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Uniformitarianism
Laws of nature DON’T change with time
Past events explained & estimated by modern processes
“The Present is the Key to the Past”
Uniformitarianism
sedimentary rocks form in horizontal layers parallel to Earth’s surface and will remain that way unless disturbed
Original HorizontalityOriginal Horizontality
CLAY
SILT
CLAY
SAND
SILT
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
He is
Can you infer who is older?
3 Ways to Determine Relative Time………?
Coming Right Up!!
In undisturbed sedimentary rock or lava flows:
Law of Superposition
Top = youngest layer
In undisturbed sedimentary rock or lava flows
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 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
Examples of Cross-Cutting Example of Cross-cutting
Extrusion - igneous rock that forms when lava flows on Earth’s surface and hardens
Example of Cross-cuttingExample of 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 FragmentsSediments 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 SuperpositionGraded Bedding - coarse, heavy particles - bottom layer
Exceptions to Law of SuperpositionRipple-Marks - tops of ripple marks point
Exceptions to Law of SuperpositionCross-Beds - curved at bottom layer, cut off at the top
Unconformity
Angular unconformity
Gaps or missing layers in the rock record due to erosion
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
Radioactive isotope- an isotope that undergoes radioactive
decay
- used to determine absolute age
Examples:
Uranium 238 and Carbon 14
Radioactivityrelease of high-energy particles from
unstable atoms, maybe cancer-causing
Radioactive Decayradioactive 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 : 0 3/4 1/2 : 1/2
1/4
7/8
1/8
15/16
1/16
31/32
1/32
Since the Half - Life of EverySince the Half - Life of Every Isotope is ….Isotope is ….
1. Unique1. Unique2. Always the same time2. Always the same time3. Unaffected by:3. Unaffected by:
Mass, Volume, Mass, Volume, Temperature & PressureTemperature & Pressure
We can use it to find:We can use it to find:
TTHHE E
AABBSSOOLLUUTTEE
AAGGEE
OOFF
RROOCCKKSS!!!!!!
Uranium 238Uranium 238 Lead 206 Lead 206
Half-Lives of Half-Lives of Special IsotopesSpecial Isotopes
4.5 x 104.5 x 109 9 yearsyears
(4,500,000,000 years)(4,500,000,000 years)
Carbon 14Carbon 14 Nitrogen Nitrogen 14 14 5.7 x 105.7 x 103 3 yearsyears
(5,700 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
1
2
3
4
5
25
50
12.5
6.25
6000
9000
12,000
15,000
100 3000
DECAY TIME (YEARS)
MA
SS
OF
UN
KO
WN
(G)
3000
6000
9000
12000
15000
NOW LET’S GRAPH THE HALF-LIFE OF THE UNKNOWN
0
1 Half - L
ife
2 H - L 3
4
200
100
0
0 Half - L
ives
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
100
200
50
25
11,400
17,100
22,800
28,500
400 5,700
DECAY TIME (YEARS)
MA
SS
OF
C14 (G
)
5,700
11,400
17,100
22,800
28,500
NOW LET’S GRAPH THE HALF-LIFE OF CARBON 14
00 H
alf - Lives
1 Half - L
ife 2
3
4
800
400
Absolute Time vs. Relative Time
Most geologists use relative time because it’s
- less expensive
- easier to do
Absolute Dating & Geologic History
QuickTime™ and aSorenson Video 3 decompressorare needed to see this picture.
Fossils
-the remains of plants and animals that lived in the past
- mainly formed in sedimentary rock
Fossils Form in Several WaysForm 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 recognizable2. Widespread3. 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 features3. 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)
Oldest
Youngest
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
DECAY TIME (YEARS)
MA
SS
OF
UN
KN
OW
N (G
)
DECAY TIME (YEARS)
MA
SS
OF
C14 (G
)
Isotopes - different forms of the same element - normal # of protons - more than normal # of neutrons
Normal Hydrogen
-1 proton
- 0 neutrons
- 1 electronIsotope of Hydrogen
- 1 proton
- 1 neutron
- 1 electron
Radioactive Isotope of Hydrogen
1 proton
2 neutrons
1 electron
Radioactive Hydrogen H-3
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