GEO Ch 18
The principle of LATERAL continuity allows geologists to
Correlate rocks that are physically separated by large distances
How old is the dinosaur fossil bone?
D is older than C, B, and A D is younger than E
What type of unconformity is S?
Disconformity (parallel to each other but significantly different in age)
Which layer is oldest?
E
List from oldest to youngest
E P G L C H O M D J A S N K B R F
List rocks in chronological order, don't forget dike (F)
F A B C D E
Determine chronological order
F P G C B R H A M O E X D K Z J N S
So now that we know that E is the oldest layer, which layer is the next layer that was deposited after E?
G
Rhyolite #1 has 50% of Parent Isotope F remaining. How many half-lives have occurred?
1 half-life
The age of the sandstone is between _____ and _______
1.4 billion and 570 million years old
Another pair: Half-life = 25 million years 6.25% of parent remaining. How old is the felsic ash? (From table in handout) 6.25% = 4 half-lives passed 4 X 25 million = 100 million years of age
100 million years of age
If Parent isotope F has 1 half life = 100 million years, then determine the age of Rhyolite #1. # of half lives x # of years in ONE half life = age 1 X 100 million years = 100 million years
100 million years of age
A different parent-daughter pair has half-life of 50 million years. There is 25% of this parent remaining. How old is the felsic ash? 25% of parent remaining means that 2 half-lives have passed. 2 half-lives X 50 million years = 100 million years of age.
100 million years old (age of this ash layer)
Explain how you can use you answers to questions 4 + 5 to determine an age range for the dinosaur fossil bone?
1st- you would date layers C + E using either the K-Ar system or the U-Pb system 2nd- you would be able to place the age of the dinosaur bone between the ages of the two volcanic rocks. This is called "bracketing" and it gives us an age range for fossils in sedimentary rocks The age of the fossil will be between the ages of C + E
Rhyolite #2 has 75% daughter G and 25% Parent H. How many half-lives has the rock gone through?
2 half-lives
Astronomers think Earth formed at same time as all other rocky materials, including oldest meteorites. The oldest meteorites contain nearly equal amount both 238U and 206 Pb. What age is the Earth?
238U = parent isotope Half-life = 4.5 billion years If there are nearly equal amounts of parent and daughter isotope, we can assume there is 50% of the parent isotope remaining. If there is 50% of parent isotope remaining, then 1 half-life has passed = 4.5 billion years. If the Earth is as old as oldest meteorites, and the oldest meteorites contain nearly equal amounts of 238U and 206Pb, then the age of the Earth is approximately 4.5 billion years old.
What is the absolute age of the lava flow based on its zircon crystals? (Previous question - figured out that 1/2 of a half-life passed) The half-life = 713 million years 713/2 = 356.5 million years
356.5 million years
Half-life of Parent H/daughter G (rhyolite 2) is 200 million years. Determine age of rhyolite 2 2 X 200 million years = 400 million years of age
400 million years of age
Bottom layer is granite = crystalline rock Grey layer on top = shale = sedimentary rock What is the unconformity
Nonconformity
Layer E is granite Layer C is sedimentary rock What type of unconformity is P?
Nonconformity
What system if any can be used to determine the absolute ages of dinosaur bones that have become fossils?
None of these systems can be used. Fossil bones are no longer made of carbon. Even if they were still made of carbon, they would be too old to date with the Carbon-14 system. The other two systems can only be used on volcanic rocks, not fossil bones.
What type of unconformity is shown in this figure?
Angular Unconformity
If layers C and E were volcanic rocks, which of the three isotopic systems mentioned earlier would you be able to use to determine the age of the volcanic rock layers?
Both Potassium-Argon and Uranium-Lead
Match the description - Nonconformity
Boundary between an unlayered crystalline rock and an overlying sequence of sedimentary rock
Match the description - Disconformity
Boundary between layers of sedimentary rock that are PARALLEL to each other, but are significantly different in age
Match the description - Angular Unconformity
Boundary between sedimentary rock layers that have been TILTED and ERODED and layer HORIZONTAL layers deposited on TOP.
71% were 235U and 29% were 207Pb About how many half-lives of the 235U to 207Pb system have elapsed in the zircon crystals?
Parent isotope = 235U 1/2 of a half-life has passed
Which of the Principles of Relative Dating would you use to determine if the dike (labeled F) was younger or older than the rock layer C?
Principle of - Cross Cutting Relationships
Which Principle of Relative Dating would you use to determine whether rock layer A is younger or older than rock layer B?
Principle of - Superposition
Is the felsic ash layer the same as rhyolite #1 or rhyolite #2?
Rhyolite #1
Two students are debating the age of the dinosaur bone. Which one is correct?
Student 2: The bone is between 80-90 million years old. You can't be more specific because you can't determine how fast the layers in this stratigraphic section were deposited.
Three students are debating how to date dinosaur bones. Which student is correct?
Student 3: Carbon-14 can't date things as old as dinosaurs and potassium-argon and uranium-lead only work for volcanic rocks, so none of these works.
If volcanic rock layer E is 90 Myrs old, and volcanic rock layer C is 80 Myrs old, what is the approximate age of the dinosaur bone?
The approximate age of the dinosaur bone is between 80-90 Myrs old.
How did you know the answer above was angular unconformity?
The layers beneath the angular unconformity are tilted and the layers above are horizontal.
What type of unconformity is Z?
angular
Are rhyolite #1 and #2 the same age?
no
