chapter 3- the rock and fossil record. geology study of earth’s history
TRANSCRIPT
Chapter 3- The Rock and Fossil
Record
Geology
Study of
Earth’s history
• Many Geologists had different theories about our changing Earth
•2 Main Theories:
–Uniformitarianism
–Catastrophism
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
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
Charles Lyell
• 1830s
• Challenged principle of catastrophism
• No evidence of catastrophes, obvious evidence of uniformitarianism
• The 2 scientists who finally convinced the public of uniformitarianism were:
1. James Hutton
2. Charles Lyell
Uniformitarianism Wins!!
James Hutton Charles Lyell
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.
• 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
Paleontology
• Science of studying fossils to learn about past life on Earth
Relative Dating
•Finding an estimated age of objects on Earth by comparing it with rocks and fossils.
Superposition• Fossils/Rocks closer to Earth’s
surface will be younger than Fossils/Rocks found closer to Earth’s center.
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
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.
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.
• 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.
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
Missing Pieces of the Record
•Missing rock layers create gaps in rock layer sequences called unconformities.
3 Types of Unconformities:
1. Disconformity
2. Nonconformity
3. Angular unconformity
• Disconformity- Sequence of parallel rock is missing- hard to see but very common
• 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)
• 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.
Absolute Dating• A very accurate way of dating rocks and fossils.
-Geologists do this by using Isotopes and Radioactive Decay
Radioactive Decay
An unstable atom turns into a stable atom .
When this happens, electrons are released.
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.
• In other words….An element changes forms over time helping scientist accurately date things
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.
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)
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.
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?
Fossils•A fossil is any naturally preserved evidence of life. You can find fossils in:–Rocks–Amber–Ice–Tar
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
Fossilization
Formation of fossils
1. Mummification
• Found in dry places because most bacteria can not survive in these places
2. Preservation in Amber
• Amber is hardened tree sap
• Amber traps insects and preserves them
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
Found in LaBrea Tar Pits near Los Angeles
4 - Freezing
• Low temperatures protect and preserve organisms and keep out bacteria
5 - Petrification
• Mineral solutions replace organic materials
• 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)
Petrified Wood- It is all made of minerals now
6 - Imprints
• Made in soft mud or clay and preserved in sedimentary rock
7 – Molds and Casts
• Formed when sediments fill an imprint and then cement to form rocks with the reverse impression of the organism
8 - Coprolites
• Fossilized waste materials
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
Pleiosaur Gastroliths
Geologic Time Scale
•GTS-Divides Earth’s 4.6 billion year history into time intervals (4 Eons)
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
Divisions of Time
•Eons•Eras• Periods• Epochs
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
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
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
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
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
Quaternary Period
• 1.8 Ma to present
• Begins with last Ice Age
• 2 epochs:1. Pleistocene
2. Holocene
Holocene Epoch
• Began 11,500 years ago
• Modern humans
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!!!!!
