Uniformitarianism
“The Present is the Key to the Past”
Laws of nature DON’T change with time
Past events explained & estimated by modern processes
Uniformitarianism
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Original Horizontality
sedimentary rocks form in horizontal layers
parallel to Earth’s surface and will remain that
way unless disturbed
SILT
SAND
CLAY
SILT
CLAY
Horizontal Layers of the Grand Canyon
• Relative Time – determination of the age of
a rock or geologic event by comparing with
other rocks/geologic events
Can you infer who is older?
He is
3 Ways to Determine
Relative Time………?
Coming Right Up!!
Law of Superposition
In undisturbedsedimentary
sedimentary rock
or lava
flows flows:
In undisturbed
rock
or lava
Top = youngest layer
Bottom = oldest layer
Law of Cross-Cutting
Relationships
Any feature that cuts across a rock or layer of
sediment is younger than the rock or layer it cuts
Example of CrossCutting
Intrusion
Intrusion- igneous
- igneousrock
rockthat
thatforms
formswhen
whenmagma
magma
squeezes
squeezesbetween
betweenexisting
existingrock
rockand
andhardens
hardens
Examples of Cross-Cutting
Example of Cross-cutting
Extrusion - igneous rock that forms when lava
flows on Earth’s surface and hardens
Example
Exampleof
ofCross-cutting
Cross-cutting
Inclusion - pieces of older rock trapped within
younger rock
Examples of Cross-Cutting
Faults, joints, tilts, and veins also follow the
law of cross--cutting relationships
Law of Included Fragments
Sediments are older than the cement
and rock they are part of
The pebble is
older than the
conglomerate rock
it is found in.
However, there are
exceptions to these
laws…….
Exceptions to Law of
Superposition
Rock layers can be overturned, older layers
pushed on top of younger layers
THEREFORE, geologists use these clues to
find the original position of rock layers…
Exceptions to Law of
Superposition
Graded Bedding - coarse, heavy particles - bottom la
Exceptions to Law of
Superposition
Ripple-Marks - tops of ripple marks point
Exceptions to Law of
Superposition
Cross-Beds - curved at bottom layer, cut off at the
Unconformity
Gaps or missing layers in the rock record due to ero
Angular
unconformity
Formation of an Unconformity
Formation of an Unconformity
Formation of an Unconformity
Formation of an Unconformity
Formation of an Unconformity
Picture of Unconformity
• Absolute Time – finding an exact date for
rocks or geologic events in YBP (years before
present)
Born 2004 or
1 YBP
Born 1922 or
83 YBP
How do geologists measure
Absolute Time?
1. Radioactive Decay
2. Biological Clocks - Tree rings,
coral growth rings, glacial lake
layers of sediment
Before you measure
ABSOLUTE TIME you
must understand the
following terms…..
ISOTOPES
- alternate forms of an
element
- some are RADIOACTIVE
Radioactivity
release of high-energy particles from unstable
atoms,
maybe cancer-causing
Radioactive isotope
- an isotope that undergoes radioactive decay
- used to determine absolute age
Examples:
Uranium 238 and Carbon 14
Radioactive Decay
radioactive isotopes give off radioactive particles
until they become stable isotopes (new elements)
Example:
Uranium - 238
Radioactive Isotope
Radioactive Decay
Lead-206
Stable Isotope
Amount of time it takes 1/2 the atoms in a given
sample to go through radioactive decay is called it’s
Half - Life
Let’s look at the half - life of
Uranium 238
U238 - Radioactive
Pb206 - Stable decay product
1/4
1:0
1/8
1/2 : 1/2
1/32
???
1/16
7/8
3/4
15/16
31/32
Since the Half - Life of Every
Isotope is ….
1. Unique
2. Always the same time
3. Unaffected by:
Mass, Volume,
Temperature & Pressure
We can use it to find:
THE
ABSOLUTE
AGE
OF
ROCKS!!!
