbed strata, the layer on the bottom is
oldest, those above are younger.
Original Horizontality
• Sediments are generally deposited as
horizontal layers.
Lateral Continuity
• Sediment layers extend laterally in all
direction until they thin & pinch out as
they meet the edge of the depositional
basin.
Charles Lyell
• 1st Principles of Geology text
- included description and use of
> principles of cross-cutting relationships
> principles of inclusions
• relative time tools
Cross-cutting Relationships
That which cuts through is younger than the
Object that is cut
dike cuts through
granite is cut
Relative Ages of Lava Flows and Sills
Principle of Inclusions
• Inclusions (one rock type contained in another rock type) are
older than the rock they are embedded in. That is, the younger
rock contains the inclusions
Principle of Inclusions
Faunal/Floral Succession
•• Fossil assemblages (groupings of fossils)
succeed one another through time.
• Correlationrelating rocks in one location to those in
another using relative age stratigraphic
principles
- Faunal Succession
-- Superposition
-- Lateral Continuity
-- Cross-cutting
Unconformities
•
surfaces
represent a long time.
a time when rocks were not
deposited or
a time when rocks were
eroded
Hiatus
the gap in time represented
in the rocks by an unconformity
3 kinds
Angular Unconformity
Nonconformity
Disconformity
Disconformities
A surface of erosion or non-deposition between
Parallel sedimentary rock beds of differing ages.
Angular Unconformities
• An angular unconformity is an erosional surface on tilted
or folded strata, over which younger strata have been deposited.
Nonconformities
A nonconformity is an erosional surface on igneous or
metamorphic rocks which are overlain by sedimentary rocks.
Breakout in to groups and discuss the sequence
observed here
Age Estimates of Earth
Counting lifetimes in the Bible
Comparing cooling rates of iron pellets.
Determine sedimentation rates & compare
Estimate age based on salinity of the ocean.
all age estimates were off by billions of years
some were more off than others!
Absolute Dating Methods
Radioactive Decay sequences
acts as an atomic clock
we see the clock at the end of its cycle
analogous to starting a stopwatch
allows assignment of numerical dates to
rocks.
> Radioactive isotopes change (decay) into
daughter isotopes at known rates.
rates vary with the isotope
++ e.g., 235 U , 40 K , 14C, etc.
Decay
unstable nuclei in parent isotope emits
subatomic particles and transform into
another isotopic element (daughter).
does so at a known rate, measured in the
lab
• Half-life
The amount of time needed for one-half of a
radioactive parent to decay into daughter
isotope.
Assumptions?-you bet
Cross-checks ensure validity of method.
Rate of Decay
t0
All atoms are parent isotope or some
known ratio of parent to daughter
t1
1 half-life period has elapsed, half of the
material has changed to a daughter
isotope (6 parent: 6 daughter)
t2
2 half-lives elapsed, half of the parent
remaining is transformed into a daughter
isotope (3 parent: 9 daughter)
t
3 half-lives elapsed, half of the parent
remaining is transformed into a daughter
3 isotope (1.5 parent: 10.5 daughter)
We would see the rock at this point.
Radioactive Isotopes
• analogous to sand in an hour glass
- we measure how much sand there is
> represents themass of elements
- we measure the ratio of sand in the bottom to sand in the top
- at the end (present)
> daughter (b) and parent (t)
- we know at what rate the sand falls into the bottom
> the half life of the radioactive element
- how long would it take to get the amount sand in the observed
ratio starting with all of it in the top?
100
Parent
Daughter
50
25
13
time----------->
Five Radioactive Isotope Pairs
Isotopes
Parent
Daughter
Half-Life
(Years)
Effective Dating Range
of Parent (Years)
Uranium 238 Lead 206
4.5 billion
10 million to
4.6 billion
Uranium 235 Lead 207
704 million
Thorium 232 Lead 208
14 billion
48.8 billion
Rubidium 87 Strontium 87
4.6 billion
10 million to
4.6 billion
Potassium 40 Argon 40
1.3 billion
100,000 to
4.6 billion
Minerals and
Rocks That Can
Be Dated
Zircon
Uraninite
Muscovite
Biotite
Potassium feldspar
Whole metamorphic
or igneous rock
Glauconite
Muscovite
Biotite
Hornblende
Whole volcanic rock
Radiocarbon and Tree- Ring Dating Methods
••
Carbon-14 dating is based on the
ratio of C-14 to C-12 in an organic
sample.
Valid only
only for
for samples
samples less
less than
than 70,000
70,000
>> Valid
years old.
old.
years
Living things
things take
take in
in both
both isotopes
isotopes of
of
>> Living
carbon.
carbon.
When the
the organism
organism dies,
dies, the
the "clock"
"clock" starts.
starts.
>> When
Method can be validated by cross-checking with tree
rings
Carbon 14 Cycle