Chapter 3- The Rock and Fossil Record. Geology Study of Earth’s history.
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Transcript of Chapter 3- The Rock and Fossil Record. Geology Study of Earth’s history.
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Chapter 3- The Rock and Fossil
Record
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Geology
Study of
Earth’s history
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• Many Geologists had different theories about our changing Earth
•2 Main Theories:
–Uniformitarianism
–Catastrophism
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Uniformitarianism• Developed by:
James Hutton• 1788• Stated that the key to
understanding Earth’s history was all around us – the geologic processes that we observe today have always occurred.
• Ex: erosion, climate change
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Catastrophism
• Principle that states that all geologic change occurs suddenly and rarely
• Allows scientists to believe that the Earth isn’t as old as Hutton suggested
• Ex: asteroid impacts, earthquakes, volcanic eruptions
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Charles Lyell
• 1830s
• Challenged principle of catastrophism
• No evidence of catastrophes, obvious evidence of uniformitarianism
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• The 2 scientists who finally convinced the public of uniformitarianism were:
1. James Hutton
2. Charles Lyell
Uniformitarianism Wins!!
James Hutton Charles Lyell
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Extinction of Dinosaurs• Even though
Uniformitarianism is the main belief how earth changes, it does not rule out that some events in Earth’s history were caused by catastrophes.
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• Extinction of the Dinosaurs
–A big asteroid was thought to have struck the Earth with such force, causing dirt to completely block sun-light changing the climate to inhabitable for plant and animal life.
Example
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Paleontology
• Science of studying fossils to learn about past life on Earth
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Relative Dating
•Finding an estimated age of objects on Earth by comparing it with rocks and fossils.
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Superposition• Fossils/Rocks closer to Earth’s
surface will be younger than Fossils/Rocks found closer to Earth’s center.
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Disturbing Forces
• The law of superposition only holds true IF there have been no disturbing forces such as:– Earthquakes– Magma intrusions– Folding or tilting of rock layers
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The Geologic Column
• A collection of Fossil/Rock sequences from all over the world
• Geologists use the geologic column to compare to other rock sequences.
• This can help geologist determine the age and other information about a certain area.
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Disturbed Rock Layers
• Faults- A break in Earth’s crust, that force the crusts to slide opposite of each other.
• Intrusion- Molten rock (from metamorphic stage) pushes up between existing rock layers.
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• Folding- When Earth’s layers bend and buckle from internal forces such as tectonic plates
• Tilting- Internal forces slant rock layers instead of folding them.
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Law of Cross-Cutting Relationships
All of these disturbances are younger than the rock layers they affect…the rock layers had to have been there already for the change to take place
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Missing Pieces of the Record
•Missing rock layers create gaps in rock layer sequences called unconformities.
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3 Types of Unconformities:
1. Disconformity
2. Nonconformity
3. Angular unconformity
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• Disconformity- Sequence of parallel rock is missing- hard to see but very common
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• Nonconformity- Sedimentary rock layers lie on top of an eroded surface on non-layered igneous rock or metamorphic rock. (layers on top of non-layered rock)
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• Angular Unconformity- Exists between horizontal rock layers and eroded tilted/folded rock layers. The tilted or folded layers were eroded before horizontal layers formed above them.
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Absolute Dating• A very accurate way of dating rocks and fossils.
-Geologists do this by using Isotopes and Radioactive Decay
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Radioactive Decay
An unstable atom turns into a stable atom .
When this happens, electrons are released.
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Radioactive Decay
• Because radioactive decay occurs at a steady pace, scientists can use the relative amounts of stable and unstable atoms present in an object to determine the object’s age.
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• In other words….An element changes forms over time helping scientist accurately date things
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Radiometric Dating• Method of using radioactive
decay to determine how old a rock is.
• Ratio of parent to daughter material or unstable to stable isotopes found.
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Ex: Say it takes 10,000 yrs for parent material in a rock to decay….and you find equal
amounts of parent material and daughter material in the sample…
This means that ½ of the original material has decayed making the
rock 10,000 years old. (1/2 life)
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Half-life• If ¼ of your sample is parent material
and ¾ is daughter material and you know that it takes 10,000 years to decay ½ of the original sample, how old is the rock?
