Using Grammatical Signals Suitable to Patterns of Idea Development
Geol162 geologic time
1. eological 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
3. 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.
4. 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.
5. Geologic Time and the "geologic column"
• Developed usingusing logical rules relative
Developed logical rules to establish to establish
sequences of events
relative sequences of events
- superposition
- cross-cutting relationships
- original horizontality
- lateral continuity
• Added to as new information is obtained and
refined is refined
data
- Use of fossils for correlation and age determination
•
Numerical Dates attached to strata after the
- development of Radiometric techniques
Still being refined as more information
becomes available
8. Relative Dating Methods
• determines the relative sequence of events.
> which came first, which came last.
> no numeric age assigned
• 6 Relative age principles:
> Superposition > Original Horizontality,
> Lateral continuity > Cross-cutting Relationships
> Inclusions > Fossil succession.
Those in yellow are most useful
9. 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
10. Law of Superposition
• In undisturbed strata, the layer on the bottom is
In undisturbed strata, the layer on the bottom is
oldest, those above are younger.
11. 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.
12. Charles Lyell
• 1st Principles of Geology text
- included description and use of
> principles of cross-cutting relationships
> principles of inclusions
• relative time tools
15. 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
18. • Correlation-
relating rocks in one location to those in
another using relative age stratigraphic
principles
- Faunal Succession
- Superposition
-
- Lateral Continuity
-
- Cross-cutting
-
19. 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 uncon-
formity
3 kinds
Angular Unconformity
Nonconformity
Disconformity
20. Disconformities
A surface of erosion or non-deposition between
Parallel sedimentary rock beds of differing ages.
21. Angular Unconformities
• An angular unconformity is an erosional surface on tilted
or folded strata, over which younger strata have been deposited.
23. Breakout in to groups and discuss the sequence
observed here
24. 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!
25. 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
+
+ 235 40 14
e.g., U , K, C, etc.
26. 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.
27. Rate of Decay
All atoms are parent isotope or some
t 0 known ratio of parent to daughter
1 half-life period has elapsed, half of the
t 1 material has changed to a daughter
isotope (6 parent: 6 daughter)
2 half-lives elapsed, half of the parent
t 2 remaining is transformed into a daughter
isotope (3 parent: 9 daughter)
3 half-lives elapsed, half of the parent
remaining is transformed into a daughter
t 3 isotope (1.5 parent: 10.5 daughter)
We would see the rock at this point.
28. Radioactive Isotopes
• analogous to sand in an hour glass
- we measure how much sand there is
> represents the mass 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 remaining
Parent
50 Daughter
25
13
time----------->
29. Five Radioactive Isotope Pairs
EffectiveDating Range Minerals and
Isotopes Half-Life of Parent (Years) Rocks That Can
Parent Daughter (Years) Be Dated
Uranium 238 Lead 206 4.5 billion 10 million to Zircon
4.6 billion Uraninite
Uranium 235 Lead 207 704 million
Muscovite
Thorium 232 Lead 208 14 billion 48.8 billion Biotite
Potassium feldspar
Rubidium 87 Strontium 87 4.6 billion 10 million to Whole metamorphic
4.6 billion or igneous rock
Potassium 40Argon 40 1.3 billion 100,000 to Glauconite
4.6 billion Muscovite
Biotite
Hornblende
Whole volcanic rock
30. 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 for samples less than 70,000
> Valid only for samples less than 70,000
years old.
years old.
> Living things take in both isotopes of
> Living things take in both isotopes of
carbon.
carbon.
> When the organism dies, the "clock" starts.
> When the organism dies, the "clock" starts.
Method can be validated by cross-checking with tree
rings
32. Recognizing Patterns of change
Walther's Law
• The vertical sequence is repeated by the horizontal
sequence
- walking from A to B to C to the Coast you would encounter the
rocks that would be encountered by drilling a core into the
earth at any point (A, B, or C)
33. Facies Diagram
• distribution of lithofacies (rock-types)
- these are associated with their respective EOD
• biofacies are similar but refer to fossils rather than
rock types
34. Eustasy, relative sea-level, and relative position
of lithofacies
• Eustasy= changes in volume of water in ocean
• lithofacies depend on
- sea-level
- land level
- geometry of coast
- sediment supply
Vail Curve
• an attempt at global
• correlation of
lithologies
- for better production
- of petroleum resources
35. Rock designations
• Rock units called Lithostratigraphic units
- described in terms of Group, Formation, & Member
> each term has specific meanings in geological parlance
• Formation
- a mappable lithostratigraphic unit
> has a location for identifying the type-section
> has a rock designation describing the lithology
- sometimes not all the same lithology
> in which case the term "Formation" takes the place of lithologic
type
• Groups are composed of several formations
• Members are distinctive units within a formation
- group is largest and contains formations and members
- formations are next and contain members
36. Fundamental lithological units
Formation- a rock layer with distinctive
characteristics that is mappable over a large are at
“typical” map scales
1:62,500 or more commonly 1:24,000
Formations have Members
smaller layers that are unique that are not mappable
over larger areas and won’t show up at typical map scales
Groups have formations; formations have members
