do now: discuss with your table: where in this picture would you expect to find the oldest rocks?...
TRANSCRIPT
DO NOW: Discuss with your table: Where in this picture would you expect to find the OLDEST rocks? Where would you expect to find the NEWEST rocks? What kinds of events might change that?
Chapter 3The Rock and Fossil Record
Section 1: Earth’s Story and Those Who
Listened
The Rock and Fossil Record
•Geology- Study of Earth’s history
Paleontology• The science of studying fossils to learn
about past life on Earth.
How does Earth Change?
Most changes on the Earth are slow – erosion, deposition, pressure, melting.
Some changes are quick – volcanos, earthquakes, asteroid impacts,
Chapter 3The Rock and Fossil Record
Section 2: Relative Dating: Which Came
First?
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.
The Law of Superposition
Disturbing Forces• The law of superposition does not
always hold true!
• Sometimes there are disturbing forces that cause rocks to tilt, fold, or get flipped upside down.
• This can be due to:– Earthquakes– Magma intrusions– Folding or tilting of rock layers
Law of Crosscutting RelationshipsAny rock layer that cuts across any other rock layer
is younger than the one it cuts across.What can cut across a rock layer?
E is older than B
F, A, and C are older than B
Intrusions – magma can seep in between rocks and then harden to intrusive igneous rock
Faults – cracks in rocks along which the rocks move
Disturbed Rock Layers
1.) Fault- A break in Earth’s crust, that force the crusts to slide opposite of each other.
2.) Intrusion- Molten rock pushes up between existing rock layers.
3.) Folding- When Earth’s layers bend and buckle from internal forces such as tectonic plates
4.) Tilting- Internal forces slant rock layers instead of folding them.
Disturbed Rock Layers
•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.
Disturbed Rock Layers
The Geologic Column• A collection of undisturbed fossil/rock
layers from all over the world with oldest rocks at the bottom.
• An “IDEAL” picture of what rocks would be present if there had been no disturbing forces.
• Geologists use it to compare to other rock sequences and find out what’s missing.
Index FossilsIndex fossils are fossils of organisms
that lived for a short period of time all over the world.
If you find an index fossil in a piece of rock, you know how old that rock is.
Common Index Fossils
Trilobites are fossils that are found in the MAUV LIMESTONE layer of the Grand Canyon. Trilobites are known to have only existed on Earth 543 to 505 million years ago, so the Mauv Limestone must be 543-505 million years old!
This layer of Bryce Canyon is called the Winsor Layer. It contains fossils of cephalopods which were only on Earth from 199 to 145 million years ago. So, the Winsor Layer is 199-145 million years old.
Missing Pieces of the Record
•Missing rock layers create gaps in rock layer sequences called unconformities.
•Unconformity - a break in the geologic record created when rock layers are eroded or when sediment is not deposited for a long period of time.
3 Types of Unconformities:
1. Disconformity
2. Nonconformity
3. Angular unconformity
1.) Disconformity- Sequence of parallel rock is missing! It is hard to see but very common.
3 Types of Unconformities
Disconformity
2.) Nonconformity - Sedimentary rock layers lie on top of an eroded surface of a non-layered igneous or metamorphic rock.
• Layers are on top of non-layered rock
3 Types of Unconformities
Nonconformity
3.) Angular Unconformity - exists between horizontal rock layers and eroded tilted or folded rock layers.
–The tilted or folded layers were eroded before horizontal layers formed above them.
3 Types of Unconformities
Angular Unconformity
Angular Unconformity
Chapter 3The Rock and Fossil Record
Section 3: Absolute Dating: A Measure of
Time
Absolute Dating•Absolute Dating -A very accurate way of dating and measuring the age of rocks and fossils.
–Geologists do this by using Isotopes and Radioactive Decay
Isotopes
• Isotopes are unstable forms of elements.
• They change, at a predictable pace, into stable forms of the elements.
• Some isotopes change quickly and some change slowly.
• When the isotope changes, it’s called radioactive decay.
Radioactive Decay
• An unstable atom turns into a stable atom.