• 20,000 years old because 10,000 decayed twice is
10,000 x 2 = 20,000• 1 half life is 10,000….2 half-lives is
20,000…..3 half-lives is 30,000.
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Elements used in Radiometric Dating
• Uranium-238 decays to lead-206…1/2 life is 4.5 billion years
• Potassium 40 decays to Argon and Calcium ….1/2 life of 100,000 years
• Carbon-14 decays to carbon-12 with a ½ of 5,000 years
– ** In a rock carved village that was said to have been built 2,500-600 years ago, which type of radiometric dating would you use to investigate the burial mounds?
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Fossils•A fossil is any naturally preserved evidence of life. You can find fossils in:–Rocks–Amber–Ice–Tar
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Fossils can:– indicate changes in the environment
–give us a time frame for the life span of certain plants and animals (index fossils are used) ex. Phacops lived in shallow oceans YRS ago
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Fossilization
Formation of fossils
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1. Mummification
• Found in dry places because most bacteria can not survive in these places
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2. Preservation in Amber
• Amber is hardened tree sap
• Amber traps insects and preserves them
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3. Tar Seeps/Pits
• Thick petroleum oozes to Earth’s surface and traps animals
• You can see the fossils of ice age animals from 10,000 to 40,000 years old, such as a saber tooth Tiger
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Found in LaBrea Tar Pits near Los Angeles
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4 - Freezing
• Low temperatures protect and preserve organisms and keep out bacteria
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5 - Petrification
• Mineral solutions replace organic materials
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• Permineralization- when minerals fill in pore space of an organism’s tissues and is preserved
• Petrification- When the organism is completely replaced by minerals (petrified wood is actually stone)
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Petrified Wood- It is all made of minerals now
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6 - Imprints
• Made in soft mud or clay and preserved in sedimentary rock
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7 – Molds and Casts
• Formed when sediments fill an imprint and then cement to form rocks with the reverse impression of the organism
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8 - Coprolites
• Fossilized waste materials
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9 - Gastroliths
• Some animals have stones in their digestive system
• These stones help them break their food into smaller parts
• The stones become gastroliths when the organism is dead
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Pleiosaur Gastroliths
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Geologic Time Scale
•GTS-Divides Earth’s 4.6 billion year history into time intervals (4 Eons)
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What Determines a New Era?
The geologic column is divided into EONS, ERAS, PERIODS, PERIODS, and EPOCHS based on:
1. Major changes in Earth’s surface
2. Major changes in climate
3. Major changes in the type of organisms
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Divisions of Time
•Eons•Eras• Periods• Epochs
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Hadean Era = PreCambrian Time
• Begins with formation of Earth
• Ends about 542 Ma (‘mega annum = 1M years)
• 88% of Earth’s history
• Blue-green algae, marine worms, jellyfish, single celled organisms
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Paleozoic Era
542 Ma to 251 Ma Begins with dramatic increase in plant and
animal species Ends with Pangaea Divided into 7 periods
1. Cambrian2. Ordovician3. Silurian4. Devonian5. Mississippian Carboniferous6. Pennsylvanian Carboniferous7. Permian
Ends with 90% marine and 70% land extinction
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Mesozoic Era
251 Ma to 65.5 Ma Pangaea breaks up, Sierra Nevada and
Andes form “Age of Reptiles” 3 periods:
1. Triassic
2. Jurassic
3. Cretaceous
Ends with mass extinction – KT Boundary
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Cenozoic Era
• 65.5 Ma to present
• Continents move to present day positions
• Alps, Himalayas form
• “Age of Mammals”
• 2 periods:1. Tertiary
2. Quaternary
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Tertiary Period
• 65.5 Ma to 1.8 Ma
• Begins at KT Boundary
• Ends at last Ice Age
• 5 epochs:1. Paleocene
2. Eocene
3. Oligocene
4. Miocene
5. Pliocene
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Quaternary Period
• 1.8 Ma to present
• Begins with last Ice Age
• 2 epochs:1. Pleistocene
2. Holocene
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Holocene Epoch
• Began 11,500 years ago
• Modern humans
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Earth’s Clock
• If Earth was 12 hours old, 1 hour=383million years, 1 minute= 6.4 million years and 1 second = 106,000 years!!! In the scheme of things, a human life span would be less than an eye blink long!!!!!
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