Geologic Dating - Red Hook Central School District
Geologic
Time
GEOLOGIC
TIME
FOSSILS CAN ONLY BE
FOUND IN sedimentary
ROCKS.
WHY IS THIS STATEMENT FALSE: “THE
CAVEMAN HAD
DINOSAUR FOR BREAKFAST.”?
Man and Dinosaur never lived at the same time
FAULTS ARE ALWAYS
(OLDER, YOUNGER)
THAN THE ROCKS THEY CUT THROUGH.
Which is older:
F or S
How do you know?
IF A GEOLOGIST FINDS AN IGNEOUS LAYER,
HOW CAN SHE DETERMINE IF THE LAYER IS
AN INTRUSION OR AN EXTRUSION?
INTRUSIONS
contact metamorphism on all sides
CONTACT METAMORPHISM
Contact metamorphism: occurs at the border where magma touches solid rock- at this boundary, the rock is turned into metamorphic rock through contact with the heat of the magma.
Use the ESRT to determine the type of sedimentary rock, then the metamorphic rock table to determine what the sedimentary rock turns in to at this contact boundary
EXTRUSIONS
no contact metamorphism on top
Is “H” an intrusion or extrusion?
How can you tell?
H is an intrusion.
Contact metamorphism on top
What is an unconformity?
a buried erosional surface
How does it complicate the relative dating of rock layers?
a part of the rock record is missing
A RECORD OF UPLIFT, EROSION,
AND DEPOSITION
What processes could lead to an unconformity?
weathering and erosion
Using the diagram to the left, identify where the unconformity is located by drawing an arrow and writing the word “unconformity” next to it.
Evolution
Explain the theory of evolution.
organisms adapt to their environment in order to survive
Evolution
Explain how the fossil record supports this theory can see that different organisms have changed over time
ABSOLUTE DATING WITH RADIOACTIVE ISOTOPES
Half-Life
A half-life is the amount of time necessary for one-half of a radioactive, unstable substance to become not radioactive, stable.
Half life depends on the element that is unstable- some radioactive isotopes stabilize quickly, some take much longer
RADIOACTIVE ISOTOPES FREQUENTLY
USED IN RADIOMETRIC DATING
EXAMPLE
RADIOACTIVE ISOTOPE = URANIUM,
STABLE ISOTOPE= LEAD
(IN OTHER WORDS, URANIUM TURNS INTO LEAD)
HALF-LIFE TIPS
Radioactive Isotope
Carbon-14
Potassium
Uranium-238
Rubidium-87
Circumstance to Use
Anything organic or of relatively recent origin- Use if a fossil of any kind is mentioned in a question!
For rocks more than 70,000 years in age
Relatively middle-aged rocks
A rock of ancient origin
DATING WITH RADIOACTIVITY
Dating with Carbon-14
When an organism dies, the amount of carbon-14 it contains decreases at a predictable rate
By comparing the ratio of carbon-14 to
Nitrogen-14 in a sample, absolute age can be determined.
What can be done to change the half-life of a radioactive isotope?
Why is this important?
NOTHING!
it is reliable to calculate age
If there is a 100g sample of C 14 , how many grams of
C 14 would remain after three half-lives?
How long would this take? Show all work.
100g 50g 25g
12.5g
3 half-lives x (5.7 x 10 3 ) =1.71 x 10 4
= 17,100 years