• Unstable = parent
• Stable = daughter
Radioactive Decay
•Because radioactive decay occurs at a steady pace, scientists can use the relative amounts of stable daughter and unstable parent atoms present in an object to determine the object’s age.
Radiometric Dating
•Using radioactive decay to determine how old a rock is
•Scientists determine a ratio of the unstable isotope is present compared to how much of the stable isotope is present.
•In other words….–An element changes forms over time helping scientist accurately date things
Half LifeThe amount of time it takes
for one half of the parent isotope to turn into daughter
isotope
Newly formed rock = 100% parent
After 1 half life = 50% parent
EXAMPLEThe element we measured has
a half life of 10,000 years.
This rock is newly formed
This rock is one half life old because half of it has changed to daughter. The rock is 10,000 years old.
This rock is two half lives old because ¾ of it has changed to daughter. The rock is 20,000 years old.
This rock is three half lives old because 7/8 of it has changed to daughter. The rock is 30,000 years old.
This rock is four half lives old because 15/16 of it has changed to daughter. The rock is 40,000 years old.
ExampleThe half life of the element we
measured is 8 years.
• If ¼ of your sample is parent material then ______ is daughter material.
• If ¼ of the your sample is parent material, how many half lives has it been through?
0 years 8 years 16 years 24 years 32 years
3/4
2
ExampleThe half life of the element we
measured is 2000 years.
• If 1/16 of your sample is parent material then how many half lives has it been through? ___________
• How old is it? ______________
0 years 2000 years 4000 years 6000 years 8,000 years
4
4 x 2000 = 8000 years
ExampleIf the mineral you’re studying has a half life of 12,000 years, identify the fraction of parent and daughter isotopes and the ages of each of these rock samples.
Examples of 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 – 1/2 life of 5,000 years
Chapter 3The Rock and Fossil Record
Section 4: Looking At Fossils
Fossils• Any naturally preserved evidence of
life. • Fossils can indicated changes in the
environment and can give us a time frame for the life span of certain plants and animals
FossilizationFormation of fossils
1. Mummification
• In dry areas organisms can die and be preserved because of low humidity and 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/Tar 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 bacteria out
5.) Petrification
• Minerals replace an organism’s tissues.• Petrified wood is actually stone and
fossilized.
Petrified Wood- It is all made of minerals now
6.) Imprints
•Made in soft mud or clay and preserved in sedimentary rock.
7.) Casts
• Formed when sediments fill an imprint (mold) and then cement to form rocks with the reverse impression of the organism
8.) Coprolites
•Fossilized waste materials
9.) Gastroliths
• Fossilized stones from inside an organism’s digestive system to help break food into smaller parts.
–The stones become gastroliths when the organism is dead
Pleiosaur Gastroliths
Chapter 3The Rock and Fossil Record
Section 5 – Time Marches On
Geologic Time Scale
•Divides Earth’s 4.6 billion year history into time intervals (4 Eons)
Divisions of Time•Eons Eras Periods Epochs
BiggestSmallest
What Determines a New Era?
• The geologic column is divided into EONS, ERAS, PERIODS, & EPOCHS based on major changes in:1. Earth’s surface2. Climate3. Type of organisms
The Phanerozoic Eon is all of the time that Earth has been here – 4.6B years.
It’s divided into:1. Paleozoic Era – 251M-542M years ago2. Mesozoic Era – 65M-251M years ago3. Cenozoic Era – 65Myears ago to today
Paleozoic Mesozoic Cenozoic
542MYA 251MYA 65MYA TODAY
Paleozoic Era• 542 to 251 million years ago
• Begins - with dramatic increase in plant and animal species.
• Ends - with landform called Pangaea and mass extinction of 90% marine species and 70% land species.
Pangea
Mesozoic Era
•251 to 65.5 million years ago
•Known as “Age of Reptiles”– Ex. Dinosaurs
•Pangaea breaks up
Cenozoic Era
•65.5 million years ago to present
•“Age of Mammals”•Continents move to present day positions.
