Sedimentary rocks form in horizontal layers parallel to Earth

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Original Horizontality
Sedimentary rocks form in horizontal layers
parallel to Earths surface and will remain that
way unless disturbed
SILT
SAND
CLAY
SILT
CLAY
EARTH'S SURFACE
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Horizontal Layers of the Grand Canyon
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• 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
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3 Ways to Determine Relative Time? Coming
Right Up!!
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Law of Superposition
In undisturbed sedimentary rock or
lava flows
In undisturbed sedimentary rock or lava flows
Top youngest layer
Bottom oldest layer
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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
youngest layer
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Example of Cross-Cutting
Intrusion - igneous rock that forms when magma
squeezes between existing rock and hardens
Intrusion - igneous rock that forms when magma
squeezes between existing rock and hardens
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Example of Cross-cutting
Examples of Cross-Cutting
Extrusion - igneous rock that forms when lava
flows on Earths surface and hardens
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Example of Cross-cutting
Example of Cross-cutting
Inclusion - pieces of older rock trapped within
younger rock
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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.
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However, there are exceptions to these laws.
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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
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Unconformity
Gaps or missing layers in the rock record due to
erosion
Angular unconformity
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Picture of Unconformity
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• 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
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How do geologists measure Absolute Time?

• Radioactive Decay
• 2. Biological Clocks - Tree rings, coral growth
  rings, glacial lake layers of sediment

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Before you measure ABSOLUTE TIME you must
understand the following terms..
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ISOTOPES - alternate forms of an element -
some are RADIOACTIVE
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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
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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
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Amount of time it takes 1/2 the atoms in a given
sample to go through radioactive decay is called
its
Half - Life
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We can use it to find
THE ABSOLUTE AGE OF ROCKS!!!
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Unknown Radioactive Isotope
half - life 3000 years
sample size 200 g
Lets
complete the table below.
Half Life Remaining Unknown Radioactive Isotope (g) Number of Years
0 200 0
1
2
3
4
5
3000
100
50
6000
25
9000
12.5
12,000
6.25
15,000
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NOW LETS GRAPH THE HALF-LIFE OF THE UNKNOWN
200
















100
0 Half - Lives
MASS OF UNKOWN (G)
















1 Half - Life
2 H - L
3
4
0
3000
6000
9000
12000
15000
0
DECAY TIME (YEARS)
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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
2
3
4
5
400
5,700
200
11,400
100
17,100
50
22,800
25
28,500
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NOW LETS GRAPH THE HALF-LIFE OF CARBON 14
800
















400
0 Half - Lives
















MASS OF C14 (G)
1 Half - Life
2
3
4
5,700
11,400
17,100
22,800
28,500
0
DECAY TIME (YEARS)
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Absolute Time vs. Relative Time
Most geologists use relative time because its
- less expensive - easier to do
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Fossils

• the remains of plants and animals that lived in
  the past
• mainly formed in sedimentary rock

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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
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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
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Fossils form in Several Ways
5. Trace fossils - fossil evidence of animal
movement Ex. Trails, footprints, burrows
borings
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Evolution
- process of change that produces new life forms
over time
- fossils provide evidence of evolution
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Natural Selection

• theory of evolution
• best adapted organisms will survive in large
  numbers and pass on these adaptations to
  their offspring

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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
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