CHAPTER 5: THE ROCK AND FOSSIL RECORD
SECTION 1: (PG. 112)
- geology is the study of Earth’s history.
- James Hutton published “Theory of the Earth” which spoke of unifromitarianism. This means that the same geologic processes occuring today occurred long ago. “The present is the key to the past.”
- Catastrophism is all geologic change occurs suddenly. By catastrophes.
- Scientists believed catastrophism because they thought Earth was only thousands of years old. This was not enough time for uniformitarianism to occur.
- Charles Lyell published “Principles of Geology” (3 volumes) which reintroduced uniformitarianism. He challenged catastrophism successfully.
-Result: Most change is gradual and uniform, but catastrophes do occur.
SECTION 2: (PG. 115)
- “Relative Dating: Which came first?”
- Relative Dating is determining whether an object or event is older or younger than other objects or events.
- Imagine if you never cleaned out your locker. You kept throwing items in there until it was packed all the way to the top. The most recent items would be on top and the oldest on bottom. Just like rock layers! (Superposition)
- Superposition is younger rocks lie above older rocks in undisturbed sequences. “ Younger over older.”
- Some layers are disturbed by forces within the Earth. Older layers can be pushed above younger ones. The geologists look at the geologic column for help.
- Geologic column is the ideal sequence of rock layers that contains known fossils and rock formations on Earth arranged from oldest to youngest.
-Read page 117.
- Fault, intrusion, folding, tilting-disruptions in the rock layers
-Sometimes rock layers can be missing which creates gaps in rock layer sequences called uncomformities. (represents a missing part)-or missing time. Nondeposition and erosion can cause this.
- Most are formed by both nondeposition and erosion.
- 3 categories of uncomformities: discomformities, nonconformities, and angular unoconformities.
SECTION 3: ABSOLUTE DATING: A MEASURE OF TIME (PG. 120)
- Absolute dating is the process of establishing the age of an object, such as a fossil or rock layer, by determining the number of years it has existed.
WAYS TO DETERMINE ABSOLUTE DATING OF FOSSILS AND ROCKS:
1. Analyze radioactive isotopes (most often used )
- isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. Most are stable, but some are unstable, which means they change forms(radioactive).
- Radioactive Decay: process of radioactive isotopes breaking down into stable
isotopes of other elements.
- Scientists use the relative amounts of stable and unstable isotopes present to
determine the age.
- Radiometric Dating: determining the absolute age of a sample based on the
ratio of parent material to daughter material.
- parent material is the original isotope. The decayed parent materials becomes
daughter material. The amount of parent material left aids in the accurate aging
of the object because scientists know how quick or slow isotopes decay into
another.
- Half-life: the amount of time it takes for one-half of radioactive sample to decay.
- the older the rock the more daughter material there is.
Types of Radiometric Dating
-Uranium-Lead Method is used for rocks more than 10 million years old. Its half-life is 4.5 billion years. Uranium decays into lead.
- Potassium -Argon Method- Potassium has a half-life of 1.3 billion years. It decays to Argon and Calcium. Argon is the daughter material. This method is used to date rocks older than 100,000 years.
- Carbon-14 Method- Carbon is found in 3 forms:
1. Carbon-12
2. Carbon-13
3. Carbon-14
They combine with oxygen to form carbon dioxide used in photosynthesis. As long as plants are alive, a constant carbon-14 to carbon-12 is taken in. Once they die, no new carbon is taken in. Carbon-14 begins to decrease as the plant or animal decays. Half-life is 5,730 years. Used for the last 50,000 years.
SECTION 4: LOOKING AT FOSSILS (PG. 124)
- fossil: any naturally preserved evidence of life.
- preserved organisms are the easiest fossils to identify.
-Fossils in rocks- hard part of the organism’s body is buried under sediment and preserved.
- Mineral Replacement:
permineralization is the process which minerals fill in pore spaces of an organism’s
tissues.
petrification is the organism’s tissues are completely replaced by minerals.
-Fossils in Amber: organism gets stuck in sap and covered by more sap. It then hardens.
-Mummification: organisms die in dry places and they dry out so quickly that there is not time for their soft parts to decay.
-Frozen fossils: frozen specimens due to freezing.
-Fossils in Tar: natural tar pits.
OTHER FOSSILS
- Trace fossils are any naturally preserved evidence of an animal’s activity (tracks)
Tracks are filled with sediment and hardened
- Burrows are forms of trace fossils. An animals shelter can be filled with sediment.
-Coporlites (“dung stone” Greek word) preserved feces from animals.
-Molds an Casts
Mold is a cavity in the ground or rock where a plant or animal was buried.
Cast is an object created when sediment fills a mold and becomes rock.
FOSSILS TELL ABOUT THE PAST
-Changes in Environments: Iowa is said to have once been covered by a shallow sea.
- Changes in life: Superposition helps in this.
FOSSILS HELP THE DATING OF ROCKS
-Index fossils: fossils of organisms that lived during a relatively short, well-defined time span.
-Geologists can tell what part of the geologic column the fossil came from.
SECTION 5: TIME MARCHES ON (PG. 129)
- Grand Canyon shows lots of layers. Best place to look at Earth’s history.
- Geologic time scale: scale that divides Earth’s 4.9 billion year history into distinct intervals of time
Eons are the largest divisions of geologic time. They are broken down into eras.
Eras are the 2nd largest divisions and they are broken down into periods.
Periods are the 3rd largest divisions and they are broken down into epochs.
Epochs are the 4th largest divisions. The smallest.
PHANEROZOIC EON
Paleozoic Era: means “old life.” Lasted 540-248 million years ago. First era well represented by fossils. Beginning of this era there was no land organisms. The middle of this era plants developed. At the end of this era there were amphibians living on land and insects. This era ended with a mass extinction. 90% of the species perished.
Mesozoic Era: means “middle life.” This era lasted 248-65 million years ago. This era is known as the Age of Reptiles. Dinosaurs inhabited land and water. Later on birds and small mammals began to evolve. By the end 50% of species and dinosaurs became extinct.
Cenozoic Era: means “recent life.” This era began about 65 million years ago and continues to the present. We live in this era. This era is known as the Age of Mammals.