Half-Lives of Special
Isotopes
Uranium 238
Lead 206
4.5 x 109 years
(4,500,000,000 years)
Carbon 14
Nitrogen 14
5.7 x 103 years
(5,700 years)
Unknown Radioactive Isotope
half - life = 3000 years
sample size = 200 g
Let’s complete the table below.
Half Life
Remaining Unknown
Radioactive Isotope (g)
Number of Years
0
200
0
100
3000
50
6000
25
9000
4
12.5
12,000
5
6.25
15,000
1
2
3
NOW LET’S GRAPH THE HALF-LIFE OF THE UNKNOW
200
3
2 H-L
1 Half - Life
0 Half - Lives
4
MASS OF UNKOWN (G)
100
0
DECAY TIME (YEARS)
Carbon14
half - life = 5,700 years
sample size = 800 g
Complete the following table.
Half Life
Remaining
Radioactive C14 (g)
Number of Years
0
800
0
1
400
5,700
2
200
11,400
100
17,100
4
50
22,800
5
25
28,500
3
NOW LET’S GRAPH THE HALF-LIFE OF CARBON 14
800
2
1 Half - Life
0 Half - Lives
MASS OF C14 (G)
400
3
4
DECAY TIME (YEARS)
Absolute Time vs. Relative Time
Most geologists use relative time because it’s
- less expensive
- easier to do
Absolute Dating & Geologic
History
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Fossil
s
-the remains of plants and animals that
lived in the past
- mainly formed in sedimentary rock
Fossils Form in Several
Ways
Form in several ways
1. Original remains - unchanged
remains
of a plant or animal
Ex. Dinosaur bones/teeth
2. Replaced remains - soft parts of
original animal replaced by
minerals
Ex. Petrified wood
Fossils Form in Several
Ways
3. Mold - fossilized shell or bone
dissolves
and leaves a hollow
depression in a rock
Ex. Ferns, leaves, or fish
4. Cast - new mineral material fills a
mold
Ex. Shellfish
Fossils form in Several
Ways
5. Trace fossils - fossil evidence of
animal
movement
Ex. Trails, footprints, burrows &
borings
Evolution
- process of change that produces new life
forms over time
- fossils provide evidence of evolution
Natural
Selection
-theory of evolution
-best adapted organisms will survive in
large
numbers and pass on these
adaptations to their offspring
Index Fossils
- special fossils that give the relative age of
the rocks that contain them
Index fossils must:
1. Be recognizable
2. Widespread
3. Exist for a short time
Key Bed
- single rock layer that is recognizable,
widespread, and exists for a short time
Correlation
- matching rock layers from one area to
another
This can be done in several ways:
1. “Walking the outcrop”
2. Matching similar rock features
3. Index fossils
Correlating or matching index fossils from
three outcrops.
Correlate the 3 rock sequences to make
a single rock column for the geologic
history of the region. (oldest on bottom)
Youngest
Oldest
No place on the earth has all the rocks formed
through time. So geologists use correlation to
make a single rock column that shows the entire
history of the earth.
• Correlation means “to match”
• Geologists match all of the following:
–
–
–
–
rock types
rock sequences
ash layers from volcanic eruptions
index fossils
Page 8 & 9 of the Reference Tables.
Each black
vertical bar
represent the
existence of a
group of
organisms.
Extinction
First
appearance
MASS OF UNKNOWN (G)
DECAY TIME (YEARS)
MASS OF C14 (G)
DECAY TIME (YEARS)
Isotopes - different forms of the same element
- normal # of protons
- more than normal # of neutrons
Normal Hydrogen
-1 proton
- 0 neutrons
- 1 electron
Isotope of Hydrogen
- 1 proton
- 1 neutron
- 1 electron
Radioactive Isotope of Hydrogen
1 proton
2 neutrons
1 electron
Radioactive Hydrogen
H-3