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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 EARTH'S SURFACE
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Horizontal Layers of the Grand Canyon
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Can you infer who is older?
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 Top = youngest layer
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 Earth’s 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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Radioactive isotope - an isotope that undergoes radioactive decay
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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Uranium - 238 Radioactive Isotope Radioactive Decay
radioactive isotopes give off radioactive particles until they become stable isotopes (new elements) Example: Uranium Radioactive Isotope Radioactive Decay Lead 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 it’s 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
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 200 1 2 3 4 5 3000 100 50 6000 25 9000 12.5 12,000 6.25 15,000
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MASS OF UNKOWN (G) DECAY TIME (YEARS)
NOW LET’S GRAPH THE HALF-LIFE OF THE UNKNOWN 200 100 MASS OF UNKOWN (G) 0 Half - Lives 1 Half - Life 2 H - L 3 4 3000 6000 9000 12000 15000 DECAY TIME (YEARS)
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Remaining Radioactive C14 (g)
Carbon half - life = 5,700 years sample size = 800 g Complete the following table. Half Life Remaining Radioactive C14 (g) Number of Years 800 1 2 3 4 5 400 5,700 200 11,400 100 17,100 50 22,800 25 28,500
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MASS OF C14 (G) DECAY TIME (YEARS)
NOW LET’S 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 DECAY TIME (YEARS)
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Absolute Time vs. Relative Time
Most geologists use relative time because it’s - 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
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 Index fossils must: 1. Be recognizable 2. Widespread
- 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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