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 3 The 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 3 The 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.
The Law of Superposition • 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 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 Relationships Any rock layer that cuts across any other rock layer is youngerthan the one it cuts across. What can cut across a rock layer? Intrusions– magma can seep in between rocks and then harden to intrusive igneous rock Faults– cracks in rocks along which the rocks move F, A, and C are older than B E is older than B
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.
Disturbed 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.
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
6 What is the story here?
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 Fossils Index 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: • Disconformity • Nonconformity • Angular unconformity
1.) Disconformity- Sequence of parallel rock is missing! It is hard to see but very common. 3 Types of Unconformities
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
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
5 What is the story here?
Chapter 3 The 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 Life The 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
EXAMPLE The element we measured has a half life of 10,000 years. This rock is four half lives old because 15/16 of it has changed to daughter. The rock is 40,000 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 one half life old because half of it has changed to daughter. The rock is 10,000 years old. This rock is newly formed
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? 32 years 16 years 8 years 0 years 24 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? ______________ 4000 years 2000 years 8,000 years 0 years 6000 years 4 4 x 2000 = 8000 years
Example If 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.